A Large-Deformation Phase-Field Model for Characterizing Electro-Chemo-Mechanical Failure Mechanisms at Li/Solid Electrolyte Interface

Abstract

All-solid-state-batteries (ASSBs) are considered the future replacements for traditional lithium-ion batteries, thanks to their superior energy density and enhanced safety. The ionic conductivity of solid electrolytes (SEs) could be improved to match that of liquid electrolytes through careful material design and optimization. However, the relatively sluggish kinetics at the interface between at the active material (lithium metal or cathode materials) and the SE interface remain a hurdle for the wider adoption of ASSBs. The SE’s contact with the anode and cathode materials is inferior compared to that of its liquid counterpart, and improper working conditions can easily lead to the formation of many voids and pores. This results in an increased interface resistance and a deteriorated power density of the ASSB cell. The mechanical contact between the anode material and the SE is more significant due to the larger volumetric expansion and contraction during cycling compared to the cathode side. In addition, high stack pressure and plating current density can also induce the penetration of lithium dendrites. Therefore, it is vital to explore the stability envelope for the anode-SE interface, namely determining the operation condition under which void formation and dendrite growth could be suppressed for the effective design and utilization of ASSBs.Pressure and current density are two controllable condition parameters in the operation of ASSBs1, 2. High current density can lead to the formation of numerous voids at the Li/SE interface during stripping, as well as the initiation and growth of lithium metal dendrite within the SE. This results in increased polarization and potential battery failure. Applying stack pressure to the ASSB can stabilize the Li/SE interface during manufacturing and operation, as the creep of Li metal is believed to occur once its stress surpasses a certain threshold. The plastic ‘flow’ of Li metal could help to hinder the growth of voids2 and keep good contact condition. However, at an excessively large pressure3, 4, dendrites can penetrate the SE, which cause a short-circuit in the ASSB and lead to failure5. Therefore, an electro-chemo-mechanical model that can describe the large-deformation mechanical properties of Li metal and its coupled mechanisms with the reactions at the Li/SE interface is essential for determining the stability envelope for stable stripping and plating of lithium in the ASSBs.In this work, a phase-field electro-chemo-mechanical model is proposed, in which the coupling of void diffusion, lattice annihilation, stripping and plating reactions, and mechanical properties of lithium metal are comprehensively described. The first contribution of this work is a comprehensive summary of the mechanical properties of the lithium metal under different temperatures and deformation rates, generating a unified deformation-mechanism map for the general battery manufacturing and characterizing community. Based on this map, our phase-field electro-chemo-mechanical model is developed to include important features below. The diffusion of vacancies and Li sites in the lithium metal is considered to simulate the lattice annihilation and the void formation during stripping.The flow of lithium metal caused by creeping or plasticity is incorporated in the kinetical equation of the order parameters.The plating and stripping kinetics are described by the modified Butler-Volmer equation in which the effect of vacancies is considered The general theory proposed in this work can simulate the electro-chemo-mechanical effects at different operation conditions for Li metal or other Li alloy anode materials, which is believed to be a powerful tool for the effective design, manufacturing and management of next-generation batteries.