Fabrication of thin solid electrolytes containing a small volume of an Li3OCl-type antiperovskite phase by RF magnetron sputtering

Abstract

Several attempts to synthesize Li₃OCl – a lithium-rich antiperovskite compound envisaged as a potential solid electrolyte material for lithium metal batteries – have been reported, but few have yielded convincing results. There are two key challenges associated with this synthesis: the thermodynamic instability of Li₃OCl at room temperature and its extreme hygroscopicity. Therefore, the likelihood of inadvertently forming the structurally similar thermodynamically stable hydroxide halide compound Li₂OHCl is very high. In this report, we demonstrate the stabilization of a small volume fraction of antiperovskite phase with the characteristics expected for Li₃OCl in ∼0.5 to ∼1 μm films fabricated from a Li₂O + LiCl powder target by RF magnetron sputtering. Measures were taken to minimize the presence of moisture at all stages of synthesis and characterization. X-ray diffraction (XRD) experiments showed that reaction between the precursor phases occurred within the growing films to form a volume of antiperovskite phase with an identical lattice parameter to that predicted for cubic Li₃OCl. This antiperovskite phase decomposed into Li₂O and LiCl upon annealing at moderate temperatures. Characterization by Fourier transform infrared spectroscopy (FT-IR) confirmed the absence of O–H bonding in the films, providing further evidence that the antiperovskite phase was Li₃OCl rather than Li₂OHCl. Deposition of films with similar thicknesses from an Li₂OHCl powder target was also performed for comparison. While FT-IR results showed that O–H bonding was present in these films, a small volume fraction of an antiperovskite phase with identical lattice parameter to Li₂OHCl was only detected after heating the films to ∼100 °C. Owing to the low phase purities of films deposited from both target types, the Li⁺ conductivities were found to be on the order of 10⁻⁸ S cm⁻¹. For Li₂OHCl in particular, it is expected that further optimization of the processing conditions will lead to a significant increase in Li⁺ conductivity. This is the first reported attempt to synthesize lithium-rich antiperovskite compounds by RF magnetron sputtering.