(134a) Identifying Defect Sites in Lithium-Ion Battery Materials: Local Disorder in LiVPO4F and Its Influence On Bulk Properties

The transition metal fluorophosphate LiVPO₄F is a promising electrode material for lithium-ion batteries that is capable of operating both as a high-voltage positive electrode (4.25 V, V^III/IV) and a negative electrode (1.8 V, V^II/III).^1,2 While its Tavorite-like structure has been solved by a combination of X-ray and neutron diffraction, solid-state nuclear magnetic resonance (NMR) measurements reveal a significant extent of local disorder that has not been observed at longer length scales with scattering techniques.  In particular, solid-state single-pulse ⁷Li NMR spectra acquired under conditions of ultra-fast (64 kHz) magic-angle-spinning (MAS) reveal that LiVPO₄F exhibits several additional ⁷Li NMR signals of unknown origin that can account for up to 20% of the total lithium content. Electrochemical measurements in Swagelok-type cells with lithium metal as the negative electrode establish that materials with different populations of unknown lithium sites exhibit different cycling behavior, including differing extents of irreversible capacity and polarization.  2D ⁷Li{⁷Li} dipolar correlation NMR measurements establish the sub-nanometer proximities of the unknown lithium sites with respect to the LiVPO₄F site, proving unambiguously that the unknown sites are local defects within the LiVPO₄F structure and are not associated with separate impurity phases.  In combination with magnetic measurements, variable-temperature NMR measurements reveal that the paramagnetic ⁷Li NMR shifts of the different lithium sites vary linearly with the bulk magnetic susceptibility, further confirming that the sites are defects within the LiVPO₄F structure.  The results provide a general strategy for identifying lithium defect sites in lithium-ion battery materials and highlight the importance of understanding local disorder and its relationship to macroscopic material and device properties.

¹J.-M. Ateba Mba, C. Masquelier, E. Suard, L. Croguennec, Chem. Mater. 2012, 24, 1223 – 1234. 

²J.-M. Ateba Mba, L. Croguennec,  N.I. Basir, J. Barker, C. Masquelier, J. Electrochem. Soc. 2012, 159, A1171-A1175.