Electrochemical Performance of Chemically and Solid State-Derived Chevrel Phase Mo₆T₈ (T = S, Se) Positive Electrodes for Sodium-Ion Batteries

Chevrel phases, or CPs (Mo₆T₈; T = S, Se), can accommodate cations (Li⁺, Mg²⁺ etc.) within the Mo₆T₈ open framework at room temperature due to their unusually high electronic conductivity and ionic mobility and are hence proposed as positive electrodes for secondary batteries. However, cation insertion into Mo₆T₈ generates strong repulsion between the cation–cation or cation–Mo atoms, leading to partial charge trapping within the Mo₆T₈ structure. The present work examines CPs as positive electrodes for sodium-ion batteries. In this regard, ternary CPs of Cu_<i>x</i>Mo₆S₈ and Cu_<i>x</i>Mo₆Se₈ phase were prepared by solution chemistry and high energy mechanical milling (HEMM) routes, respectively, followed by acid leaching of copper. X-ray diffraction and scanning electron micrographs revealed the formation of 1–1.5 μm size cuboidal Cu_1.8Mo₆S₈ particles, whereas, HEMM of CuSe, MoSe₂ and Mo powder followed by heating leads to the formation of Cu₂Mo₆Se₈ phase. Results from cyclic voltammetry and galvanostatic cycling of Na/Mo₆S₈ and Na/Mo₆Se₈ cells within 1.2–2.2 V versus sodium revealed that two-step sodiation/desodiation reaction occurs with a gradual capacity fade due to Na-ion trapping within two terminal compositions, Na_<i>x</i>Mo₆T₈ (T = S, Se; <i>x</i> ∼ 1 and 3). Electrochemical impedance spectroscopy at ∼0.1 V intervals during the sodiation/desodiation process illustrates that partial Na-ion trapping resulted in an increase in charge transfer resistance, <i>R</i>_e, due to the formation of stable Na_∼1Mo₆S₈ phase after the first charge cycle. However, charge trapping continues to occur during the first and second cycles in the case of Mo₆Se₈ phase. Nevertheless, the ease of fabrication, stable capacity, and high Coulombic efficiency render Mo₆T₈ (T = S, Se) as promising Na-ion positive electrodes for stationary electrical energy storage (EES) applications