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The unexpected formation of lithiated-silicon sponge during cyclic charge of lithium-ion batteries with silicon anodes has been observed in recent reports. The severe hollowing of silicon electrodes leads to an irreversible deformation and the generation of new surfaces, which greatly affects the structural integrity and cycling stability of the cells. However, the understanding of lithiated-silicon sponge still remains as a great challenge. In this work, a coupled mechanical-electrochemical model based on the phase-field method is proposed, which can describe the void evolution and the surface instability in Si electrodes. The model combines a description of the lithium diffusion, vacancy nucleation and annihilation, and an elastic-plastic constitutive model with concentration-dependent material properties. The simulation results reveal that the void within/on the silicon electrode tends to grow toward/along the electrode surface and toward other voids, leading to the electrode surface disruption and void merging, as well as the intra-electrode tunnel formation and electrolyte intrusion. Importantly, it is found that mechanical pressure can not only suppress the growth of a single void but also prevent the linkage and merging between multiple voids. The proposed theoretical framework and the findings of this work provide an in-depth understanding of the complicated mechanical-electrochemical coupling behavior of Si electrodes. [ABSTRACT FROM AUTHOR]