Trace Cu Doping Enabled High Rate and Long Cycle Life Sodium Iron Phosphate Cathode for Sodium-Ion Batteries

Na4Fe3(PO4)2(P2O7) (NFPP) is currently receiving a lot of attention, as it combines the advantages of NaFePO4 and Na2FeP2O7 in terms of cost, energy density, and cycle stability. However, the issues of intrinsic poor electronic conductivity and difficult high-purity preparation may impede its practical application. Herein, the pivotal role of Cu doping in strengthening the polyanion structure and improving its electrochemical properties is comprehensively investigated. It is found that trace Cu doping not only expands the lattice volume of NFPP but also suppresses the formation of the inactive NaFePO4 impurity phase. In addition, Cu doping can effectively reduce the structural variations of NFPP during sodiation/desodiation processes (2.02%) while decreasing the band gap and lowering the ion mobility energy barrier (from 0.46 to 0.426 eV). Consequently, the Cu-doped NFPP electrode exhibits superior rate capability and long-term cycling performance. The computational simulations reveal a strong electronic interaction between Fe and Cu that tunes the electron localization and distribution, and additional Na+ transport channels can be created in NFPP by distorting the [PO4] units adjacent to the doping site, which provides a reference to enhance the performance of NFPP and reveals the great application potential of NFPP materials.