Simultaneous Anionic and Cationic Redox in the Mo₃S₁₁ Polymer Electrode of a Sodium-Ion Battery

Using the density functional theory (DFT) calculations, we investigate the molybdenum sulfide polymer (Mo₃S₁₁) as an electrode for the sodium-ion battery. The ionic ordering of Na_xMo₃S₁₁ in the ground-state structures is determined by the DFT method. During the intercalation process of Na ions, we find that the Na_xMo₃S₁₁ structure exhibits a two-step reaction pathway involving both cationic and anionic redox reactions for Mo and S, respectively. In the first step, an initial anionic redox (S₂)^2- → S^2- (1 ≤ x ≤ 4) occurs, while in the second step, both anionic and cationic redoxes of (S₂)^2- → S^2- and Mo⁴⁺ → Mo³⁺ (4 < x ≤ 17) occur simultaneously. In total, the Na_xMo₃S₁₁ electrode can store up to 17 Na ions with a predicted capacity of 711 mA h/g. Moreover, a semiconductor-to-metal transition is observed during the cationic/anionic redox due to the appearance of mid-gap states. Mo₃S₁₁ thus is predicted to be a promising one-dimensional polymer electrode for the sodium-ion battery.