Evolution of lithium ordering with (de)-lithiation in β-LiVOPO4: insights through solid-state NMR and first principles DFT calculations. Issue 11 (3rd March 2020)

• Record Type: Journal Article
• 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.
• Is Part Of: Journal of materials chemistry. Volume 8:Issue 11(2020)
• Page Start: 5546
• Page End: 5557
• Publication Date: 2020-03-03
• Subjects: Materials -- Research -- Periodicals; Chemistry, Analytic -- Periodicals; Environmental sciences -- Research -- Periodicals; 543.0284
• DOI: 10.1039/d0ta00121j
• Languages: English
• ISSNs: 2050-7488

Abstract

Abstract : Solid-state NMR combined with DFT calculations are used to characterise metastable phases formed during (electro)chemical lithiation of β-(Li)VOPO4 . Abstract : 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 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-Li2 VOPO4 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-Li2 VOPO4 phase from amongst several DFT optimised structures. In contrast, electrochemically lithiated β-Li2− x VOPO4 discharged to 1.6 V exhibits, in addition to β-Li2 VOPO4, a β-Li1.5 VOPO4 phase. The existence of β-Li1.5 VOPO4 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-Li1.5 VOPO4 phase and β-Li2 VOPO4 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.