Silicon electrodes for lithium-ion batteries exhibit a theoretical capacity ten times higher than that of graphite electrodes currently used in commercial systems. When lithiated/delithiated, silicon experiences stresses on the order of 1 GPa, a cause of poor cyclability and dissipation unfavorable to energy storage. When monophasic lithiation takes place after the first charging cycle, mechanical stresses affect lithium insertion through the chemical potential of the diffusing species (Larché-Cahn theory). In a first study we have investigated experimentally the coupling under homogeneous lithiation conditions by imposing an external mechanical loading on an amorphous silicon thin-film electrode at various lithiation rates, and by measuring its effect on the electrochemical response of the battery cell [1]. This mechanical loading made it possible to directly probe the contribution of mechanics to the lithiation/delithiation processes, and we showed that the instantaneous change in the cell voltage and its subsequent relaxation are well captured by a viscoplastic flow rule [2;3]. In the present numerical, modeling and experimental work, we aim to model the effect of an external mechanical load on the two-phase process that occurs during the first lithiation, involving the progressive invasion of the pristine Si phase by an amorphous lithiated LixSi phase. Our experiments show that when undergoing such a phase-transformation, the very same external mechanical loading as that applied during homogeneous lithiation leads to a very different response of the silicon electrode voltage. To rationalize these observations, we first use an Arbitrary Lagrangian-Eulerian method to solve a sharp-interface model of the phase transformation in the one-dimensional thin film setting. A finite-element numerical implementation is formulated with an implicit time scheme. This sharp-interface model is then employed to calibrate a phase-field model inspired from [4], which is more amenable to simulating complex phase-front geometries, as a mere numerical regularization of the sharp-interface model.