Simulation of Li Plating in Si/Graphite Composite Electrodes

Kurzfassung

Li-ion batteries play a key role especially for electric vehicles and portable electronics. However, additional improvements are needed, for example, to achieve the fast charging criteria given by the automotive industry. The performance characteristics of the batteries such as high energy or power density can be tuned by the electrode microstructure, composition, or choice of materials. One such promising material for the negative electrode is Silicon: Si exhibits a high theoretical capacity and is very abundant. On the other hand, Si shows a large volume expansion and low Li mobility. Thus, to take advantage of the high theoretical capacity and to limit the deformation during cycling, Si is mixed with Graphite to produce more practical Si/Graphite composite electrodes. In order to increase the cycle life of Si containing electrodes, it is critical to trace the degradation processes responsible for their performance loss. One major aging mechanism causing fast degradation and fundamental safety risks is Li plating. This deposition of a metallic Li phase on the surface of Si/Graphite anodes is barely studied in the literature yet crucial to improve the performance and safety of state-of-the-art Li-ion batteries. In our contribution we present simulation results of Si/Graphite composite electrodes including models for Li plating on Graphite and Si particles in 3D microstructure-resolved simulations. While focusing on the differing lithiation behaviors of Graphite and Si, the findings are validated with experimental results from our project partners. More specifically, we compare our simulations to multiple complementary techniques such as neutron depth profiling (NDP), post-mortem glow discharge optical emission spectroscopy (GD-OES) depth profiling, and X-ray diffraction analysis (XRD). We examine commercial and self-manufactured full- and half-cells containing varying amounts of Si or SiC blend. Since inhomogeneities in the amount of Li plating were observed, studies on simplified half-cells are conducted to clarify the impact of relevant material parameters. Furthermore, we developed a homogenized p2D model of composite electrodes which also includes the volume changes during lithiation and delithiation of representative Si particles and the effect on transport processes. Not only provides the complementarity of sophisticated experimental and simulative studies a better understanding of how Li plating takes place in Si containing electrodes, but also enables an improved possibility to optimize the design of Si/Graphite composite electrodes.