Single- to Few-Layer Nanoparticle Cathode Coating for Thiophosphate-Based All-Solid-State Batteries

Bulk-type solid-state batteries (SSBs) composed of lithium thiophosphate superionic solid electrolytes (SEs) and high-capacity cathode active materials (CAMs) have recently attracted much attention for their potential application in next-generation electrochemical energy storage. However, compatibility issues between the key components in this kind of battery system are difficult to overcome. Here, we report on a protective cathode coating that strongly reduces the prevalence of detrimental side reactions between CAM and SE during battery operation. This is demonstrated using preformed HfO₂ nanoparticles as a secondary particle coating for a layered Ni-rich oxide CAM, LiNi_0.85Co_0.1Mn_0.05O₂ (NCM85). The preparation of a stable dispersion of the HfO₂ nanoparticles enabled the deposition of a uniform coating of thickness ≤11 nm. When incorporated into Li₆PS₅Cl-based, pellet-stack SSBs, the coated NCM85 showed superior performance in terms of reversibility, cell capacity, longevity, and rate capability over its uncoated counterpart. The effectiveness of the protective coating in mitigating electro-chemo-mechanical degradation was investigated using a suite of physical and electrochemical characterization techniques. In addition, the adaptability to wet processing of the coated NCM85 is demonstrated in slurry-cast SSBs and liquid-electrolyte-based Li-ion cells.