Sodium and Manganese Stoichiometry of P2‐Type Na 2/3 MnO 2

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) (t2g (3) -eg (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 Na2/3 MnO2 having different Mn stoichiometry, and investigate how the structural and stoichiometric difference influences electrochemical reaction. The distorted Na2/3 MnO2 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 Na2/3 MnO2 . Furthermore, reversible phase transitions correlated with CJTD are found upon (de)sodiation for distorted Na2/3 MnO2 , providing a new insight into utilization of the Mn(III/IV) redox couple for positive electrodes of Na-ion batteries.