Off-Stoichiometry of Sodium Iron Pyrophosphate as Cathode Materials for Sodium-Ion Batteries with Superior Cycling Stability

As one of the important devices for large-scale electrochemical energy storage, sodium-ion batteries have received much attention due to the abundant resources of raw materials. However, whether it is a base station power source, an energy storage power station, or a start-stop power supply, long energy cycle life (more than 5000 cycles), high stability, and safety performance are application prerequisites. Regrettably, currently, few sodium-ion batteries can meet this requirement, mainly due to shortcomings in positive electrode performance. We report a sufficiently stable sodium-ion battery cathode material, Na₂Fe_0.95P₂O₇, that retains 97.5% capacity after 5000 charge/discharge cycles. The use of nonstoichiometry in the lattice enables simultaneous modification of the crystal and electronic structure, promoting Na₂Fe_0.95P₂O₇ to be extremely stable while still being able to achieve a capacity of 92 mAh g^-1 and stable cycling at high temperatures up to 60 °C. Our results confirm the positive effect of nonstoichiometric ratios on the performance of Na₂Fe_0.95P₂O₇ and provide a reliable idea to promote the practical application of sodium-ion batteries.