Multifunctional ion-conductive polymer coatings for high-performance sulfide solid-state batteries with Ni-rich cathodes†

Sulfide-based solid-state batteries (SSBs) are emerging as a top contender for next-generation rechargeable batteries with improved safety and higher energy densities. However, SSBs with Ni-rich cathode materials such as LiNi_0.82Mn_0.07Co_0.11O₂ (NMC82) exhibit several chemomechanical challenges at the cathode–electrolyte interface, such as contact loss and solid-electrolyte decomposition, resulting in poor interfacial Li⁺ ion transport. To overcome these challenges, we used polymerized ionic liquids (PIL) as coatings at the NMC82 cathode surface, with and without incorporating a lithium salt. The thin Li⁺ ion-conductive Li–PIL nanocoating shows excellent compatibility with sulfide solid electrolytes and enables efficient Li⁺ transfer over the cathode–solid electrolyte interface, as demonstrated by 2D solid-state exchange NMR. It also improves contact retention between the cathode–solid electrolyte particles and mitigates electrolyte oxidation-induced degradation. This is reflected in the electrochemical performance of coated NMC82 in sulfide SSBs, where both a higher rate performance (190 mA h g⁻¹vs. 163 mA h g⁻¹ for uncoated at 0.1C) and a remarkable capacity retention of 82.7% after 500 cycles at 0.2C and ambient conditions (20 °C) are observed. These results emphasize the effectiveness of PILs with Li salts as multifunctional coatings that enable high-performance sulfide-based SSBs with Ni-rich cathode materials at ambient temperature.