(696j) A First-Principle Study on the Structure and Properties of Na-Se Alloys for Sodium Ion Battery

While selenium (Se) is recognized as a promising cathode material for sodium-ion battery (NIB), fundamental understanding of the Na-Se electrochemical reactions upon sodiation is in its infancy. On the basis of density functional theory and ab initio molecular dynamics simulations, we evaluate single Na incorporation in the crystalline Se and investigate the formation of Na-Se alloys in terms of structural evolution and energetics, along with their mechanical and diffusion properties. Our calculations clearly evidence that the inserted Na atom energetically prefers a heptagonal interstitial site, while cleaving the Se network bonds. We also found that the inserted Na atoms undergo migration with low energy barrier of 0.16 (0.05) eV by jumping to adjacent heptagonal sites through intrachain (interchain) paths. When the Na content is sufficiently high, alloying between Na and Se becomes energetically favorable as evidenced by the negative mixing enthalpy, with the most stable state obtained at 66 at.% of Na. Our calculations also show that the increasing degree of sodiation leads to disintegration of the chain-like bonded Se network into small chain. Lastly, we will discuss the influence of alloy composition on the diffusivity of Na and Se and mechanical properties in amorphous Na_nSe.