High-performance CoSbS-based Na-ion battery anodes

Abstract CoSbS and its hard carbon-containing nanocomposite (CoSbS@hard-C) are prepared via simple solid-state reactions, and their potential as Na-ion battery (NIB) anode materials is investigated. The electrochemical phase change mechanism of CoSbS during Na insertion/extraction is thoroughly investigated using various ex situ analytical tools. During Na insertion, the CoSbS undergoes a topotactic transformation owing to the formation of a NaxCoSbS (x ≤ 1.6), and then, it is converted into Na3Sb, Na2S, and Co in the fully Na-inserted state via a conversion reaction. Conversely, during Na extraction, Na3Sb transforms to Sb, which alloys with Co to form CoSb. In the fully Na-extracted state, CoSb and S from Na2S are recombined into CoSbS. Owing to the interesting reaction mechanism of CoSbS, the electrochemical performance of the CoSbS and CoSbS@hard-C anodes is excellent. Specifically, when CoSbS is used as a NIB anode, the topotactic transformation between CoSbS and NaxCoSbS (x ≤ 1.6) leads to stable cycling behavior of the NIB and a volumetric capacity of ~480 mAh cm−3 is retained after 100 cycles. Additionally, the NIB with the CoSbS@hard-C anode presents highly reversible and stable capacity (~570 mAh cm−3 after 150 cycles) and fast rate capability (~500 and ~450 mAh cm−3 at 2C and 5C, respectively), which can be attributed to the presence of uniformly dispersed small (5–8 nm) CoSbS nanocrystallites in the hard carbon matrix. Therefore, CoSbS can be utilized as a new anode material for high-performance NIBs.