Revealing the Structure of Solid Electrolyte Thin Films to Enable Lithium Metal Batteries

Advanced batteries containing a lithium metal anode (LMA) are needed to enable the clean economy and meet ambitious climate targets. Currently, LMA batteries are limited by dendrite propagation due to the non-uniform plating and stripping of Li, which is detrimental to cell performance and rate capability. The amorphous solid electrolyte (SE), lithium phosphorus oxynitride (LiPON) is unique in demonstrating stable Li plating/ stripping at appreciable rates, but its synthesis requires costly vacuum deposition methods. In general, glassy SEs are advantageous due to their chemical stability, ductility, and resistance to dendrite formation, therefore new glassy SEs produced by scalable techniques are desirable. Our work uses aqueous precursor solutions which are rapidly condensed during spin coating and gently annealed to produce dense, lithium-containing oxide films without long-range order. As the bulk ionic conductivity (σ) of LiPON is ~10^-6 S cm^-1at 1 μm thick, we propose significantly thinner films need 10^-7S cm^-1 – current materials fall far short of this value. Methods to increase the σ of these lithium-containing oxide glasses include tuning composition and the degree of crystallinity via thermal processing conditions, but these parameters have been challenging to probe in thin-film format.<br/>Thus, the local structure of these types of glasses is likely to play an important role in their σ which can only be probed via pair-distribution-function (PDF) analysis. Correlating the local structure to the σ processed at different annealing temperatures will reveal the structure-property relationship of these SEs. Preliminary X-ray total scattering measurements (XTS) were carried out using a grazing incidence (GI) setup. PDF analysis of the lithium-containing oxide films revealed that they were nanocrystalline, containing ~1 nm domains. The displayed local order highlights the importance of PDF analysis as lab XRD showed the film to be completely amorphous, whereas GI-XTS reveals the structure to have nanocrystalline domains likely embedded in a glass matrix.<br/>Lithium-containing oxide films have been successfully spin-coated and characterised using cross-sectional SEM which reveals a continuous film. Initial optimisation of processing parameters has yielded an σ of 10^-7S cm^-1. Electrochemical tests of these glassy films deposited onto a current collector in an anode-free cell will be used to evaluate their ability to facilitate stable deposition and stripping of Li at appreciable current densities, in order to enable fast charging. As only limited chemical compositions and processing conditions of these glassy SEs have been reported, with no corresponding structural studies, our work will provide insights to enable the optimisation and discovery of scalable glassy SEs for advanced batteries.