Interphase Formation at the Interface between Solid Electrolytes and Lithium Metal Anodes in Solid-State Batteries

Abstract

The growing demand for energy storage systems, driven by the need for the “energy transition”, highlights the importance of efficient energy storage. Conventional lithium-ion batteries with liquid electrolytes are approaching their theoretical physicochemical limits and are subject to safety con-cerns due to risk of leakage and high flammability. To overcome these limitations, solid-state bat-teries with solid electrolytes are explored as an alternative that offer greater safety and a wider operation temperature range. When combined with a lithium metal anode, solid-state batteries can achieve the required energy and power densities. However, the reactivity of lithium metal presents safety challenges such as side reactions and den-drite formation. Most solid electrolytes decompose upon contact with lithium, forming different interphases depending on the electrolyte composition. Understanding the interphase formation at the lithium|solid electrolyte interface is crucial for developing protective measures. In addition, studying the kinetic growth of interphase formation helps in estimating the lifetime of the battery. This dissertation focuses on the characterization of interphase formation between solid electrolytes and lithium metal. The interphase formation of recently developed solid electrolytes with high ionic conductivities, which are required for industrial applications, is investigated. No prior information on the reduction stability has been available for these electrolytes. Techniques such as in situ X-ray photoelectron spectroscopy and impedance spectroscopy are used to investigate the chemical sta-bility towards lithium, the interphase evolution, and the cell resistance. In Publication 1, the reduction stability of halide solid electrolytes Li3MCl6 (M = In, Y) is investi-gated. They are found to be unstable toward lithium and form the corresponding metal upon contact. When electronically conducting decomposition products are formed during reduction, the inter-phase grows continuously, which is detrimental for cell performance. In order to use both the halide solid electrolytes and lithium metal, the use of Li6PS5Cl as a protective layer between both is pro-posed. The interfacial resistance of Li3InCl6|Li6PS5Cl is negligible and does not affect the perfor-mance of the cell, making it suitable for industrial applications. The thiophosphate solid electrolyte Li7SiPS8 studied in Publication 2 also decomposes by reduction. Although no elemental silicon or an Li-Si alloy is formed, the interphase grows continuously, show-ing that electronically conducting pathways are nevertheless formed during decomposition. Unfor-tunately, the component causing the electronic conductivity could not be determined. However, the continuously forming, highly resistive interphase shows that the solid electrolyte is unsuitable for application with direct contact to a lithium metal anode. The influence of the lithium metal surface on the growth kinetics of the Li|Li6PS5Cl interphase evolution is investigated in Publication 3 using impedance spectroscopy. It is found that the pas-sivation layer present on commercial lithium foils negatively impacts the overall resistance of the cell and the growth kinetics of the interphase. Cells utilizing passivated lithium exhibit non-self-limited interphase growth, in contrast to cells built with freshly prepared lithium for which inter-phase formation ceases rapidly within 9 hours after contact. Based on this result, reservoir-free cells are proposed to avoid the drawbacks caused by the passivation layer of commercial lithium foils. Overall, this dissertation extends the knowledge of interfacial stability in different solid electrolyte classes. The findings are relevant for the use of lithium metal as anode material and the development of protective strategies for solid electrolytes unstable against lithium. In addition, insights into the influence of the processing history of cell components on cell performance are obtained, facilitating the commercialization of SSBs.