Multiple Redox Modes in the Reversible Lithiation of High-Capacity, Peierls-Distorted Vanadium Sulfide

Vanadium sulfide VS₄ in the patronite mineral structure is a linear chain compound comprising vanadium atoms coordinated by disulfide anions [S₂]^2-. ⁵¹V NMR shows that the material, despite having V formally in the d¹ configuration, is diamagnetic, suggesting potential dimerization through metal-metal bonding associated with a Peierls distortion of the linear chains. This is supported by density functional calculations, and is also consistent with the observed alternation in V-V distances of 2.8 and 3.2 Å along the chains. Partial lithiation results in reduction of the disulfide ions to sulfide S^2-, via an internal redox process whereby an electron from V⁴⁺ is transferred to [S₂]^2- resulting in oxidation of V⁴⁺ to V⁵⁺ and reduction of the [S₂]^2- to S^2- to form Li₃VS₄ containing tetrahedral [VS₄]^3- anions. On further lithiation this is followed by reduction of the V⁵⁺ in Li₃VS₄ to form Li_3+xVS₄ (x = 0.5-1), a mixed valent V⁴⁺/V⁵⁺ compound. Eventually reduction to Li₂S plus elemental V occurs. Despite the complex redox processes involving both the cation and the anion occurring in this material, the system is found to be partially reversible between 0 and 3 V. The unusual redox processes in this system are elucidated using a suite of short-range characterization tools including ⁵¹V nuclear magnetic resonance spectroscopy (NMR), S K-edge X-ray absorption near edge spectroscopy (XANES), and pair distribution function (PDF) analysis of X-ray data.