Graphene-Encapsulated FeS2 in Carbon Fibers as High Reversible Anodes for Na+ /K+ Batteries in a Wide Temperature Range.

Developing low cost, long life, and high capacity rechargeable batteries is a critical factor towards developing next-generation energy storage devices for practical applications. Therefore, a simple method to prepare graphene-coated FeS2 embedded in carbon nanofibers is employed; the double protection from graphene coating and carbon fibers ensures high reversibility of FeS2 during sodiation/desodiation and improved conductivity, resulting in high rate capacity and long-term life for Na+ (305.5 mAh g-1 at 3 A g-1 after 2450 cycles) and K+ (120 mAh g-1 at 1 A g-1 after 680 cycles) storage at room temperature. Benefitting from the enhanced conductivity and protection on graphene-encapsulated FeS2 nanoparticles, the composites exhibit excellent electrochemical performance under low temperature (0 and -20 °C), and temperature tolerance with stable capacity as sodium-ion half-cells. The Na-ion full-cells based on the above composites and Na3 V2 (PO4 )3 can afford reversible capacity of 95 mAh g-1 at room temperature. Furthermore, the full-cells deliver promising discharge capacity (50 mAh g-1 at 0 °C, 43 mAh g-1 at -20 °C) and high energy density at low temperatures. Density functional theory calculations imply that graphene coating can effectively decrease the Na+ diffusion barrier between FeS2 and graphene heterointerface and promote the reversibility of Na+ storage in FeS2 , resulting in advanced Na+ storage properties.