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

Jahn-Teller distorted Mn(III) (t2g3eg1) 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 Na2/3Mn0.9Ti0.1O2 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-Na2/3Mn0.9Ti0.1O2 upon Na+ ions extraction/insertion. Such Jahn-Teller distorted Mn assisted Na migration enables that the orthorhombic Na2/3Mn0.9Ti0.1O2 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 Na2/3Mn0.9Ti0.1O2. 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.