Unraveling Atomically Irreversible Cation Migration in Sodium Layered Oxide Cathodes.

Transition metal (TM)-based layered oxides NaTMO (TM = Fe, Ni, Co, Mn, etc.) have been intensively pursued as high-capacity cathode materials for Na-ion batteries. Nevertheless, they still suffer from fast capacity loss and voltage decay, as a result of the layered structure instability upon extended electrochemical cycling. The mechanism underlying such instability remains poorly understood. Here we unravel the TM migrations and structural evolution of a quaternary NaNiCoMnFeO compound during electrochemical cycling using atomic-resolution electron microscopy and associated spectroscopies. We discover successive migrations of TM ions to Na layers that account for structure and performance degradations. The Fe ions migrate into the interstices of both tetrahedra and octahedra of the layers; on the contrary, the Ni ions migrate predominantly in the octahedral ones, and the Mn and Co ions mostly remain in the TM layers. Direct atomic-level observations of the TM migration process upon cycling offer deep insight into designing high-capacity and long-life span cathode materials for sodium-ion batteries.