Enhancing the Cycling Stability of Amorphous-Based LiNiO₂-Li₂MnO₃-Li₂SO₄ Positive Electrodes: Insights from In Situ Transmission Electron Microscopy.

Abstract

Li-rich layered oxides offer high capacities for solid-state batteries. However, they suffer from sluggish ion transport, interfacial side reactions, and structural instability. Herein, we address these limitations using an amorphous-based LiNiO₂-Li₂MnO₃-Li₂SO₄ positive electrode that couples an S-enriched amorphous matrix with Ni-enriched nanocrystalline domains. A two-step process, in which LiNiO₂ and Li₂MnO₃ react prior to the addition of Li₂SO₄, suppresses Ni/Mn segregation and eliminates highly oxidized Li-Ni-O domains formed during charge-discharge reactions. In situ scanning transmission electron microscopy with electron energy-loss spectroscopy directly visualizes the evolution of the Li distribution and electronic states of O and Ni during cycling, confirming that both the nanoparticles and the amorphous matrix are electrochemically active, whereas localized deep charging is absent. The ductile amorphous-crystalline framework accommodates reversible volume changes without cracking and interfacial delamination. Consequently, sulfide-based solid-state cells obtained using the two-step synthesis route delivered 173 mAh/g initially and retained 79% capacity after 300 cycles at an aggressive 4.6 V vs Li cutoff voltage without any positive electrode coating. These findings demonstrate that cation homogeneity, nanoscale particle-size control, and flexibility provided by the amorphous matrix act synergistically to enhance the potential of Li-rich positive electrodes in solid-state batteries. The synthesis strategy and mechanistic insights presented herein provide a clear roadmap for the design of robust, high-capacity amorphous-based positive electrodes with extended lifetimes.