Structurally\nModulated Na<sub>4–<i>x</i></sub>Fe<sub>3–<i>x</i></sub>V<sub><i>x</i></sub>(PO<sub>4</sub>)<sub>2</sub>P<sub>2</sub>O<sub>7</sub> by Vanadium\nDoping for Long-Life Sodium-Ion Batteries

Phosphate-pyrophosphate iron sodium (Na₄Fe₃(PO₄)₂P₂O₇, denoted\nNFPP)\nis a viable cathode material for sodium-ion batteries (SIBs) due to\nits low cost, environmental friendliness, and high structural stability.\nHowever, the limiting factors for the cycle stability and rate capabilities\nare attributed to the low mobility and insufficient electronic conductivity\nof the Na ions. In this work, vanadium (V)-doped NFPP, nanoproducts\nwith carbon layer encapsulation, are prepared by a spray-drying method.\nAfter optimizing the doping amount of V, the Na_3.94Fe_2.94V_0.06(PO₄)₂P₂O₇ (denoted as NFPP-2 V) cathode material displays an\ninitial 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\ndischarge specific capacity of 99.6 mA h g^–1 and\nstill retains 81.65% after 10,000 cycles. We also coupled hard carbon\nwith this NFPP-2 V cathode to assemble a pouch cell, which can exhibit\nexcellent performance. Therefore, trace amount of V doping is an excellent\nprocess for the production of pure-phase NFPP, which provides a new\nstrategy for future large-scale production.