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Recently, dual-ion cells based on the anion intercalation into a graphite positive electrode have been proposed as electrochemical energy storage devices. For this technology, in particular electrolytes which display a high stability vs. oxidation are required due to the very high operation potentials of the cathode, which may exceed 5 V vs. Li/Li+. In this work, we present highly promising results for the use of graphite as both the anode and cathode material in a so-called “dual-graphite” or “dual-carbon” cell. A major goal for this system is to find suitable electrolyte mixtures which exhibit not only a high oxidative stability at the cathode but also form a stable solid electrolyte interphase (SEI) at the graphite anode. As an electrolyte system, the ionic liquid-based electrolyte mixture Pyr14TFSI-LiTFSI is used in combination with the SEI-forming additive ethylene sulfite (ES) which allows stable and highly reversible Li+ ion and TFSI− anion intercalation/de-intercalation into/from the graphite anode and cathode, respectively. By addition of ES, also the discharge capacity for the anion intercalation can be remarkably increased from 50 mA h g−1 to 97 mA h g−1. X-ray diffraction studies of the anion intercalation into graphite are conducted in order to understand the influence of the electrolyte additive on the graphite structure and on the cell performance.

The high-voltage spinel cathode material LiNi0.5Mn1.5O4 (LNMO) with its high energy per mass and volume is widely considered as promising alternative to the cobalt-containing state-of-the-art layered cathode materials. LNMO is usually charged/discharged at high voltage to make use of Ni-redox at the 4.7 V plateau, while after insertion of additional lithium the utilization of excess Li at the low voltage region of LNMO (

The economical storage of electric energy at a large scale is one of the main technological challenges of this century. The state-of-the-art lithium ion batteries (LIBs) has several advantages as, e.g., high energy efficiency and safety. However, to improve the energy storage capabilities, the scientific community is intensively exploring various alternative rechargeable battery concepts as, e.g., lithium metal-based all-solid-state batteries (ASSBs), lithium/sulfur batteries (LSBs) or magnesium/sulfur batteries (MSBs) that could outperform LIBs in different aspects [1, 2]. This goes often along with the promise of a very high theoretical energy per mass or volume. The theoretical values, however, exclude numerous parameters relevant for the practical battery cell and, thus, might translate very differently into practically achievable energy values [3]. The discharge voltages achieved in practice and especially the required amount of inactive materials on stack level can vary substantially between the different cell chemistries. Consequently, the theoretical energy values, which are commonly quoted for emerging battery chemistries (mostly at the material level), could highly overestimate the realistic potential of these systems in comparison to established LIBs [3]. In this study, we calculated the energy for several battery technologies on stack and cell level, including LIBs, ASSBs, LSBs and MSBs to evaluate their practical specific energy (Wh/kg) and energy density (Wh/L) [3]. In order to provide a high comparability, the energy values are calculated in six successive steps, each adding further weight and volume of inactive material, like different amounts of electrolyte, inactive materials in the electrodes, separator, and an 18650 cell housing [3]. With a special emphasis on the amount of electrolyte and the volume expansion during charge and discharge, this study illustrates the impact these cell components have on the practical energy values in the individual cell chemistries. Thereby, the most critical cell components for achieving higher energy values are identified for each battery system [3]. By providing insights how to calculate the energy values of the different battery technologies with different necessary assumptions made evident, this study aims for more transparency and reliability in the comparison of the different cell chemistries. [1] J.W. Choi, D. Aurbach, Nat. Rev. Mater., 1 (2016) 16013. [2] T. Placke, R. Kloepsch, S. Dühnen, M. Winter, J. Solid State Electr., 21 (2017) 1939-1964. [3] J. Betz, G. Bieker, P. Meister, T. Placke, M. Winter, R. Schmuch, Adv. Energy Mater., (2018) 1803170.

The improvement in energy density of lithium ion batteries (LIBs) is primarily focused on the increase of the specific capacity of active materials and the cell voltage.[1] Transition metal (TM) oxide-based cathodes materials reach a high specific capacity through the optimization of the chemical composition with a higher Ni content and can be cycled up to 4.6 V.[2] However, the electrochemical performance of Ni-rich cathode materials depends strongly on their physical properties such as particle size, specific surface area or surface characteristics. The poor rate capability and capacity retention as well as the high first cycle irreversible capacity loss (ICL) and the low volumetric energy density of electrodes have to be improved by specific design of the particle shape. A spherical morphology with a narrow size distribution is the preferred particle shape and can only be synthesized during the co-precipitation of TM-hydroxides or -carbonates by a continuously stirred tank reactor (CSTR). Additionally, the CSTR-process allows a further improvement of the cathode materials during the synthesis of core-shell and full concentration-gradient particles.[2] In this work, the design and the operating principle of a novel Couette-Taylor-Flow-Reactor (CTFR) is evaluated regarding the synthesis of spherical shaped Ni-rich TM-precursors. The CTFR offers a continuous process and is constructed by two co‐axial arranged cylinders with a small gap and a low working volume. The rotational motion of the inner cylinder induces a turbulent Taylor‐vortex flow. Above the suitable Taylor number (Ta), a periodic and stable turbulent fluid motion is formed, which increases the fluid shear and the local concentration of educts and promotes the agglomeration of precipitates to uniform spherical particles with narrow particle size distribution.[3] Based on the co-precipitation of Ni-rich TM-precursors by a continuous CTFR, the influence of the reaction conditions on the properties of TM-precursor is investigated. The rotational speed, residence time, temperature and pH-value were varied in a broad range. The growth behavior of the particles, regarding the morphology, particle size distribution, tap density and chemical composition of TM-precursors as well as for lithiated cathode materials are investigated. The electrochemical properties of the corresponding cathode materials with their unique particle morphology in dependence of the co-precipitation conditions are comprehensively evaluated for finding the optimal synthesis conditions. [1] J. Betz, G. Bieker, P. Meister, T. Placke, M. Winter, R. Schmuch, Adv. Energy Mater. 2018, 1803170/1-18. [2] H. Li, P. Zhou, F. Liu, H. Li F. Cheng, J. Chen, Chem. Sci, DOI: 10.1039/c8sc03385d. [3] J.-M. Kim, S.-M. Chang, J.-H. Chang, W.-S- Kim, Colloids and Surfaces A: Physicochem. Eng. Aspects (2011) 384, 31-39.

The broad commercial application of high capacity silicon (Si)-based anodes for lithium ion batteries (LIBs) is still hindered by their poor cycling stability caused by large volume changes upon lithiation/delithiation and by the associated continuous electrolyte decomposition and active lithium loss. One suitable strategy to improve the electrochemical performance is to develop improved electrolyte formulations resulting in an efficient solid electrolyte interphase (SEI) formation. A recent study [1] has shown that the addition of 2 wt-% of 3-methyl-1,4,2-dioxazol-5-one (MDO) to a propylene carbonate (PC)-based electrolyte can significantly improve the cycling stability of lithium nickel manganese cobalt oxide (NMC532)/graphite full cells. Since electrolytes, based only on PC, do not lead to the formation of a stable SEI at the anode, the addition of MDO can be linked to an increased SEI stability, which is necessary for a high cycling stability. In this study, we develop a systematic approach to investigate the electrochemical performance of MDO and its various side chain modified derivatives as additives for carbonate-based electrolytes in LIB cells with silicon-carbon (Si-C) composite anodes. Our results show that the addition of MDO improves the cycling stability of Si-C/NMC622 cells in comparison to the state-of-the-art additives, such as vinylene carbonate (VC) and fluoroethylene carbonate (FEC). The systematic evaluation of the electrochemical performance of MDO and its analogues, such as 3-phenyl-1,4,2-dioxazol-5-one (PDO), was done using a cycling protocol adapted from Tornheim et al [2]. This method enables the concurrent determination of the electrode capacity retention and area specific impedance (ASI). Post mortem analysis is performed in conjunction with computational studies to understand the decomposition mechanism. [1] S. Röser, A. Lerchen, L. Ibing, X. Cao, J. Kasnatscheew, F. Glorius, M. Winter, R. Wagner, Chem. Mater. 2017, 29, 7733-7739. [2] A. Tornheim, C. Peebles, J. A. Gilbert, R. Sahore, J. C. Garcia, J. Bareño, D. P. Abraham, J. Power Sources 2017, 365, 201-209.

Lithium ion batteries (LIBs) are the dominating energy storage devices in the field of portable consumer electronics. They are also considered to be the most promising technology for the application in electric vehicles due to their high energy density. Nonetheless, their power and energy density need to be further improved to meet the challenging requirements for automotive applications, such as extended driving ranges and fast charging capabilities. [1,2] The partial substitution of the commonly used anode material graphite with silicon (Si) depicts one possible approach to increase the energy density significantly, since Si offers a higher specific capacity compared to graphite. However, an insufficient capacity retention of Si-based anodes during cycling represents a major challenge, regarding the commercial application. The poor cycling performance is caused by enormous volume changes, accompanying the lithiation/de-lithiation process of Si, resulting in the deterioration of the electrode due to pulverization of Si particles and contact loss between the active material and the current collector. Additionally, the consumption of active lithium and electrolyte during the formation of a new passivating solid electrolyte interphase (SEI) in every cycle contributes to a strong capacity decay. [3] The performance of Si-based anodes can be improved by embedding Si in different matrices, e.g. graphite or amorphous carbon. Furthermore, the alloying of Si with an inactive metal, such as iron (Fe) can significantly increase the performance by forming electrochemical inactive metal silicides that can stabilize the whole structure and suppress volume changes. [4, 5] In this contribution, we present the synthesis of Fe/Si-alloys via a simple high energy ball milling process, using elemental Fe and Si as the precursor materials. The influence of different Fe:Si ratios, the addition of carbon, as well as the influence of different heat-treatment conditions on the structure and electrochemical performance are investigated. Therefore, the synthesized active/inactive Si/FexSiy-composites are analyzed by X-ray diffraction and scanning electron microscopy equipped with energy dispersive X-ray spectroscopy in order to identify the formed intermetallic phases, their morphology and elemental distribution within the material. Nitrogen adsorption experiments are carried out to determine the specific surface area of Si/FexSiy-composites. To evaluate the suitability of the composites as high-energy anode in LIBs, their electrochemical performance is characterized regarding their long-term cycling stability and rate capability. References 1 Placke, T.; Kloepsch, R.; Dühnen, S.; Winter, M. Lithium ion, lithium metal, and alternative rechargeable battery technologies: The odyssey for high energy density. Journal of Solid State Electrochemistry 2017, 21, 1939-1964. 2 Andre, D.; Hain, H.; Lamp, P.; Maglia, F.; Stiaszny, B. Future high-energy density anode materials from an automotive application perspective, Journal of Materials Chemistry A 2017, 5, 17174-17198. 3 Wu, H.; Cui, Y. Designing nanostructured Si anodes for high energy lithium ion batteries. Nano Today 2012, 7, 414–429. 4 Besenhard, J. O.; Yang, J.; Winter, M. Will advanced lithium-alloy anodes have a chance in lithium-ion batteries? Journal of Power Sources 1997, 68, 87–90. 5 Obrovac M. N., Chevrier V. L. Alloy Negative Electrodes for Li-Ion Batteries, Chemical Reviews 2014, 114, 11444-11502

