EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Metallic lithium electrodes hold promise for increasing the energy density of Li-ion batteries, and when used in conjunction with solid electrolytes, adverse safety implications associated with dendrite formation in organic liquid electrolytes can be overcome. To better understand the stability of solid electrolytes when in contact with lithium and the reactions that occur, requires experimental approaches to access the chemistry of the buried electrode-electrolyte interfaces. All solid-state batteries are typically assembled and studied within ‘inert’ glovebox environments, however in practice trace contaminants alter the surfaces of battery materials with carbonate/hydroxide surface layers often formed. Such contamination has been observed to affect the cycling performance of sulphide-based electrolytes increasing interfacial resistance, acting as a barrier to lithium-ion transport. Additionally, these contaminants react with cycled, disassembled surfaces studied by ex-situ methods masking the true nature of reactions occurring at the interface.
We thus present an experimental approach for forming and cycling cells in an ultra-high vacuum environment, in the absence of these trace contaminants. Cell preparation consists of evaporation of an ultra-thin (≈ 30 to 60 nm), X-ray transparent lithium film, to study the Li-electrolyte interface. Such a cell, in conjunction with hard X-ray photoelectron spectroscopy (HAXPES) allows operando measurements of the electrode-electrolyte interface. The long inelastic mean free path of photoelectrons through alkali metals allows photoelectrons to escape through the Li electrode. By changing the incident X-ray energy the photoelectron escape depth varies, providing a depth resolved study of the interfacial layering between the electrolyte and the electrode. Herein, we discuss the detection of different chemical species formed at different charge states and different locations within the solid-electrolyte interphase whilst cycling, including reactive intermediates which cannot be observed by ex-situ studies. This approach is expected to elucidate the nature of the interfacial layers and degradation processes occurring in promising, argyrodite-type electrolytes.