Improvement of Na Ion Electrode Activity of Metal Oxide via Composite Formation with Metal Sulfide
2017
The composite formation with a conductive metal sulfide domain can provide an effective methodology to improve the Na-ion electrode functionality of metal oxide. The heat treatment of TiO₂(B) under CS₂ flow yields an intimately coupled TiO₂(B)–TiS₂ nanocomposite with intervened TiS₂ domain, since the reaction between metal oxide and CS₂ leads to the formation of metal sulfide and CO₂. The negligible change in lattice parameters and significant enhancement of visible light absorption upon the reaction with CS₂ underscore the formation of conductive metal sulfide domains. The resulting TiO₂(B)–TiS₂ nanocomposites deliver greater discharge capacities with better rate characteristics for electrochemical sodiation–desodiation process than does the pristine TiO₂(B). The ²³Na magic angle spinning nuclear magnetic resonance analysis clearly demonstrates that the electrode activities of the present nanocomposites rely on the capacitive storage of Na⁺ ions, and the TiS₂ domains in TiO₂(B)–TiS₂ nanocomposites play a role as mediators for Na⁺ ions to and from TiO₂(B) domains. According to the electrochemical impedance spectroscopy, the reaction with CS₂ leads to the significant enhancement of charge transfer kinetics, which is responsible for the accompanying improvement in electrode performance. The present study provides clear evidence for the usefulness in composite formation between the semiconducting metal oxide and metal sulfide in exploring new efficient NIB electrode materials.
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