Since Sony introduced the Lithium Ion Battery (LIB) in 1991, an enormous effort has been made to further improve the performance of this battery system[1-2]. Nowadays, the research in LIBs is driven by the transition from combustion-based transportation to electric engines and the global energy revolution[3-5]. Environmental friendly cell chemistries are thus of particular interest in terms of recycling and safety reasons, especially for storage of renewable energies. In this connection, dual-graphite batteries receive increasing attention due to the use of graphitic carbon for both the negative and positive electrodes, i.e., free of transition metals[6]. A prerequisite for further improvements is clearly the understanding of the ongoing chemistry in these complex systems. In the last years, solid-state Nuclear Magnetic Resonance (NMR) has proven to be a very powerful technique for the investigation of the underlying chemistry of a vast number of materials used in LIBs[7-8]. Since NMR is not restricted to crystalline samples also highly disordered and amorphous materials can be studied. Furthermore, solid-state NMR provides quantitative measurements, is chemically specific and therefore extremely suitable for the investigation of the electrochemical processes in battery systems[7-8]. Also, the recent development of in situ or in operando measurements makes it highly beneficial for battery research[7-9]. In this contribution, we present the (i) first 119Sn NMR spectroscopic characterization of the Li-Sn system for the subsequent determination of Sn-containing battery materials, (ii) in operando determination of high surface area lithium (HSAL) on lithium metal electrodes, (iii) investigations of the charge/discharge mechanisms in S/PAN, and finally (iv) insights in the anion intercalation in dual-graphite cells. References: [1] J. Cabana, L. Monconduit, D. Larcher, M. R. Palacín, Advanced Materials 2010, 22, E170-E192. [2] G. E. Blomgren, Journal of The Electrochemical Society 2017, 164, A5019-A5025. [3] M. V. Reddy, G. V. Subba Rao, B. V. R. Chowdari, Chemical Reviews 2013, 113, 5364-5457. [4] D. Andre, S.-J. Kim, P. Lamp, S. F. Lux, F. Maglia, O. Paschos, B. Stiaszny, Journal of Materials Chemistry A 2015, 3, 6709-6732. [5] B. Scrosati, J. Garche, Journal of Power Sources 2010, 195, 2419-2430. [6] T. Placke, A. Heckmann, R. Schmuch, P. Meister, K. Beltrop, M. Winter, Joule 2018, 2, 2528-2550. [7] O. Pecher, J. Carretero-González, K. J. Griffith, C. P. Grey, Chemistry of Materials 2017, 29, 213-242. [8] S. Haber, M. Leskes, Advanced Materials 2018, 0, 1706496. [9] S. A. Kayser, A. Mester, A. Mertens, P. Jakes, R.-A. Eichel, J. Granwehr, Physical Chemistry Chemical Physics 2018, 20, 13765-13776.

Active cathode materials in lithium ion batteries are often based on transition metals like cobalt or nickel. High cost and toxicity as well as the mining and fabrication under ethically questionable conditions are serious drawbacks of these metals used in state-of-the-art materials. On the route towards a “greener” and more sustainable lithium ion technology, alternatives such as organic and hybrid inorganic/organic active materials obtained increasing attraction in the last years. [1] One promising hybrid material class are metal-organic frameworks (MOFs). Due to their high surface area and well-defined porosity as well as their flexible and designable structure MOFs have received increasing attention over the last decades for different potential applications like gas storage or catalysis. More recently, this promising class of materials has also been investigated for energy storage devices as electrode active material. [2] MOFs consist of inorganic metal-oxo clusters (called secondary building unit, SBU in short) coordinated to multivalent rigid organic molecules (often referred as “linker”), which can be modified with a variety of functional groups forming a crystalline porous structure. The well-defined pore size is tailorable by the length of the linker molecules leading to a high flexibility accessible for reversible insertion and removal of guest molecules. The highly porous crystalline structure, especially their large pore size, make them interesting for reversible cation or anion storage, e.g. in the dual-ion battery concept. [3] Furthermore, multivalent metal ions in the SBU as well as organic linker molecules can act as redox-active sites, leading to a promising active material. [4] Porphyrin-based organic derivates, which occur. e.g. in human blood and vitamin B12, are well known for their catalytic- and redox-activity. In the present work, we synthesize various porphyrin-based MOFs with different coordinated metals, which are successfully applied as an energy storage material in a lithium metal cell and characterized with respect to the structural and surface properties. Combining the redox-active porphyrin derivate, Tetrakis(4-carboxyphenyl)porphyrin (TCPP), and a redox-active metal(oxo-)cluster, a non-toxic and environmentally friendly cathode material was achieved. Constant current cycling and cyclic voltammetry studies reveal a high and reversible redox activity. Using suitable methods such as X-ray diffraction (XRD), the redox reaction behavior of the metal-organic framework and the structural properties of the MOFs were investigated upon charge/discharge operation. Furthermore, the influence of different conductive salts and solvents on the electrochemical performance were analyzed. References: [1] D. Larcher; J.-M. Tarascon; Towards greener and more sustainable batteries for electrical energy storage. Nature Chemistry 2015; 7; 19-29. [2] Wang, L.; Han, Y.; Feng, X.; Zhou, J.; Qi, P.; Wang, B., Metal–organic frameworks for energy storage: Batteries and supercapacitors. Coordination Chemistry Reviews 2016, 307, 361-381. [3] Aubrey, M. L.; Long, J. R., A Dual−Ion Battery Cathode via Oxidative Insertion of Anions in a Metal–Organic Framework. Journal of the American Chemical Society 2015, 137 (42), 13594-13602. [4] D'Alessandro, D. M., Exploiting redox activity in metal-organic frameworks: concepts, trends and perspectives. Chemical Communications 2016, 52, 8957-8971.

The class of Li-Mn-rich layered transition metal oxides (LMR-NCM) is one of the most promising candidates for cathode materials in advanced lithium ion batteries (LIBs) due to their high specific discharge capacity of up to 300 mAh/g and specific energy up to 1000 Wh/kg at material level.[1-4] Furthermore, those materials contain a high content of low-cost manganese due to the addition of a Li2MnO3 component compared to standard layered transition metal oxides, which at the same time enables the access of additional capacity. Nevertheless, a strong capacity and voltage fade as well as a poor rate capability plagues the class of LMR-NCM materials. The pronounced reversible oxygen redox activity (causing the voltage hysteresis), the transition metal migration and a phase transition to a spinel-like phase (causing the voltage fade) add to an even stronger fade of the specific energy and inhibit the commercialisation of these cathode materials so far. To closely study the underlying phenomena, LMR-NCM oxides with different morphologies are synthesized. Therefore, the syntheses via coprecipitation in a Couette-Taylor flow reactor and in a conventional flask are compared. The constant monitoring of critical parameters during the synthesis by means of the reactor leads to dense spherical particles with a narrow particle size distribution. These characteristics offer a spherical cathode material with enhanced capacity retention due to a more compact structure with a lower surface area. Also, taking the voltage fade into consideration allows a more objective evaluation of LMR-NCM oxides materials especially in direct comparison with state-of-the-art NCM-based cathodes. [1] E. M. Erickson, F. Schipper, T. R. Penki, J.-Y. Shin, C. Erk, F.-F. Chesneau, B. Markovsky, D. Aurbach J. Electrochem. Soc. 2017, 164, A6341 – A6348. [2] B. Qiu, M. Zhang, L. Wu, J. Wang, Y. Xia, D. Qian, H. Liu, S. Hy, Y. Chen, K. An, Y. Zhu, Z. Liu, Y. S. Meng Nat. Commun. 2016, 7, 12108. [3] D. Andre, S.-J. Kim, P. Lamp, S. F. Lux, F. Maglia, O. Paschos, B. Stiaszny J. Mater. Chem. A 2015, 3, 6709 – 6732. [4] T. Placke, R. Kloepsch, S. Dühnen, M. Winter J. Solid State Electrochem. 2017, 21, 1939 – 1964.

Following up the great success of lithium ion batteries (LIBs) in the field of consumer electronics in the recent decades, LIBs are today also widely used as energy storage devices for various types of electric vehicles (EVs). However, it is believed that a substantial breakthrough of the electromobility can only take place, if EVs are able to achieve driving ranges of more than 500 km and, therefore, gaining greater consumer acceptance. To achieve these driving ranges, the energy content of the used LIBs needs to be increased to values of 350 Wh/kg and 750 Wh/L at the cell level. These improvements can be achieved by replacing the state-of-the-art graphite anode by high-capacity anode materials like silicon (Si), which is able to provide a nearly 10 times higher specific capacity compared to graphite.[1] However, due to huge volume changes of up to 300% upon lithiation/de-lithiation, Si-based anodes often suffer from poor cycling performance related to cracking and pulverization of the Si particles and continuous solid electrolyte interphase (SEI) re-formation. Typically, these issues are addressed by decreasing the size of the active material particles to the nanoscale (i.e. nanoparticles, nanowires or thin films) or the fabrication of composites like intermetallic Si phases or Si/carbon composite materials to decrease the overall volume changes.[2] Although these strategies are reported to be effective to improve the performance of Si electrodes, it was revealed that the dominating failure mechanism of full cells containing Si-based negative electrodes lies in the consumption active lithium from the cathode related to continuous electrolyte reduction at the Si negative electrode. The addition of appropriate electrolyte additives such as fluoroethylene carbonate (FEC) or vinylene carbonate (VC) can significantly improve the capacity retention and Coulombic efficiency of Si-based full cells by forming a more stable SEI on the Si negative electrode.[3, 4] Recently, Krause et al. demonstrated that the addition of carbon dioxide in the form of dry ice significantly improved the performance of Si-based full cells and even outperformed the most commonly used additive FEC.[5] However, the exact working mechanism of the carbon dioxide additive and especially the reductive decomposition reactions including the resulting morphology and/or chemical composition of the formed SEI layer are still uncertain. Within this study, thin film Si electrodes prepared via magnetron sputtering are employed as model electrodes (i.e. no binder or conductive additive) therefore, possible effects of the carbon dioxide can be correlated to the Si active material. These Si thin film electrodes are coupled with NMC-111 positive electrodes to Si/NMC-111 full cells, which are used for the electrochemical investigations. As the solubility of carbon dioxide in the organic carbonate based electrolytes is rather low, diethylpyrocarbonate is added to the baseline electrolyte, as pyrocarbonates are known as carbon dioxide generators and, therefore, the carbon dioxide is released in-situ within the assembled full cell. The addition of the diethylpyrocarbonate electrolyte additive leads to significantly improved performance of the Si/NMC-111 full cells in terms of Coulombic efficiency and capacity retention full cells during prolonged cycling compared to the baseline electrolyte. The chemical composition of the SEI layer formed on the Si negative electrodes is investigated by means of X-ray photoelectron spectroscopy (XPS). The XPS investigations reveal that the improved performance of the carbon dioxide containing cells is related the improved passivation of the negative electrode by the formation of mainly lithium carbonate at the Si surface. Reference s : [1] D. Andre, H. Hain, P. Lamp, F. Maglia, B. Stiaszny, Future high-energy density anode materials from an automotive application perspective, Journal of Materials Chemistry A, 5 (2017) 17174-17198. [2] M.N. Obrovac, V.L. Chevrier, Alloy Negative Electrodes for Li-Ion Batteries, Chemical Reviews, 114 (2014) 11444-11502. [3] R. Nölle, A.J. Achazi, P. Kaghazchi, M. Winter, T. Placke, Pentafluorophenyl Isocyanate as an Effective Electrolyte Additive for Improved Performance of Silicon-Based Lithium-Ion Full Cells, ACS Applied Materials & Interfaces, 10 (2018) 28187-28198. [4] M. Klett, J.A. Gilbert, S.E. Trask, B.J. Polzin, A.N. Jansen, D.W. Dees, D.P. Abraham, Electrode Behavior RE-Visited: Monitoring Potential Windows, Capacity Loss, and Impedance Changes in Li1.03(Ni0.5Co0.2Mn0.3)0.97O2/Silicon-Graphite Full Cells, Journal of The Electrochemical Society, 163 (2016) A875-A887. [5] L.J. Krause, V.L. Chevrier, L.D. Jensen, T. Brandt, The Effect of Carbon Dioxide on the Cycle Life and Electrolyte Stability of Li-Ion Full Cells Containing Silicon Alloy, Journal of The Electrochemical Society, 164 (2017) A2527-A2533.

