Structural Complexity and Evolving Lithium-Ion Dynamics within the Cathode Material LiFeV2O7 Revealed by Diffraction and Solid-State NMR

7Li solid-state nuclear magnetic resonance (ssNMR) reveals unexpected structural complexity and substantive changes in the local dynamics of lithium exchange in monoclinic LiFeV2O7 as a function of electrochemical lithium insertion. The one-dimensional (1D) NMR spectrum revealed a multiplicity of peaks beyond the three expected, which prompted a further structural investigation. A new sample was synthesized through a solid-state reaction, which revealed a change in the crystal structure. The single-crystal refinement showed an arrangement where the O8, V2, and V6 positions shift in a way that changes the geometry of the vanadium sites. With this new configuration, lithium sites are no longer equivalent, providing a reason why there are extra signals in the 7Li ssNMR spectrum of the title compound. Ex situ 7Li magic angle spinning ssNMR experiments were used to track structural changes of the LiFeV2O7 electrode during electrochemical cycling. A new lithium arrangement was observed during the lithium insertion process, which occurs at a similar point of the electrochemical process as a notable increase of the lithium-ion dynamics as observed by 2D EXSY experiments. 7Li selective inversion (SI) experiments were measured over a temperature range of 303–318 K to quantify the exchange rates and energy barriers of ion mobility for each exchange pair present in the structure. In general, the activation energy increases as a function of the lithiation, suggesting that the lithium vacancies play a significant role in the current dynamics.