Structurally Modulated Na_4-x Fe_3-x V _x (PO₄)₂P₂O₇ by Vanadium Doping for Long-Life Sodium-Ion Batteries

Phosphate-pyrophosphate iron sodium (Na4Fe3(PO4)(2)P2O7, denoted NFPP) is a viable cathode material for sodium-ion batteries (SIBs) due to its low cost, environmental friendliness, and high structural stability. However, the limiting factors for the cycle stability and rate capabilities are attributed to the low mobility and insufficient electronic conductivity of the Na ions. In this work, vanadium (V)-doped NFPP, nanoproducts with carbon layer encapsulation, are prepared by a spray-drying method. After optimizing the doping amount of V, the Na3.94Fe2.94V0.06(PO4)(2)P2O7 (denoted as NFPP-2 V) cathode material displays an initial reversible specific capacity of 123.4 mA h g(-1) at 0.1C in SIBs. Even at 20C, the NFPP-2 V cathode shows a reversible discharge specific capacity of 99.6 mA h g(-1) and still retains 81.65% after 10,000 cycles. We also coupled hard carbon with this NFPP-2 V cathode to assemble a pouch cell, which can exhibit excellent performance. Therefore, trace amount of V doping is an excellent process for the production of pure-phase NFPP, which provides a new strategy for future large-scale production.