Enhanced polysulfides Adsorption and Conversion for High Coulombic Efficiency Sodium‐Ion Batteries

Polysulfides: Transition metal sulfides has a low Coulombic efficiency in sodium storage due to the formation of the intermediate product polysulfide and chain reactions. We designed a hybrid with synergistic sodium storage and polysulfide adsorption capabilities to effectively improve the initial coulombic efficiency and cyclic stability of CoS₂.

Abstract

Transition metal sulphides based on conversion reaction mechanism have attracted much attention as anode materials for sodium-ion batteries owing to their theoretical specific capacity and potential long lifespan. The unsatisfactory Coulombic efficiency, however, inhabits the cycling lifespan due to the intermediate polysulfide formation (Na₂S _x ; 4≤x≤6) and chain reactions. To enhance the polysulfide polysulfide adsorption capability and capacity, a hybrid with synergistic sodium storage and polysulfide adsorption capability was designed, in which massive CoS₂ nanoparticles (CoS₂ NP) were utilized as electrons supplier to enhance the adsorption energy of three-dimensional nitrogen-doped carbon (3D-NC) framework towards sodium polysulfide. Combining the experimental and computational results, the encapsulated CoS₂ can dramatically optimize the local electronic structure and function as a powerful electron reservoir to increase the effective adsorption to Na₂S _x (4≤x≤6). Meanwhile, the electron transferring between CoS₂ NP and 3D-NC can facilitate the conversion of Na₂S to Na₂S _x . An ultra-high initial Coulombic efficiency of 98.05 % and 98.38 % can be achieved at current densities of 0.06 A g⁻¹ and 0.4 A g⁻¹, respectively. Beneficial from the high reversible capability, a superior cycling lifespan is obtained up to 800 long cycles at current densities of 0.6 A g⁻¹.

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