Development of a Fabrication Process for Antiperovskite Li₃OCl Thin Films

Research interest in lithium-rich antiperovskites for use as electrolytes in all-solid-state batteries has grown significantly over the past decade due to the relatively low costs of these materials and predictions of promising electrochemical stability [1]. Among them, the lithium hydroxide halide compounds Li_3-xOH_xA, where A is usually Cl or Br, have received the most attention. These compositions have demonstrated stability against lithium metal but relatively low ionic conductivities (~10^-6 S cm^-1) [2]. Several attempts have been made to fabricate dehydrated compounds [1], most notably Li₃OCl, owing to the possibility of superior electrochemical performance. However, fabrication of Li₃OCl has proved challenging due to its hygroscopicity and thermodynamic instability. It is therefore likely that most synthesis attempts have inadvertently formed more stable hydrated compounds such as Li₂OHCl.<br/><br/>We have investigated the fabrication of Li₃OCl thin films by RF magnetron sputtering to try to overcome this synthesis challenge. The fabrication of Li₃OCl by pulsed laser deposition (PLD) has been reported previously [3], but there are currently no reports on the use of RF magnetron sputtering for this purpose. Like PLD, RF sputtering enables reaction between precursor compounds in a high purity atmosphere, making it ideally suited to the synthesis of hydrogen-free materials. However, it has the additional benefit of being deployable at an industrial scale. We have produced sputtering targets composed of uncompacted mixtures of Li₂O and LiCl powders which we sputtered in an atmosphere containing argon and oxygen gas. The use of sputtering conditions typical for other battery materials such as LiPON and LiCoO₂ caused rapid reaction between the precursor compounds within the target, forming a molten phase without any applied heating. By optimizing the values of applied RF power and process gas pressure the target was stabilized against melting, giving a repeatable deposition process. Other deposition conditions (LiCl to Li₂O ratio, substrate material and argon to oxygen gas ratio) were optimized along with the post-deposition annealing conditions to promote the formation of a crystalline Li₃OCl phase. <br/><br/>The initial results are promising, and we show that an antiperovskite phase exists within our films along with small fractions of secondary phases (as determined by X-ray diffraction), including the Li₂O and LiCl precursors. We also show that the concentrations of H⁺ and OH^- determined by secondary ion mass spectrometry and Fourier-transform infrared spectroscopy are notably lower than in an Li₂OHCl pellet, and the overall composition measured by energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy is close to the targeted stoichiometry. We have developed a fundamental understanding of the processing-composition-structure relationships pertaining to the sputter deposition of thin films from LiCl and Li₂O precursors, which will prove useful in future work on optimizing the electrochemical performance of these thin films.<br/>[1] Dawson JA, Famprikis T, Johnston KE, Anti-perovskites for solid-state batteries: recent developments, current challenges and future prospects. Journal of Materials Chemistry A, 2021, 9, 18746-18772. doi: 10.1039/D1TA03680G<br/>[2] Lee HJ, Darminto B, Narayanan S, Diaz-Lopez M, Xiao A, Chart Y, et al. A comprehensive investigation of Li-ion conductivity in lithium hydroxy halide antiperovskite solid electrolytes. ChemRxiv. Cambridge: Cambridge Open Engage; 2021; This content is a preprint and has not been peer-reviewed.<br/>[3] Lü, X., Howard, J. W., Chen, A., Zhu, J., Li, S., Wu, G., Dowden, P., Xu, H., Zhao, Y., Jia, Q. (2016). Antiperovskite Li3OCl Superionic Conductor Films for Solid-State Li-Ion Batteries. Adv. Sci., 3: 1500359. doi: 10.1002/advs.201500359