NaNbV(PO 4 ) 3 : Multielectron NASICON-Type Anode Material for Na-Ion Batteries with Excellent Rate Capability.

NASICON-type NaNbV(PO 4 ) 3 electrode material synthesized by the Pechini sol-gel technique undergoes a reversible three-electron reaction in a Na-ion cell which corresponds to the Nb 5+ /Nb 4+ , Nb 4+ /Nb 3+ , and V 3+ /V 2+ redox processes and provides a reversible capacity of 180 mAh·g -1 . The sodium insertion/extraction takes place in a narrow potential range at an average potential of 1.55 V versus Na + /Na. Structural characterization by operando and ex situ X-ray diffraction disclosed the reversible evolution of the NaNbV(PO 4 ) 3 polyhedron framework during cycling, while XANES measurements in the operando regime confirmed the multielectron transfer upon sodium intercalation/extraction into NaNbV(PO 4 ) 3 . This electrode material demonstrates extended cycling stability and excellent rate capability maintaining the capacity value of 144 mAh·g -1 at 10 C current rates. It can be regarded as a superior anode material suitable for application in high-power and long-life sodium-ion batteries.