Solid-state ⁷Li NMR studies of inverse spinel LiNi_xCo_1-xVO₄ cathode materials

Solid-state ⁷Li NMR spectroscopy revealed different Ni/Co distributions in an inverse spinel structure of LiNi_xCo_1-VO₄ cathode materials prepared by either a high-temperature solid-state reaction method (abbreviated as the HT method) or a low-temperature solution co-precipitation method (abbreviated as the LT method). Solid-state ⁷Li NMR measurements confirmed that the lithium nuclear spin was dominated by a chemical shift anisotropy (CSA) interaction compounded with a small second-order quadrupolar interaction. Ni/Co distribution and inhomogeneity of chemical shift tensors (δ_CSA and η_CSA constants) associated with crystalline defects are accountable for the variation in spread of the MAS spinning side-band manifolds between nickel uptake and preparation methods. The NMR study also revealed that the HT method yielded broader Ni/Co distributions with greater lithium defects, while the LT method gave much more homogeneous Ni/Co distributions with smaller defects. The NMR results were consistent with XRD data that showed a gradual expansion in the unit-cell lattice with increasing Co content. All cells suffered a large irreversible loss in overall capacity in the first cycle and became stable in terms of cycle efficiency during later cycling. We have found that LiNi_xCo_1-xVO₄ cathode materials prepared by the LT method with more homogeneous Ni/Co distribution and smaller crystalline defects offer a small advantage in capacity over the HT method.