(358g) Charge Transport Mechanism in Sodium Selenide (Na2Se) for Sodium-Selenium Batteries: A First Principles Study

Sodium−selenium (Na−Se) batteries are recognized as promising energy storage systems because of their high theoretical volumetric capacity (~ 3250 mAh/cm³), which is of particular importance for portable devices and electric vehicles (EVs) with the limited battery packing space. Sodium selenide (Na₂Se) is the final discharge product for the Na−Se batteries and is expected to limit the overall battery conductivity because of the insulating nature. To effectively utilize the active materials, it is necessary to have a better understanding of the charge transport mechanism in Na₂Se. Based on first-principles simulations, we first evaluate the contribution of potential charge carriers (ionic and electronic defects) to the conductivity in Na₂Se by calculating the formation energies and diffusion barriers. Negatively charged sodium vacancies (V_Na^-) and positive sodium interstitials (Na_i⁺) are found to be the defects with the lowest formation energy (0.80 eV) in Na₂Se, followed by hole polarons (h_p⁺) with a formation energy of 1.95 eV. Our simulations demonstrate that while the V_Na^- has a higher diffusion barrier than the h_p⁺ (0.25 eV vs. 0.11 eV), its contribution to the conductivity is the most dominant, on the order of 10^-15 S/cm, due to the high concentration. On the other hand, the electronic conductivity mediated by the h_p⁺ is predicted to be negligible because of its low concentration. Once the vacancy−polaron (V_Na^- − h_p⁺) complex forms, the energy cost to break the pair is predicted to be 0.78 eV, implying that the charge transport could be hindered due to its charge neutrality. Our calculations also illustrate that compressive strain (which is potentially induced by surrounding host materials) could reduce both ionic and electrical conductivity of Na₂Se. Lastly, we will discuss a possible strategy to modify the ionic and electronic conductivities in Na₂Se by controlling the strain.