Muon Spin Relaxation Studies of Lithium Nitridometallate Battery Materials: Muon Trapping and Lithium Ion Diffusion

Lithium nitride has a unique layered structure and the highest reported Li+ ion conductivity for a crystalline material. The conductivity is highly anisotropic, with an intralayer contribution within the graphitic [Li2N] planes dominant at ambient temperature. In this paper transverse- and zero-field muon spin relaxation (mu SR) studies on Li3N and two novel paramagnetic derivatives Li3-x-yNixN with x = 0.36 and 0.57 are reported. These new materials have potential as anodes in rechargeable lithium batteries. The decrease in the muon depolarization rate observed above 180 K for the three materials is shown to arise from motional narrowing due to intralayer Li+ diffusion. The increase in the measured activation energy with x for Li3-x-yNixN suggests that the reduction in the layer spacing that results at high substitution levels is responsible for raising the energy barrier to Li+ jumps, despite the concomitant expansion of the [Li2N] plane. In addition, the onset of interlayer diffusion appears at lower temperatures in Ni-substituted derivatives than in the parent Li3N. The muons themselves are quasi-static, most probably located in a 4h site between the [Li2N] plane and the Li(1)/Ni layer. This is similar to the Li+ interstitial position identified by molecular dynamics simulations as an intermediate for an exchange mechanism for interlayer diffusion. Finally, mu SR gives no evidence for the formation of the muonium equivalent of the hydrogen defects thought to play an important role in intralayer diffusion in Li3N. These results demonstrate that mu SR can be used to obtain diffusion coefficients and activation energies for Li+ transport even in paramagnetic materials where NMR Studies are complicated by strong interactions with the electronic moments.