Fabrication routes for thin-film solid-state batteries
Keywords: solid electrolyte
antiperovskite
RF magnetron sputtering
lithium cobalt oxide
cathode material
thin film battery
solid state battery

Thin-film solid-state batteries have been studied since the 1950s, but it was the demonstration of remarkable cycling stability in an all-thin-film Li/LiPON (lithium phosphorus oxynitride)/LCO (LiCoO2) cell in the mid-1990s that captured the attention of the solid-state battery research community. Although these cells were commercialized on a small scale, their utility is limited by their low areal capacities. Nevertheless, there is a growing realization that thin-film processing techniques such as RF (radio frequency) magnetron sputtering offer various advantages over the traditional powder processing routes commonly used to fabricate solid-state cell components.
The first investigation reported here looked at a strategy to improve the performance of the “traditional” all-thin-film cell by optimizing the processing of the LCO cathode. Most previous studies on thin-film LCO cathodes have reported similar processing conditions, and these are not optimal for large-scale manufacturing. In this work, the process pressure used for RF magnetron sputtering was reduced while the power density was increased relative to typical conditions from the literature. Several other changes were made, including the omission of an oxygen gas flow during deposition, until ~3 μm thick films well-crystallized in the targeted HT-LCO (high temperature LCO) structure were achieved at a deposition rate 3-4 times higher than obtained using the reference conditions. Characterization by X-ray diffraction (XRD) revealed strong parallel alignment of the (104), (101) and (110) planes with the film surface, as desired for efficient lithium transfer to the electrolyte. Notably, a maximum areal discharge capacity of 172 μAh cm-2 was measured during galvanostatic cycling between 3.0 and 4.3 V at a current density of 18 μA cm-2, which is one of the highest values reported. This can be attributed to the greater film thicknesses reported here compared to those of most previous studies (generally below 1 μm) and the use of processing conditions optimized both for manufacturability and electrochemical performance.
The second investigation looked at the synthesis of Li3OCl, a member of the recently discovered class of lithium-rich antiperovskite electrolyte materials. This compound has attracted interest because of its predicted stability in contact with lithium metal anodes, raising the prospect of future application in all-solid-state lithium metal batteries. However, its thermodynamic instability at room temperature and extreme hygroscopicity appear to preclude synthesis by traditional solid state and melt processing routes since the more stable Li2OHCl compound is likely to form instead. This work attempted to synthesize Li3OCl using RF magnetron sputtering, taking advantage of the moisture-free environment and high cooling rates associated with this technique. Sputtering was performed from an equimolar powdered mixture of the precursor compounds Li2O and LiCl. After optimizing the sputtering conditions for target stability, XRD, Fourier transform infrared spectroscopy and electrochemical impedance spectroscopy were used to show that a small volume fraction of a phase with the properties expected for Li3OCl was present in the as-deposited films. The results were compared to those of films deposited from Li2OHCl powder to prove that Li2OHCl had not been produced inadvertently. Both sets of films exhibited low ionic conductivities (on the order of 10-8 S cm-1) owing to their low antiperovskite phase purities. Nevertheless, an increased understanding of the processing-structure-properties relationships pertaining to these compounds allowed several promising directions for future research to be proposed.