“Dual-graphite” cells use graphite as intercalation host at both electrodes, where the energy storage mechanism is based on the intercalation of lithium ions into the negative and the intercalation of electrolyte anions into the positive electrode. In this contribution, the electrochemical intercalation of bis(trifluoromethanesulfonyl)imide (TFSI) anions from an ionic liquid-based electrolyte (Pyr14TFSI) into graphite was studied using ex situ and in situ X-ray diffraction. We prove that the TFSI intercalated graphite exists in a series of staged phases. As for this electrode material the discharge capacity can be strongly influenced by the cut-off potential and temperature, the stage transitions and compositions of the TFSI intercalated graphite were determined in dependency of different temperatures and cut-off potentials. From the experimental data, the periodic repeat distance of the graphite intercalation compound, the TFSI gallery height and the maximum stoichiometries of C n + TFSI− at these different conditions are calculated and discussed.

Dual-graphite cells have been proposed as electrochemical energy storage devices using graphite as both, the negative and positive electrode. In this setup, the electrolyte cations intercalate into the graphite anode and the electrolyte anions intercalate into the graphite cathode during charge. On discharge, cations and anions are released back into the electrolyte. In this contribution, the electrochemical intercalation of the bis(trifluoromethanesulfonyl) imide anion (TFSI-) into various graphites as cathode from an ionic liquid-based electrolyte, namely N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide (Pyr14TFSI), was studied. The focus of this investigation is on the influence of the graphite properties such as the particle size and surface characteristics, i. e. the ratio of the basal plane to “non-basal plane” surface area, on the electrochemical performance of TFSI- anion intercalation.

Electrochemical energy storage devices utilizing graphitic carbons as positive electrode material have been proposed as "dual-ion cells". In this type of electrochemical cell, the electrolyte does not only act as a charge carrier, but additionally as the active material. In the charge process, lithium ions are inserted/intercalated or deposited into/on the negative electrode, e. g. Li4Ti5O12, graphite or metallic lithium, and the electrolyte anions are intercalated into the graphite positive electrode. In the discharge process, both lithium ions and anions are released back into the electrolyte. We report herein on the intercalation of bis(trifluoromethanesulfonyl) imide anions (TFSI−) into a graphite cathode from an ionic liquid-based, namely N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide (Pyr14TFSI), electrolyte. We studied the influence of the graphite characteristics, such as the particle size distribution, specific surface area and ratio of basal plane to "non-basal plane" surface areas, on the electrochemical behavior of TFSI− anion intercalation. The rate performance, long-term cycling behavior and stability of dual-ion cells at high temperatures (60°C) are also discussed. A specific discharge capacity exceeding 100 mAh g−1 can be achieved at discharge rates of up to 10C, when operating at 60°C.

Electrochemical energy storage systems using graphite as both the negative and the positive electrode have been proposed as “dual-graphite cells”. In this kind of electrochemical system, the electrolyte cations intercalate into the negative electrode and the electrolyte anions intercalate into the positive electrode, both based on graphite, during the charging process. On discharge, cations and anions are released back into the electrolyte. So far, the systems proposed in literature are primarily based on Li+and PF6-intercalation/de-intercalation into/from graphite from non-aqueous organic solvent based electrolytes. As the positive electrode potential during charging always exceeds 4.2 Vvs.Li/Li+, the organic electrolyte starts to decompose at these highly oxidizing conditions resulting in insufficient discharge/charge efficiencies. The replacement of organic solvent by ionic liquids (ILs) leads an increased stability of the electrolyte towards oxidation and thus to remarkably higher efficiencies as well as an increased cycling stability. In fact, ionic liquids provide extended anodic electrochemical stability and in addition, no solvent co-intercalation occurs in parallel to anion intercalation at high potentials.Here, we present highly promising results for “dual-ion cells” based on a graphite cathode and an ionic liquid based electrolyte, namely N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr14TFSI). As the compatibility of this IL with graphite anodes is poor, alternative anodes such as metallic lithium or lithium titanate (Li4Ti5O12, LTO) are used. Consequently, the “dual-graphite” cell is renamed to “dual-ion” cell. In addition, the calculation of the specific energy of these systems will be in the focus of the discussion.

For graphitic carbons as anode materials in lithium ion batteries, the morphology and chemistry of the graphite surface have a significant impact on the formation of the solid electrolyte interphase (SEI), the corresponding irreversible charge losses, and the overall electrochemical anode performance. In this work the effects of graphite surface modification, induced by an elevated temperature treatment, on the SEI formation are discussed in details. Morphology changes due to burn-off of carbon are investigated by Raman spectroscopy and nitrogen adsorption measurements, which are not only used to calculate the BET specific surface area but also for the estimation of the absolute and relative extents of the basal plane surface area and the “non-basal plane surface” area. In particular, the relation of the first cycle irreversible charge loss to the change of surface morphology, especially to the quantitative amounts of the different types of surfaces is highlighted.

The continuous optimization of lithium ion batteries regarding the energy density, cycle life and safety has enabled their widespread application various fields, such as for electric vehicles. To satisfy the ever-increasing demands on both high gravimetric and volumetric energy density, high voltage and high capacity materials are being intensively studied. Promising candidates are the Ni-rich layered LiNixCoYMn1-x-yO2 materials. High Ni content LiNixCoyMn1-x-yO2 materials are attracting more attention in the last years due to their high specific capacity. Compared to the other commercial cathode materials, e.g. LiFePO4, LiCoO2 and Li-rich materials, the high Ni content LiNixCoyMn1-x-yO2 materials such as LiNi0.5Co0.2Mn0.3O2 (NMC-532) possess high specific capacity ca. 200 mAh g-1 at a cut-off potential of 4.6 V vs. Li/Li+.(1) However, the oxidative decomposition of conventional carbonate-based electrolytes at high voltages ( > 4.5 V vs. Li/Li+) and the deterioration of the active material particles result in rapid capacity fading. Different strategies are known to improve the cycle life of the high voltage cathode material NMC-532. One opportunity is to protect the cathode surface by inorganic coatings. Su et al. demonstrated the use of an Al203 coating which resulted in a significant improvement of the cycle life of NMC-532 by minimizing the structural transformation (2). A further option to improve the cycle life of NMC-532 is to use electrolyte additives (3). On the basis that the added additive decomposed prior to the electrolyte components, a cathode electrolyte interface (CEI) layer is formed on the cathode particles. This CEI layer on the cathode can prevent the electrochemical and chemical oxidation of the electrolyte. In this work, we present the combination of both an inorganic coating and the use of electrolyte additives to significantly improve the cycle life of NMC-532/graphite LIB full cells at 4.6 V vs. Li/Li+. References: (1)Yan, G., et al. (2015). "Effects of 1-propylphosphonic acid cyclic anhydride as an electrolyte additive on the high voltage cycling performance of graphite/LiNi0.5Co0.2Mn0.3O2 battery." Electrochimica Acta 166: 190-196. (2)Su, Y., et al. (2015). "Enhancing the High-Voltage Cycling Performance of LiNi0.5Mn0.3Co0.2O2 by Retarding Its Interfacial Reaction with an Electrolyte by Atomic-Layer-Deposited Al2O3." ACS Applied Materials & Interfaces 7(45): 25105-25112. (3)Lee, S. H., et al. (2018). "[4,4′-bi(1,3,2-dioxathiolane)] 2,2′-dioxide: A novel cathode additive for high-voltage performance in lithium ion batteries." Journal of Power Sources 378: 112-118.

Further enhancements regarding the energy density and specific power of lithium ion batteries are absolutely necessary in order to satisfy the increasing requirements for automotive applications e.g. extended driving ranges. One way to realize such improvements, depicts the replacement of commonly used carbon-based anode materials with high capacity anodes.1 In this regard silicon (Si) containing composites are considered promising candidates for the replacement of carbonaceous anode materials due to the significantly higher specific capacity of Si. However, the use of Si is still hindered by several challenges that have to be overcome for a successful application. One major issue of Si-based anode materials is the strong capacity decay due to the huge volume changes (~ 300%) of Si during the lithiation/de-lithiation process, leading to an recurring solid electrolyte interphase (SEI) reformation, which results in the ongoing consumption of active lithium from the cathode.2 One concept to alleviate the detrimental effects previously mentioned and to boost the performance of such anode materials, comprises the combination of Si with a matrix material. The general idea behind this approach is to combine Si with a second phase that enhances the mechanical stability and can buffer the volumetric changes of Si and thus, enables the formation of a stable SEI.3,4 In this study, we present a silicon iron (Fe) composite that contains two phases, a crystalline Si phase and a second, intermetallic FexSiy-phase. The applied synthesis route yields materials that combine a porous structure with the aforementioned matrix approach. This composite design is believed to have a beneficial effect on the capacity retention during cycling since it may buffers the volumetric changes of the Si and simultaneously provides extra space for the volume changes. The applied synthesis route contains a ball-milling and washing step, and subsequently the addition of a thin carbon coating. The composites, synthesized this way, are investigated via scanning electron microscopy, energy dispersive x-ray spectroscopy, x-ray diffraction and thermogravimetric analysis in order to characterize their structure, morphology and composition. Moreover, electrochemical studies on the long-term cycling and rate performance with regard to the application as anodes in lithium ion batteries are conducted. Thereby, this work focuses on the influence of a high temperature treatment, as well as the influence of the Fe to Si ratio on the electrochemical performance. Furthermore, the role of the stabilizing FexSiy matrix phase in the composite is investigated regarding the question whether this phase is inactive towards lithiation or if it contributes to the lithiation/de-lithiation capacity of the composite. References 1 Placke, T.; Kloepsch, R.; Dühnen, S.; Winter, M. Lithium ion, lithium metal, and alternative rechargeable battery technologies: The odyssey for high energy density. Journal of Solid State Electrochemistry 2017, 21, 1939-1964. 2 Wu, H.; Cui, Y. Designing nanostructured Si anodes for high energy lithium ion batteries. Nano Today 2012, 7, 414–429. 3 Besenhard, J. O.; Yang, J.; Winter, M. Will advanced lithium-alloy anodes have a chance in lithium-ion batteries? Journal of Power Sources 1997, 68, 87–90. 4 Park, C.-M.; Kim, J.-H.; Kim, H.; Sohn, H.-J. Li-alloy based anode materials for Li secondary batteries. Chemical Society reviews 2010, 39, 3115–3141.

