Solid-state batteries (SSBs) are attracting significant attention due to their potential to surpass lithium-ion batteries in energy density, safety, and manufacturing simplicity. Lithium metal anodes, with their high theoretical specific capacity (3861 mAh g⁻¹) and low redox potential (-3.04 V), offer substantial advantages, similarly for sodium metal anodes in sodium-based SSBs. Reservoir-free cells (RFCs) represent a promising approach, eliminating the need for handling reactive alkali metal foils by depositing the metal onto a current collector (CC) during the first charging step. This increases energy density, simplifies fabrication, and improves safety. However, a major challenge lies in controlling the morphology and microstructure of the plated alkali metal at the CC|SE interface. While various strategies exist to control the morphology, the microstructure (grain size and orientation) remains largely unknown and is expected to significantly influence the anode performance by affecting factors like pore formation and inhomogeneous plating. Another critical unknown is the impact of alkali metal grain growth during room temperature storage. While typically metals show microstructural changes at homologous temperatures (TH) of 0.4–0.6, the high TH values for Li and Na at room temperature suggest potential microstructural evolution over time. Electron backscatter diffraction (EBSD) is ideally suited for analyzing metal microstructure, providing quantitative information about grain size, orientation, grain boundaries, dislocations, and strain. However, applying EBSD to alkali metals is challenging due to the formation of thin degradation layers that mask the diffraction patterns. This paper addresses this challenge by developing a protocol for analyzing lithium and sodium microstructure using EBSD, including ex situ and in situ analysis of electrodeposited films in RFCs.

Previous research has focused on strategies to control the morphology of electrodeposited alkali metal films in RFCs, including specialized plating protocols, engineered CC materials, seed layers, and the application of pressure during plating. However, the microstructure of these films, specifically grain size and orientation, has remained largely uncharacterized. Studies on electrodeposited silver show microstructural changes during room temperature storage, but it's unclear whether this applies to lithium and sodium. Previous work on lithium suggests that its microstructure is tunable by thermal processing, indicating slow grain coarsening. Existing EBSD analyses of lithium foils are limited, and no data exist for sodium or electrodeposited alkali metals. This lack of microstructural characterization hinders a complete understanding of the relationships between microstructure and electrochemical performance in RFCs. This paper aims to bridge this knowledge gap.

The researchers developed a comprehensive protocol for analyzing the microstructure of lithium and sodium, both as foils and electrodeposited films in RFCs, using EBSD. This protocol emphasizes inert or high-vacuum conditions at all stages to prevent oxidation and other artifacts. Cryogenic focused ion beam (FIB) cutting and polishing were employed to prepare samples for EBSD analysis, minimizing potential artifacts from heating and annealing. Four types of alkali metal foils were prepared: reference lithium (R-Li), reference sodium (R-Na), quenched lithium (Q-Li), and quenched sodium (Q-Na). The microstructure of these foils was initially characterized using scanning electron microscopy (SEM) to determine apparent grain size. Subsequently, EBSD analysis was performed on both the surface and cross-sections of these foils. To study electrodeposited films, cross-sectional EBSD analysis was performed on lithium films deposited at Cu|Li7Ta0.5La3Zr2O12 (Cu|LLZO) and stainless-steel|Li7P3S11Cl (SS|LiPSCI) interfaces, and sodium films deposited at a carbon-coated Al|Na3Zr2Si2PO12 (Al|NZSP) interface. In situ EBSD analysis was also conducted during electrodeposition and electrodissolution to observe dynamic microstructural changes. The researchers utilized a high-resolution field-emission SEM equipped with an EBSD detector. EBSPs were recorded under optimized conditions. Data processing involved indexing via a Hough algorithm and dynamic simulated pattern matching to improve accuracy. Various mathematical refinement steps were implemented to enhance map quality. Cryogenic transfer systems were used to minimize exposure to air during sample preparation and analysis. Electrochemical deposition was carried out using potentiostats at controlled temperature and pressure, and impedance measurements were used to confirm the formation of alkali metal layers.

The study revealed that thermal processing significantly influences the microstructure of both lithium and sodium foils. Q-Li exhibited smaller grains (~10–50 μm) compared to R-Li (~100–300 μm). Similarly, Q-Na displayed a wider grain size distribution (~200–600 μm) compared to R-Na (close to millimeter range). Importantly, no significant grain growth was observed in either Q-Li or Q-Na during room temperature storage, despite their high homologous temperatures. EBSD analysis confirmed these findings, revealing large grains in both the reference and quenched metal foils. For the electrodeposited films, large grain sizes (10–150 μm) were observed, with grain boundaries predominantly perpendicular to the CC|SE interface, unlike metal foils where grain boundaries are randomly oriented. In situ EBSD experiments revealed dynamic grain coarsening during lithium electrodeposition, with smaller grains merging to form larger ones. This process was attributed to the movement of grain boundaries. During sodium electrodissolution, pore formation occurred predominantly within the grains, not at grain boundaries, indicating faster vacancy diffusion along grain boundaries. The absence of significant grain growth during room-temperature storage for both foils and electrodeposited films suggests that the microstructure attained during plating processes is essentially static after deposition, highlighting the importance of process parameters.

The findings address the research question by demonstrating the significant influence of microstructure on the performance of alkali metal anodes in RFCs. The observed large grain sizes in electrodeposited films and the preferential orientation of grain boundaries are critical for understanding the electrochemical properties. The in situ EBSD analysis reveals the dynamic nature of grain growth during electrodeposition and the preferential pore formation within grains during dissolution. These observations have significant implications for optimizing the electrochemical performance of RFCs. The development of a reliable EBSD analysis protocol for alkali metals opens up new avenues for investigating microstructure-property relationships. Controlling the grain size and orientation could potentially lead to improved battery performance. The preferential pore formation within grains, not at grain boundaries, is counterintuitive but emphasizes the role of fast vacancy diffusion along grain boundaries in maintaining interfacial stability.

This work establishes a reliable protocol for characterizing the microstructure of lithium and sodium metal in anode-free solid-state batteries. The results demonstrate the significant impact of deposition parameters and inherent material properties on the resulting grain size and orientation. The in situ observations provide insights into dynamic grain coarsening and pore formation mechanisms. Future research should focus on strategies to control grain size and orientation during electrodeposition, potentially through tailored impurities, seed layers, or geometric confinements, to optimize electrochemical performance. Further studies are needed to fully elucidate the relationship between microstructure, plating parameters, and electrochemical performance.

While the cryogenic FIB preparation minimized artifacts, it cannot entirely eliminate the possibility of minor microstructural alterations. The in situ EBSD analysis was limited to specific cell configurations and plating conditions. The study primarily focused on the microstructure; further investigations are needed to correlate these microstructural features directly with long-term cycling performance and other crucial battery metrics. The analysis of the electrodeposited films is limited by the relatively small size of the analyzed cross-sections in relation to the typical size of the electrode.