New binder-free metal phosphide-carbon felt composite anodes for sodium-ion battery

Metal phosphides are promising anode candidates for sodium-ion batteries (SIBs) due to their high specific capacity and low operating potential but suffer from poor cycling stability caused by huge volume expansion and poor solid-state ion transfer rate. Herein, a new strategy to grow a new class of mesoporous metal phosphide nanoarrays on carbon felt (CF) as binder-free anodes for SIBs is reported. The resultant integrated electrodes demonstrate excellent cycling life up to 1000 times (>90% retention rate) and high rate capability of 535 mAh g at a current density of 4 A g. Detailed characterization reveals that the synergistic effect of unique mesoporous structure for accommodating huge volume expansion during sodiation/desodiation process, ultrasmall primary particle size (≈10 nm) for providing larger electrode/electrolyte contact area and shorter ion diffusion distance, and 3D conductive networks for facilitating the electrochemical reaction, leads to the extraordinary battery performance. Remarkably, a full SIB using the new CoP/CF anode and a NaV(PO)F cathode delivers an average operating voltage of ≈3.0 V, a reversible capacity of 553 mAh g, and very high energy density of ≈280 Wh kg for SIBs. A flexible SIB with outstanding mechanical strength based on this binder-free new anode is also demonstrated