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Electrochemical Performance of Chemically and Solid State-Derived Chevrel Phase Mo6T8 (T = S, Se) Positive Electrodes for Sodium-Ion Batteries
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DOI：10.1021/jp509057w.png)
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Abstract
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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 CuxMo6S8 and CuxMo6Se8 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 mu m size cuboidal Cu1.8Mo6S8 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 1.22.2 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, NaxMo6T8 (T = S, Se; x similar to 1 and 3). Electrochemical impedance spectroscopy at similar to 0.1 V intervals during the sodiation/desodiation process illustrates that partial Na-ion trapping, resulted in an increase in charge transfer resistance, R-e, due to the formation of stable Na similar to 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 = 8, Se) as promising Na-ion positive electrodes for stationary electrical energy storage (EES) applications.
Keywords:
CLUSTER CHALCOGENIDES MO6X8
ENERGY-STORAGE
HIGH-CAPACITY
PHYSICAL-PROPERTIES
AB-INITIO
LITHIUM
INTERCALATION
MECHANISM
CATHODE
LI
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