Jahn–Teller type small polaron assisted Na diffusion in NaMnO2 as a cathode material for Na-ion batteries

Abstract

Na-ion diffusion kinetics is one of the main bottlenecks that limit the charge/discharge rate of the electrode materials in Na-ion batteries. In this paper, we identified that Na-ion diffusion is assisted by the Jahn–Teller (JT) type small polaron in the NaMnO₂ compound from density functional theory calculations. The structure of NaMnO₂ is modeled using two extreme cases of JT distorted structures, namely z-direction elongated E-NaMnO₂ and compressed C-NaMnO₂, both of which may occur in the structure of NaMnO₂ at environmental temperatures. The performance of the NaMnO₂ cathode material is evaluated. The average charge/discharge potentials are calculated to be 3.00 and 2.91 V for E-NaMnO₂ and C-NaMnO₂, respectively. The band gaps of E-NaMnO₂ and C-NaMnO₂ are 1.04 and 0.15 eV, indicating that the NaMnO₂ material composed of both E-NaMnO₂ and C-NaMnO₂ components is a small gap semiconductor. Ultra-low Na-ion migration energy barriers are observed. The lowest energy barriers for dilute Na-ion/vacancy diffusion in the fully charged/discharged state Na₀MnO₂/NaMnO₂ are found to be 0.29 and 0.36 eV, respectively. Interestingly, a strong coupling effect between Na-ions and JT-type polarons has been found in NaMnO₂. The low Na-vacancy migration energy barrier is a result of the collaborative migration of the JT-type polarons and the Na-vacancies. Considering the good stability, suitable extraction potential, excellent electronic conductivity, and outstanding ionic transport, NaMnO₂ is shown to have potential applications as a cathode material for Na-ion batteries.