Imaging the Microstructure of Sodium Metal in "Anode-free" Solid-State Batteries Using EBSD

The use of sodium metal as an electrode material can increase the energy and power density of solid-state batteries. However, challenges such as contact loss and dendrite growth hinder its use in their current anode-free solid-state batteries. While most attempts to improve battery performance involve introducing interlayers or modifying the solid electrolyte material, the microstructure of the metal anode has not been considered in the literature so far.
To date, the microstructure of electrodeposited sodium metal, i.e., its grain size distribution, shape and orientation is unknown, and a suitable characterization route is yet missing. However, the influence of the sodium metal microstructure on the electrochemical performance of the electrode, including the available discharge capacity, is expected to be huge.^[1] Hence, analysis of the microstructure and its influence on the performance of electrochemically deposited sodium metal layers is a key requirement.
We have established a highly reproducible protocol for characterizing the size and orientation of metal grains in differently processed sodium samples using a combination of focused-ion beam (FIB) techniques and electron-backscatter diffraction (EBSD) with high spatial resolution.^[2] After ruling out grain growth during storage at room temperature, electrodeposited films at Al|Na_3.4Zr₂Si_2.4P_0.6O₁₂ interfaces were characterized. The analyses show very large grain sizes within these films and a clear preferential orientation of grain boundaries. It was demonstrated in an in situ EBSD experiment that pore formation during anodic dissolution occurs primarily at the interface within grains rather than at grain boundaries. Our methodology and initial results open up a new research field for improving the performance of high-capacity metal electrodes.
^[1] T. Krauskopf, F. H. Richter, W. G. Zeier, J. Janek, Physicochemical Concepts of the Lithium Metal Anode in Solid-State Batteries. Chem. Rev. 2020, 120, 15, 7745–7794.
^[2] Fuchs, T., Ortmann, T., Becker, J. et al. Imaging the microstructure of lithium and sodium metal in anode-free solid-state batteries using electron backscatter diffraction. Nat. Mater. (2024).