In recent years, the demand for lithium ion batteries (LIBs) significantly increases due to the wide range of application, from consumer electronics to automotive industry. The improvement of the LIB’s energy density is one of the key factors to the mass market penetration of electric mobility. Currently, LIBs are applied in hybrid (HEV), plug-in hybrid (PHEV) and fully electric (EV) vehicles. Particularly, layered lithium Ni-Co-Mn oxides LiNixCoyMnzO2 (NCM, x + y + z = 1) cathode materials are the most promising candidates for LIBs due to their high capacity, lower cost and enhanced stability compared with LiCoO2.[1] Although the state-of-the-art NCM cathode material is NCM-523, the trend is going towards a higher energy density. Recently, the focus of research is on the cathode materials with high nickel content. For example NCM-811 with a Ni content of 80% can deliver a discharge capacity of 200 mAh/g at 4.3 V. At the same time, the high Ni content causes some disadvantages, such as an unstable electrode structure and rapid capacity fading. One of the challenges is the synthesis and stabilization of Ni-rich materials. In order to avoid cation mixing, an excess of lithium must be ensured. Consequently, the unreacted lithium on the surface of the active material can react with water and CO2 from the air. As a result of the contamination, LiOH and L2CO3 are formed on the surface of the active material. These residues can form an insulating layer of LiF in contact with the electrolyte, which leads an increased internal resistance of the electrode. Furthermore, the highly reactive Ni4+ ions in the delithiated state of the electrode of Ni-rich cathode materials can oxidize the electrolyte and result in active material loss.[2] In this contribution, we investigate the effect of surface modification of NCM-811 via an inorganic coating on the electrochemical performance. The inorganic coating acts as protective layer on the surface of the active material, preventing the direct contact of the cathode material with the electrolyte. Moreover, it suppresses the side reactions on the particle surface. The effect of the coating on the structural and electrochemical properties was investigated in NCM-811/graphite full cells. The cells with coated electrodes were significantly improved with respect to their long-term cycling performance compared to the uncoated electrodes (Figure 1). [1] S.-K. Jung, H. Gwon, J. Hong, K.-Y. Park, D.-H. Seo, H. Kim, J. Hyun, W. Yang and K. Kang, Advanced Energy Materials, 2014, 4, 1300787. [2] W. Liu, P. Oh, X. Liu, M.-J. Lee, W. Cho, S. Chae, Y. Kim and J. Cho, Angewandte Chemie International Edition, 2015, 54, 4440-4457. Figure 1: Cycling performance of uncoated and coated NCM-811/graphite LIB full cells at room temperature using a voltage window of 2.5 – 4.3 V. Three formation cycles at 0.1C and following cycling at 0.5C rate. Figure 1

In order to increase the energy density of lithium-ion batteries (LIB) one promising approach is rooted in the increase of the nickel content in layered oxide cathode materials (LiNixMnyCo1-x-yO2, x ≥ 0.6, LiNMC) for an application as positive electrode due to the high specific discharge capacity at moderate upper cutoff voltages below 4.4 V vs. Li/Li+.[1, 2] However, the insufficient cycling stability as well as significant safety concerns restricted this material class from industrial application so far. For that reason, new electrolyte compositions including specifically designed additives are of great importance to enable a sufficient electrochemical performance of Ni-rich NCM materials.[3] In this work, we introduce a low molecular weight, low cost and non-toxic chemical compound as electrolyte additive for the application in graphite/LiNi0.8Mn0.1Co0.1O2 (NMC811) cells.[4] The addition of only 0.5 wt.% of the additive into the control electrolyte (1M LiPF6 in EC / EMC 3:7, by weight) significantly increases the reversible capacity as well as the 1st cycle Coulombic efficiency (CE) improving the capacity retention of the standard full cell system between 2.8 - 4.3 V. Irreversible lithium loss in the negative electrode was found to be the predominant factor for the full cell capacity fade and the poor cycling performance of the control electrolyte. Inductively coupled plasma-mass spectrometry (ICP-MS) and X-ray photoelectron spectroscopy (XPS) analysis reveals the contribution of the electrolyte additive in the solid electrolyte interface (SEI) formation on the negative electrode as well as in the cathode electrolyte interface (CEI) formation on the positive electrode. As a result, less active lithium loss during cycling is present in the additive containing electrolyte which leads to an impressive increase in capacity retention of 77%. In addition, a comprehensive performance study of state of the art electrolyte additives such as triphenylphosphine (TPP), vinylene carbonate (VC) and diphenyl carbonate (DPC) is presented and compared to the invented electrolyte additive, demonstrating the outstanding working ability of this compound in graphite/NMC811 cells. To the best of our knowledge the presented cycling results outperform the so far reported electrolyte additives in the field. Reference: [1] J. Kasnatscheew, M. Evertz, R. Kloepsch, B. Streipert, R. Wagner, I. Cekic Laskovic, M. Winter, Energy Technology, 5 (2017) 1670-1679. [2] H.-J. Noh, S. Youn, C.S. Yoon, Y.-K. Sun, Journal of Power Sources, 233 (2013) 121-130. [3] A.M. Haregewoin, A.S. Wotango, B.J. Hwang, Energy & Environmental Science, 9 (2016) 1955-1988. [4] K. Beltrop, S. Klein, R. Nölle, A. Wilken, J.J. Lee, J. Reiter, L. Tao, C. Liang, M. Winter, X. Qi, T. Placke, Chemistry of Materials, (2018), submitted. Figure 1

Lithium ion batteries (LIBs) are considered as key technology for stationary energy storage systems and especially as power supply for electric vehicles (EVs). Even though LIBs are already used in EVs, there is a need of further improvements of the LIBs to achieve driving ranges of more than 500 km, what is considered to be a value for greater consumer acceptance of EVs. To reach this goal, the specific energy and energy density of LIBs need to be increased to approximately 350 Wh kg-1 and 750 Wh L-1 at cell level, respectively. The use of alternative anode materials to replace the state-of-the-art graphite anode, is considered as an efficient strategy to increase the energy density of LIBs. Silicon (Si) turns out to be the most promising material for advanced anodes in LIBs, as it offers a nearly 10 times higher specific capacity compared to graphite. However, the implementation of electrode materials containing contents of more than 5-10% Si in commercial LIBs is still hampered by huge volume changes leading to a continuous solid electrolyte interphase (SEI) re-formation, loss of active lithium and, therefore, to a poor capacity retention.[1] The application of nano-sized Si materials like nanoparticles, nanowires or thin films is reported to significantly improve the performance of Si based anodes, due to better accommodation of the huge volume changes upon lithiation/de-lithiation. Additionally, Si thin film electrodes exhibit improved specific capacities in comparison to typical composite-based electrodes, because of the absence of inactive components like binder and conductive additives.[2] An additional approach to improve the performance of Si based electrodes is the addition of additives to the electrolyte. Electrolyte additives like fluoroethylene carbonate (FEC) and vinylene carbonate (VC) are commonly known to enhance the capacity retention of Si electrodes by forming a more stable SEI, thus, preventing ongoing electrolyte decomposition and continuous active lithium loss.[3] Isocyanate compounds are able to undergo reductive polymerization and, therefore, may be considered as effective electrolyte additives for Si anodes. Actually, several isocyanates were reported to function as effective film-forming additives for graphite-based negative electrodes.[4] Within this work, magnetron sputtering was utilized for the preparation of thin film Si anodes, which contain neither a binder nor a conductive agent. Therefore, the effect of the electrolyte additive can be directly related to the Si active material. Since it was recently reported, that lithium consumption related to SEI reformation is the main failure mechanism of lithium ion full cells containing a Si anode[5], the electrochemical performance of these Si thin film electrodes was investigated in Si/NMC-111 full cells using different electrolyte formulations. DFT calculations (HOMO/LUMO energies) were performed prior to electrochemical investigations for reductive and oxidative stability predictions of the electrolyte solvent and additive molecules. The addition of the pentafluorophenyl isocyanate electrolyte additive leads to an increased Coulombic efficiency and a significantly enhanced capacity retention of the LIB full cells during prolonged cycling, in comparison to the baseline electrolyte. Post-mortem investigations of the negative Si electrodes by means of scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were performed to study the SEI layer, formed in the different electrolyte formulations. The enhanced cycling performance of the full cells can be correlated to an improved SEI formation. Reference s : [1] D. Andre, H. Hain, P. Lamp, F. Maglia, B. Stiaszny, Future high-energy density anode materials from an automotive application perspective, Journal of Materials Chemistry A, 5 (2017) 17174-17198. [2] M.N. Obrovac, V.L. Chevrier, Alloy Negative Electrodes for Li-Ion Batteries, Chemical Reviews, 114 (2014) 11444-11502. [3] S. Dalavi, P. Guduru, B.L. Lucht, Performance Enhancing Electrolyte Additives for Lithium Ion Batteries with Silicon Anodes, Journal of The Electrochemical Society, 159 (2012) A642-A646. [4] C. Korepp, W. Kern, E.A. Lanzer, P.R. Raimann, J.O. Besenhard, M.H. Yang, K.C. Möller, D.T. Shieh, M. Winter, Isocyanate compounds as electrolyte additives for lithium-ion batteries, Journal of Power Sources, 174 (2007) 387-393. [5] M. Klett, J.A. Gilbert, S.E. Trask, B.J. Polzin, A.N. Jansen, D.W. Dees, D.P. Abraham, Electrode Behavior RE-Visited: Monitoring Potential Windows, Capacity Loss, and Impedance Changes in Li1.03(Ni0.5Co0.2Mn0.3)0.97O2/Silicon-Graphite Full Cells, Journal of The Electrochemical Society, 163 (2016) A875-A887.

Lithium ion batteries (LIBs), as the state-of-the-art energy storage technique, possess high energy and power density. The target to decrease the emission of carbon dioxide, released by the combustion of fossil fuels promotes the market of electric vehicles (EVs) or hybrid electric vehicles (HEVs), resulting in a high demand for LIBs. LIBs are expected to be the most promising candidate to replace the fossil fuels in conventional means of transportation [1]. LiNixMnyCo1-x-yO2 (x ≤ 0.5) has been widely applied as the state-of-the-art cathode material for lithium ion batteries, with the application in E-bikes or xEVs. However, the application in competitive xEVs requires high specific capacity as well as high working potential (> 4.5 V vs. Li/Li+). Therefore, Ni-rich or high-voltage NMC cathode materials will be established as future cathode materials, offering higher discharge capacities at equivalent cut-off potentials. Certainly, as the operation voltage or Ni content increases, not only the intrinsic stability of the layered oxides decreases due to the higher delithiation degree, but also the oxidative decomposition of the electrolyte by e.g. chemical reaction with highly reactive Ni4+ species on the cathode surface becomes more severe. These side reactions can promote the loss of active material (Ni4+ → Ni2+) and the formation of a thick layer of decomposition products on the electrode surface, resulting in an overall impedance increase. Furthermore, Ni-rich layered oxides particles tend to form micro-cracks, revealing the pristine active material and further accelerating capacity fading, due to ongoing electrolyte decomposition. The use of electrolyte additives to prevent these cathode fading mechanisms is one promising approach to improve the capacity retention and cell performance [2,3]. A variety of electrolyte additives to act as a film forming agent to hinder the fading of different cathode materials have been reported in literature so far. These additives are oxidized prior to the blank electrolyte components and in situ form a protective layer on the surface of the electrode [4]. Within this work, several new compounds were synthesized and evaluated as possible electrolyte additives for NMC/graphite cells, to address the previously mentioned fading mechanisms. The novel compounds were characterized towards their reductive and oxidative stability on active electrode materials, as opposed to commonly used inactive materials (e.g. Pt, or glassy carbon).The addition of these nitrogen-based electrolyte additives lead to an increased Coulombic efficiency and enhanced capacity retention during long-term cycling in comparison to the baseline electrolyte. The working mechanism was tried to elucidate using different ex situ analytical techniques. Post-mortem investigations of the extracted electrolyte and the cathode surface were performed to study the cathode electrolyte interphase (CEI) layer formed by the addition of these additives. The improved cycling performance of these additives in LIB full cells can be correlated to the formation of a passivation film on the cathode surface. References [1] J.M. Tarascon, M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature 414 (2001) 359–367. [2] R. Jung, M. Metzger, F. Maglia, C. Stinner, H.A. Gasteiger, Oxygen Release and Its Effect on the Cycling Stability of LiNixMnyCozO2 (NMC) Cathode Materials for Li-Ion Batteries, J. Electrochem. Soc. 164 (2017) A1361-A1377. [3] J. Kasnatscheew, M. Evertz, B. Streipert, R. Wagner, S. Nowak, I. Cekic Laskovic, M. Winter, Changing Established Belief on Capacity Fade Mechanisms, J. Phys. Chem. C 121 (2017) 1521–1529. [4] Y. Dong, B.T. Young, Y. Zhang, T. Yoon, D.R. Heskett, Y. Hu, B.L. Lucht, Effect of Lithium Borate Additives on Cathode Film Formation in LiNi0.5Mn1.5O4/Li Cells, ACS Appl. Mater. Interfaces 9 (2017) 20467–20475.

