Structure and Properties of Na-Se Alloys for Sodium Ion Battery: A First-Principles Study

Abstract

While selenium (Se) is recognized as a promising cathode material for sodium-ion battery (NIB), a fundamental understanding of the Na-Se electrochemical reactions upon sodiation is in its infancy. Based on density functional theory and ab initio molecular dynamics simulations, we evaluate single Na incorporation in the crystalline Se and also discuss the formation of Na-Se alloys in terms of structural evolution and energetics, along with their mechanical and diffusion properties. Our calculations clearly show that the inserted Na atom energetically prefers a heptagonal interstitial site, while cleaving the Se network bonds primarily due to the charge transfer from Na to the surrounding Se network. 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. We also found that increasing degree of sodiation leads to the disintegration of the chain-like bonded Se network into small chain. Lastly, we will address the influence of alloy composition on the diffusivity of Na and Se and mechanical properties in amorphous Na n Se.