Operando Mapping of Li Concentration Profiles and Phase Transformations in Graphite Electrodes by Magnetic Resonance Imaging and Nuclear Magnetic Resonance Spectroscopy

An innovative combined magnetic resonance imaging and nuclear magnetic resonance methodology, which enables the visualization and spatially resolved quantification of the lithiation/delithiation process in porous Li-ion battery electrodes in real time, is reported. We demonstrate that polarization of the thick graphite electrode correlates with the appearance of energy barriers during the graphite phase transformations and the resulting significant reduction of the lithium chemical diffusion, which must be addressed in any attempt at the fast charging of Li-ion batteries. It is also shown that a portion of the transported Li+ is temporarily stored in the electrode surface film prior to its intercalation into graphite, indicating reversible lithium storage within the surface electrolyte interface. The inclusion of a short current reversal, which could be considered as part of a shaped charging process, facilitates the complete lithiation of thick graphite electrodes, an attractive strategy for increasing usable cell capacities.