New-type NASICON-Na4FeV(PO4)3 cathode with high retention and durability for sodium ion batteries

Na3V2(PO4)3 is a promising potential cathode for sodium-ion batteries owning to its stable three-dimensional structure framework. However, from the perspective of environmental protection, the substitution of vanadium with low cost and low toxicity elements is pressing to further boost its application in large-scale energy storage production. To reduce the content of V in Na3V2(PO4)3 and increase the transfer number of sodium ions, ball-milling assisted sol-gel routine is employed to prepare Na4FeV(PO4)3@C sodium-rich cathode, which delivers an initial charge capacity high to 175.6 mAh g−1 with high coulombic efficiency of 99% from 1.3 to 4.3 V, deriving from V(II)/V(III), Fe(II)/Fe(III), V(III)/V(IV) and V(IV)/V(V) redox couples. The cathode shows long-life span with an excellent retention of 96.8% after 800 cycles at 5C from 1.8 to 3.8V. Solid-state 23Na nuclear magnetic resonance reveals that the sodium ions at 8-coordinated Na2 sites in Na4FeV(PO4)3@C show faster extraction/insertion dynamics during electrochemical cycling. X-ray diffraction and time-of-flight neutron powder diffraction results demonstrate that the electrochemical process undergoes a reversible solid solution reaction with stable structure, resulting in fast rate performance and excellent cyclic retention. The composition modulation of the sodium rich material and the cycling mechanisms obtained from this study would contribute a great insight for the sodium energy storage with improved performances.