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Ni-rich layered oxides, in a general term of Li(Ni

Replacement of graphite with alloying and conversion materials, having high specific capacity, has emerged as versatile route to increasing the energy density of Li-ion batteries. A key challenge is the large volume change in these materials, which leads to an unstable solid electrolyte interphase (SEI). The use sacrificial electrolyte additives, such as fluoroethylene-carbonate (FEC), has been established as an effective strategy for considerably improving cycling stability, but a mechanistic understanding of the underlying processes has been lacking so far. Here, we present an in-depth chemical and morphological study of the FEC-based interphase on graphite and SnO2–graphite model electrodes. We found that the FEC decomposition products aggregate first into spherical particles, whose growth depends on the cell medium and follows the laws of crystal-growth theory, before forming a continuous carbonate-rich film. The discrimination of the chemical composition of the FEC-derived particles from the rest of the electrode was obtained by X-ray photoemission electron microscopy (XPEEM) due to the high lateral resolution of this technique. The obtained understanding of SEI formation in fluorine-rich electrolytes should help to guide future designs of sacrificial fluorine-based additives.

Improving the interfacial stability between the electrode and the electrolyte at high voltage is a key to successfully obtain high energy-density Li-ion batteries. Therefore, this study is dedicated to a novel multifunctional electrolyte additive, methoxytriethyleneoxypropyltrimethoxysilane (MTE-TMS), able to stabilize the interface of both Ni-rich layered LiNi0.85Co0.1Mn0.05O2 (NCM851005) cathode and graphite anode in a full-cell. Electrochemical tests reveal that the addition of 1 wt% MTE-TMS significantly improves the long-term cycling stability of the graphite‖NCM851005 full-cell, with an achieved maximum capacity of 198 mAh g−1 and its excellent capacity retention of 84% after 100 cycles at C/5 using upper voltage cut-off of 4.3 V vs Li+/Li. In contrast, the standard electrolyte in absence of MTE-TMS leads to a rapid performance fade. The significantly improved electrochemical performance is attributed to the formation of a stable surface protective film at both the cathode and the anode surfaces upon long-term cycling in elevated voltage window, and thus suppressing the electrolyte decomposition at and structural degradation of both cathode and anode, resulting as well in reduced transition metal transfer between the two electrodes.

Rendering the solid electrolyte interphase and the inter-particle connections more resilient to volume changes of the active material is a key challenge for silicon electrodes. The slurry preparation in a buffered aqueous solution offers a strategy to increase the cycle life and capacity retention of silicon electrodes considerably. So far, studies have mostly been focused on a citrate buffer at pH = 3, and therefore, in this study a series of carboxylic acids is examined as potential buffers for slurry preparation in order to assess which chemical and physical properties of carboxylic acids are decisive for maximizing the capacity retention for Si as active material. In addition, the cycling stability of buffer-containing electrodes was tested in dependence of the buffer content. The results were complemented by analysis of the gas evolution using online electrochemical mass spectrometry in order to understand the SEI layer formation in presence of carboxylic acids and effect of high proton concentration.

Preparing aqueous silicon slurries in presence of a low-pH buffer improves the cycle life of silicon electrodes considerably because of higher reversibility of the alloying process and higher resilience towards volume changes during (de)alloying. While the positive effects of processing at low pH have been demonstrated repeatedly, there are gaps in understanding of the buffer’s role during the slurry preparation and the effect of buffer residues within the electrode during cycling. This study uses a combination of soft and hard x-ray photoelectron spectroscopy to investigate the silicon particle interface after aqueous processing in both pH-neutral and citrate-buffered environments. Further, silicon electrodes are investigated after ten cycles in half-cells to identify the processing-dependant differences in the surface layer composition. By tuning the excitation energy between 100 eV and 7080 eV, a wide range of probing depths were sampled to vertically map the electrode surface from top to bulk. The results demonstrate that the citrate-buffer becomes an integral part of the surface layer on Si particles and is, together with the electrode binder, part of an artificial solid-electrolyte interphase that is created during the electrode preparation and drying.

Lithium-ion batteries based on single-crystal LiNi

The mass loading of Si–graphite electrodes is often considered as a parameter of secondary importance when testing their electrochemical performance. However, if a sacrificial additive is present in the electrolyte to improve the electrochemical performance, the electrode loading becomes the battery cycle-life-determining factor. The correlation between mass-loading, electrolyte additive, and binder type was investigated by analyzing the cycling behavior of Si–graphite electrodes, prepared with water-based binders, with mass loading ranging from 3 to 9.5 mg cm−2 and cycled with FEC electrolyte additive, while keeping electrolyte amount constant. A lower loading was obtained by keeping slurry preparation steps unchanged from binder to binder and resulted in a longer lifetime for some of the binders. When the final loading was kept constant instead, the performance became independent of the binder used. Since such results can lead to the misinterpretation of the influence of electrode components on the cycling stability (and to a preference of one binder over another in our case), we propose that a comparison of long-term electrochemical performance data of Si–graphite electrodes needs to be always collected by using the same mass-loading with the constant electrolyte and additive.

