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Nanosized, carbon-coated LiFePO4(LFP) is a promisingcathode for Li-ion batteries. However, nano-particles are problematicfor electrode design, optimized electrodes requiring high tap densities,good electronic wiring, and a low tortuosity for efficient Li diffusionin the electrolyte in between the solid particles, conditions thatare difficult to achieve simultaneously. Using in situenergy-dispersive X-ray diffraction, we map the evolution of theinhomogeneous electrochemical reaction in LFP-electrodes. On the firstcycle, the dynamics are limited by Li diffusion in the electrolyteat a cycle rate of C/7. On the second cycle, there appear to be tworate-limiting processes: Li diffusion in the electrolyte and electronicconductivity through the electrode. Three-dimensional modeling basedon porous electrode theory shows that this change in dynamics canbe reproduced by reducing the electronic conductivity of the compositeelectrode by a factor of 8 compared to the first cycle. The poorerelectronic wiring could result from the expansion and contractionof the particles upon cycling and/or the formation of a solid-electrolyteinterphase layer. A lag was also observed perpendicular to the directionof the current: the LFP particles at the edges of the cathode reactedpreferentially to those in the middle, owing to the closer proximityto the electrolyte source. Simulations show that, at low charge rates,the reaction becomes more uniformly distributed across the electrodeas the porosity or the width of the particle-size distribution isincreased. However, at higher rates, the reaction becomes less uniformand independent of the particle-size distribution. [ABSTRACT FROM AUTHOR]