Mapping the total lithium inventory of Li-ion batteries

Li-ion battery charging speed is limited by Li+ mass transport in the electrolyte and active materials, leading to spatiotemporal concentration gradients that cripple rate capabilities. Optimization of Li transport through porous composite electrodes is limited by the difficulty of speciating and mapping Li at the micron scale inside the dense, opaque, and typically paramagnetic matrix. Here, we report how Li dynamics can be quantitatively tracked in real time, inside an operational cell, using simultaneous X-ray diffraction and X-ray fluorescence. This approach differentiates Li in the liquid electrolyte and Li intercalated into the active material, allowing the transport in each phase to be fully decoupled for the first time. This generally applicable strategy is exploited to reveal the spatiotemporal gradients in total Li inventory created during different charging algorithms, determine maximum effective (dis)charge rates, and diagnose the physical origins of cell performance loss.