A First-Principles Based Study of Fundamental Thermodynamic, Kinetic and Mechanical Properties of Lithium and Its Alloys

All-solid-state batteries are increasingly being recognized as a viable high-energy density alternative to commercial lithium-ion batteries. Lithium alloys play a crucial role in enabling uniform plating and stripping of lithium metal in all-solid-state batteries. The specific role metal additions play in morphological evolution at the anode remains poorly understood, however, their ability to form alloys with lithium appears to be crucial. Despite their increased importance, many fundamental properties of lithium-metal alloys remain poorly characterized and understood. In this contribution, we report on a systematic first-principles study of fundamental thermodynamic, kinetic and mechanical properties of lithium and several important Li-M alloys (M = Mg, Ag, Zn, Al, Ga, In, Sn, Sb and Bi). Most lithium-metal alloys prefer perfect stoichiometric intermetallic compounds, with the exception of Li-Mg, which forms a solid solution. We show that pure lithium and its alloys have an unusually flat energy landscape along paths that connect the BCC crystal structure to close-packed structures such as FCC and HCP. Only when the concentration of the alloying element increases does the energy landscape become progressively stiffer. We predict exceptionally low migration barriers for lithium diffusion in pure Li and many Li-intermetallics comparable to superion conductors. The concentration of vacancies crucial for mediating substitutional diffusion, however, is predicted to be very low in metallic Li and most Li-M intermetallics, resulting in low diffusion coefficients despite the unusually low migration barriers. We identify several intermetallic compounds, such as B32 LiAl and LiGa, D0₃ Li₃Sb and Li₃Bi, which favor structural vacancies at the higher ends of their voltage stability windows that, coupled with very low migration barriers, show exceptionally high Li mobilities. We believe our results are relevant to experimental researchers and the large community of battery modelers of morphological evolution and are invaluable in directing the rational design of all-solid-state batteries.