Lithium exchange across a lithium-less coating for high energy cathodes

Surface degradation is a major limitation in the utilization of high energy cathodes. Cathode coatings provide a promising route for achieving interfacial stability. In this case, there is interest in developing coatings which will provide efficient ion transport across the cathode-electrolyte-interface (CEI). Aluminum based coatings, particularly metal fluorides, have shown great promise for stabilizing the high voltage cathode, LiNi0.5Mn1.5O4. Despite the clear practical advantages of coatings, the chemical basis for their improved performance remains ambiguous. Here we present an in-depth investigation of a nominal AlF3 coating on LiNi0.5Mn1.5O4 based on solid state NMR spectroscopy to determine the composition and function of the Al/F coating layer. NMR results, supported by X-ray photoelectron spectroscopy and electron microscopy, reveal that the actual composition of the deposited layer is an amorphous Al–O–F phase. Isotope exchange is used to follow the spontaneous exchange of lithium ions across the CEI, revealing improved transport in coated cathodes. Finally, NMR experiments provide evidence for insertion of lithium ions in the Al/F coating following electrochemical cycling. The results suggest that lithium insertion into a nominally lithium-less coating plays a role in its improved performance, highlighting lithium content as an important factor in designing beneficial coatings for higher energy cathodes.