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

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.