A phase-field model, accounting for large elasto-plastic deformation, is developed to study the evolution of phase, morphology and stress in crystalline silicon (Si) electrodes upon lithium (Li) insertion. The Li concentration profiles and deformation geometries are co-evolved by solving a set of coupled phase-field and mechanics equations using the finite element method. The present phase-field model is validated in comparison with a non-linear concentration-dependent diffusion model of lithiation in Si electrodes. It is shown that as the lithiation proceeds, the hoop stress changes from the initial compression to tension in the surface layer of the Si electrode, which may explain the surface cracking observed in experiments. The present phase-field model is generally applicable to high-capacity electrode systems undergoing both phase change and large elasto-plastic deformation. As a promising anode material for lithium (Li)-ion batteries, 1-3 the theoretical Li capacity of Silicon (Si) is 4200 mAh/g (corresponding to the lithiated phase of Li 4.4 Si), which is one order of magnitude larger than the commercialized graphite anode.1,2 Recent experiments revealed that the lithiation of crystalline Si (c-Si) occurs through a two-phase mechanism, i.e., growth of lithiated amorphous Li x Si (aLi x Si, x ∼ 3.75) phase separated from the unlithiated c-Si phase by a sharp phase boundary of about 1 nm thick.4-8 An abrupt change of Li concentration across the amorphous-crystalline interface (ACI) gives rise to drastic volume strain inhomogeneity. The resulting high stresses induce plastic flow, fracture, and pulverization of Si electrodes, thereby leading to the loss of electrical contact and limiting the cycle life of Li-ion batteries. [9][10][11] Electrochemically driven mechanical degradation in high-capacity electrodes has stimulated enormous efforts on the development of chemo-mechanical models to understand how the stress arises and evolves in lithiated Si electrodes.12-15 These chemo-mechanical models often treated the lithiation-induced stress as the diffusion-induced stress by considering Li diffusion in a solid-state electrode that results in the change of composition from its stoichiometric state. Deviation from stoichiometry usually results in a volume change that generates stress if the Li distribution is non-uniform. Early chemo-mechanical models only involved a unidirectional coupling. Namely, the diffusioninduced mechanical stress was considered, whereas the effect of mechanical stress on diffusion was ignored. Both experimental and computational studies, however, have shown that the mechanical stresses play an important role in the lithiation kinetics of Si electrodes. 4,6,16,17 Recently, fully coupled chemo-mechanical models were developed to incorporate the mechanical stress into the chemical potential. [18][19][20][21][22] In these models, the local stress modulates lithiation kinetics (reaction rate and diffusivity), 4,6,23 and in turn, lithiation kinetics regulates the stress generation in lithiated ...