Chemistry of Defects in Crystalline Na₂Se: Implications for the Na–Se Battery
2020
Liu, Zhixiao | Hu, Wangyu | Deng, Huiqiu
Selenium (Se) is a promising cathode material for next-generation alkali metal–ion batteries due to its high volumetric capacity and good conductivity. Using a first-principles approach, the present study systematically studies the electronic structure and the chemistry of defects of the final discharge product Na₂Se for Na–Se batteries. It is found that Na₂Se is insulating for free electron transfer. The charged native defects can act as carriers to transfer charges through crystalline Na₂Se. The hole polaron p⁺ has a very low diffusion barrier of 76 meV. However, p⁺ makes a minor contribution to the conductivity of Na₂Se due to its high formation energy of 1.92 eV. In the Na₂Se crystal, the negatively charged Na vacancy (VNₐ–) is the dominant defect with a formation energy of 0.71 eV, and its diffusion barrier is 258 meV which is much lower than that for other vacancies and interstitial defects. According to the present theoretical study, VNₐ– is the main charge carrier, and the corresponding ionic conductivity is 10–¹³ S cm–¹ at the room temperature. Such a low conductivity can limit the utilization of the active material and reduce the discharge voltage of Na–Se batteries, especially for Na–Se batteries based on the solid–solid reaction mechanism.
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