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100%-Si / NCM lithium-ion full cell investigation based on columnar silicon anodes with a tailored microstructure
Active materials for Lithium-ion batteries

The great demand of high volumetric energy density (>1000 Wh/L) for energy storage systems in electric vehicles and mobile devices pushes both academia and industry tremendously to outperform state-of-the-art (SoA) lithium-ion batteries (LIB) (~650 Wh/L). The limiting component in SoA-LIBs is the graphite anode comprising restricted volumetric capacity (740 mAh cm-3). With its high abundance and volumet-ric energy density (2190 mAh cm-3 in lithiated state, for Li15Si4), silicon keeps being one of the most at-tractive anode materials to compensate the above requirement.
Up to date, various structural designs, especially nano-architectures, are employed for silicon to resolve the issues like material pulverization, loss of electrical contact and evolution/accumulation of internal stress due to huge volumetric fluctuation (~400%) of silicon during (de-)lithiation. Although these chal-lenges are partially mitigated via nanodesigning of silicon, the practical aspect towards its implementation in a real battery cell system is often overlooked, where high initial coloumbic efficiency (ICE >90%), tap density, areal capacity (>2.0 mAh cm-2) and rate capability (>1.0 mA cm-2) are required. On the other hand, our columnar silicon film anode system (>5 µm), which is manufactured via a scalable PVD process, has adequate surface-to-volume ratio providing high ICE while its underlying structured current collector ensures intimate adhesion and electrical contact.
In this work, we consider NCM/Si cell systems not only in coin but also multi-layered pouch cells by di-verse aspects like monitoring thickness change over state of charge (SoC), post-mortem analysis, dilation behavior at various electrochemical balancing factors (n/p:1.1-3.3) during the battery operation to meet the requirements of the next generation LIBs. An optimum balancing factor (oversized anode) leads to significant reduction of volume expansion without sacrificing cycling stability, if the full cell potential is adjusted accordingly. In addition, by controlling the surface morphology of Cu substrates via laser-structuring and electrochemical deposition (ECD) of Cu dendrites, we investigate the optimum surface roughness and contact area for on-growing silicon film. Moreover, applying ECD with optimized parame-ters (size and distribution of dendrites) for a blank Cu foil with high tensile properties mitigates the plastic deformation of such thin substrates (~10 µm), preserving original thickness of the electrode which is again crucial for an outstanding volumetric energy density.

This work has received funding from the Federal Ministry of Education and Research (BMBF), support code 03XP0254 (“KaSiLi”) as part of the ExcellBatMat competence cluster.

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Thomas Abendroth, Jonas Schlaier, Oliver Lohrberg, Susanne Dörfler, Holger Althues, Stefan Kaskel

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