The increasing demand for large scale energy storage capabilities causes a big field of research to make the battery systems safer and environmentally benign. A promising approach is the replacement of conventional volatile and highly flammable organic solvent-based electrolytes by ionic liquids (ILs) as the main component of the electrolyte. ILs exhibit excellent properties such as a broad electrochemical stability window, a high thermal stability, non-flammability and a high ionic conductivity.[1] A further prevention of pollution is the application of carbon-based, transition metal-free electrode materials. Recently, “dual-ion” and “dual-graphite” cells were introduced which are composed of a lithium-incorporating anode and an anion incorporating graphite-based cathode.[2][3] During charge the ions intercalate into the electrode active materials and are released back into the electrolyte during discharge (see Figure 1). Since the cathode potential reaches and may even exceed 5 V vs. Li/Li+, electrolytes that exhibit a high stability vs. oxidation are necessary. For that reason, ILs with their versatile properties were introduced and investigated. The state of the art electrolyte for this system is composed of a solution of lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) in N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide (Pyr14TFSI). In order to increase the specific capacity of this system, one main strategy is to focus on the intercalation of smaller anions than TFSI into the graphite structure. In this context, the major challenge is the synthesis of ILs or mixtures of ILs with organic electrolytes consisting of small anions to provide an electrochemically stable electrolyte. Interestingly, not only the size and shape of the intercalated anions plays a major role in view of onset potential for anion uptake as well as discharge capacity. Instead, also more complex coherences such as ion association, self-aggregation of ions as well as solvent co-intercalation have to be taken into consideration. In this work, we investigate the electrochemical intercalation of anions with different shapes and sizes from IL based electrolytes into a graphite-based cathode. Against this background, the influence of side chain modifications within the anion structure as well as the influence of the used cation within the conductive salt (Li+/K+) on the intercalation behavior was studied using different electrochemical analysis techniques. Therefore, in this contribution we will give new insights to the question if the anion size is the main issue to tailor the reversible capacity for anion intercalation. Furthermore, the influence of lithium plating behavior in different electrolyte systems on the overall cell performance as well as the effect of adding small amounts of different additives such as foreign cations and different conductive salts into the baseline electrolyte is discussed. Figure 1: Schematic illustration of the operating principle of a "dual-ion cell”.[2] 1] A. Lewandowski, A. Swiderska-Mocek, Journal of Power Sources, 194 (2009) 601-609. [2] T. Placke, P. Bieker, S.F. Lux, O. Fromm, H.W. Meyer, S. Passerini, M. Winter, Zeitschrift für Physikalische Chemie, 226 (2012) 391-407. [3] K. Beltrop, P. Meister, S. Klein, A. Heckmann, M. Grünebaum, H.-D. Wiemhöfer, M. Winter, T. Placke, Electrochimica Acta, 209 (2016) 44-55. Figure 1

Lithium consuming parasitic reactions which induce active lithium loss (ALL) are a major reason for capacity fading in lithium ion batteries (LIBs).[1, 2] As a consequence of capacity fading, the usable energy density is reduced. Typically, the ALL is set equal to the accumulated irreversible capacity (sum of the differences between charge and discharge capacity in each cycle, AIC). However, regarding the AIC it is known that it includes all parasitic reactions which generate a parasitic current. Hence, the AIC cannot be directly equated with ALL. Due to this, we proposed a novel approach in the following named as IRLC method, which is based on the remaining active lithium content of the cell, in order to determine the ALL with a higher accuracy. Therefore, a cell with a three electrode set-up is cycled and afterwards the positive electrode is de-lithiated with help of the former reference electrode in order to obtain the remaining active lithium content. Finally, the ALL can be calculated by using the remaining and the initial active lithium content. In order to evaluate the IRLC method we investigated different anode materials. It can be shown that within the first cycles the ALL can be described moderately well with help of AIC. However, with increasing cycle number the difference between ALL and AIC increases, most likely attributed to the fact that the impact of SEI formation and repair becomes smaller in comparison to other parasitic reactions which do not consume lithium. Furthermore, one additional advantage of the IRLC method is that it allows a statement whether the reduction of lithium content is crucial for capacity fading or whether the fading is related to host material losses or other degradation mechanisms e.g. a higher charge transfer resistances due to a growing SEI film. 1. Krueger, S., et al., How Do Reactions at the Anode/Electrolyte Interface Determine the Cathode Performance in Lithium-Ion Batteries? Journal of the Electrochemical Society, 2013. 160(4): p. A542-A548. 2. Vetter, J., et al., Ageing mechanisms in lithium-ion batteries. Journal of Power Sources, 2005. 147(1-2): p. 269-281.

Macroporous Ge with a hexagonal-like morphology is prepared by magnesiothermic reduction of GeO2 (corundum boat, 5%H2/Ar, 650 °C, 7 h).

Graphitized carbon blacks have shown a more promising electrochemical performance than the non-treated ones when being applied in small amounts as conductive additives in composite cathode electrodes for lithium ion batteries, due to the absence of surface functional groups which contribute to detrimental side-reactions with the electrolyte. Here, we report that at high potentials of4.5 V vs. Li/Li(+), graphitic structures in carbon black can provide host sites for the partially reversible intercalation of electrolyte salt anions. This process is in analogy to the charge reaction of graphite positive electrodes in dual-ion cells. A standard furnace carbon black with small graphitic structural units, as well as slightly and highly graphitized carbon blacks, were characterized and analyzed with regard to anion intercalation. A LiPF6 containing organic solvent based electrolyte as well as a state-of-the-art ionic liquid based electrolyte composed of LiTFSI in PYR14TFSI were applied. The intercalation of both PF6(-) and TFSI(-) could be confirmed by cyclic voltammetry in electrodes made of carbon blacks. When exposed to high potentials, carbon blacks experienced strong activation in the 1st cycle, which promotes the perception for anion intercalation, and thus increases the anion intercalation capacity in the following cycles. The specific capacity from anion intercalation was evaluated by constant current charge-discharge cycling. The obtained capacity was proportional to the graphitization degree. As anion intercalation might be accompanied by decomposition reactions of the electrolyte, e.g., by co-intercalation of solvent molecules, it could induce the decomposition of the electrolyte inside the carbon and thus degradation of the carbon black graphitic structure. In order to avoid side reactions from surface groups and from anion intercalation, the thermal treatment of carbon blacks must be optimized.

As one of the most promising anode materials for the next generation high performance lithium-ion batteries, silicon exhibits not only a low lithiation potential of 0.2V vs. Li/Li+, but also the highest theoretical capacity (4200 mA h g-1), which is more than one order of magnitude higher than the current commercial graphite anode material (~370 mA h g-1). However, despite of the advantages, there are several crucial problems such as, tremendous volume changes (≥300%) during Li+ insertion/extraction and low electronic conductivity which impede the commercial application of silicon anodes [1-2]. In order to moderate the drastic volume changes of silicon and some other draw backs, three main approaches have been developed. The first important approach is the use of nanoscaled particles instead of normal materials. Meanwhile, another useful strategy is to design new structures (porous, nanoweb, and hierarchical) which can supply enough free space for the expansion of silicon. Additionally, some groups [3-5] tried to introduce a volumetric stable high electrical conductivity second phase into the host matrix, which can act as a buffer to reduce the volume changes of silicon and maintain the electrode integrity. For example, Jia et al[6], proposed a porous NiSi2/Si/Carbon core-shell structured anode material by using a facile ball milling and chemical vapor deposition (CVD) method, which resulted in a stable capacity of 1272 mA h g-1 for 200 cycles (at 1C) and a reversible capacity of 740 mA h g-1 (at 5C). However, despite of the superior electrical performance, the exact role of the NiSi2phase in the composite during the discharge/charge process is still unclear. As a unique elementary selective technique, X-ray absorption spectroscopy experiments were performed on porous NiSi2/Si composite electrodes in various states of charge during the 1st lithiation and de-lithiation steps. It is observed that the NiSi2 phase shows a strong metal-metal bond character and no clear changes can be observed in XANES in fig.1 during lithiation and de-lithiation. The variation of the number of nearest neighbors of the Ni atom for the 1st coordinate Ni-Si shell and σ2 in the 1st cycle, both determined by refinement, demonstrates that NiSi2 can partially react with lithium during discharge and charge. A partially reversible non-stoichiometric compound NiSi2-y is formed during cell operation, the crystal structure of which is the same as that of the NiSi2 phase. It can be concluded that NiSi2in the composite not only accommodates the pronounced volume changes caused by the lithium uptake into silicon, but also contributes to the reversible capacity of the cell. Reference [1] B.A. Boukamp, G.C. Lesh, R.A. Huggins, Journal of The Electrochemical Society 128 (1981) 725-729. [2] M. Winter, J.O. Besenhard, M.E. Spahr, P. Novák, Advanced Materials 10 (1998) 725-763. [3] H. Ma, F. Cheng, J.Y. Chen, J.Z. Zhao, C.S. Li, Z.L. Tao, J. Liang, Advanced Materials 19 (2007) 4067-4070. [4] H. Kim, B. Han, J. Choo, J. Cho, Angewandte Chemie International Edition 47 (2008) 10151-10154. [5] Y. Yu, L. Gu, C. Zhu, S. Tsukimoto, P.A. van Aken, J. Maier, Advanced Materials 22 (2010) 2247-2250. [6] H. Jia, C. Stock, R. Kloepsch, X. He, J.P. Badillo, O. Fromm, B. Vortmann, M. Winter, T. Placke, ACS Applied Materials & Interfaces 7 (2015) 1508-1515. Figure 1

The redox-amphoteric character of graphitic carbon makes it possible to intercalate a broad range of anions and cations. These graphite intercalation compounds (GICs) are divided into two types: donor-type GICs, which host cations and acceptor-type GICs, which are formed by anions. A well-known application for a donor-type GIC is lithium-intercalated graphite (LiC6) as anode material in lithium-ion batteries. Besides this example, also acceptor-type GICs are known as active material in batteries. In this context, Placke et al.1 presented the “dual-ion cell”, that also deals with anion intercalation into the graphite structure. During charge anions and cations are inserted into the respective electrode and are released back into the electrolyte during discharge. In the charged state the cathode reaches a potential of 5 V (vs. Li/Li+). In order to avoid electrolyte degradation, an ionic liquid mixture is chosen as electrolyte. It is composed of N-butyl-N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl) imide (Pyr14TFSI) and lithium bis(trifluoromethanesulfonyl) imide (LiTFSI). The focus of this work is to investigate the influence of different graphite characteristics on the intercalation of the bis(trifluoromethanesulfonyl) imide anion into the graphite structure. For that purpose, nitrogen adsorption measurements were performed in order to determine the porosity of the graphite samples and X-ray diffraction was carried out to identify the crystallite size. Both structural parameters are correlated to the electrochemical performances. 1. Placke, T.; Bieker, P.; Lux, S. F.; Fromm, O.; Meyer, H. W.; Passerini, S.; Winter, M., Dual-ion cells based on anion intercalation into graphite from ionic liquid-based electrolytes. Zeitschrift für Physikalische Chemie 2012, 226, 391-407.

The next generation of lithium-ion batteries (LIBs) for the application in miniaturized electronic devices and traction batteries need further development of new active materials with higher specific capacities to increase the resulting energy and power of the battery cells. In particular, new anode materials with high specific capacities and improved cycling performance are of great interest.[1] Since silicon provides a very high gravimetric and volumetric capacity, a low operation potential and low costs, it is widely regarded as the most promising anode material in LIBs for the future.[1] However, major issues arise from its poor capacity retention due to mechanical degradation caused by the large volume expansion during lithium insertion/extraction. In recent years, several attempts have been undertaken to overcome this problem.[2] Compared to bulk silicon anodes, which contain at least 10 wt.% of inactive binders and conductive additives, silicon thin film anodes contain no inactive materials, giving an increased specific capacity.[3] In this work, silicon-carbon thin films have been prepared by magnetron sputtering. The silicon-carbon thin films have been prepared by alternatively sputtering layers of silicon and carbon (layered structure of silicon and carbon) or by co-sputtering silicon and carbon (mixed silicon-carbon layers). The characterization of the electrodes was performed by focused ion beam-scanning electron microscopy (FIB-SEM), Raman spectroscopy and electrochemical impedance spectroscopy. Furthermore, the influence of the carbon content and layered/non-layered silicon-carbon structure was thoroughly investigated in cycling experiments compared to the bare thin film silicon electrodes. Literature: [1] R. Wagner, N. Preschitschek, S. Passerini, J. Leker, M. Winter, J Appl Electrochem. 43 (2013) 481-496. [2] M.N. Obrovac, L.J. Krause, J Electrochem Soc. 154 (2007) A103-A108. [3] U. Kasavajjula, C. Wang, A.J. Appleby, J Power Sources, 163 (2007) 1003-1039.

