Morphological Instability of Lithium Electrodeposition Induced by Elastic Stress-Driven Diffusion

Abstract

Lithium metal negative electrodes for liquid cells have high theoretical capacity, but suffer from unstable metal interfaces during charging. Here a model for morphology evolution during plating is presented that focuses on the interface instability during plating at low to moderate current density. It is proposed that the instability is caused by stress due to excess Li atoms incorporated in the metal during electrodeposition. The resulting stress gradients drive diffusion along the lithium surface that amplify surface roughness. The instabilty is modeled by extending the Asaro-Tiller-Grinfel’d diffusional instability for stressed surfaces of elastic solids to interfaces between elastic layers, namely that between lithium metal and the solid-electrolyte interphase (SEI). The assumption of elastic deformation is valid for lithium deposits less than about 1 μm thick. Linear stability analysis revealed that the model predicts the correct scaling between instability length scale and stress: for elastic stress of order 1 MPa as found experimentally, the spacing between interface protrusions is a few μm, similar to typical distances between whiskers on deposit surfaces. The model suggests that the instability can be suppressed by use of elastically stiff current collector substrates.