Abstract
Lithiated thin films are necessary for the fabrication of novel solid-state batteries, including the electrodes and solid electrolytes. Physical vapour deposition and chemical vapour deposition can be used to deposit lithiated films. However, the issue of conformality on non-planar substrates with large surface area makes them impractical for nanobatteries the capacity of which scales with surface area. Atomic layer deposition (ALD) avoids these issues and is able to deposit conformal films on 3D substrates. However, ALD is limited in the range of chemical reactions, due to the required volatility of the precursors. Moreover, relatively high temperatures are necessary (above 100 °C), which can be detrimental to electrode layers and substrates, for example to silicon into which the lithium can easily diffuse. In addition, several highly reactive precursors, such as Grignard reagents or n-butyllithium (BuLi) are only usable in solution. In theory, it is possible to use BuLi and water in solution to produce thin films of LiH. This theoretical reaction is self-saturating and, therefore, follows the principles of solution atomic layer deposition (sALD). Therefore, in this work the sALD technique and principles have been employed to experimentally prove the possibility of LiH deposition. The formation of homogeneous air-sensitive thin films, characterized by using ellipsometry, grazing incidence X-ray diffraction (GIXRD), in situ quartz crystal microbalance, and scanning electron microscopy, was observed. Lithium hydride diffraction peaks have been observed in as-deposited films by GIXRD. X-ray photoelectron spectroscopy and Auger spectroscopy analysis show the chemical identity of the decomposing air-sensitive films. Despite the air sensitivity of BuLi and LiH, making many standard measurements difficult, this work establishes the use of sALD to deposit LiH, a material inaccessible to conventional ALD, from precursors and at temperatures not suitable for conventional ALD.
Highlights
While the development of electric motors and semiconductor devices is progressing, the pressure on battery development is increasing correspondingly
Thermodynamic considerations demonstrate that lithium hydride and the concomitant reaction byproduct butanol are more stable than lithium hydroxide
The results indicate that an Atomic layer deposition (ALD) process occurred with a definite growth per cycle, albeit its saturation has yet to be determined
Summary
While the development of electric motors and semiconductor devices is progressing, the pressure on battery development is increasing correspondingly. The advent of pocket hand-held devices places even stricter demands on the safety of rechargeable batteries. Low conformity leads to low surface area and thick films are needed to avoid pinholes. The whole concept of a solid-state battery needs to be reconsidered, if we wish to surpass the capacity of current liquid-electrolyte batteries. While a niche use can be found for wholly nanoscale batteries, such as a nanoscale batteries for nanoscale transistors, the scaling issue needs to be addressed for more general applicability. To meet this challenge, the use of atomic layer deposition (ALD) has been proposed [2,3]. The inherent conformity of ALD allows for thinner, conformal, pin-hole free films [4,5]
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