A major disadvantage of anode materials, in particular high-capacity materials like silicon, is the high irreversible lithium loss attributed to solid electrolyte interphase (SEI) formation triggered by electrolyte decomposition. For this reason, pre-lithiation of anode materials is getting more attention. However, therefore the exact amount of irreversible lithium loss is needed. Even if the SEI formation presents the major part of lithium loss, there are some other parasitic reactions which can lead to irreversible lithium loss, e.g., positive electrode degradation or the reaction of lithium ions with water. In addition, there are also some parasitic reactions which do not change the active lithium content of the cell but still possess an impact on the capacity or are able to transfer lithium ions out of the electrolyte into the electrodes. Typically, the irreversible lithium loss is set equal with the accumulated irreversible capacity (sum of the differences between charge and discharge capacity in each cycle). It can be shown that this approach possesses a relatively low accuracy and one can only get an acceptable estimation of the first cycles. With increasing cycle number, the determination of the irreversible lithium loss with help of the accumulated irreversible capacity becomes more inaccurate because the impact of SEI formation and regeneration becomes smaller in comparison to other parasitic reactions. In this work, a novel approach is investigated in order to determine the irreversible lithium loss with high accuracy. Therefore, “pseudo full cells” are built, while the term “pseudo full cell” refers to a cell with an oversized cathode (in terms of capacity). These cells are cycled like “real” full cells and afterwards the positive electrode is de-lithiated with help of the reference electrode in order to determine the remaining active lithium content of the cell. Finally, the exact amount of irreversible lithium loss can be determined as difference between initial and remaining active lithium content.

The increasing demand for large scale energy storage capabilities causes a big field of research to make the battery systems safer and environmentally benign. A promising approach is the replacement of conventional volatile and highly flammable organic solvent-based electrolytes by ionic liquids (ILs) as the main component of the electrolyte. ILs exhibit excellent properties such as a broad electrochemical stability window, a high thermal stability, non-flammability and a high ionic conductivity.[1] A further prevention of pollution is the application of carbon-based, transition metal-free electrode materials. Recently, “dual-ion” and “dual-graphite” cells were introduced which are composed of a lithium-incorporating anode and an anion incorporating graphite-based cathode.[2][3] During charge the ions intercalate into the electrode active materials and are released back into the electrolyte during discharge (see Figure 1). Since the cathode potential reaches and may even exceed 5 V vs. Li/Li+, electrolytes that exhibit a high stability vs. oxidation are necessary. For that reason, ILs with their versatile properties were introduced and investigated. The state of the art electrolyte for this system is composed of a solution of lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) in N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide (Pyr14TFSI). In order to increase the specific capacity of this system, one main strategy is to focus on the intercalation of smaller anions than TFSI into the graphite structure. In this context, the major challenge is the synthesis of ILs or mixtures of ILs with organic electrolytes consisting of small anions to provide an electrochemically stable electrolyte. Interestingly, not only the size and shape of the intercalated anions plays a major role in view of onset potential for anion uptake as well as discharge capacity. Instead, also more complex coherences such as ion association, self-aggregation of ions as well as solvent co-intercalation have to be taken into consideration. In this work, we investigate the electrochemical intercalation of anions with different shapes and sizes from either IL or organic based electrolytes into a graphite-based cathode. Against this background, the influence of side chain modifications within the anion structure on the intercalation behavior was studied using different electrochemical analysis techniques. Therefore, in this contribution we will give new insights to the question if the anion size is the main issue to tailor the reversible capacity for anion intercalation. [1] A. Lewandowski, A. Swiderska-Mocek, Journal of Power Sources, 194 (2009) 601-609. [2] T. Placke, P. Bieker, S.F. Lux, O. Fromm, H.W. Meyer, S. Passerini, M. Winter, Zeitschrift für Physikalische Chemie, 226 (2012) 391-407. [3] S. Rothermel, P. Meister, G. Schmuelling, O. Fromm, H.-W. Meyer, S. Nowak, M. Winter, T. Placke, Energy & Environmental Science, (2014). Figure 1

Currently, graphitic carbon is still the most commonly used anode material for commercial lithium ion batteries (LIBs) due to its relatively high specific capacity, high structural stability and low operation potential.(1) For graphitic materials, it is widely accepted that the surface chemistry and morphology of the prismatic surfaces have a significant impact on the reactivity in LIBs. Current strategies for optimization of this type of anode material, e.g. graphite surface modifications, focus on the improvement of the irreversible capacity in the first cycle and rate performance.(2) Regarding the need for high capacity anode materials in order to further increase the specific energy (Wh kg-1) and energy density (Wh L-1) of LIBs, silicon is one of the most promising candidates. Among different alloying elements, such as tin or germanium, silicon displays the highest reversible capacity of ca. 3,500 mAh g-1 and a relatively low lithiation potential of ca. 0.3 V vs. Li/Li+. However, drawbacks arise with the use of silicon, which are mainly related to the severe volume changes (up to 300%) during the lithium uptake/release process and the low intrinsic electronic conductivity.(3) The large stress and strain during lithiation/de-lithiation may lead to particle cracking and even particle pulverization resulting in a contact loss between the electrode components as well as into an ongoing formation of the solid electrolyte interphase (SEI) and an increase in the cell impedance.(3, 4) Several strategies have been developed in order to reduce the detrimental effects of the large volume changes of silicon and to suppress the side reactions with the electrolyte and thus to improve the long-term cycling stability (Figure 1).(3) One main approach is to reduce the silicon particle size to the nanometer range. Another strategy is to design porous structures, e.g. macroporous (pore width ˃ 50 nm) or mesoporous materials (2 to 50 nm), that can offer sufficient local void space to absorb the volume expansion.(5) A further strategy to improve the electrochemical performance of silicon is the formation of multiphase composite materials, i.e. by introducing components, which display no or only less volume changes as well as preferentially a high electronic conductivity. The idea of this approach is to use the host matrix to buffer the volume changes of silicon and thus to maintain the electrode integrity and electronic network. Examples are silicon-carbon composite materials and silicon-based alloys or composite heterostructures, such as Si/SiOx/carbon composites, or intermetallic compounds like Si/TiSi2 or NiSi alloys.(5, 6) Figure 1. Strategies to improve high-capacity anode materials for LIBs. Besides alloying-type anode materials, also conversion-type materials, such as transition metal oxides like Fe3O4 and MnO, which store charge via a conversion reaction (MOx + 2xLi ⇌ M + xLi2O), are of growing interest for next generation LIBs.(7) This is related to their high specific capacity of ca. 800 to 1200 mAh g-1 and the broad range of material combinations (oxides, sulfides, etc.). However, these materials typically suffer from a large irreversible capacity in the first charge/discharge cycle and from a relatively poor cycling stability.(7) Here, we report on the improvement of graphite-based anode materials as well as high-capacity materials based on alloying and conversion mechanisms for application in LIBs. The latter ones include the optimization of group IV elements such as silicon and germanium (porous and core-shell structures) and conversion materials like ZnFe2O4, with a special focus on the metal-ion dissolution mechanism. References 1. T. Placke, V. Siozios, R. Schmitz, S. F. Lux, P. Bieker, C. Colle, H. W. Meyer, S. Passerini and M. Winter, Journal of Power Sources, 200, 83 (2012). 2. T. Placke, V. Siozios, S. Rothermel, P. Meister, C. Colle and M. Winter, Zeitschrift für Physikalische Chemie (2015). 3. M. N. Obrovac and V. L. Chevrier, Chemical Reviews, 114, 11444 (2014). 4. M. Winter, Zeitschrift Für Physikalische Chemie-International Journal of Research in Physical Chemistry & Chemical Physics, 223, 1395 (2009). 5. W.-J. Zhang, Journal of Power Sources, 196, 13 (2011). 6. H. Jia, C. Stock, R. Kloepsch, X. He, J. P. Badillo, O. Fromm, B. Vortmann, M. Winter and T. Placke, ACS Applied Materials & Interfaces, 7, 1508 (2015). 7. M. V. Reddy, G. V. Subba Rao and B. V. R. Chowdari, Chemical Reviews, 113, 5364 (2013). Figure 1

Based on its redox-amphotheric character, graphitic carbon possesses the capability to store a broad range of different cations and anions within its graphene sheets. These materials are called graphite intercalation compounds (GIC). In case of intercalating cations, these compounds are defined as acceptor-type GICs and in case of intercalating anions they are called donor-type GICs[1-4]. One of the best known examples for an acceptor-type GIC is the storage of lithium ions within the graphitic host material, which is used as anode in conventional lithium-ion batteries. In recent publications, we reported an electrochemical energy storage system using both, the donor-type and acceptor-type mechanism in order to store anions and cations into the graphitic lattice structure[5, 6]. We presented this so-called “dual-graphite cell” which is composed of cost-efficient, safe and eco-friendly materials as a promising candidate for a stationary energy storage plant. The active material of both, the anode and cathode consists of graphitic carbon. Moreover, the ionic liquid Pyr14TFSI (N-butyl-N-methyl-pyrrolidiniumin bis(trifluoromethanesulfonyl) imide) in combination with the conductive salt LiTFSI (lithium bis(trifluoromethanesulfonyl) imide) is used as a non-flammable and thus safe electrolyte. During charge, cations and anions, stemming from the electrolyte, are intercalated into the graphitic lattice structure of negative and positive electrode, respectively. During discharge, they are released back into the electrolyte. Hence, the electrolyte can be considered as active material, as it does not only work as charge carrier for lithium ions[4, 7]. Here, we present the synthesis of spherical graphite particles, their physical as well as their electrochemical characterization. Carbon spheres (CS) of different sizes were produced by a hydrothermal method based on glucose as a cheap, renewable and environmentally friendly precursor. Then, the carbon spheres were heat-treated at 1200 °C in order to decompose the remaining functional groups of the glucose structure and in order to obtain a soft carbon structure. Afterwards, the soft carbon spheres (SCS) were finally graphitized at temperatures of 2100 °C, 2400 °C and 2800 °C. The aim of this work is the investigation of the graphitization degree and the influence of the structure changes regarding to the electrochemical performance in dual-graphite cells. References [1] W. Rüdorff, U. Hofmann, Zeitschrift für anorganische und allgemeine Chemie, 238 (1938) 1. [2] M.S. Dresselhaus, G. Dresselhaus, Advances in Physics, 51 (2002) 1-186. [3] J.R. Dahn, J.A. Seel, Journal of the Electrochemical Society, 147 (2000) 899-901. [4] T. Placke, O. Fromm, S.F. Lux, P. Bieker, S. Rothermel, H.W. Meyer, S. Passerini, M. Winter, Journal of the Electrochemical Society, 159 (2012) A1755-A1765. [5] S. Rothermel, P. Meister, O. Fromm, J. Huesker, H.W. Meyer, M. Winter, T. Placke, ECS Transactions, 58 (2014) 15-25. [6] S. Rothermel, P. Meister, G. Schmuelling, O. Fromm, H.-W. Meyer, S. Nowak, M. Winter, T. Placke, Energy & Environmental Science, (2014). [7] T. Placke, P. Bieker, S.F. Lux, O. Fromm, H.W. Meyer, S. Passerini, M. Winter, Zeitschrift für Physikalische Chemie, 226 (2012) 391-407. Figure 1

