Evolution of lithium ordering with (de)-lithiation in β-LiVOPO₄: Insights through solid-state NMR and first principles DFT calculations

The lithium-ion battery cathode material β-VOPO₄ is capable of intercalating more than one Li ion per transition metal ion due to the accessibility of both the V⁵⁺/V⁴⁺ and V⁴⁺/V³⁺ redox couples at ∼4.5 V and ∼2.3 V vs. Li, respectively, giving a theoretical capacity greater than ∼300 mA h g^-1. The ability to perform full and reversible two Li-ion intercalation in this material, however, has been a matter of debate and the poor crystallinity of the fully lithiated phase has thus far precluded its complete structural characterisation by conventional diffraction-based methods. In this work, ⁷Li and ³¹P NMR spectroscopy, in combination with first principles DFT calculations, indicate that chemical lithiation results in a single phase β-Li₂VOPO₄ exhibiting a complex Li ordering scheme with lithium ions occupying multiple disordered environments. 2D NMR ⁷Li correlation experiments were used to deduce the most likely Li ordering for the β-Li₂VOPO₄ phase from amongst several DFT optimised structures. In contrast, electrochemically lithiated β-Li_2-xVOPO₄ discharged to 1.6 V exhibits, in addition to β-Li₂VOPO₄, a β-Li_1.5VOPO₄ phase. The existence of β-Li_1.5VOPO₄ is not reflected in the flat galvanostatic charge and discharge curves nor is evident from diffraction-based methods due to the very close structural similarity between the β-Li_1.5VOPO₄ phase and β-Li₂VOPO₄ phases. We demonstrate that solid state NMR spectroscopy, in combination with DFT results, provides a powerful tool for identifying intermediate states formed during charge/discharge of these complex phosphates as these phases can be distinguished from the end member phases primarily by the nature of the lithium ordering.