Charge Relaxation within Silicon/Graphite Anodes – A Multi-Method Study

As silicon/graphite (SiG) composites are more commonly used as the anode active material in commercial Li-ion batteries, investigation of the (de-)lithiation behavior of the blended anodes becomes increasingly important. In this study, the charge redistribution between graphite and silicon was investigated in graphite-NMC 622 and SiG (23 wt.-% Si) – NMC 622 bilayer pouch cells using in situ and operando X-ray diffraction (XRD). In addition to XRD, ex situ and in situ optical microscopy (IOM), as well as microstructural resolved simulations using digital twins of the cells, were used. Different SOC values (0%, 25%, 50%, 75%, and 100%) and two different C-rates (0.1C and 0.5C) were compared in cells during operation and in the relaxed state. Insights into the relaxation process at 75% SOC were gained by tracking of the charge redistribution in IOM cells. Ex situ optical microscopy measurements reinforced the findings of the IOM measurements. Both XRD and optical microscopy showed the disappearance of charge in the graphite component of the SiG anode during the relaxation period (≥24h) at SOC ≤75%, indicating a redistribution of Li from graphite into Si in the anode. The simulations allowed tracking of the concentration of Li in both active material components, verifying the observations on the charge relaxation processes observed in the XRD and microscopy experiments. The gained insights can support a better understanding of aging of blended SiG anodes during operation

Simulation of Li Plating in Si/Graphite Composite Electrodes

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