Hybrid Functional Study on Small Polaron Formation and Ion Diffusion in the Cathode Material NaMn(SO).

The crystal structure, electronic structure, and diffusion mechanism of Na ions in the cathode material NaMn(SO) are investigated based on the Heyd-Scuseria-Ernzerhof hybrid density functional method. The simultaneous motion model of polaron-sodium vacancy complexes was used to reveal the diffusion mechanism of Na ions in this material. Polaron formation at the Mn third-nearest neighbor to the Na vacancy was found. Two crossing and two parallel elementary diffusion processes of the polaron-Na vacancy complex were explored. The most preferable elementary diffusion process has an activation energy of 852 meV, which generates a zigzag-like pathway of Na-ion diffusion along the [001] direction in the whole material. Possessing a voltage of 4.4 V and an activation energy of 852 meV, NaMn(SO) is expected to be a good cathode material for rechargeable sodium ions.