Plastic Deformation of Lithium Metal and Impact on Dendrite Growth Process

Abstract

Lithium metal has attracted interest as an alternative anode material for lithium ion batteries due to its extremely low reduction potential (-3.04V with respect to standard hydrogen electrode) and very high theoretical specific capacity (3860mAh/g of Li). However, lithium dendrite growth is a major challenge preventing the widespread commercial use of lithium thin films as lithium ion battery anodes. It has been observed experimentally that stiff protective layers are capable of preventing the growth of dendritic protrusions. In a previous article[1], the present authors investigated a dendritic protrusion into a solid polymer electrolyte/separator region. Under the assumption of elastic deformation, stress fields and their impact on the current distribution were calculated. It was found that operation at low current density did not lead to any dendrite growth. Also the stress field under the assumption of linear elastic deformation significantly exceeded the yield strength of both lithium metal and polymer electrolyte region. Contrary to the results of this previous analysis, there are several reports of lithium dendrite growth in systems where PEO-based polymers have been used as the electrolyte[2]. Elucidation of the dendrite growth mechanism within these Li/PEO systems is the main aim of the present work, which extends the earlier model by considering i) a more realistic elastic-plastic description of lithium metal and PEO electrolyte deformation, and ii) operation at current densities close to the limiting current, at which significant concentration and potential gradients evolve according to concentrated solution theory, promoting dendrite growth through higher total current density at dendrite protrusion peaks than valleys. An electrolyte phase with a higher elastic modulus can suppress this growth by i) reducing reaction current density at the peak through increased compressive stress, and ii) reducing variations in ionic conduction path lengths by decreasing dendritic protrusion height. Based on these two mechanisms, the minimum value of the elastic modulus needed for the electrolyte phase to prevent growth of dendritic protrusions is estimated.