Lithium-metal batteries offer substantial advantages over lithium-ion batteries in terms of gravimetric and volumetric energy densities. However, their widespread practical use is hindered by safety concerns, often attributed to the poor stability of the metallic lithium interface, where electrochemical impedance spectroscopy (EIS) can provide crucial information. The EIS spectra of metallic lithium electrodes proved to be more complex than expected, especially when studying thin lithium metal foils. Here, it is identified that charge-transfer impedance becomes one of the main components of the EIS spectra, the magnitude of which is found to be strongly dependent on the native passivation layer of metallic lithium and on the nature of electrolyte. "Asymmetricity" of the EIS spectra in symmetric cells when separated the working and counter electrode contributions to the total impedance using three-electrode cells is also identified.

Developing a simple, cheap, and scalable synthetic method for the fabrication of functional nanomaterials is crucial. Carbon-based nanowire nanocomposites could play a key role in integrating group IV semiconducting nanomaterials as anodes into Li-ion batteries. Here, we report a very simple, one-pot solvothermal-like growth of carbonaceous germanium (C-Ge) nanowires in a supercritical solvent. C-Ge nanowires are grown just by heating (380–490 °C) a commercially sourced Ge precursor, diphenylgermane (DPG), in supercritical toluene, without any external catalysts or surfactants. The self-seeded nanowires are highly crystalline and very thin, with an average diameter between 11 and 19 nm. The amorphous carbonaceous layer coating on Ge nanowires is formed from the polymerization and condensation of light carbon compounds generated from the decomposition of DPG during the growth process. These carbonaceous Ge nanowires demonstrate impressive electrochemical performance as an anode material for Li-ion batteries with high specific charge values (>1200 mAh g–1 after 500 cycles), greater than most of the previously reported for other “binder-free” Ge nanowire anode materials, and exceptionally stable capacity retention. The high specific charge values and impressively stable capacity are due to the unique morphology and composition of the nanowires.

The identification of sulfur host materials, which can overcome issues associated with the insulating nature of sulfur and the shuttling of soluble lithium polysulfides, is crucial in order to realize the practical applications of lithium–sulfur (Li–S) batteries. Therefore, the development of conductive electrode architectures with novel, porous structures is essential to improve the electrochemical performance of Li–S cells. To this end, carbon inverse opals (IOs) were prepared using sacrificial polystyrene spheres, directly on a current-collecting substrate, and were infilled with sulfur via a solution infiltration method. The electrochemical performance of S-infilled carbon IOs (S-CIO), prepared with spheres of different diameters, is evaluated through cyclic voltammetry and galvanostatic cycling. We demonstrate that S-CIO provide long cycle life and stable specific charge, when tested as cathode materials for Li–S batteries. IO samples, prepared with smaller spheres (100 and 200 nm diameters), achieve significantly higher specific charge values than samples prepared with larger spheres. The electrochemical performance of our S-CIO is compared with conventional slurry electrodes containing CIO powder. Mechanical stress during slurry preparation, grinding or ball-milling of material, destroys the IO structure, resulting in worse electrochemical performance as compared to as deposited IO samples. Consequently, this report offers insight into the importance of retaining and optimizing the structure of conductive S-hosts for improving electrochemical performance of Li–S cells.

Developments in lithium-ion batteries for energy storage are currently focused on improving energy density, increase cycle life, and reducing cost to match targets set by the automotive industry. A...

The combination of SnO2 and graphite in a composite electrode allows obtaining a Li-ion battery negative electrode with enhanced specific charge. However, the reported cycling stability of these el...

