Mechanical Properties of Alkali Metal Anodes of Rechargeable Batteries

Metallic anodes have the potential to enable batteries with enormous capacities. Indeed, lithium metal has the highest theoretical capacity, lowest density, and most negative electrochemical potential of known anode materials for rechargeable batteries. However, dendrites of lithium can form during cycling, which lead to significant safety issues that have precluded their practical deployment. Sodium metal anodes have similar safety concerns but have recently received increased attention due to their natural abundance, relatively low cost, and potential for grid-scale energy storage. Potassium metal anodes are still very much in a developmental phase but have promise stemming from their potential for simple cell design with cheap fabrication, earth abundance, and relatively low cost. While the electrochemistry of these systems has received extensive study, at the heart of many practical issues in these systems lies a mechanics of materials problem. Specifically, as atoms are deposited and stripped during electrochemical cycling, the material deforms, generating stresses under constraint. These stresses can result in fracture, delamination, detachment, and/or unstable deformation (e.g., the formation of dendritic structures) of the electrodes, diminishing their capacity or leading to severe safety issues. Likewise, the stresses in turn may affect the electrode kinetics, the growth morphology under cycling, and/or the integrity of the contact at the anode/solid-state-electrolyte interface. Studies that provide deeper understanding of mechanics in these systems may thus play a key role in mitigating or even preventing many of these issues. For instance, modelling studies have suggested that the mechanical properties of the anode materials (relative to the separators or solid-state electrolytes with which they are in contact) are key in guiding the suppression of dendritic growth during electrochemical operation. Experimental studies have shown that the morphology of Li during electrochemical deposition depends on external pressures applied to battery stacks, as does the propensity for maintaining interfacial contact in all-solid-state batteries. As such, prior to real applications, a comprehensive understanding of the mechanical properties of these materials is vital. To this end, through nanoindentation, microhardness, and bulk testing, this talk will present experimental studies of the mechanical properties of metallic Li, Na, K anodes, including how these properties vary with loading rate, representative size scale, and temperature. These properties will be connected to implications in terms of potential battery performance, e.g., in preventing dendritic structures, detachment between solid-state electrolytes and metallic anodes, etc. Overall, this talk will provide insight into guiding the design of battery materials, architectures, and electrochemical (e.g., charging) conditions that mitigate unstable growth of alkali metal anodes during electrochemical cycling.