Complex defect chemistry of hydrothermally-synthesized Nb-substituted β′-LiVOPO₄

Lithium vanadyl phosphate (LiVOPO₄) is a next-generation multielectron battery cathode that can intercalate up to two Li-ions per V-ion through the redox couples of V⁴⁺/V³⁺ and V⁵⁺/V⁴⁺. However, its rate capacity is undermined by the sluggish Li-ion diffusion in the high-voltage region (4 V for V⁵⁺/V⁴⁺ redox). Nb substitution was used to expand the crystal lattice to facilitate Li-ion diffusion. In this work, Nb substitution was achieved via hydrothermal synthesis, which resulted in a new, lower symmetry β′-LiVOPO₄ phase with preferential Nb occupation of one of the two V sites. This phase presents complex defect chemistries, including cation vacancies and hydrogen interstitials, characterized by a combination of X-ray and neutron diffraction, elemental and thermogravimetric analyses, X-ray absorption spectroscopy, and magnetic susceptibility measurements. The Nb-substituted samples demonstrated improved capacity retention and rate capabilities in the high-voltage region, albeit an enlarged voltage hysteresis related to a partial V⁴⁺/V³⁺ redox reaction, as evidenced by ex situ X-ray absorption spectroscopy and pair distribution function analysis. This work highlights the importance of understanding the complex defect chemistry and its consequence on electrochemistry in polyanionic intercalation compounds.