Eldfellite NaV(SO4)2 as a versatile cathode insertion host for Li-ion and Na-ion batteries

Abstract

In search of high energy density cathode materials, the eldfellite mineral-type NaV^III(SO₄)₂ compound has been theoretically predicted to be a promising cathode insertion host for sodium-ion batteries. Synergizing computational and experimental investigations, the current work introduces NaV^III(SO₄)₂ as a novel versatile cathode for Li-ion and Na-ion batteries. Prepared by a low temperature sol–gel synthesis route, the eldfellite NaV(SO₄)₂ cathode exhibited an initial capacity approaching ∼79% (vs. Li⁺/Li) and ∼69% (vs. Na⁺/Na) of the theoretical capacity (1e⁻ ≅ 101 mA h g⁻¹) involving the V³⁺/V²⁺ redox potential centered at 2.57 V and 2.28 V, respectively. The bond valence site energy (BVSE) approach and DFT-based calculations were used to gain mechanistic insight into alkali ion migration and probe the redox center during (de)insertion of Li⁺/Na⁺ ions. Post-mortem and electrochemical titration tools revealed the occurrence of a single-phase (solid-solution) redox mechanism during reversible Li⁺/Na⁺ (de)insertion into NaV^III(SO₄)₂. With the multivalent vanadium redox center, eldfellite NaV^III(SO₄)₂ forms a new cathode insertion host for Li/Na-ion batteries with potential two-electron uptake.