Sodium and Manganese Stoichiometry of P2‐Type Na_2/3MnO₂

To realize a reversible solid‐state Mn III/IV redox couple in layered oxides, co‐operative Jahn–Teller distortion (CJTD) of six‐coordinate Mn III (t 2g 3 –e g 1 ) is a key factor in terms of structural and physical properties. We develop a single‐phase synthesis route for two polymorphs, namely distorted and undistorted P2‐type Na 2/3 MnO 2 having different Mn stoichiometry, and investigate how the structural and stoichiometric difference influences electrochemical reaction. The distorted Na 2/3 MnO 2 delivers 216 mAh g −1 as a 3 V class positive electrode, reaching 590 Wh (kg oxide) −1 with excellent cycle stability in a non‐aqueous Na cell and demonstrates better electrochemical behavior compared to undistorted Na 2/3 MnO 2 . Furthermore, reversible phase transitions correlated with CJTD are found upon (de)sodiation for distorted Na 2/3 MnO 2 , providing a new insight into utilization of the Mn III/IV redox couple for positive electrodes of Na‐ion batteries.