Microstructural Analysis of Deposited Alkali Metal in “Anode-Free” Solid-State Batteries

Abstract

Lithium metal as an anode material can greatly boost energy and power density in solid-state batteries.[1] However, challenges like local contact loss and dendrite growth limit the applicable current density. It was shown previously that the lithium microstructure can greatly influence the electrochemical performance of lithium, e.g. the stripping capacity until pore formation occurs.[2]Solid-state-batteries (SSBs) manufactured without the use of an alkali metal reservoir are under intense investigation, as they may enable unmatched energy densities without the need to handle reactive and inherently unsafe alkali metal foils. In these reservoir-free cells (RFCs) the alkali metal is usually stored in the discharged cathode active material upon assembly and thus only deposited at the anode current collector within the first charging step. However, despite its expected influence on the electrochemical performance, the microstructure of freshly deposited lithium has yet to be analyzed. Up to now, the lack of a combination of suitable preparation and analyses tools restrict any study of the microstructure of thin alkali metal films.Within this study, we designed a workflow to reliably analyze the microstructure of thermally processed lithium and sodium films. Using a combination of cryogenic focused ion beam (FIB) preparation and electron back-scatter diffraction (EBSD), we were able to exclude any recrystallization during room temperature storage or FIB preparation. Therefore, this newly developed workflow was further used to study the microstructure of otherwise buried alkali metal films, which were deposited in so called “anode-free” or reservoir free cell designs.Our analyses show that the grain size of plated alkali metal films is independent on the microstructure of used current collector and generally consists of very large grains, albeit not as large as as-bought foil. Even more interesting, every single deposited film consists solely of columnar grains with grain boundaries oriented perpendicular to the solid electrolyte interface, marking a clear difference to using an alkali metal foil as the anode. Our results are confirmed at two different RFCs, namely Cu|LLZO|Li and Steel|LPSCl|Li. Generally, we believe our research on metal anodes will open up the possibility of tackling more advanced scientific questions previously impossible to answer.(1) Krauskopf, T., Richter, F. H., Zeier, W. G. & Janek, J. Physicochemical Concepts of the Lithium Metal Anode in Solid-State Batteries. Chem. Rev. 120, 7745–7794 (2020).(2) Singh, D. K., Fuchs, T. et al. Origin of the lithium metal anode instability in solid-state batteries during discharge. Matter 6, 1463–1483 (2023).