Unraveling the Microstructural Complexities in Electro deposited Alkali Metal Films

 

---

Introduction

The evolution of solid-state batteries stands at the forefront of energy storage research, primarily due to their potential for higher energy densities and increased safety compared to conventional lithium-ion batteries. A pivotal aspect of solid-state batteries is the interface between the alkali metal anode and the solid electrolyte (SE). This article re-examines a key study utilizing Electron Backscatter Diffraction (EBSD) to analyze the microstructure of alkali metal films deposited on various reference cells. By exploring not only the microstructural characteristics but also the implications for battery performance, this discussion aims to provide fresh insights into the interaction dynamics at the electrolyte-anode interface and underscores the essential role of microstructuring in improving battery technology.

---

Overview of Electrodeposition and Cell Preparations

Preparative Techniques

Recent advancements in preparing reference cells, specifically SS|LPSCl|Li, Cu|LLZO|Li, and Al|NZSP|Na cells, showcase the current state of solid electrolytes coupled with alkali metal anodes. Each cell configuration employs a unique combination of current collectors (CCs) and solid electrolytes (SEs) to enable the distinct electrodeposition of lithium or sodium.

• SS|LPSCl|Li: Stainless steel as the current collector with a lithium phosphorous oxynitride solid electrolyte.
• Cu|LLZO|Li: Copper used alongside a lithium lanthanum zirconate (LLZO) electrolyte.
• Al|NZSP|Na: Aluminum paired with a sodium zirconium silicate composite.

Each configuration serves as a platform to investigate the deposition process for alkali metals.

---

Methodology of EBSD Analysis

Cross-sectional Imaging

Employing focused ion beam (FIB) techniques, the study captures cross-sectional EBSD images that reveal important microstructural information. The analysis protocol is detailed in the figures presented in the original research, which illustrates the transformation of the metals during the plating process.

• Current and Pressure Conditions:
  • Lithium was electrodeposited at 100 µA cm−2 with pressures of 15 MPa and 5 MPa for SS|LPSCl and Cu|LLZO, respectively.
  • Sodium deposition at 300 µA cm−2 employed a carbon-coated aluminum interface at 3 MPa.

These meticulously chosen conditions promote uniform metal film deposition, enhancing the homogeneity crucial for battery performance.

---

Microstructural Insights from EBSD Data

Analysis of Grain Structures

The resultant EBSD analysis reveals intriguing patterns regarding grain size and orientation. Here are some notable findings:

• Grain Size Comparison:

  • Lithium films at both SS|LPSCl and Cu|LLZO interfaces exhibit larger average grain sizes (20-100 µm and 10-100 µm, respectively) compared to conventional alkali metal foils, suggesting that electrodeposited films may not undergo substantial growth at room temperatures.
• Grain Orientation:
  • A remarkable observation is that all grain boundaries are aligned perpendicularly to the CC|SE interface, resulting in a distinctly different microstructure from that of as-built foils, which typically have randomly oriented grains.

This orientation may influence subsequent electrochemical performance by affecting pore formation during discharge cycles.

---

Pore Formation and Microstructural Evolution During Cycling

In Situ Analysis: A Novel Approach

Recent in situ EBSD studies during charging and discharging phases unveil dynamic microstructural changes:

• Lithium and Sodium Deposition Dynamics:

  • Detecting microstructural evolution during lithium plating and sodium stripping, the process reveals how boundary diffusion plays a significant role in avoiding dendritic growth, crucial for enhancing cycle stability.
• Pore Formation Mechanism:
  • Analyzing sodium stripping, notable pore genesis predominantly occurs within grains rather than at grain boundaries—indicating fast vacancy movement through grain boundaries, which can lead to inefficient interfacial contact.

This behavior elucidates the need for controlling grain structures to minimize pore development and maximize anode efficacy.

---

Conclusion

The contemporary study of microstructural evolution in electrodeposited alkali metal films has illuminated the pivotal role of grain structure and orientation on battery performance, particularly through the lens of EBSD analysis. Key insights reveal that while substantial interfacial defects like pores can arise from microstructural evolution, the alignment of the grains serves as a promising avenue for refining solid-state battery design. Further research should delve into the implications of these findings on long-term cycle stability and the scaling of solid-state battery technologies. Ultimately, understanding these microstructural traits could lead to the development of next-generation batteries with significantly improved safety and energy efficiency.