Dynamic surface phases controlling asymmetry of high-rate lithiation and delithiation in phase-separating electrodes

Li-ion battery electrodes exhibit a poorly understood resistance increase during high-rate lithiation, which is rarely observed during delithiation. This hysteresis, often attributed to the solid-state diffusion effect, has not been clearly understood. Here, we unambiguously prove that Li transfer kinetics at the surface determines the overall hysteresis. We use operando X-ray microscopy to observe the surface and bulk Li compositions in [100]-oriented LixFePO4 sub-micron particles over a wide range of currents. The experiments reveal how surface and bulk phase separation strongly depend on the direction and magnitude of the current, thereby reconciling contradictions among existing models. The dynamical asymmetry between fast lithiation and delithiation is attributed to autoinhibitory (negative self-feedback on reaction rate) Li-rich and autocatalytic (positive self-feedback) Li-poor domains, respectively. These domains are stabilised in proximity to the active crystal surface. Stabilising electro-autocatalytic surface phases is a promising strategy to enhance the rate capability of Li-ion batteries, as well as lithionic memristors for information storage.