Reversible multielectron redox activity of the anti-NASICON-type phosphate LiNbV(PO4)3 towards lithium and sodium intercalation

Abstract

The LiNbV(PO₄)₃ phosphate with the anti-NASICON structure (a = 12.126(1) Å, b = 8.6158(4) Å, c = 8.6959(6) Å, V = 908.5(1) ų, S.G. Pbcn) has been synthesized using a Pechini sol–gel process. It exhibits reversible multielectron transitions versus Li and Na anodes. In a Li half-cell, it supports a 4e⁻ transfer due to the activation of the Nb⁵⁺/Nb³⁺ and V⁴⁺/V²⁺ redox couples, being the first example of 4d metal redox transitions within the anti-NASICON framework confirmed by XANES measurements. X-ray diffraction performed in ex situ and operando regimes disclosed a single-phase mechanism of lithium (de)intercalation. In a Na half-cell, the material demonstrates reversible uptake of 2.77 Na⁺ ions. Density functional theory calculations revealed percolation barriers of ∼0.5–0.7 eV for Na⁺ hopping, thus supporting the activation of Na⁺ ion diffusion in the NbV(PO₄)₃ framework. This study introduces a new approach to improve anti-NASICON-structured electrode materials by utilizing redox transitions of 4d elements for energy storage.