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 Nb 5+ /Nb 4+, Nb 4+ /Nb 3+, and Cr 3+ /Cr 2+ redox couples in NASICON-structured NaCrNb(PO 4 ) 3, thus introducing a novel three-electron reaction anode for sodium ion batteries with a high specific capacity. Carbon-coated NaCrNb(PO 4 ) 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 Cr 3+ /Cr 2+ reversible transition confirmed by Cr K -edge XANES measurements. Operando powder X-ray diffraction showed that sodium insertion–extraction in NaCrNb(PO 4 ) 3 proceeds through a combination of single-phase and biphasic processes. A differential scanning calorimetry study revealed the excellent thermal stability of NaCrNb(PO 4 ) 3, exceeding that of hard carbon anodes. Attractive electrochemical properties of NaCrNb(PO 4 ) 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.