Amorphous Ni‐Rich Li(Ni1−x−yMnxCoy)O2–Li2SO4 Positive Electrode Materials for Bulk‐Type All‐Oxide Solid‐State Batteries

All‐solid‐state batteries attract significant attention owing to their potential to realize an energy storage system with high safety and energy density. In this paper, a mechanochemical synthesis of novel amorphous positive electrode materials of the Ni‐rich LiNi1−x−yMnxCoyO2 (NMC)–Li2SO4 system suitable for oxide‐type all‐solid‐state batteries is reported. Through the mechanochemical treatment with Li2SO4, excellent formabilities of the electrode materials as those of ductile solid electrolytes are obtained. Owing to the deformability of the active material, a good electrode/electrolyte interface is provided simply by pressing at room temperature. In all‐oxide solid‐state cells using 80NMCs·20Li2SO4 (mol%) positive electrode materials, the cell capacity increases with the Ni content in the NMC. The all‐solid‐state cell using the 80NMC811·20Li2SO4 positive electrode active material exhibits a high capacity larger than 250 mAh g−1 in a voltage range of 1.6–4.8 V versus Li at 100 °C. Furthermore, bulk‐type all‐oxide solid‐state batteries (Li4Ti5O12/80NMC532·20Li2SO4 (mol%)) successfully function as secondary batteries with excellent cycle performances.