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

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, Na₃V_1.75Fe_0.25(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 Na₃V_1.75Fe_0.25(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.