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The interfacial decomposition products forming the so-called solid-electrolyte interphase (SEI) significantly determine the destiny of a Li-ion battery. Ultimate knowledge of its detailed behavior and better control are required for higher rates, longer life-time, and increased safety. Employing an electrochemical surface force apparatus, it is possible to control the growth and to investigate the mechanical properties of an SEI in a lithium-ion battery environment. This new approach is here introduced on a gold model system and reveals a compressible film at all stages of SEI growth. The demonstrated methodology provides a unique tool for analyzing electrochemical battery interfaces, in particular in view of alternative electrolyte formulations and artificial interfaces. B.M. acknowledges travel support by InnoEnergy Ph.D. School. The authors thank Carsten Dosche for additional XPS support. Furthermore, the funding of the Escalab250 Xi XPS spectrometer at Carl von Ossietzky University of Oldenburg by the Deutsche Forschungsgemeinschaft (INST 184/106-1 FUGG and INST 184/144-1 FUGG) is acknowledged. B.M. and H.W.C. performed the SFA experiments. C.M. and Q.H. helped with the experiments and produced thin films. B.M., Q.H., and M.O. provided XPS measurements. M.V. and F.U.R. conceived and designed the experiments. M.S. designed and carried out theoretical kinetic analysis. All authors analyzed data, discussed the results, co-wrote the paper, and commented on the manuscript.

The identification, fine‐tuning, and process optimization of appropriate hole transporting layers (HTLs) for organic solar cells is indispensable for the production of efficient and sustainable functional devices. In this study, the optimization of a solution‐processed molybdenum oxide (MoOx) layer fabricated from a combustion precursor is carried out via the introduction of zirconium and tin additives. The evaluation of the output characteristics of both organic photovoltaic (OPV) and organic light emitting diode (OLED) devices demonstrates the beneficial influence upon the addition of the Zr and Sn ions compared to the generic MoOx precursor. A dopant effect in which the heteroatoms and the molybdenum oxide form a chemical identity with fundamentally different structural properties could not be observed, as the additives do not affect the molybdenum oxide composition or electronic band structure. An improved surface roughness due to a reduced crystallinity was found to be a key parameter leading to the superior performance of the devices employing modified HTLs.

The unsatisfactory behaviour of the solid electrolyte interphase (SEI) on silicon-based anodes still prevents the commercial introduction of silicon in lithium-ion batteries – a milestone which would be a huge leap forward towards a sustainable energy use.[1] Our group has recently developed an electrolyte mixture improving the cycling performance of a silicon-based lithium-ion battery. For understanding the stabilizing effect of this novel electrolyte on the SEI, advanced characterization techniques had to be employed, such as a modified Surface Forces Apparatus (SFA) and synchrotron-based Hard X-ray Photoelectron Spectroscopy (HAXPES). So far, the SFA was used to research fundamental biologic and solution based interfacial processes.[2] Here we present a detailed investigation of the SEI on a silicon anode by designing a battery SFA cell for in-situ electrochemical measurements. This enabled us to monitor the SEI thickness during the different stages of the SEI formation on both silicon and graphite surfaces. Additionally, we report the mechanical properties of the SEI for different electrolytes which can also be addressed by the SFA setup. HAXPES data collected on a modified SEI on silicon based anodes reveal a complex nanostructure and provide an additional insight into the stabilization mechanism of the electrolyte mixture. Coin-cell testing confirm the stabilizing effect of the novel electrolyte mixture on the battery performance. The electrochemical battery SFA will be an important tool for evaluating novel electrolyte compositions and their effect on the mechanical properties and growth mechanism of the interfacial SEI. The introduction of this in-situ battery test cell for SFA opens the way for both fundamental interfacial insights as well as applied material studies in battery research. REFERENCES 1. E.C. Evarts, To the limits of lithium, Nature, 2015, 526, 593-595. 2. J. Israelachvili et al., Recent advances in the surface forces apparatus (SFA) technique, Rep. Prog. Phys., 2010, 73, 1-16. Figure 1

