Flexible electrode constructed by encapsulating ultrafine VSe2 in carbon fiber for quasi-solid-state sodium ion batteries

Sodium ion batteries (SIBs) based on quasi-solid-state electrolyte are a new type of energy device that exhibit high energy and security. However, the large-volume expansion and sluggish kinetics of the anode materials during the ion insertion/extraction remains a particular challenge for solid-state SIBs. In this work, flexible binder-free films composed of ultrafine VSe 2 particles wrapped in N-doped carbon nanofibers (VSe 2 /NCNFs) are prepared as the anode of a quasi-solid-state SIBs. The ultra-small VSe 2 nanoparticles allow for rapid Na ion migration, and the highly graphitized carbon fibers provides a fast electronic transmission channel and stable host to relive the volume expansion of VSe 2 during cycles. Foremost, the elastic coordination between VSe 2 /NCNFs and gel electrolyte can further buffer the overall volume expansion of VSe 2 /NCNFs. Moreover, the flexible freestanding VSe 2 /NCNFs anode combine with gel polymer in the sodium ion half-cells deliver a high reversible capacity of 420.8 mA h g −1 at a current density of 0.05 A g −1 , excellent rate capability of 278.1 mA h g −1 at 5 A g −1 , and 206.8 mA h g −1 at 5 A g −1 after 10,000 cycles. The sodium storage mechanism of VSe 2 /NCNFs electrodes during the charge/discharge process is also explored by ex situ XRD in the first time.