Lithium transport from operando microscopy: Kinetic trapping in lithium manganese oxide

Diffusion coefficients of Lithium in Lithium Manganese Oxide (Li x Mn 2 O 4 , LMO) battery electrodes determined by common electrochemical techniques vary by up to ten orders of magnitude. A reliable determination of the transport coefficients would be of utmost importance. In this work, operando optical microscopy is applied to measure the chemical diffusion coefficient in nanometric films of LMO based on the machine-learned optical response of the material. This way, major errors of common electrochemical methods are avoided. Most remarkably, the measured bulk diffusion coefficient is orders of magnitude faster than determined by potentiostatic intermittent titration. It is quantitatively demonstrated that this discrepancy is likely due to a common misinterpretation in many electrochemical studies of LMO that assume linear interface kinetics to be a diffusion process. Besides the bulk Li diffusion coefficient, the kinetic barriers at phase interfaces and the dependences on grain size, concentration and temperature are quantified. The grain boundary diffusion coefficient is shown to be two orders of magnitude higher than bulk diffusion at room temperature. The compositional dependence of diffusivity shows a variation over three orders of magnitude. A maximum in the concentration-dependent activation energy is observed at half-filling.