Revealing and Controlling Dynamic Lithium Insertion Pathway at High Rate via In Situ XRD and X-Ray Microscopy

Lithium-ion insertion kinetics fundamentally hinges upon phase transformation behavior during (dis)charging. At high c-rates, kinetic hysteresis is amplified and phase evolution becomes heterogeneous and unpredictable. For instance, discharging becomes more sluggish than charging for most of the battery electrodes. In addition, single-phase LiNi_xMn_yCo_zO₂(NCM) undergoes phase separation behavior. Here, we developed an operando soft x-ray microscopy to track the lithium transport and phase evolution on the surface and bulk of individual battery particles over a wide range of cycling rates (0.01 – 10C). Our result unambiguously reveals that dynamic asymmetry between fast charging and discharging originates from auto-inhibitory Li-rich and autocatalytic Li-poor surface domains, respectively. In addition, we developed synchrotron-based operando fast XRD to track phase evolution during fast cycling. We electrochemically manipulate the lithium-ion concentration distribution within NCM particles to effectively promote solid-state lithium diffusion. Our method succeeded in redirecting the unwanted phase separation at a fast rate to solid-solution behavior. Our work lays the groundwork for developing high-power applications and ultrafast charging protocols.