Vacancy-Enabled O3 Phase Stabilization for Manganese-rich Layered Sodium Cathodes.

Manganese-rich layered oxide materials hold great potential as low-cost and high-capacity cathodes for Na-ion batteries. However, they usually form P2 phase and suffer from fast capacity fade. In this work, an O3 phase sodium cathode has been developed out of a Li and Mn-rich layered material by leveraging the creation of transition metal (TM) and oxygen vacancies and the electrochemical exchange of Na and Li. The Mn-rich layered cathode material remains primarily O3 phase during sodiation/desodiation and can have a full sodiation capacity of ~220 mAh/g. It delivers ~160 mAh/g specific capacity between 2-3.8V with >86% retention over 250 cycles. Systematic characterizations and computational studies revealed that the TM and oxygen vacancies pre-formed in the sodiated material enables a "reversible" migration of TMs from the TM layer to the tetrahedral sites in the Na layer upon de-sodiation and sodiation. The migration creates metastable states, leading to increased kinetic barrier that prohibits a complete O3-P3 phase transition, hence stabilizing the structure and battery performance. This work provides a critical insight on the role of vacancies in TM migration and stabilization of phase transition, and sheds light on the future development of high performance and stable cathode materials for Na-ion batteries and beyond.