Solid-State Reaction Heterogeneity during Calcination of Lithium-Ion Battery Cathode

Li-ion batteries, nickel-rich cathodes, synthesis during calcination, phase transitions with solid-state reaction, spatial distribution of local chemical compositions within the particles. During solid-state calcination, with increasing temperature, materials undergo complex phase transitions with heterogeneous solid-state reactions and mass transport. Precise control of the calcination chemistry is therefore crucial for synthesizing state-of-the-art Ni-rich layered oxides (LiNi1−x−yCoxMnyO2, NRNCM) as cathode materials for lithium-ion batteries. Although the battery performance depends on the chemical heterogeneity during NRNCM calcination, it has not yet been elucidated. Herein, through synchrotron-based in-situ structural analyses, gas analysis, X-ray microscopy, mass spectrometry microscopy, we provide a reaction map for Ni-rich layered oxides (LiNi1−x−yCoxMnyO2, NRNCM) cathodes during their calcination, which includes dehydration of the precursors, the insertion of ambient oxygen, and the insertion of solid-state Li+2-O2− after Li2CO3 thermal decomposition. The temperature-dependent reaction kinetics, the diffusivity of solid-state lithium sources, and the ambient oxygen determine the favorable aerobic decomposition of particle shell maintaining layered structure while the anaerobic decomposition of the particle core to lithium-blocking Ni-O rocksalt. Additionally, we found that the variations in the reducing power of the transition metals (i.e., Ni, Co, and Mn) determine the local structures at the nanoscale. The investigation of the reaction mechanism via imaging analysis provides valuable information for tuning the calcination chemistry and developing high-energy/power density lithium-ion batteries.