Conditions Extrêmes et Matériaux : Haute Température et Irradiation
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2015

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R.J.Messinger, M.Ménétrier, E.Salager, A.Boulineau, M.Duttine, D.Carlier, J.M.Ateba Mba, L.Croguennec, C.Masquelier, D.Massiot, M.Deschamps, 'Revealing Defects in Crystalline Lithium-Ion Battery Electrodes by Solid-State NMR: Applications to LiVPO4F', Chem. Mat. 27(15) 5212–5221 (2015) doi:10.1021/acs.chemmater.5b01234

Identifying and characterizing atomic-scale defects in crystalline solids is a challenging problem, particularly for lithium-ion intercalation materials, which often exhibit multiple oxidation and spin states as well as local ordering of lithium and charges. Here, we reveal the existence of characteristic lithium defect environments in the crystalline lithium-ion battery electrode LiVPO4F and establish the relative proximities between them. Well-crystallized LiVPO4F samples were synthesized with the expected tavorite-like structure, as established by X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) measurements. Solid-state 7Li nuclear magnetic resonance (NMR) spectra reveal unexpected paramagnetic 7Li environments that can account for up to 20% of the total lithium content. Multi-dimensional and site-selective solid-state 7Li NMR experiments using finite-pulse radio-frequency-driven recoupling (fp-RFDR), which recouple through-space 7Li magnetic dipolar interactions, establish unambiguously that the unexpected lithium environments are associated with defects within the LiVPO4F crystal structure. The lithium defects exhibit local electronic environments that are distinct from lithium ions in the crystallographic LiVPO4F site, which result from altered oxidation and/or spin states of nearby paramagnetic vanadium atoms. The results provide a general strategy for identifying and characterizing sub-nanometer-scale defects in lithium-containing crystalline solids, including paramagnetic materials with short 7Li NMR relaxation times on the order of milliseconds.