Slurry preparation in a citric-acid-buffered aqueous solution at low pH has been established as a viable strategy for tackling the poor capacity retention of silicon electrodes. A number of studies ascribed the improved capacity retention to the formation of a silyl ester between the Si surface and carboxymethyl cellulose (CMC-Na). Most recent findings suggest that the citric acid itself interacts with the Si surface. Moreover, cross-linking reactions between the carboxylic acid and the binder can occur. In order to provide a comprehensive overview and to gain a better understanding of the reactions on the Si surface during slurry preparation, we review here previous results and interpretations and revisit earlier infrared (IR) studies, whose findings we link to our own IR studies of the impact of the slurry components, individually and combined. Specifically, we studied the interactions between the carboxylic acid, CMC-Na and Si particles, with the aim to clarify the effects of different amounts of carboxyl groups in carboxylic acids, namely glycolic, malic and citric acids with 1, 2 and 3 carboxyl groups, respectively. Furthermore, we demonstrate that the capacity retention of Si electrodes can be improved considerably with any of the acids studied.

Detrimental side-reactions of Ni-rich cathode active materials (CAMs) with the electrolyte have historically impeded the extension of the utilized voltage window to higher upper cut-off voltages. Doping and coating approaches are studied widely to further improve these materials and to reduce the intensity of bulk and surface degradation but suffer from poor control of film thickness and homogeneity, leading to partial doping of the bulk. We herein report the singular effect of a tantalum oxide (Ta2O5) thin film on Li[Ni0.8Mn0.1Co0.1]O2 (NMC811), generated by atomic layer deposition, offering the possibility of a high-level homogeneity and thickness control. After chemical analysis using X-ray photoelectron spectroscopy the composition of the deposited thin film is identified as a lithium tantalate chemistry (LiTaO3). At an early degradation stage, clear improvements directly attributed to the coating, such as suppressed exothermic side-reactions (−51%), reduced released gas amounts (−14.8%) and less charge-transfer resistance growth (2× lower) are observed. However, at an advanced degradation stage, the materials show similar cycle life, as well as similar gassing behavior and an even higher charge-transfer resistance growth as compared to the uncoated material. This study highlights the necessity of bulk stabilization and identifies the effect of surface coatings on undoped NMC811 without any doping influence.

Control of electrode-electrolyte interfacial reactivity at high-voltage is a key to successfully obtain high-energy-density lithium-ion batteries. In this study, 2-aminoethyldiphenyl borate (AEDB) is investigated as a multifunctional electrolyte additive in stabilizing surface and bulk of both Ni-rich LiNi0.85 Co0.1 Mn0.05 O2 (NCM851005) and graphite electrodes in a cell operated with elevated upper cutoff voltage of 4.4 V vs. Li+ /Li. The presence of AEDB in a full-cell inhibits structural degradation of both cathode and anode materials, suppressing crack formation, and reduces metal dissolution at the cathode and metal deposition at the anode. As a consequence, the interfacial resistance is significantly reduced. Moreover, this is a case where "the whole is greater than the sum of the parts": the effect of AEDB in half-cells is rather modest, whereas in full-cells its addition results in tremendous performance improvement. The graphite‖NCM851005 full-cell in the presence of AEDB has a capacity retention of 88 % after 100 cycles, even when the upper cutoff voltage is set to 4.35 V, corresponding to 4.4 V vs Li+ /Li, whereas with standard electrolyte under the same conditions it is only 21 %. The study shows a simple and easy approach to using Ni-rich cathodes in an extended voltage window and demonstrates the importance of full-cell testing for electrolyte additive selection.

Carbon inverse opals (IOs) were prepared via a facile synthesis approach using a sucrose-based precursor and polystyrene (PS) spheres as a sacrificial template. During IO preparation, polymer spheres are typically removed by dispersion in organic solvents, such as toluene or tetrahydrofuran. In this study, carbon IOs are prepared with and without removal of PS spheres by toluene to determine the influence of template removal prior to high-temperature treatment on the morphology and chemistry of the resulting carbons. Properties of samples are compared through a systematic investigation by electron microscopy, Fourier-transform infrared spectroscopy and Raman spectroscopy. We demonstrate that a commonly used processing step—polymer sphere template chemical removal—does not make any significant difference to the IO morphology. A correlation of Raman spectroscopy with SEM imaging and TGA analysis indicates that carbon IOs prepared without the solvent-treatment step are more ordered than samples prepared with this processing step. The key finding of this report is the simplified IO synthesis procedure, which can be adapted to the preparation of IOs of other materials besides carbon.

Silicon (Si) is a promising candidate to enhance the specific charge of graphite electrode, but there is no consensus in the literature on its cycling mechanism. Our aim in this study was to understand Si electrochemical behavior in commercially viable graphite/Si electrodes. From the comparison of three types of commercial Si particles with a producer-declared particle sizes of 30–50 nm, 70–130, and 100 nm, respectively, we identified the presence of micrometric Si agglomerates and the Si micro- and mesoporosity as the main physical properties affecting the cycling performance. Moreover, ex situ SEM, XRD, and Raman investigations allowed us to understand the lithiation/delithiation mechanism for each type of Si particles. For nanoscale Si particles, the entire Si particle is utilized, resulting in high specific charge, and the stress induced by the formation of Li15Si4 alloy upon deep lithiation is well managed within the Si mesoporosity. This leads to reversible cycling behavior and, thus, to good cycli...

