Polyelectrolyte Complex Coating for Mitigating Decomposition at Argyrodite and Conductive Carbon Interfaces in Solid‐State Batteries

Sulfide‐based solid electrolyte batteries (SEBs), which are an important type of solid‐state battery, show strong potential for commercializing solid‐state battery technology in large scale with high energy density. For delivering high capacity, maximum utilization of cathode active materials is a prime criterion, which can be attained using carbon additives to ensure electronic connectivity of all cathode particles. Fibrous carbon additives such as vapor‐grown carbon fibers (VGCFs) are often preferred in SEBs. However, degradation of sulfide‐based solid electrolytes such as Li 6 PS 5 Cl (LPSCl) at the interfaces with cathode active material and VGCF lowers cell capacity. Coating of carbon surfaces is a viable method to mitigate electrolyte decomposition. Here, we report a new polyelectrolyte‐based coating on VGCFs as protective interlayer for LiIn|LPSCl|LPSCl‐NCM‐VGCF ( LiIn SEB NCM ) cells. We use cyclic voltammetry to evaluate oxidation of electrolyte at the VGCF interface along with galvanostatic charge–discharge cycling. The polymer coating decreases argyrodite oxidation at the VGCF|LPSCl interface and improves cycling capacity. An interplay between coating thickness and aggregation of VGCF fibers is observed, which leads to an optimum of polymer coating layers to maximize cycling performance.