Elucidation of the Jahn-Teller effect in a pair of sodium isomer

Jahn-Teller distorted Mn(III) (t_2g³e_g¹) ions play a key role in the performance of manganese-based layered oxides. Here we show that there is an obvious relationship between the Jahn-Teller distortion of a trivalent manganese and the electrochemistry in a pair of Na isomer, namely orthorhombic and hexagonal P2-type Na_2/3Mn_0.9Ti_0.1O₂ having the same composition. It is found that more reversible phase transformations, higher working voltage and faster Na diffusion correlated with Jahn-Teller effect are found for distorted P′2-Na_2/3Mn_0.9Ti_0.1O₂ upon Na⁺ ions extraction/insertion. Such Jahn-Teller distorted Mn assisted Na migration enables that the orthorhombic Na_2/3Mn_0.9Ti_0.1O₂ delivers a high specific capacity of 204.0 mA h g⁻¹ with a 2.7 V average working voltage, reaching 550 Wh Kg⁻¹ with both better cycle stability (a capacity retention of 82.3% after 100 cycles) and enhanced rate capability (97.8 mA h g⁻¹ cycled at 10C) in Na cell in contrast with undistorted Na_2/3Mn_0.9Ti_0.1O₂. This strategy for understanding the Jahn-Teller effect of P2-type compounds at orbital energy level grasps new insight into designing high energy density positive electrode materials for Na-ion batteries.