Silicon (Si) is a promising additive for enhancing the specific charge of graphite negative electrodes in Li-ion batteries. However, Si alloying with lithium leads to an extreme volume expansion and in turn to rapid performance decline. Here we present how controlling the lithiation depth affects the performance of graphite/Si electrodes when different lithiation cutoff potentials are applied. The relationship between Si particle size and cutoff potential was investigated to clarify the interdependence of these two parameters and their impact on the performance of Si-containing graphite electrodes. For Si with a particle size of 30–50 nm, Li15Si4 is only formed for the potential cutoff of 5 mV vs Li+/Li, whereas using a higher cutoff of 50 mV has no impact on the performance. For larger Si nanoparticles, 70–130 nm in size, Li15Si4 is already formed at 50 mV. However, in these larger particles only 70% of the Si initially participates in the lithiation, independent of the cutoff potential (5 or 50 mV), and...

Symmetric cells comprising either a pair of positive (sulfur), blocking or negative (lithium) electrodes were investigated by means of electrochemical impedance spectroscopy, to break down the complexity of the lithium–sulfur chemistry and to understand how the individual cell components influence the transport properties of a full cell. We thoroughly analyzed the ageing of the negative electrode (lithium metal) interface under open circuit conditions as a function of the nature of species present in the electrolyte, including the influence of dissolved binder and/or long-chain polysulfides. In a similar manner, we investigated the positive electrode interface. Herein, the objective was to understand the impact of an individual electrode component on the impedance response of the symmetric cell, and, more importantly, to discern the role of the sulfur in charge transfer reaction(s).

Thin polymeric membranes such as Celgard are commonly used as separators in Li-ion batteries to ensure high volumetric energy density. Independently, for silicon-based electrodes fluoroethylene carbonate (FEC) is often added to the electrolyte to improve the cycling stability of the cell. Here we demonstrate that, counterintuitively, this separator–electrolyte combination negatively affects the performance of graphite–Si electrodes in half-cells. In a statistical evaluation of the cycling behavior of C–Si electrode cells with various separators and either with or without FEC addition, we show that by improving the solid electrolyte interphase on the silicon particles, FEC addition leads to inhomogeneous current distribution in the electrodes, therefore favoring lithium dendrite growth and leading to irreversible failure with Celgard. In contrast, self-recovery is observed with simple glass-fiber separators. Without FEC, neither dendrites nor failure are observed, but cells with Celgard suffer from poorer electrochemical performance, due to clogging by the thick polymeric layer formed using standard electrolytes, than cells with thicker and hydrophilic separators.

High-energy-density lithium-rich materials are of significant interest for advanced lithium-ion batteries, provided that several roadblocks, such as voltage fade and poor energy efficiency are removed. However, this remains challenging as their functioning mechanisms during first cycle are not fully understood. Here we enlarge the cycling potential window for Li1.2Ni0.13Mn0.54Co0.13O2 electrode, identifying novel structural evolution mechanism involving a structurally-densified single-phase A’ formed under harsh oxidizing conditions throughout the crystallites and not only at the surface, in contrast to previous beliefs. We also recover a majority of first-cycle capacity loss by applying a constant-voltage step on discharge. Using highly reducing conditions we obtain additional capacity via a new low-potential P” phase, which is involved into triggering oxygen redox on charge. Altogether, these results provide deeper insights into the structural-composition evolution of Li1.2Ni0.13Mn0.54Co0.13O2 and will help to find measures to cure voltage fade and improve energy efficiency in this class of material., Practical application of high-energy-density lithium-rich materials remains a challenge due to issues including voltage fade and poor energy efficiency. Here the authors report a novel densified phase together with a trick to recover capacity in these materials that could help in curing their practical limitations.

