Time-Resolved and Site-Specific Insights into Migration Pathways of Li+ in α-Li3VF6 by 6Li 2D Exchange MAS NMR

Abstract

Two-dimensional (2D) exchange nuclear magnetic resonance (NMR) spectroscopy carried out under magic angle spinning (MAS) conditions is ideally suited to study site-specific Li diffusion parameters of cathode materials required for the target-oriented development of so-called high-energy density 4 V-lithium-ion batteries. In the present study, we took advantage of Li NMR hyperfine shifts to record temperature-variable 1D and mixing-time dependent 2D exchange MAS 6Li NMR spectra on α-Li3VF6 serving as both a potential cathode material as well as an application-oriented model substance with three magnetically inequivalent Li sites. By comparing the NMR results with structural details of the material we were able to obtain detailed insights into the migration pathways and Li exchange rates which are of the order of some hundreds of Li jumps per second at approximately 340 K. Site-specific Li jump rates τ−1 reveal the electrochemically active sites and provide information how to modify the material in order to increase its relatively low Li diffusivity found at room temperature.