LiFePO4 is widely considered to be a promising cathode material for Li-ion rechargeable batteries. The lithiation process in LiFePO4− nano-particles proceeds via a coherent solid-solid phase transforma- tion between the LiFePO4 (LFP-phase) and FePO4 (FP-phase). We develop a continuum phase eld model for solid-solid phase trans- formations in single-LiFePO4−nano-particles, embedded in an elasti- cally soft electrolyte-phase. The model-description explicitly includes anisotropic (orthorhombic) and inhomogeneous elastic e ects, result- ing from coherency strain, as well as anisotropic (1D) Li-di usion in- sight the nano-particles. The moving LFP/FP-phase boundary is mod- eled as a di use interface of nite width. Here, with the inclusion of the elastically soft electrolyte-phase, we consider also non-rectangular shaped LiFePO4− particles in a fully anisotropic 3D-framework. The resulting model is employed to investigate e ects of the nano-particle’s size and shape on the kinetics of FP to LFP phase transformations, relating to single particle charge rates. In respective model studies, we nd that in at particles the transformation velocity increases over more than two orders of magnitudes with decreasing particle-hight. We show that in contrast to Cahn-Hilliard-type models this behavior can be regularized in Allen-Cahn-type phase- eld formulations.