Realizing Three-Electron Redox Reactions in NASICON-Type NaCrNb(PO₄)₃ for Sodium Ion Battery Applications

Sodium ion battery (SIB) technology offers an attractive alternative to rechargeable batteries in large-scale applications. To meet the practical demands, it is essential to develop stable electrode materials with a high capacity and high energy density. Herein, we demonstrate a successful activation of the Nb5+/Nb4+, Nb4+/Nb3+, and Cr3+/Cr2+ redox couples in NASICON-structured NaCrNb(PO4)3, thus introducing a novel three-electron reaction anode for sodium ion batteries with a high specific capacity. Carbon-coated NaCrNb(PO4)3 synthesized using the Pechini sol–gel method demonstrates the reversible capacity of 162 mA h·g–1 at the 1C rate and good cycling stability. This is the first example of NASICON-type anode material with the Cr3+/Cr2+ reversible transition confirmed by Cr K-edge XANES measurements. Operando powder X-ray diffraction showed that sodium insertion–extraction in NaCrNb(PO4)3 proceeds through a combination of single-phase and biphasic processes. A differential scanning calorimetry study revealed the excellent thermal stability of NaCrNb(PO4)3, exceeding that of hard carbon anodes. Attractive electrochemical properties of NaCrNb(PO4)3, its high thermal and cyclic stability, and the scalable synthesis of this material demonstrate its potential as an anode for use in sodium ion batteries.