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

Summary:

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 volumetric energy density (2190 mAh cm-3 in lithiated state, for Li15Si4), silicon keeps being one of the most attractive anode materials to compensate the above requirement.
Up to date, various structural designs, especially nanoarchitectures, 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 challenges are par-tially mitigated via nanodesigning of silicon, the practical aspect towards its implementation in a real bat-tery 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 colum-nar 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 diverse aspects like effect of silicon loading on electrode deformation, post-mortem analysis, cycling performance at various electrochemical balancing factors (n/p:1.1-3.0) to meet the requirements of the next generation LIBs. An optimum balancing factor of 2.0 (oversized anode) leads to the volumetric energy density above 850 Wh L-1 (at 0.1 C) and stable cycling over 100 cycles in pouch cells, if the full cell potential is adjusted accordingly. Moreover, a tremendous rate performance is observable up to 5 C delivering energy densities of 450 Wh L-1. In addition, by controlling the surface morphology of Cu substrates via laser-structuring and electrochemical deposition (ECD) of Cu dendrites, we demonstrate the optimum surface roughness and contact area for on-growing silicon film on 18 µm Cu foils, which provides a capacity retention >90% after 50 cycles. Applying ECD with optimized parameters (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 mechanical integrity 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”).