In the field of energy there is a continuous drive to increase available capacity. To satisfy this growing need for energy storage, the capacity available for Li-ion batteries should increase appropriately. For the cathode of LIB's a continuous exploration of new materials is in effect, while for the construction of the anode, carbonaceous materials have been in use since the first LIB’s commercialized by Sony in 1991. This material was not chosen because of its high capacity (approx. 372 mAh/g), but because of its stability against degradation due to a low volume expansion (10%). [1,2] In the differentiation towards new elements as to replace graphitic carbon as the anode in LIB's, silicon stands out due to its high volumetric (8330 mAh/cm³) and gravimetric (3579 mAh/g) capacity. This large increase in capacity being a full order of magnitude can be understood by looking at the reaction mechanism as Si undergoes an alloying reaction with Li rather than an intercalation reaction such as with carbonaceous materials. Unfortunately there is also a negative side to this alloying reaction which has prohibited its use in commercial batteries up until this point. Namely the anode material expands widely in volume (up to 275%). A bulk layer of Si would crack up when cycling the battery. A method to circumvent this volume-expansion problem exists, consisting of nanosizing the Si to spherical particles of a few 100 nanometres. These particles survive cycling during charge and discharge cycles, yet reveal a new problem. Due to the large volume changes no stable SEI can be formed on the particles. As with each volume expansion the SEI would crack and reveal new exposed silicon surface to the electrolyte. Thus each cycle more SEI is formed and lithium is irreversibly consumed. Using this silicon in a full cell would quickly deplete the available lithium. In order for silicon to be considered an alternative for commercial batteries a solution to this problem must be found. [3-6] In this work the authors demonstrate a novel way to combine the high capacity of silicon and the stable surface of carbon. The route of preference for this goal is the synthesis of a core/shell particle. However conventional core/shell particles would have the same problem as the SEI formation. Namely the shell would crack with each volume expansion rendering the protective surface ineffectual. To alleviate this problem free space must be introduced in the core/shell particle into which the silicon can expand freely to allow for the large volume expansion. This has been accomplished by the conformal coating of a sacrificial alumina layer on top of the silicon particles using a surfactant controlled synthesis step via the thermal decomposition route of a suitable aluminium precursor. Controlling the reaction time and precursor concentration allows for optimal control of the resulting coating so this can be matched to accommodate the volume expansion. On top of these silicon alumina core/shell particles a carbon layer is deposited by firing an aliphatic precursor. Conformal coating is achieved by dispersion in solution using the precursor as a surfactant. In a final step the alumina layer can be removed by a mild acidification. This technique results in a core/void/shell particle which can absorb the volume expansion internally and present a stable SEI forming carbon surface to the electrolyte. Electrochemical characterizations were carried out on the coin cell level showing a decrease in the irreversible capacity on the first cycles as well as enhanced capacity retention during lifetime testing. Total capacity was less than for pure silicon based anodes due to the need to incorporate inactive material in the formation of the core/shell particles. The novelty of this research lies in the use of alumina as the sacrificial layer, thus requiring only a mild acid treatment. As previous core/shell designs have made extensive use of TEOS based silica growth, thus requiring an aggressive hydrogen fluoride. In contrast the here proposed route requires only hydrogen chloride which does not corrode the underlying silicon particles. [7,8] [1] T. Nagaura and K. Tozawa, Prog. Batteries Sol. Cells. 9, 209 [2] J.L. Tirado, Mater. Sci. Eng. R. 40, 103 [3] A. N. Dey, J Electrochem. Soc. 118(10), 1547 [4] Tian et. al., J. Electrochem. Soc. 156(3) A187-A191 [5] Besenhard et. al., J. Power Sources 67, 87 [6] Winter et. al., Adv. Mat. 10, 725 [7] Hiu et. Al., Nano Lett. 2012, 12, 904−909 [8] Cui et. Al., Nature Nanotech 2012, 7, 310-315

Electrolyte decomposition and the formation of the solid-electrolyte interface SEI depends critically on the used active materials, electrolytes or electrolyte additives. A better knowledge of the initial steps of SEI formation is thus necessary to cope with new active anode materials or new electrolytes. The initial wetting as well as the further SEI growth are critical for the stability of the applied battery system. Here we present the use of different surface sensitive techniques to address the SEI formation. Next to Synchrotron-based high-energy or hard x-ray XPS (HAXPES) or AFM we belief that studies employing a surface force apparatus (SFA) may play a more prominent role in the future. We report first results on using an electrochemical SFA setup which was modified for addressing battery interfaces. The SFA technique detects changes in layer thickness together with the mechanical behavior of the growing surface films. The potential-induced SEI formation was followed first on Au, but also the initial wetting behavior of a EC/DEC electrolyte on model electrodes such as mica, gold or graphene was studied providing an unprecedented insight in ultrathin wetting layer formation under confined conditions.

