Initiating High-Voltage Multielectron Reactions in NASICON Cathodes for Aqueous Zinc/Sodium Batteries

Abstract

Sodium superionic conductor (NASICON) is a class of compounds with robust polyanionic frameworks and high thermal stability, which are regarded as prospective cathodes candidates for secondary batteries. However, NASICON cathodes typically have low discharge plateaus and low practical capacities in aqueous electrolytes. Here, Na3V1.75Fe0.25(PO4)2F3 is investigated as a cathode material for the aqueous zinc/sodium batteries. While the addition of F helps with the improvement of NASICON structural stability, the low-cost Fe substitution has a positive impact on the capacity increment, reaction voltage increases, and cycling stability improvement. Because the Fe3+ substitution could induce a change in the spin magnetic moments of the 3d orbitals of the VO4F2 and FeO4F2 octahedra, the 2-electron reaction of V is activated, which are V4+/V3+ and V5+/V4+ redox couples. As a result, the novel Na3V1.75Fe0.25(PO4)2F3 cathode delivers a high operating voltage of 1.7 V, a high energy density of 209 W·h·kg−1 and stable lifespan (83.5% capacity retention after 6,000 cycles at 1 A·g−1) in the aqueous zinc/sodium batteries. This research demonstrates the practicality of activating multielectron reactions to optimize the electrochemical properties of NASICON cathodes for aqueous secondary batteries.