Synchrotron Operando Depth Profiling Studies of State-of-Charge Gradients in Thick Li(Ni_0.8Mn_0.1Co_0.1)O₂ Cathode Films

Higher energy densities in rechargeable batteries can be achieved using thicker cathode films, though it is a challenging endeavor since the electrochemical performance of thick electrodes is substantially worse than that of the conventional thin electrodes due to a variety of transport limitations, which are thus far poorly understood. Operando synchrotron studies have been, for the first time, applied to thick film samples to determine the depth-dependent state of charge (SOC) distribution inside 170 micron thick Li­(Ni_0.8Mn_0.1Co_0.1)­O₂ cathode films using an unconventional radial diffraction experiment geometry, allowing the SOC to be probed with both high spatial resolution (20 microns) and high temporal resolution (hundreds of time steps) in a single experiment. The resulting data allow the evolution of vertical inhomogeneity within these thick cathode films to be determined during cycling and they reveal a number of unexpected phenomena, such as the continuation of charging at some heights within the cathode during the discharge cycle of the cell. The new availability of comprehensive depth-dependent SOC data will drive the parameterization and advancement of whole-cell models, leading to an improved understanding of large-scale transport phenomena and enhanced capabilities for the rational design of thick electrodes with improved performance.