Germanium (Ge) has received much attention when compared to other alloy anode materials like silicon [1, 2], due to its still high enough theoretical capacity of 1,600 mAh g-1 (4.4 Li+ ions per Ge atom), good lithium ion diffusivity (more than 400 times higher than in silicon at room temperature: 1.41 × 10-14 cm2 s-1 for Si and 6.51 × 10-12 cm2 s-1 for Ge), high electronic conductivity (104 times higher than silicon)[3] and a lower specific volume change during the lithium insertion/extraction process [4]. Nevertheless, the, compared to silicon, tin or other lithium storage metals, high price of germanium is the major drawback for the commercialization of this anode material.[5] Furthermore, as with silicon, tin and their derivatives [6, 7] severe particle pulverization can be triggered by a large volume change during the lithium alloying (to form LixGe) and de-alloying processes (to reform Ge), which results in electronically disconnected germanium particles as well as a continuous reactivity of the anode with the electrolyte. As a result, the electrode suffers from a rapid capacity fading upon cycling. Many approaches have been pursued to accommodate the volume changes of germanium, such as preparation of Ge/carbon composites[8], fabrication of Ge-based alloy composites (Sn22/Ge78/Carbon)[9] and designing a special morphology of germanium (e.g. nanoparticles[10], nanotubes[11] and nanowires[12]). In recent years, more attention has been paid to these anode materials to avoid the common problems of ultra-fine powders (especially those with a particle size of less than 100 nm), such as a strong aggregation tendency, which results in increased lithium ion diffusion pathways, their difficulty to handle and an enhanced binder requirement leading to a lower energy density. The basic strategy is that a porous structure can offer sufficient local void space to absorb the volume expansion of alloys like silicon, tin or germanium and thereby to improve the cycling stability. In order to buffer the large volume changes during lithiation/de-lithiation and offer an easier electrolyte penetration route, the concept of this work is to create the active material with a macro-porous structure. Moreover, the open structure of the porous particles is favorable for fast transport of Li+and gives rise to a high charge/discharge rate performance. In this work, we report a facile synthesis route, i.e. a simple heat treatment procedure, for the formation of three-dimensional porous germanium particles by a magnesiothermic reduction method (2Mg + GeO2 → 2MgO + Ge). The synthesized GeO2 which shows a hexagonal-like morphology with an average particle size of about 700 nm, as shown in the SEM and TEM images in Figure 1a and b, respectively, serves not only as the template, but also as the germanium source in the magnesium reduction procedure. XRD characterization illustrates that it is feasible to obtain germanium via a magnesium reduction reaction between Mg and GeO2. Three-dimensional porous germanium particles, which basically retain the original morphology of GeO2 (Figure 1c and d), can be obtained by this synthesis route. The porous germanium electrode displays a stable reversible capacity of 1,131 mAh g-1 for 200 cycles at 1C and a high rate capability of up to 5C. This great improvement can be attributed to the optimized nanostructure related to the uniformly distributed pores and very thin pore walls in the range of 30 nm. Figure 1. a) SEM and b) TEM images of GeO2; c) and d) SEM images of porous germanium. References [1] D.Y. Chen, X. Mei, G. Ji, M.H. Lu, J.P. Xie, J.M. Lu, J.Y. Lee, Angew. Chem.-Int. Edit., 51 (2012) 2409-2413. [2] H.P. Jia, P.F. Gao, J. Yang, J.L. Wang, Y.N. Nuli, Z. Yang, Adv. Energy Mater., 1 (2011) 1036-1039. [3] K.H. Seng, M.-H. Park, Z.P. Guo, H.K. Liu, J. Cho, Angew. Chem.-Int. Edit., 51 (2012) 5657-5661. [4] H. Lee, J. Cho, Nano Lett., 7 (2007) 2638-2641. [5] M.G. Kim, S. Sim, J. Cho, Adv. Mater., 22 (2010) 5154-5158. [6] M. Winter, J.O. Besenhard, J.H. Albering, J. Yang, M. Wachtler, Prog.Batteries Battery Mater. 1998, 17, 208−214. [7] T. Tabuchi, N. Hochgatterer, Z. Ogumi, M. Winter, J. Power Sources, 188 (2009) 552-557. [8] K.H. Seng, M.H. Park, Z.P. Guo, H.K. Liu, J. Cho, Angew. Chem.-Int. Edit., 51 (2012) 5657-5661. [9] H. Lee, J. Cho, Nano Lett., 7 (2007) 2638-2641. [10] G. Cui, L. Gu, L. Zhi, N. Kaskhedikar, P.A. van Aken, K. Muellen, J. Maier, Adv. Mater., 20 (2008) 3079-3083. [11] G. Cui, L. Gu, N. Kaskhedikar, P.A. van Aken, J. Maier, Electrochim. Acta., 55 (2010) 985-988. [12] M.-H. Seo, M. Park, K.T. Lee, K. Kim, J. Kim, J. Cho, Energy Environ. Sci.,4 (2011) 425-428. Figure 1

Regarding the current need for high capacity anode materials in order to further increase the specific energy (Wh kg-1) and energy density (Wh L-1) of LIBs, silicon is one of the most promising candidates. Among different alloying elements, such as tin or aluminum, silicon displays the highest theoretical capacity of 4,200 mAh g-1 (practical: ca. 3,500 mAh g-1) and a relatively low lithiation potential of 0.2 V vs. Li/Li+, compared to the conventional carbonaceous anode materials, displaying a relatively low specific capacity (372 mAh g-1), which cannot meet the growing demand for the next-generation high performance lithium-ion batteries. However, there are some critical drawbacks arising with the use of silicon, which are mainly related to the severe volume changes (up to 300%) during the lithium insertion/extraction process and the low intrinsic electronic conductivity.[1,2] The large stress and strain during lithiation/de-lithiation may lead to particle cracking and even particle pulverization. The volume changes of alloying materials in general also lead to a contact loss between the active material particles and the conductive carbon and/or current collector as well as to an ongoing formation of the solid electrolyte interphase (SEI; dynamic process of breaking off and reforming) and an increase in the overall resistance, thus resulting in an enhanced capacity fading during charge/discharge cycling.[3,4] Several strategies have been developed in order to reduce the detrimental effects of the large volume changes of silicon and to suppress the side reactions with the electrolyte and thus to improve the long-term cycling stability.[3] One main approach is to reduce the silicon particle size to the nanometer range, while several material structures, including e.g. nanowires[5], nanospheres[6] and nanotubes[7], have been proposed. In addition, another strategy is to design porous structures, e.g. macroporous (pore width ˃ 50 nm) or mesoporous materials (pore width of 2 to 50 nm), that can offer sufficient local void space to absorb the volume expansion of silicon and thus to enhance the cycling stability. A further main strategy to improve the electrochemical performance of silicon is the formation of multiphase composite materials, i.e. by introducing a second or more components, which display no or only less volume changes as well as preferentially a high electronic conductivity. The idea of this approach is to use the host matrix to buffer the volume changes of silicon and thus to maintain the electrode integrity and electronic network. Examples are silicon-carbon composite materials, e.g. silicon particles dispersed in a carbon composite matrix[8], and silicon-based alloys or composite heterostructures, such as Si/SiOx/carbon composites[9], or intermetallic compounds like Si/TiSi2[10] or NiSi alloys[11]. In this work, we report a facile synthesis route for a porous core-shell NiSi2/Si/carbon composite material and its electrochemical characterization as anode material for lithium-ion batteries. The as-prepared composite material consists of NiSi2, silicon and carbon phases, in which the NiSi2 phase is embedded in a silicon matrix having homogeneously distributed pores, while the surface of this composite is coated with a carbon layer (Figure 1). The electrochemical characterization shows that the porous and core-shell structure of the composite anode material can effectively absorb and buffer the immense volume changes of silicon during the lithiation/de-lithiation process. The obtained NiSi2/Si/carbon composite anode material displays an outstanding electrochemical performance, which gives a stable capacity of 1,272 mAh g-1for 200 cycles at a charge/discharge rate of 1C. The simple and high-yield synthesis process, using cheap and abundant raw materials, makes it more competitive for industrial applications. References [1] M.N. Obrovac, L. Christensen, Electrochem. Solid-State Lett., 7 (2004) A93-A96. [2] Martin Winter, Jürgen O. Besenhard, M.E. Spahr, P. Novák, Adv. Mater, 10 (1998) 725-763. [3] W.J. Zhang, J. Power Sources, 196 (2011) 13-24. [4] M. Winter, Z PHYS CHEM., 223 (2009) 1395-1406. [5] L.F. Cui, R. Ruffo, C.K. Chan, H. Peng, Y. Cui, Nano Lett., 9 (2009) 491-495. [6] H. Kim, M. Seo, M.H. Park, J. Cho, Angew. Chem.-Int. Edit., 49 (2010) 2146-2149. [7] M.H. Park, M.G. Kim, J. Joo, K. Kim, J. Kim, S. Ahn, Y. Cui, J. Cho, Nano Lett., 9 (2009) 3844-3847. [8] J. Yang, B.F. Wang, K. Wang, Y. Liu, J.Y. Xie, Z.S. Wen, Electrochemical and Solid State Letters, 6 (2003) A154-A156. [9] Y.S. Hu, R. Demir-Cakan, M.M. Titirici, J.O. Muller, R. Schlogl, M. Antonietti, J. Maier, Angew. Chem.-Int. Edit., 47 (2008) 1645-1649. [10] S. Zhou, X. Liu, D. Wang, Nano Lett., 10 (2010) 860-863. [11] H.Y. Lee, Y.L. Kim, M.K. Hong, S.M. Lee, J. Power Sources, 141 (2005) 159-162. Figure 1. (a-c) TEM images of the NiSi2/Si composite material; (d) TEM image of the NiSi2/Si/carbon composite material. Figure 1

Graphitic carbon provides a redox-amphoteric character, which leads to the capability to intercalate a broad range of different cations and anions between its planar graphene sheets, resulting in so-called graphite intercalation compounds (GICs). Graphite intercalation compounds, formed by cations are called donor-type GICs and those intercalated by anions are called acceptor type GICs [1-4]. A well-known application for a donor-type GIC is lithium-intercalated graphite (LiC6) as the anode material in lithium-ion batteries. In recent publications, we reported an electrochemical energy storage system using graphite as both anion and cation intercalation host. During charge, cations and anions, stemming from the electrolyte, are intercalated into the graphitic lattice structure of negative and positive electrode, respectively. During discharge, they are released back into the electrolyte. Hence, the electrolyte can be considered as active material, as it does not only act as charge carrier for lithium ions [4, 5]. Since a high oxidative stability of the electrolyte is mandatory, it is most likely composed of an ionic liquid and a suitable conductive salt, possessing the same anion as the ionic liquid. This system has been proposed as dual-graphite cell [6]. The electrolyte of the dual-graphite system used in this work is composed of the ionic liquid N-butyl-N-methyl-pyrrolidinium bis(trifluoromethanesul-fonyl) imide (Pyr14TFSI) using lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) as conductive and electro-active salt. This work deals with the synthesis of spherical graphite particles, their physical characterization and the application as active material in dual-graphite cells. First, carbon spheres, possessing different sizes, are synthesized by a hydrothermal bottom-up synthesis from glucose as precursor. Afterwards, a heat treatment at 1200 °C under argon atmosphere was performed in order to decompose the glucose structure to a soft carbon structure. Finally, the resulting soft carbon spheres were heat treated at different temperatures above 2000 °C in order to investigate the graphitization degree as well as the influence of the carbonaceous material structure to the electrochemical performance for application in a dual-graphite cell. The structure of the novel materials was characterized by X-ray diffraction, Raman spectroscopy, specific surface area investigations, scanning electron microscopy and electrochemical measurements. References [1] W. Rüdorff, U. Hofmann, Zeitschrift für anorganische und allgemeine Chemie, 238 (1938) 1. [2] M.S. Dresselhaus, G. Dresselhaus, Advances in Physics, 51 (2002) 1-186. [3] J.R. Dahn, J.A. Seel, Journal of the Electrochemical Society, 147 (2000) 899-901. [4] T. Placke, O. Fromm, S.F. Lux, P. Bieker, S. Rothermel, H.W. Meyer, S. Passerini, M. Winter, Journal of the Electrochemical Society, 159 (2012) A1755-A1765. [5] T. Placke, P. Bieker, S.F. Lux, O. Fromm, H.W. Meyer, S. Passerini, M. Winter, Zeitschrift für Physikalische Chemie, 226 (2012) 391-407. [6] S. Rothermel, P. Meister, G. Schmuelling, O. Fromm, H.-W. Meyer, S. Nowak, M. Winter, T. Placke, Energy & Environmental Science, 2014, 7(10), 3412-3423. Figure 1

