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

摘要

A combined NMR and neutron diffraction study has been carried out on three Li_(3-x-y)Cu_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-y)Ni_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.