Direct Observation of Li-Ion Transport Heterogeneity Induced by Nanoscale Phase Separation in Li-rich Cathodes of Solid-State Batteries.

Li-rich layered oxide (LLO) cathode materials with high specific capacities could significantly enhance the energy density of all-solid-state lithium batteries (ASSLBs). However, the specific practical capacities of LLO materials in ASSLBs are extremely low due to poor initial activation. Here, scanning transmission electron microscopy with in situ differential phase contrast imaging was first used to study the initial activation mechanism of Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 . Li-ion transport heterogeneity was observed in LLO grains and across the LLO/Li 6 PS 5 Cl interface, due to the coexistence of the nanoscale Li 2 MnO 3 and LiNi 1/3 Co 1/3 Mn 1/3 O 2 phases. Consequently, the severely constrained activation of Li 2 MnO 3 during the first charging could be attributed to a nanoscale phase separation in LLO, hindering Li-ion transport through its particles, and causing high impedance in the Li 2 MnO 3 domain/Li 6 PS 5 Cl interface. This study could facilitate interface design of high-performance LLO-based ASSLBs.