In order to meet the requirements of high energy density as well as high power performance, a worldwide effort has been made to develop novel high-performance electrode materials to keep pace with the fast increasing market demands. Various compounds that form intermetallic phases (so-called “alloys”) with lithium have been proposed as alternative anode materials, such as silicon (Si) [1-2], tin (Sn) [3] and germanium (Ge) [4]. Compared to Si, less attention has been paid so far on Ge due to its higher costs. Nevertheless, its good lithium diffusivity (400 times higher than in silicon) and high electronic conductivity (ca. 104 times higher than silicon) enable it as a promising anode material for the next-generation LIBs. Currently, researchers report on different structures of Ge-based materials, such as nanotubes [5], nanowires [6], porous structures [7] and Ge-based composite materials [4]. So far, only a few studies focus on GeO2 as anode material. GeO2 is of interest for application as negative electrode material due to its high theoretical reversible capacity (1125 mAh g-1 based on 4.25 mol Li per mol Ge), low operation voltage (0.7 V to 0 V vs. Li/Li+) and higher thermal stability compared to Ge.[8] The reaction of GeO2 with Li involves two steps: 1) GeO2 + 4Li à Ge + 2Li2O; 2) xLi + Ge ⇌ LixGe (x ≤ 4.25). While the first reaction can be considered as to a large part electrochemically irreversible, resulting in a large irreversible charge loss by formation of Li2O, the lithiation/de-lithiation reaction in step 2) can be regarded as reversible, providing the discharge capacity of GeO2. The high irreversible capacity during the first lithiation process is also well-known for SnO2-based anodes and is one of the main reasons hindering so far the application in commercial lithium-ion batteries.[9] In general, the synthesis of GeO2 can be achieved by different methods, including hydrolysis reactions from a Ge precursor, chemical vapor deposition (CVD) and sputter processes. In this work, novel mesoporous three-dimensional GeO2 was successfully synthesized by a facile one-step synthesis method followed by mixing with graphene using a spray drying process. The as-prepared GeO2 shows a bean-like morphology with a particle size of 400 to 500 nm in length and 200 to 300 nm in diameter (Figure 1a-d). In a further step, we take the advantages of graphene (low-dimensional carbon material with high electronic conductivity; excellent mechanical properties; graphene can contribute to the capacity) to further enhance the electrochemical performance of b-GeO2. As illustrated in Figure 2, it can be clearly observed that the GeO2 particles are tightly anchored on or wrapped within the graphene sheets. Notably, during the mixing process, GeO2 particles are still homogenously dispersed within or firmly encapsulated by the graphene sheets. The b-GeO2 without any additional conductive surface layer shows a high reversible capacity for lithium storage of 845 mAh g-1 after 100 cycles, with nearly no capacity fading. When graphene was employed to be mixed with GeO2 via a spray drying method, the electrochemical performance is further significantly improved. The b-GeO2/graphene composite electrode gives a higher de-lithiation capacity of 1,021 mAh g-1 for 200 cycles at 0.2C and a high rate capability of up to 5C. This superior electrochemical performance benefits from its uniformly dispersed mesoporous structure, in which the GeO2 particles are homogeneously distributed on or within the conductive graphene sheets. Figure 1. Low-resolution (a, b) and high-resolution-resolution (c, d) SEM images of b GeO2. Figure 2. (a, b) SEM images of the b-GeO2/graphene composite prepared by spray drying method. References [1] Y. Yu, L. Gu, C. Zhu, S. Tsukimoto, P.A. van Aken, J. Maier, Adv. Mater., 22 (2010) 2247-2250. [2] H.P. Jia, P.F. Gao, J. Yang, J.L. Wang, Y.N. Nuli, Z. Yang, Adv. Energy Mater., 1 (2011) 1036-1039. [3] M. Winter, J.O. Besenhard, J.H. Albering, J. Yang, M. Wachtler, Prog. Batteries Battery Mater. 1998, 17, 208−214. [4] K.H. Seng, M.H. Park, Z.P. Guo, H.K. Liu, J. Cho, Angewandte Chemie (International ed. in English), 51 (2012) 5657-5661. [5] M.H. Park, Y. Cho, K. Kim, J. Kim, M. Liu, J. Cho, Angew. Chem.-Int. Edit., 50 (2011) 9647-9650. [6] C.K. Chan, X.F. Zhang, Y. Cui, Nano Lett., 8 (2008) 307-309. [7] L.C. Yang, Q.S. Gao, L. Li, Y. Tang, Y.P. Wu, Electrochem. Commun., 12 (2010) 418-421. [8] Y. Kim, H. Hwang, K. Lawler, S.W. Martin, J. Cho, Electrochimica Acta, 53 (2008) 5058-5064. [9] J.S. Chen, C.M. Li, W.W. Zhou, Q.Y. Yan, L.A. Archer, X.W. Lou, Nanoscale, 1 (2009) 280-285. Figure 1

The intercalation of anions into graphite forming so-called acceptor-type graphite intercalation compounds (GICs) [1] was presented for the first time by Rüdorff and Hofmann [2] in 1938. This storage mechanism was adopted and modified by McCullough et al. [3, 4] and Carlin et al. [5, 6] in their work about “dual-graphite” cells. Further studies on this topic were carried out by Seel and Dahn [7, 8], using a mixture of organic solvents and LiPF6. In this type of cells, a storage mechanism which is different from lithium-ion cells is shown, where upon charge the lithium ions intercalate into the graphitic anode, whereas the anions are simultaneously intercalated into the graphitic cathode. The reverse processes take place during discharge. Thus, the ions are released back into the electrolyte. However, since the use of organic solvent-based electrolytes, which suffer from the highly oxidizing conditions, yields in electrolyte degradation, only an insufficient coulombic efficiency was achieved. To face this problem, Read et al. [9] investigated an electrolyte composed of LiPF6 and a fluorinated carbonate (monofluoroethylene carbonate, FEC) with an improved oxidation stability. Despite this improved anodic stability, only an insufficient efficiency of around 96% was achieved. In recent publications [10, 11], we reported about an electrochemical energy storage system using an ionic liquid-based electrolyte as well as a lithium metal anode and a graphite cathode. To characterize the storage mechanism in this system we introduced the term “dual-ion” cell. Considering an identical storage mechanism presented in the references 3-9, one may conclude that the dual-graphite cell is just a special case of the dual-ion technology owing to an exchange of the anode material. Generally, ionic liquids possess different properties depending on the choice and combination of cation and anion. In particular, the ionic liquid N-butyl-N-methyl-pyrrolidinium bis(trifluoro-methanesulfonyl) imide (Pyr14TFSI) shows a high anodic stability, thus being an ideal candidate for dual-ion cells. Our studies have shown a high coulombic efficiency exceeding 99% for dual-ion cells utilizing a mixture of 1M LiTFSI and Pyr14TFSI. Due to the broad electrochemical stability window of Pyr14TFSI, the issue of graphite exfoliation arose by the application of this electrolyte in dual-graphite cells. Rothermel et al. [12] were able to solve this problem by the addition of the SEI-forming additive ethylene sulfite (ES). Based on the environmental, safety and cost benefits (e.g. no use of transition metals, but cheap graphite, non-flammability of the ionic liquid electrolyte), this technology might be a candidate for stationary energy storage. One strategy to improve the discharge capacity as well as the specific energy of the system is the usage of smaller anions. Due to the decreased dimensions, the graphite host structure is capable to accommodate a higher amount of anions and therefore, a higher discharge capacity is obtained. In addition, the replacement of the anion leads also to a change of the stage number of the GIC. In this contribution, the electrochemical performance of dual-ion cells using ionic liquid-based electrolytes with different-sized anions will be evaluated. In particular, the influence of the upper cut-off potential on the discharge capacity as well as the coulombic efficiency will be shown. Furthermore, a correlation between discharge capacity and the anion size will be discussed. As example the anion fluorosulfonyl-(trifluoromethanesulfonyl) imide (FTFSI-, Figure 1) will be presented. References [1] M.S. Dresselhaus, G. Dresselhaus, Advances in Physics, 51 (2002) 1-186. [2] W. Rüdorff, U. Hofmann, Zeitschrift für anorganische und allgemeine Chemie, 238 (1938) 1. [3] F.P. McCullough, A.F. Beale, U.S. Pat. 4,865,931, (1989). [4] F.P. McCullough, A. Levine, R.V. Snelgrove, U.S. Pat. 4,830,938, (1989). [5] R.T. Carlin, H.C. Delong, J. Fuller, P.C. Trulove, Journal of the Electrochemical Society, 141 (1994) L73-L76. [6] R.T. Carlin, H.C. de Long, J. Fuller, W.J. Lauderdale, T. Naughton, P.C. Trulove, C.S. Bahn, Materials for Electrochemical Energy Storage and Conversion - Batteries, Capacitors and Fuel Cells. Symposium, (1995). [7] J.A. Seel, J.R. Dahn, Journal of the Electrochemical Society, 147 (2000) 892-898. [8] J.R. Dahn, J.A. Seel, Journal of the Electrochemical Society, 147 (2000) 899-901. [9] J.A. Read, A.v.W. Cresce, M. Ervin, K. Xu, Energy & Environmental Science, 7 (2014) 617-620. [10] T. Placke, S. Rothermel, O. Fromm, P. Meister, S.F. Lux, J. Huesker, H.-W. Meyer, M. Winter, Journal of the Electrochemical Society, 160 (2013) A1979-A1991. [11] T. Placke, O. Fromm, S.F. Lux, P. Bieker, S. Rothermel, H.W. Meyer, S. Passerini, M. Winter, Journal of the Electrochemical Society, 159 (2012) A1755-A1765. [12] S. Rothermel, P. Meister, G. Schmuelling, O. Fromm, H.-W. Meyer, S. Nowak, M. Winter, T. Placke, Energy & Environmental Science, (2014). Figure 1

Tin is able to lithiate and delithiate reversibly with a high theoretical specific capacity, which makes it a promising candidate to supersede graphite as the state-of-the-art negative electrode material in lithium ion battery technology. Nevertheless, it still suffers from poor cycling stability and high irreversible capacities. In this contribution, we show the synthesis of three different nano-sized core/shell-type particles with crystalline tin cores and different amorphous surface shells consisting of SnOx and organic polymers. The spherical size and the surface shell can be tailored by adjusting the synthesis temperature and the polymer reagents in the synthesis, respectively. We determine the influence of the surface modifications with respect to the electrochemical performance and characterize the morphology, structure, and thermal properties of the nano-sized tin particles by means of high-resolution transmission electron microscopy, x-ray diffraction, and thermogravimetric analysis. The electrochemical performance is investigated by constant current charge/discharge cycling as well as cyclic voltammetry.

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