Olivine LiFePO4 is believed to form two-phase equilibrium between the FePO4 (FP) and LiFePO4(LFP) phases in large particles. However, in rechargeable batteries, LFP is nanosized to overcome the low electric conductivity. The phase transformation behavior of nanosized LFP remains poorly understood. To address this issue, we have developed models and an electrochemical simulation framework for Li intercalation dynamics at particle/microstructural level to examine the phase transformation dynamics of LFP nanoparticles. The charge/discharge processes of Li-ion batteries involve several coupled physical mechanisms that occur within complex electrode microstructures. To simulate the electrochemical processes at particle/microstructural level, we developed an innovative technique that allows us to efficiently simulate electrochemical (de)intercalation processes at length scales ranging from electrode particle to complex electrode microstructures. In this presentation, we will discuss the smoothed-boundary-method electrochemical simulation framework, which straightforwardly couples the multiple physical mechanisms in the different phases of a composite electrode and incorporates the complex electrode geometries, as well as its applications. For demonstration, we used an experimentally obtained LiCoO2 cathode microstructure and corresponding material properties as simulation input [1]. We applied this simulation framework to investigate the phase-separation dynamics of LFP-nanoparticle electrodes. We first simulated a two-particle system to elucidate the detailed interparticle interactions upon (de)lithiation [2], and identified a unique process of interparticle phase separation. The simulation was extended to a collection of a modest number of LFP nanoparticles, where the particles were not contacting one another and formed a dilute agglomerate [3]. The simulation revealed a group-by-group interparticle phase separation process at low currents. In a further extension, we investigated the dynamics in a densely packed LFP-nanoparticle agglomerate [3], in which direct contact between particles was taken into account. In the dense agglomerate, we found that interparticle phase separation can still prevail at high currents due to the direct Li redistribution across particle interfaces. Finally, we included the consideration of particle-size dependent equilibrium potential into the model [4]. The simulation showed an asymmetry of phase transformation between lithiation and delithiation of an electrode at low currents. Acknowledgement: This material is based upon work supported as part of NECCES, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Numbers DE-SC0001294 and DE-SC0012583. [1] H.-C. Yu et al., in preparation. [2] B. Orvanonos et al., J Electrochem Soc, 162 (2015) A965 [3] B. Orvananos et al., Electrochimca Acta, 137 (2014) 245 [4] B. Orvananos et al., submitted to J Electrochem Soc.