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

The lithium-ion battery cathode material beta-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 similar to 4.5 V and similar to 2.3 V vs. Li, respectively, giving a theoretical capacity greater than similar to 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, Li-7 and P-31 NMR spectroscopy, in combination with first principles DFT calculations, indicate that chemical lithiation results in a single phase beta-Li2VOPO4 exhibiting a complex Li ordering scheme with lithium ions occupying multiple disordered environments. 2D NMR Li-7 correlation experiments were used to deduce the most likely Li ordering for the beta-Li2VOPO4 phase from amongst several DFT optimised structures. In contrast, electrochemically lithiated beta-Li2-xVOPO4 discharged to 1.6 V exhibits, in addition to beta-Li2VOPO4, a beta-Li1.5VOPO4 phase. The existence of beta-Li1.5VOPO4 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 beta-Li1.5VOPO4 phase and beta-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.