Solid-state batteries promise higher energy density and improved safety, yet loss of contact at the anode–solid electrolyte interface during cycling remains a major barrier to commercialization. Experiments show that externally applied stack pressure can mitigate contact loss, highlighting the importance of interfacial mechanics. However, the coupled mechanisms governing interface evolution are not yet fully understood. Current literature points to two key physical effects: the inhomogeneous plating and stripping driven by the interfacial current density and the viscoplastic flow of the lithium anode under mechanical stress. Understanding their interaction motivates the development of numerical models for interface dynamics. We present a finite element electro-chemo-mechanical model incorporating these two phenomena. The model extends a nonlinear continuum mechanics framework that decomposes deformation multiplicatively into elastic and inelastic components, thereby accounting for large volume changes during (de-)lithiation. Inhomogeneous interface evolution due to plating and stripping is modeled based on the local Butler-Volmer current density arising from interface overpotentials. The influence of interface evolution on solid mechanics is fully integrated within the numerical framework through a dedicated inelastic deformation component. Moreover, our model incorporates large-deformation anode viscoplasticity as an extension of standard hyperelasticity formulations, accommodating various hyperelastic constitutive relations, viscoplastic flow rules, and material hardening models. Following recent literature on lithium electrodeposition [2], we represent the viscoplastic behavior as transversely isotropic to capture the columnar structure of electrodeposited lithium. The developed framework enables systematic studies of interfacial phenomena under realistic battery-operating conditions, resolving the interaction between stack pressure and electrochemical cycling. It provides a foundation for understanding interface stability mechanisms and can readily incorporate additional physical effects relevant to solid-state battery performance.