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.