Simulation of Li Plating in Si/Graphite Composite Electrodes

Abstract

It is a common perception that the demand for high-performance batteries is constantly increasing. While the primary characteristics depend on the application, decisive criteria are energy density, safety, cost, and sustainability of the batteries. Here, Li-ion batteries play a key role especially for electric vehicles and portable electronics. How-ever, 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 lithium 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 mod-els for Li plating on Graphite and Si particles in 3D microstructure-resolved simulations. While focus-sing on the differing lithiation behaviours of Graphite and Si, the findings are validated with experimental results from our project partners. More specifically, we compare our simulations to multiple complemen-tary techniques such as neutron depth profiling (NDP), post-mortem glow discharge optical emission spectroscopy (GD-OES) depth profiling, and X-ray diffraction analysis (XRD). Under examination are commercial and self-manufactured full and half cells containing varying amounts of Si or SiC blend. 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. The studies serve as a starting point to understand Li plating in Si containing electrodes and, eventually, the design of better Si/Graphite composite electrodes.