Na₂FeP₂O₇: A Safe Cathode for Rechargeable Sodium-ion Batteries

Vying for newer sodium-ion chemistry for rechargeable batteries, Na₂FeP₂O₇ pyrophosphate has been recently unveiled as a 3 V high-rate cathode. In addition to its low cost and promising electrochemical performance, here we demonstrate Na₂FeP₂O₇ as a safe cathode with high thermal stability. Chemical/electrochemical desodiation of this insertion compound has led to the discovery of a new polymorph of NaFeP₂O₇. High-temperature analyses of the desodiated state NaFeP₂O₇ show an irreversible phase transition from triclinic (P1̅) to the ground state monoclinic (P2₁/c) polymorph above 560 °C. It demonstrates high thermal stability, with no thermal decomposition and/or oxygen evolution until 600 °C, the upper limit of the present investigation. This high operational stability is rooted in the stable pyrophosphate (P₂O₇)^4– anion, which offers better safety than other phosphate-based cathodes. It establishes Na₂FeP₂O₇ as a safe cathode candidate for large-scale economic sodium-ion battery applications.