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

• Published in: Journal of materials chemistry. A, Materials for energy and sustainability Materials for energy and sustainability, 2020-03, Vol.8 (11), p.5546-5557
• Main Authors: Britto, Sylvia, Seymour, Ieuan D, Halat, David M, Hidalgo, Marc F. V, Siu, Carrie, Reeves, Philip J, Zhou, Hui, Chernova, Natasha A, Whittingham, M. Stanley, Grey, Clare P
• Format: Article
• Language: English
• ISSN: 2050-7488; 2050-7496
• DOI: 10.1039/d0ta00121j

The lithium-ion battery cathode material β-VOPO 4 is capable of intercalating more than one Li ion per transition metal ion due to the accessibility of both the V 5+ /V 4+ and V 4+ /V 3+ 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, 7 Li and 31 P NMR spectroscopy, in combination with first principles DFT calculations, indicate that chemical lithiation results in a single phase β-Li 2 VOPO 4 exhibiting a complex Li ordering scheme with lithium ions occupying multiple disordered environments. 2D NMR 7 Li correlation experiments were used to deduce the most likely Li ordering for the β-Li 2 VOPO 4 phase from amongst several DFT optimised structures. In contrast, electrochemically lithiated β-Li 2− x VOPO 4 discharged to 1.6 V exhibits, in addition to β-Li 2 VOPO 4 , a β-Li 1.5 VOPO 4 phase. The existence of β-Li 1.5 VOPO 4 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 1.5 VOPO 4 phase and β-Li 2 VOPO 4 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. Solid-state NMR combined with DFT calculations are used to characterise metastable phases formed during (electro)chemical lithiation of β-(Li)VOPO 4 .