Generation of highly porous Li‒Mg and Li‒Zn alloys from kinetically controlled lithiation

Abstract

Li‒Mg and Li‒Zn alloy electrodes are prepared by the kinetically controlled vapour deposition (KCVD) of Li‒Mg and Li‒Zn alloys on a temperature-controlled substrate. The mode of preparation greatly influences the microstructure, surface morphology and electrochemical properties of these Li‒Mg and Li‒Zn alloys. The KCVD technique, as applied to the generation of Li-Mg alloys, establishes the composition of each prepared alloy by independently varying the temperature of the molten lithium, the temperature of a solid magnesium surface with which the lithium interacts, and the temperature of the substrate on which a resulting intimately mixed Li-Mg mixture is deposited. Here, the required temperature for lithium induced magnesium vaporization is more than 200°C below the magnesium melting point. The effect of variable temperature control on the microstructure, morphology and electrochemical properties of the vapour-deposited alloys has been studied and the diffusion coefficients for lithium in the Li‒Mg alloy electrodes prepared by the KCVD method are found to be in the range 1.2–5.2 × 10⁻⁷ cm²/s⁻¹ at room temperature. The diffusion coefficients for moderate lithium content alloys, firstly, exceed those for standard alloy preparation and, secondly, are two to three orders of magnitude larger than those for other lithium alloy systems (e.g. 6.0 × 10⁻¹⁰cm² s⁻¹ in LiA1). A variant of the KCVD technique is used to generate highly porous Li‒Zn alloy films prepared by controlling the temperature of a molten lithium source and the rate at which a zinc source, placed in proximity to the lithium, equilibrates in temperature as the lithium interacts with the zinc. Diffusion coefficients for the Li‒Zn alloy electrodes prepared by this variant of the KCVD method vary from 10⁻⁷ to 10⁻⁹ cm² s⁻¹ at room temperature, depending on the alloy composition. This should be compared with the standard alloy whose diffusion coefficient never exceeds 10⁹ cm² s⁻¹ for any composition. These observations suggest that alloys generated using kinetically controlled lithiation display an enhanced porosity and diffusivity useful for a number of applications.