Electrochemical-mechanical model of the space charge zone at the interface

The interface between a solid electrolyte and an electrode plays an important role in determining the physical processes controlling the electrochemical performance of metal-ion batteries. In this work, we developed an electrochemical-mechanical model for the determination of net charge density, stress and electric fields in a solid electrolyte, which is in contact with an electrode, under the framework of thermodynamics and linear elasticity. Mobile species are cations, which occupy interstitial sites through the formation of Frenkel defects. Analytical solutions of net charge density, stress and electric fields are derived using the linear coupling model, which is a simplification of the nonlinear coupling system under low stress and electric fields. For a solid electrolyte sandwiched between two parallel electrodes, numerical results predict that there exists an accumulation/adsorption of a layer of charges (interstitial ions) onto the electrode, i.e., the presence of a space charge zone whose size is dependent on the electric potential and elastic constants of the solid electrolyte. Such behavior is similar to the Stern layer of a liquid electrolyte and allows for the storage of energy in a capacitive form, similar to an electrical double layer. The ratio of the nominal size of the space charge zone to the thickness of a solid electrolyte decreases as the thickness of the solid electrolyte increases. The nonlinear and coupling system developed in this work lays a foundation to analyze the interface behavior of heterogeneous structures and the effects of the space charge zone on the energy storage of multilayer structures. The approach presented in this work can be extended to investigate the multi-field coupling problems in solid oxide fuel cells, mixed halide quantum dots and transducers.