Alloying Reactions as a Means of Controlling Morphological Evolution of Lithium Metal in All Solid-State Batteries

All solid-state batteries promise significant increases in energy density because they will enable the use of lithium metal instead of graphite as anodes. However, there are significant challenges in controlling the morphology of metallic lithium during the plating and stripping of lithium between a solid electrolyte and the current collector. Metal additives that alloy with Li can facilitate the uniform deposition and stripping of metallic Li in anode-free all solid-state batteries by affecting nucleation, diffusion and growth kinetics. Very little is known about the fundamental thermodynamic and kinetic properties of lithium-metal alloys. Some alloying elements such as Mg form solid solutions with Li, while many other alloying elements, including Ag, Al, Ga, In, Zn, Sn, Sb and Bi, form a variety of intermetallic compounds. A crucial property is the mobility of Li within the intermetallic compounds that form during alloying reactions. First-principles statistical mechanics methods that rely on kinetic Monte Carlo simulations are able to elucidate diffusion mechanisms in substitutional alloys and predict the concentration dependence of diffusion coefficients. Li diffusion in solid solutions and intermetallic phases is mediated by vacancies, which in most alloys are predicted to be present at very dilute concentrations. The migration barriers for Li diffusion in most intermetallic phases is predicted to be very low, rivaling those of super-ion conductors. The complex crystal structure of most intermetallic phases leads to unusual diffusion mechanisms, including two-atom hops and multi-hop cycles to preserve long-range order. Several intermetallic compounds, such as the LiAl zintl phase, however, favor structural vacancies and have crystal structures with fully interconnected Li sublattices. This results in exceptionally high Li diffusion coefficients. Li alloys also exhibit intriguing mechanical properties due to the unusual energy surface of lithium metal along crystallographic pathways that connect the BCC crystal structure to close-packed crystal structures. First-principles calculations predict that high concentrations of alloying elements are necessary to modify the mechanical properties of lithium metal. The combination of the unique thermodynamic, kinetic and mechanical properties of Li alloys offers a rich pallet with which to control the morphological evolution of lithium metal in all solid-state batteries.