Ni-rich layer oxides Li[Ni x CoyMn1−x−y ]O2 (x ≥ 0.8, NCMs) are promising advanced cathode materials for high-energy Li-ion batteries because of their high specific capacity (≥ 200 mAh g−1) with an average discharge voltage of 3.8 V vs Li+/Li, as compared to the commercialized cathode materials (e.g. LiCoO2, LiFePO4).1 However, the instability of cathode–electrolyte interface causes the structural degradation of cathode active material and the electrolyte consumption, as well as gas evolution due to oxidative decomposition of electrolyte, resulting in a rapid capacity fading.2 Thus, improvement in the stability of cathode–electrolyte interphase is a key requirement to inhibit their structural degradation and enhance their electrochemical properties. The formation of a protective surface film via electrolyte additives is considered a cost-effective and reliable way to improve the cathode–electrolyte interfacial stability, as the stable surface film, uniformly distributed over the entire cathode surface, would prevent direct contact of oxide with the electrolyte, still allowing Li+ transport between the cathode and electrolyte.3 In this work, we report the high-performance NCM cathode through interfacial stabilization using a novel electrolyte additive. The details of surface film stability and formation mechanism, and their relation to gas evolution as well as cycling performance would be discussed. References: 1 S.-T. Myung, F. Maglia, K.-J. Park, C. S. Yoon, P. Lamp, S.-J. Kim, Y.-K. Sun, ACS Energy Lett. 2017, 2, 196 2 H. Q. Pham, E.-H. Hwang, Y.-G. Kwon, S.-W. Song, Chem. Commun., 2019, 55, 1256 3 K. Kim, Y. Kim, S. Parka, H. J. Yang, S. J. Park, K. Shin, J.-Je Woo, S. Kim, S. Y. Hong, N.-S. Choi, J. Power Sources 2018, 396, 276.

To date lithium-ion (Li-ion) batteries have been the de facto commercial battery type used in consumer electronics as well as in the first generation of electric vehicles (EVs). However, increases in the practical energy density of Li-ion batteries are too sluggish to keep pace with the demands of the next generation of portable electronics and EVs. Lithium Sulfur (Li–S) batteries are one of the most promising “beyond Li-ion” rechargeable battery systems in terms of both cost and specific energy density. (1, 2) Li–S batteries can deliver a practical specific energy density of 600 Wh/kg, which is more than double the values offered by state-of-the-art Li-ion batteries. (3, 4) However, there are still issues, which need to be addressed before widespread commercialization of Li–S batteries can be possible. Sulfur (S) and lithium sulfide (Li2S) have intrinsically low electronic conductivity. (5) Furthermore, the high-order lithium polysulfides (Li2Sx (6 < x ≤ 8)), which are formed upon first lithiation, are highly soluble in the standard liquid Li–S battery electrolyte. These various polysulfide anions are mobile and can be partially reduced and oxidized multiple times in the vicinity of the anode and cathode, respectively, leading to a so-called polysulfide shuttle phenomenon. (6) Additionally, the large volume changes (80%) which are associated with conversion reactions can lead to electrode disintegration and electrical isolation issues, resulting in severe capacity decay. (7) Various methods have been explored in an attempt to tackle these detrimental issues. Sulfur-carbon composites have been investigated to improve conductivity, following the pioneering work of the Nazar Group. (8) Efforts to reduce the polysulfide shuttle have been made via the application of various polysulfide-trapping interlayers, the use of functional separators and the exploitation of solid-state electrolytes. In this work, we detail the preparation of carbon nanospheres (CNS) with application as conductive sulfur host materials for Li–S batteries. CNS are prepared via facile hydrothermal treatment with different cost-effective precursors and the influence of each precursor on the structural and physical properties of the nanospheres is determined through examination of electron microscopy, X-ray diffraction, Raman spectroscopy, Fourier-transform infrared spectroscopy and gas adsorption data. The electrochemical performance of sulfur infilled CNS samples is evaluated through comparison of cyclic voltammetry and galvanostatic cycling data. We demonstrate that the choice of carbon precursor can have a significant influence on the performance of the CNS-based sulfur electrodes. Sulfur infilled CNS materials, which are morphologically similar, can have significantly different electrochemical response due to differences in their physical properties such as pore volume and pore structure. References: 1. A. Manthiram, Y. Fu, S.-H. Chung, C. Zu and Y.-S. Su, Chem. Rev., 114, 11751 (2014). 2. S. Urbonaite and P. Novák, J. Power Sources, 249, 497 (2014). 3. P. G. Bruce, S. A. Freunberger, L. J. Hardwick and J.-M. Tarascon, Nat. Mater., 11, 19 (2011). 4. D. Lv, J. Zheng, Q. Li, X. Xie, S. Ferrara, Z. Nie, L. B. Mehdi, N. D. Browning, J.-G. Zhang, G. L. Graff, J. Liu and J. Xiao, Adv. Energy Mater., 5, 1402290 (2015). 5. H. Chu, H. Noh, Y.-J. Kim, S. Yuk, J.-H. Lee, J. Lee, H. Kwack, Y. Kim, D.-K. Yang and H.-T. Kim, Nat. Commun., 10, 188 (2019). 6. Y. V. Mikhaylik and J. R. Akridge, J. Electrochem. Soc., 151, A1969 (2004). 7. F. Jin, S. Xiao, L. Lu and Y. Wang, Nano Lett., 16, 440 (2016). 8. X. Ji, K. T. Lee and L. F. Nazar, Nat. Mater., 8, 500 (2009).

