Role of Short-Range\nOrder on Diffusion Coefficients\nin the Li–Mg Alloy

Li–Mg alloys are important because of their beneficial\nrole\nin fostering uniform plating and stripping of lithium in all-solid-state\nbatteries. The alloy LixMg1−x forms a solid solution on the BCC crystal\nstructure when the lithium content is greater than <i>x</i> ≈ 0.3. The activation barriers of lithium and magnesium exchanges\nwith a vacancy, crucial for substitutional diffusion, are predicted\nto be exceptionally low and almost identical, with negligible dependence\non the alloy composition. The equilibrium vacancy concentration at\nroom temperature is predicted to be very low, and it also remains\nalmost constant with no dependence on Mg content in the alloy (for xLi≥0.5). Nevertheless, both experiments\nand kinetic Monte Carlo simulations indicate that the tracer diffusion\ncoefficients decrease by almost an order of magnitude with the addition\nof Mg to the alloy. In this contribution, the crucial role that chemical\nshort-range order plays in affecting the diffusion coefficients is\nstudied. Chemical short-range order is found to increase the effective\nactivation barrier for lithium and magnesium diffusion by making successive\natomic hops with vacancies correlated.