Theoretical Study of the Charge Transport Mechanism in a Sodium Selenide (Na2Se) Cathode for Sodium-Selenium Batteries

Selenium (Se) is recognized as a promising cathode material for sodium-selenium (Na-Se) batteries because of its high electrical conductivity on the order of 10 -3 S m -1 and facile sodiation kinetics. Although the terminal product, Na 2 Se, could limit the overall battery conductivity, fundamental understanding of the charge transport mechanism and the formation of final product remains largely unexplored. Based on density functional theory, we evaluate the influence of each type of defect and strain on conductivity in Na 2 Se by calculating the formation energy and mobility. We found that hole polaron (p + ) can stably form on Se ion upon initial distortion. Our calculations also show that the negatively charged sodium vacancy (V Na - ) could be the major charge transport carrier with its lowest formation energy and fast mobility. While diffusion barrier of p + is considerably smaller than that of V Na - , the electronic transport related to p + is suggested to be negligible due to its high formation energy. Once the vacancy-polaron (V Na - ˗ p + ) complex forms, the charge transport could be hindered mainly due to its charge neutrality and the relatively large binding strength of the complex. Lastly, we will also discuss the influence of compression which could be induced by significant volume change of Na-Se alloys on the conductivity of Na 2 Se.