Silicon is a long-standing candidate for replacing graphite as the active material in negative electrodes for Li-ion batteries, due to its significantly higher specific capacity. However, Si suffers from rapid capacity fading, as a result of the large volume expansion upon lithiation. As an alternative to pure Si electrodes, Si could be used, instead, as a capacity-enhancing additive in graphite electrodes. Such graphite–Si blended electrodes exhibit lower irreversible-charge losses during the formation of the passivation layer and maintain a better electronic contact than pure Si electrodes. While previous works have mostly focused on the Si properties and Si content, this study investigates how the choice of graphite matrix can alter the electrode properties. By varying the type of graphite and the Si content (5 or 20 wt%), different electrode morphologies were obtained and their capacity retention upon long-term cycling was studied. Despite unfavorable electrode morphologies, such as large void spaces and poor active-material distribution, certain types of graphites with large particle sizes were found to be competitive with graphite–Si blends, containing smaller graphite particles. In an attempt to mitigate excess void-space and inhomogeneous material distribution, two approaches were examined: densification (calendering) and blending in a fraction of smaller graphite particles. While the former approach led in general to poorer capacity retention, the latter yielded an improved Coulombic efficiency without compromising the cycling performance.

In this study, the use of silicon as a capacity-enhancing electrode additive to graphite electrodes was investigated. Based on energy-density estimations, the silicon amount was restricted to less than 20 wt%. The viability of such graphite–Si electrode blends was evaluated with regard to their cycle-life in relation to the silicon content of the electrode, showing how Si content and capacity fade are correlated. The addition of Si gradually alters the electrode morphology as shown by electrode cross-section images, and is partly responsible for the progressive capacity loss. In addition, we also demonstrate how analysis of the voltage hysteresis can provide early indications of cell failure. To complete the picture, one graphite–Si formulation was evaluated in a full-cell setup. The electrochemical study of this 3-electrode setup is complemented by a charge-injection experiment that replenishes the Li inventory and serves to pinpoint the origins of capacity fading more accurately under the cycling conditions chosen.

