Low volume expansion carbon-coated Fe₂P₄O₁₂ anode material for high-performance sodium dual-ion battery

Sodium dual-ion batteries (SDIBs) have attracted considerable attention due to their low manufacturing costs and environmentally friendly. However, the commonly used anode materials often suffer from large volume change when sodiated, leading to unsatisfactory capacity and cycling stability of SDIBs. Herein, a low volume expansion carbon-coated iron tetrametaphosphate (Fe₂P₄O₁₂@C) composite is proposed as the anode material for SDIB for the first time. During the initial discharging process, Fe₂P₄O₁₂ would convert into Na₂Fe₃(PO₄)₃ which then undergoes the subsequent desodiation/sodiation processes. It can be revealed that the tunnels of Na₂Fe₃(PO₄)₃ along the c axis are feasible for Na⁺ ions migration with a low volume change of 5.7%, while the carbon coating can effectively promote electron transfer, which endows the Fe₂P₄O₁₂@C with a high specific capacity of 343.1 mAh g⁻¹ and stable cycling performance for 400 cycles. Furthermore, a novel SDIB prototype is constructed by using Fe₂P₄O₁₂@C as anode and environmentally friendly expanded graphite as the cathode, which displays a high capacity of 230.4 mAh g⁻¹ at 2 C, excellent rate capability up to 15 C, and a long-term cycling life more than 600 cycles at 15 C, showing some superiority over most reported SDIBs.