Tracking Non-Uniform Mesoscale Transport in LiFePO4 Agglomerates During Electrochemical Cycling

State-of-charge maps of a LiFePO4 electrode in standard nonaqueous electrolyte are recorded inoperando with a resolution of tens of nanometers by using hard X-ray spectroscopic microscopy. We observe spatially inhomogeneous charging and discharging of the composite electrode, with predominately Li-rich and Li-poor micron-scale agglomerates. There appears to be no correlation between the agglomerate size and the (de)lithiation pathway. Additionally, we observe that the electrochemical heterogeneity exhibits a last to delithiate, first to lithiate behavior, in which the regions that are the last to charge are also the regions that are the first to discharge. These observations suggest that the nanoscale pore structure within the agglomerates dominates lithium-ion transport.