Advanced flexible CoSe@carbon fibers as high-performance electrode for sodium ion and lithium sulfur batteries and its micromorphology transformation.

Advanced flexible CoSe@carbon fibers (CoSe@CNFs) were synthesized through electrospinning and sintering processes, with the aim of being applied to high rate and long cycle stability of sodium ion batteries (SIBs) and lithium‑sulfur batteries (LSBs). CNFs interconnect ultrafine CoSe particles to form a coating structure, and the fibers intertwine to create a self-standing three-dimensional network. This not only helps to store electrons in electrochemical reactions but also facilitates the adsorption and desorption of Na/S species. Therefore, when directly used as an anode for SIBs, the electrode has a reversible capacity of 202.1 mAh g after 3000 cycles at a current density of 10 Ag. The microstructure changes of CoSe during the charging and discharging process were investigated in detail by TEM, and the results showed that the crystal form of CoSe changed from long range-order transforms into short-range disorder after sodiated. In addition, the electrocatalytic advantages of flexible electrodes as catalysts for lithium polysulfides can be easily demonstrated through catalytic experiments. Leveraging the morphology and inherent advantages of CoSe, the flexible CoSe@CNFs electrode demonstrates a reversible capacity of 566.8 mAh g at 5 C after 1500 cycles. Most importantly, the morphology of CoSe after the electrochemical reaction was carefully investigated, demonstrating its consistency before and after the electrocatalytic reaction. Flexible CoSe@CNFs provide diversity for the development of high rate performance SIBs and long cycle stability LSBs, and promote a deeper understanding of their morphological evolution during electrochemical reactions.