Analyses of Thick Lithium Coatings Deposited by Sputter‐Evaporation and Exposed to Air

Abstract

Lithium coatings on various substrates have numerous applications: Boron neutron capture therapy, neutron activation analysis, super‐conducting tokamak etc. Traditionally these coatings are produced by well‐known techniques such as electrochemistry and evaporation. In this work, we investigated a new method based on sputter‐evaporation, which enables thick coatings (>10 µm) to be built on various substrates within a short timeframe. In order to minimize the process time, evaporation techniques can be used but the coating quality suffers. Moreover, it is well known that the use of DC magnetron sputtering results in the deposition of good quality coatings (smoothness, density, adhesion); however, the deposition rate is low. The rationale of this work is to combine these two techniques, yielding a sputter‐evaporation process that possesses the advantages of each separate technique. Li is placed in a stainless steel crucible (cathode), and heated by the plasma generated by a magnetron discharge. The Li temperature is measured by a thermocouple welded onto the cathode and measured at different plasma power densities. The deposition rate of lithium is measured using a quartz balance and by profilometry, at several temperatures (from 0 to 580 °C). Li samples were depth‐profiled with the resonant nuclear reaction 7Li(p,γ)7. In addition to the concentration, certain characteristics like the density and the chemical reactivity of layers, are also important. Thus we have studied the evolution of the density with time, estimated by weight and profilometry measurements, and the change in morphology, by cross‐sectional scanning electron microscopy (SEM), of samples exposed to air at room temperature. The evolution of the film compounds have also been determined by X‐ray powder diffraction. These physical properties have been investigated for various bias substrates during deposition.

Current Status and Future Perspective on Lithium Metal Anode Production Methods

“…For example, [ 121,122 ] lithium layers of 10–40 µm thickness were deposited onto glass and SiO 2 substrates to investigate, by resonant nuclear reaction analysis, the Li diffusion into the substrates ( Figure 6 , left) and the lithium stability in air after being covered by a sputter‐deposited thin‐film of stainless‐steel (Figure 6, right). [ 146,147 ]…”

“…For example, [121,122] lithium layers of 10-40 µm thickness were deposited onto glass and SiO 2 substrates to investigate, by resonant nuclear reaction analysis, the Li diffusion into the substrates (Figure 6, left) and the lithium stability in air after being covered by a sputter-deposited thin-film of stainlesssteel (Figure 6, right). [146,147] Actually, lithium sputter targets and sputtering are very well known. However, to prove the viability of sputtering as a valuable tool for LMAs production, there is the need of a great fundamental improvement, particularly in the fabrication methods, configuration, and handling of Li sputter targets.…”

“…Finally, thermal evaporation has been demonstrated to be the most appropriate PVD technique for obtaining thin films of metallic lithium, and it can hold a relevant position for the future of LMBs. [147][148][149][150][151][152][153] LMAs produced by PVD in general, including evaporation, are particularly good in terms of homogeneity and conformality of the surface, with defect-free surfaces being achievable. Moreover, PVD offers really good control of the thickness for lithium layers that could be in the range from nanometers to tens of micrometers, thus, being possible to overcome the current limitation on thickness for the conventional method.…”

Physical vapour deposition of metallic lithium

“…The various possible reactions can be prevented by 'sandwiching' the Li layer between a passivation layer on the substrate side and a protective layer coating the lithium directly after evaporation. The encapsulation by means of a metallic layer to improve the chemical stability has been applied for thick Li coatings deposited by sputter evaporation [3] and for thin 6 Li targets [4]. Protective layers of stainless steel, Ni and Ti could not prevent the formation of Li compounds by the reaction with atmospheric air.…”