Structure, stoichiometry and transport properties of lithium copper nitride battery materials: combined NMR and powder neutron diffraction studies

Abstract

A combined NMR and neutron diffraction study has been carried out on three Li_3−x−yCu_xN materials with x = 0.17, x = 0.29 and x = 0.36. Neutron diffraction indicates that the samples retain the P6/mmm space group of the parent Li₃N with Cu located only on Li(1) sites. The lattice parameters vary smoothly with x in a similar fashion to Li_3−x−yNi_xN, but the Li(2) vacancy concentration for the Cu-substituted materials is negligible. This structural model is confirmed by wideline ⁷Li NMR spectra at 193 K which show three different local environments for the Li(1) site, resulting from the substitution of neighbouring Li atoms in the Li(1) layer by Cu. Since the Cu-substituted materials are only very weakly paramagnetic, variable temperature ⁷Li wideline NMR spectra can be used to measure diffusion coefficients and activation energies. These indicate anisotropic Li⁺ diffusion similar to the parent Li₃N with transport confined to the [Li₂N] plane at low temperature and exchange between Li(1) and Li(2) sites dominant at high temperature. For the intra-layer process the diffusion coefficients at room temperature are comparable to Li₃N and Li_3−x−y Ni_xN, while E_a decreases as x increases in contrast to the opposite trend in Ni-substituted materials. For the inter-layer process E_a decreases only slightly as x increases, but the diffusion coefficients at room temperature increase rapidly with x.