Rational Design of Electrolyte and Interface for High-Performance and Safer Solid-State Li Batteries

Solid-state Li metal batteries (SSLMB) gained increasing attention from researchers. Free of flammable organic liquid electrolytes by replacing solid-state electrolytes (SSE) greatly enhances battery safety, and employing Li metal anode possessing ultrahigh specific capacity and lowest redox potential delivers higher energy density than current state-of-art Li⁺ ion batteries. However, the safety crisis still exists, due to the extremely high reactivity of Li metal and the uneven plating-stripping behavior of Li anode. Unwanted side reactions pulverize SSE and electrode-electrolyte interface. Dendritic Li deposition accumulates dead Li, penetrates SSE, and eventually invokes internal short-circuit. Moreover, incomplete solid-solid contact between SSE and electrode induces severe polarization during battery operation. The poor interfacial compatibility of SSE to Li metal, both (electro-)chemically and physically, hinders the practical application of SSLMB so far.<br/>Here, we suggest some effective approaches to resolve interface issues in the SSLMB. Firstly, we prepared an artificial solid electrolyte interphase (SEI) layer using poly(vinyl alcohol) and hydroxyl-functionalized boron nitride nanosheets. The artificial SEI alleviates the side reaction between Li metal and electrolyte, which mitigates electrolyte degradation. We found that this artificial SEI also assists uniform Li plating-stripping and suppresses dendritic Li deposition, by boosting Li⁺ ion transport at the electrode-electrolyte interface.<br/>Secondly, we designed multi-layered SSE with inorganic-gel hybrid SSE and <i>in-situ</i> prepared SSE. The inorganic-gel hybrid layer supports excellent mechanical stability and fast Li⁺ ion conduction with a high Li⁺ ion transference number. The <i>in-situ</i> prepared SSE greatly improves interfacial compatibility between SSE and Li metal anode. It provides intimate contact at the interface and prevents inorganic electrolyte decomposition in the hybrid. Moreover, by comparing different layered SSEs, we found that the bulk electrolyte layer assists cycling stability, while the interfacial layer dictates the practical current density of layered SSE. When this multi-layered SSE is applied in the Li-S full cell, the layered SSE only allows polysulfide dissolution in the <i>in-situ</i> SSE layer and halts its further diffusion into the bulk electrolyte.