The importance of water on molecular ion structuring and charging mechanism of solid interfaces in room temperature ionic liquid (RTIL) is unclear and has been largely ignored. Water may alter structures, charging characteristics, and hence performance at electrified solid/RTIL interfaces and is utilized in various fields including energy storage, conversion, or catalysis. Here, atomic force microscopy and surface forces apparatus experiments are utilized to directly measure how water alters the interfacial structuring and charging characteristics of [C2mim][Tf2N] on mica and electrified gold surfaces. On hydrophilic and ionophobic mica surfaces, water-saturated [C2mim][Tf2N] dissolves surface-bound cations, which leads to high surface charging and strong layering. In contrast, layering of dry RTIL at weakly charged mica surfaces is weakly structured. At electrified, hydrophobic, and ionophilic gold electrodes, significant water effects are found only at positive applied electrochemical potentials. Here, the influence of water is limited to interactions within the RTIL layers, and is not related to a direct electrosorption of water on the polarized electrode. More generally, the results suggest that effects of water on interfacial structuring of RTIL strongly depend on both (1) surface charging mechanism and (2) interfacial wetting properties. This may greatly impact utilization/design of RTILs and surfaces for interface-dominated processes.

The interface with the electrolyte is a decisive feature in the design of composite battery electrodes, which typically contain active material particles, conductive material, and polymer binder. A detailed understanding of the electrolyte layering and wetting of materials such as graphene, graphites, metals such as aluminum or even gold, silicon, and metal oxides are ultimately important for improving crucial lithium-ion battery mechanisms. In particular the solid electrolyte interphase (SEI) and lithium-ion mobility at electrode interfaces are still not well understood. Here we use the (Electrochemical) Surface Forces Apparatus (SFA) (1) to address the wetting behavior of nano-scale-confined lithium-ion battery electrodes by battery electrolytes. After its introduction in the 1970s by Israelachvili and Adams (2), the SFA has been used to investigate interfacial phenomena in fields varying from geoscience to microbiology (3). Figure 1 shows a detail of the SFA-setup that was used for the experiments. White light is thereby guided through a flat contact area of two semi-transparent SFA disks, causing interference patterns which are analyzed using a spectrometer. Monitoring the pattern throughout the experiments allows to follow electrolyte wetting behavior in the confined contact area. Some of the investigated surfaces were mica, gold and graphene. For instance for graphene, a unique combination of rigid and mobile lithium-ion containing electrolyte layers was observed. Figure 2 shows a typical population mapping of a SFA-experiment which focused on the initial wetting behavior of a battery electrolyte (EC/DEC - LiPF6) on dry mica, gold and graphene surfaces. After an initial drop of about 1.8 Å in the inter-mirror distance, which is due to the excretion of a monolayer of adsorbed water from the initially dry interface, the wetting process of the electrolyte was only in the case of graphene able to overcome the contact force. Similar observations were made when nano-confined gaps were wetted with ionic liquids. Molecular beam interferometry is a versatile and powerful tool for studying such nanoscaled wetting processes using battery electrodes including ionic liquids or organic solvents. REFERENCES 1. M. Valtiner et al., Langmuir, 28, 13080 (2012) 2. J. N. Israelachvili and G. E. Adams, Nature, 262, 774 (1976) 3. S. Raman, M. Valtiner, et al., Nature Communications, 5, 5539 (2014) Figure 1

The identification, fine-tuning, and process optimization of appropriate hole transporting layers (HTLs) for organic solar cells is indispensable for the production of efficient and sustainable functional devices. In this study, the optimization of a solution-processed molybdenum oxide (MoOx) layer fabricated from a combustion precursor is carried out via the introduction of zirconium and tin additives. The evaluation of the output characteristics of both organic photovoltaic (OPV) and organic light emitting diode (OLED) devices demonstrates the beneficial influence upon the addition of the Zr and Sn ions compared to the generic MoOx precursor. A dopant effect in which the heteroatoms and the molybdenum oxide form a chemical identity with fundamentally different structural properties could not be observed, as the additives do not affect the molybdenum oxide composition or electronic band structure. An improved surface roughness due to a reduced crystallinity was found to be a key parameter leading to the superior performance of the devices employing modified HTLs. The work was supported by the FWO, The research Foundation of Flanders [project G041913N]. The author would also like to acknowledge Ilaria Cardinaletti and Jeroen Drijkoningen for the AFM support and Huguette Penxten for the assistance concerning the UV-Vis measurements.