Lithium Sulfur (Li–S) batteries are one of the most promising “beyond-lithium-ion” rechargeable battery systems in terms of both cost and specific energy density. (1, 2) Sulfur is one of the most abundant elements in the Earth’s crust and offers a high theoretical specific charge of 1672 mAh/g. (3) Furthermore, Li–S batteries can deliver a practical specific energy density of 400-600 Wh/kg, which is more than double the values offered by state-of-the-art lithium-ion batteries. (4, 5) However, issues related to the inherent insulating nature of S and the polysulfide shuttle continue to hinder the widespread commercialization of Li–S batteries. Consequently, there has been a significant research effort to identify S hosts that can assist in mitigating these limitations. Porous carbon materials have attracted a great deal of attention due to their excellent conductivity, high surface area and light weight. (6) Activated carbons and hollow carbon spheres (HCSs) have been shown to significantly improve the electrochemical performance of Li–S batteries in terms of specific capacity and its retention. (7-10) Inverse opal (IO) structured electrodes offer many potential benefits as a S host material. The spherical voids of an inverse opal can be filled with S and ensure good access of the electrolyte to the electrode surface. The carbon IOs may act as a highly ordered, three-dimensional, conductive scaffold to reduce lithium polysulfide dissolution into the electrolyte during cycling, thus reducing the effects of the detrimental polysulfide shuttle. Additionally, during discharge S is reduced to form Li2S and with prolonged cycling this process leads to structural instability of electrodes due to the large volume expansion (~80%) caused by S conversion into Li2S. The spherical pores of a carbon IO scaffold may constrain the volume expansion due to the formation of Li2S and therefore increase capacity retention. IOs and HCSs are typically synthesised using silica nanospheres, which require treatment in hydrofluoric acid or long-term exposure to highly basic solutions to etch away the hard template. (11, 12) In this work we detail the facile synthesis of carbon IO structured samples using polystyrene nanospheres, which are easily removed via thermal treatment, and their application as a S host material for Li–S batteries. We present a structural characterization of our carbon IO samples via analysis of Raman spectra, Fourier-transform infrared spectra and X-ray diffraction patterns. The electrochemical performance of the carbon IO S-hosts is evaluated via cyclic voltammetry, rate capability testing and long-term galvanostatic cycling tests. We demonstrate that our S infilled carbon IO cathodes are capable of delivering high specific charges with stable capacity retention, achieving a reversible capacity of ~ 750 mAh/g after the 100 cycles at a C/5 rate. The morphology of the carbon IOs after cycling will also be shown via ex-situ scanning electron microscopy. We demonstrate that by preparing a highly ordered, conductive, three dimensionally interconnected network in the form of a carbon IO and then infilling this porous scaffold with S, we can achieve specific charge values which are greater than standard S/C composite slurry electrodes. References: S. Urbonaite, T. Poux and P. Novák, Adv. Energy Mater., 5, 1500118 (2015). A. Manthiram, Y. Fu, S.-H. Chung, C. Zu and Y.-S. Su, Chem. Rev., 114, 11751 (2014). M. Barghamadi, A. Kapoor and C. Wen, J. Electrochem. Soc., 160, A1256 (2013). P. G. Bruce, S. A. Freunberger, L. J. Hardwick and J.-M. Tarascon, Nat. Mater., 11, 19 (2011). D. Lv, J. Zheng, Q. Li, X. Xie, S. Ferrara, Z. Nie, L. B. Mehdi, N. D. Browning, J.-G. Zhang, G. L. Graff, J. Liu and J. Xiao, Adv. Energy Mater., 5, 1402290 (2015). A. Fu, C. Wang, F. Pei, J. Cui, X. Fang and N. Zheng, Small, 15, 1804786 (2019). R. Elazari, G. Salitra, A. Garsuch, A. Panchenko and D. Aurbach, Adv. Mater., 23, 5641 (2011). F. Pei, T. An, J. Zang, X. Zhao, X. Fang, M. Zheng, Q. Dong and N. Zheng, Adv. Energy Mater., 6, 1502539 (2016). H. Ye, Y.-X. Yin, S. Xin and Y.-G. Guo, J. Mater. Chem. A, 1, 6602 (2013). G. Zhou, Y. Zhao and A. Manthiram, Adv. Energy Mater., 5, 1402263 (2015). A. A. Zakhidov, R. H. Baughman, Z. Iqbal, C. Cui, I. Khayrullin, S. O. Dantas, J. Marti and V. G. Ralchenko, Science, 282, 897 (1998). Y. Xia, Z. Yang and R. Mokaya, J. Phys. Chem. B, 108, 19293 (2004).

Asymmetric separators with polysulfide barrier properties consisting of porous polypropylene grafted with styrenesulfonate (PP-g-PLiSS) were characterized in lithium-sulfur cells to assess their practical applicability. Galvanostatic cycling at different C-rates with and without an electrolyte additive and cyclic voltammetry were used to probe the electrochemical performance of the cells with the PP-g-PLiSS separators and to compare it with the performance of the cells utilizing state-of-the-art separator, Celgard 2400. Overall, it was found that regardless of the applied cycling rate, the use of the grafted separators greatly enhances the Coulombic efficiency of the cell. An appropriate Li-exchange-site (-SO3(-)) concentration at and near the surface of the separator was found to be essential to effectively suppress the polysulfide shuttle without sacrificing the Li-ion mobility through the separator and to improve the practical specific charge of the cell.

Silicon as an anode material in lithium-ion batteries (LIB) has the 10-fold storage capacity of lithium ions than graphite and is thus an interesting candidate for next generation LIB.[1] However, due to the sever volume expansion during the alloying reaction of Si with Li, the physical inter-particle connections mediated by the electrode binder are broken. As a result the electrode disintegrates and its capacity fades rapidly. The mechanical and interfacial properties can be enhanced when the Si electrodes are prepared in an aqueous solution of a citric acid buffer at pH=3, instead of neutral pH, such as water.[2] This behavior was previously ascribed to the acid-catalyzed formation of a silyl ester between binder and the native silicon oxide surface.[3] However, many questions still remain unanswered, e.g. what is the impact of the pH or how does the type of acid affect the surface functionalization? Additionally, recent results suggest that citric acid itself interacts strongly with the Si surface, thus forming an artificial SEI layer.[3] In our study we are providing a deeper insight in the role of the carboxylic acids by examining a series of different carboxylic acids, namely glycolic acid (GlyAc), malic acid (MalAc) and citric acid (CitAc) (Figure 1). The carboxylic acids carry a different number of functional groups, which helps to interpret the rather complex FTIR spectra (Fig. 1a) of these silicon:acid:binder composites. The impact of the carboxylic acid on other cell components (e.g. electrolyte salt or current collectors) was investigated by on-line mass spectroscopy, electron microscopy and electrochemical techniques (e.g. Fig. 1b). Our aim is to evaluate how the capacity retention of acid-treated Si electrodes could be further improved by rational choice of the buffer chemistry and to identify the key control parameters in the functionalization process and during slurry preparation (e.g. pH and acid strength) in order to advance the slurry fabrication process and the performance of Si-containing electrodes. References [1] Obrovac, M. N.; Chevrier, V. L. Alloy Negative Electrodes for Li-Ion Batteries. Chem. Rev. 2014, 114 (23), 11444–11502. [2] Mazouzi, D.; Lestriez, B.; Roué, L.; Guyomard, D.; Electrochem. Solid-State Lett. 2009, 12 (11), A215–A218. [3] Chandrasiri, K.W., C. C. Nguyen, B. S. Parimalam, S. Jurng, B. L. Lucht, J. Electrochem. Soc. 2018, 165, A1991–A1996 Figure 1

