Imaging the Microstructure of Lithium and Sodium Metal in “Anode-Free” Solid-State Batteries using EBSD

“Anode-free” or more fittingly, metal reservoir-free cells (RFCs) have the potential of drastically improving current solid-state battery technology by achieving higher energy density, improving safety and simplifying the manufacturing process. Various strategies have been reported so far to control the morphology of electrodeposited alkali metal films to be homogeneous and dense, for example, by utilizing planar interfaces with seed interlayers or three-dimensional host structures. To date, the microstructure of such electrodeposited alkali metal, i.e., its grain size distribution, shape and orientation is unknown, and a suitable characterization route is yet to be identified. At the same time, the influence of the alkali metal microstructure on the electrochemical performance of the anode, including the available discharge capacity, is expected to be substantial. Hence, analysis of the microstructure and its influence on the performance of electrochemically deposited alkali metal layers is a key requirement to improving cell performance. This work establishes first a highly reproducible protocol for characterizing the size and orientation of metal grains in differently processed lithium and sodium samples by a combination of focused-ion beam (FIB) techniques and electron-backscatter diffraction (EBSD) with high spatial resolution. After ruling out grain growth in lithium or sodium during room temperature storage or induced by FIB, electrodeposited films at Cu|LLZO, Steel|LPSCl and Al|NZSP interfaces were then characterized. The analyses show very large grain sizes (> 100 µm) within these films and a clear preferential orientation of grain boundaries. Furthermore, metal growth and dissolution were investigated using in situ SEM analyses, showing a dynamic grain coarsening during electrodeposition and pore formation within grains during dissolution. Our methodology and results open up a new research field for the improvement of solid-state battery performance through first characterization of the deposited alkali metal microstructure and subsequently suggesting methods to control it.