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

Britto, Sylvia and Leskes, Michal and Hua, Xiao and Hébert, Claire-Alice and Shin, Hyeon Suk and Clarke, Simon and Borkiewicz, Olaf and Chapman, Karena W. and Seshadri, Ram and Cho, Jaephil and Grey, Clare P. (2015) Multiple Redox Modes in the Reversible Lithiation of High-Capacity, Peierls-Distorted Vanadium Sulfide. Journal of the American Chemical Society, 137 (26). pp. 8499-8508. ISSN 0002-7863

Abstract

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 S2–, 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 S2– to form Li3VS4 containing tetrahedral [VS4]3– anions. On further lithiation this is followed by reduction of the V5+ in Li3VS4 to form Li3+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.