Kinetically controlled lithiation: A variant of physical vapour deposition with application to lightweight alloys and lithium batteries

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Abstract
Employing a distinct variation of the physical vapour deposition technique, lithium vapour has been used to form Mg-Li alloy films whose physical structure can be modified through substrate temperature control over a considerable composition range. In addition to these Mg-Li alloy films, alloys of aluminium and copper can be prepared and modified primarily by controlling the interaction of lithium vapour with the precursor metal in the form of a cast sheet ranging in thickness from 0.02 to 0.07 in. A lithium-mediated process is found to produce a significant vaporization enhancement from the surface of the magnesium sheet at temperatures close to 200 degrees C below that required for vaporization in the absence of lithium. The interaction process not only promotes the vaporization of the magnesium but also leads to an intimate mixing of magnesium and lithium vapours. The lithium and magnesium contents of the formed vapours have been varied to produce alloy films of between 0.08 and 30wt%Li. As the vapour mixture is subsequently deposited onto a temperature-controlled substrate, the physical make-up of the films produced is modified through temperature variation. With the lowering of the substrate temperature, the microstructure of the deposited film transforms from the cubic crystalline structure characteristic of a phase-equilibrated Mg-Li alloy with greater than 26wt%Li to a densely packed fibrous columnar microstructure,and, on further cooling, to a tapered columnar microstructure with extensive voids. This latter structure may prove useful in the development of higher-efficiency lithium batteries. A cast aluminium sheet can be modified to an Al-Li alloy as an impinging lithium vapour creates an excess lithium content at the surface. The excess lithium can be removed or passed further into the aluminium employing a solid-state diffusion process, as Al-Li alloys whose lithium content ranges from 0.2 to 5 wt% are prepared. The deposition process, which requires the stringent control of the aluminium temperature over an approximately 20 degrees C range, is distinct in that it can permit the introduction of the reactive element, lithium, into an alloy near the final stage in the production of a wrought product and might also be used to replace the surface lithium lost from an alloy during heat treatment. The techniques described also appear applicable to alloy formation with additional elements soluble in lithium including copper, zinc and silver.