No abstract

Electrochemical activities of NaNi0.5Mn0.5O2 and NaCrO2, having the analogous layered structure to LiCoO2, were investigated in 1 mol dm-3 NaClO4 propylene carbonate at room temperature. Almost all sodium ions were extracted from the NaNi0.5Mn0.5O2 and NaCrO2 electrodes by galvanostatic oxidation to 4.5 V accompanied with several phase transitions. Layered NaNi0.5Mn0.5O2 electrode showed a highly reversible capacity of 185 mAh g-1 as positive electrode in Na cell in the potential region between 2.5 and 4.5 V versus Na. A NaCrO2 electrode was hardly electroactive after oxidation up to 4.5 V. When galvanostatic cycling was carried in the limited potential domain between 2 and 3.5 V, both electrodes showed discharge capacities of 100 - 120 mAh g-1 with satisfactory capacity retention. Layered LiCrO2 (R-3m) and NaCrO2 (R-3m) possess the quite similar crystal structures and the same transition metal, nevertheless they were inactive and active in Li and Na cells, respectively.

To realize a reversible solid‐state MnIII/IV redox couple in layered oxides, co‐operative Jahn–Teller distortion (CJTD) of six‐coordinate MnIII (t2g3–eg1) 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 Na2/3MnO2 having different Mn stoichiometry, and investigate how the structural and stoichiometric difference influences electrochemical reaction. The distorted Na2/3MnO2 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 Na2/3MnO2. Furthermore, reversible phase transitions correlated with CJTD are found upon (de)sodiation for distorted Na2/3MnO2, providing a new insight into utilization of the MnIII/IV redox couple for positive electrodes of Na‐ion batteries.

No abstract