The demand for cost-effective rechargeable batteries with high gravimetric and volumetric energy density will continue to grow due to the rapidly increasing integration of renewable energy into the global energy scheme. In terms of specific energy, modern high-end rechargeable battery technology is reaching its fundamental limits, and no quantum leaps are expected in the advancement of the field in the foreseeable future. Specific energy simply represents the product of the specific charge (in Ah/kg) and the average voltage of an electrochemical cell during discharge. The highest specific energy gives the combination of metallic lithium with oxygen. In fact, many scientists argue that oxygen can be taken from air and, thus, its mass does not need to be considered in terms of specific charge. If the calculation is done very optimistically, assuming the ultimate reaction 2Li + 1/2O2 = Li2O and electrochemical potentials under standard conditions, such a lithium–air cell is characterized by a theoretical potential difference of about 4 V and a specific energy of ca. 15’000 Wh/kg. Obviously, this is not true because the assumed reaction product, Li2O, is stored inside the cell and, therefore, the mass of oxygen needs to be accounted for, reducing the hypothetical specific energy to ca. 7’000 Wh/kg. Furthermore, research revealed that the final reduction product is Li2O2 rather than Li2O, which further reduces the theoretical specific energy to ca. 4’000 Wh/kg. Besides that, the thermodynamical cell voltage of the lithium-oxygen couple is slightly below 3 V in a non-aqueous environment, which brings down the numbers to the theoretical upper limit of specific energy of ca. 3’000 Wh/kg. Considering, finally, that many inactive auxiliary cell components are required, and that these, in first approximation, can be accounted for by dividing the theoretical specific energy based on the active materials by a factor of four, the expected specific energy of a hypothetical industrial lithium–air battery would be slightly below 1’000 Wh/kg. Similar considerations must be made for all other relevant battery systems to truly assess their ‘real-life’ potential. Based on this type of considerations we developed an energy-cost model, which helps us to find the relationship between cost and energy density for different battery chemistries, and enables us to predict the most promising material combinations. The possibility of Li-ion batteries to operate at higher charging voltages (>4.2 V) than commercial cells on the market today will allow to extract higher amounts of charge without compromising coulombic and voltage efficiencies. In our opinion, this is the only way to significantly increase both gravimetric and volumetric energy densities for future lithium-ion battery systems. If a battery, in the best case, should last 10 years with about 300 cycles per year and 80% charge retention at the end of its life-time, a coulombic efficiency of at least 99.99% is needed. All active materials for positive electrodes, except for the Li-insertion compounds, are far from reaching such efficiencies today. Among the wide variety of proposed positive electrode materials only few show sufficient potential for commercialization, and, clearly, Li-rich and Ni-rich positive materials are definitely the winners, with Li–S and Na-ion emerging as contestants due to the low cost and abundance of their key components. As no further significant improvements in gravimetric/volumetric energy density and cost can be achieved through new battery chemistries, engineering efforts, targeting cost reduction and safety assurance, will most likely be the main driving forces behind future rechargeable battery development. Our energy-cost model underlines the importance of considering full-cell configuration, i.e., the pairing of commercially relevant electrodes in the same cell, for arriving at reasonably reliable energy estimates, instead of continuing half-cell based research. We similarly want to highlight the importance of the full-cell perspective when investigating other cell parameters such as power, cycle-life, safety, etc., because many performance-related issues, such as, for example, transition metal leaching, are not evident in a commonly used half-cell configuration.