Insights of the Electrochemical Reversibility of P2-Type Sodium Manganese Oxide Cathodes via Modulation of Transition Metal Vacancies.

Among cathode materials for sodium-ion batteries, Mn-based layered oxides have attracted enormous attention owing to their high capacity, cost-effectiveness, and fast transport channels. However, their practical application is hindered by the unsatisfied structural stability and the deficient understanding of electrochemical reaction mechanisms. Among these issues, the research of transition metal (TM) vacancy remains highly active due to their modulation roles on the anionic redox reactions, but their effects on structural and electrochemical stability remain obscure. Herein, based on Al-substituted P2-type Na2/3MnO2, we comprehensively investigate the effects of TM vacancies on the corresponding layered oxides. With several characterization techniques such as neutron diffraction, superconducting quantum interferometry, in situ X-ray diffraction, ex situ solid-state nuclear magnetic resonance techniques, and X-ray photoelectron spectroscopy, we determined the TM vacancy content and further revealed that higher content of TM vacancies (7.8%) in the transition layer is beneficial to mitigate the structure evolutions and maintain the P2 structure during cycling in voltage range 1.5-4.5 V, while the oxides with lower content of TM vacancies (1.6%) deliver higher discharge capacity but experience complicated phase transition, including stacking faults and P2-P2' transitions. It is demonstrated that regulating the contents of TM vacancies can be utilized as an effective strategy to tune the structure stability and electrochemical performances of layered sodium oxide cathodes.