High-Resolution Electron Microscopic Characterization of Surfaces and Interfaces of Calcium in Calcium-Sulfur Batteries
Authors: Koegel Marco , Kiesl Christoph , Böck Reinhard , Meyer Jannik C. 
Published: February 2024

As we globally realize large-scale electrical energy storage coupled with the rapid increase in global electromobility, the challenge of securing a consistent supply of critical raw materials utilized in lithium-ion batteries (LIB) continues to grow. Consequently, the search for alternative systems delivering comparable energy densities has become an important topic. Multivalent metals, renowned for their long-term stability, emerge as promising candidates in this regard.

The combination of calcium and sulfur represents a potent alternative, attributed to their abundant availability and recent research delineating the appreciable stability of their electrolyte system 1. Nevertheless, to enhance fabrication efficiency, performance, and stability, a deeper understanding of the electrochemical dynamics at the anode surface, where the electrolyte interfaces with calcium, is essential. The work in the Casino project aims for an understanding and optimization of key aspects towards its application.

Outer surfaces, shared with the electrolyte system are central to battery health, dictating its longevity and efficiency, making their degradation a critical aspect affecting the overall battery degradation. Therefore, investigating all galvanic surfaces stands paramount to this study. Drawing parallels with LIB systems, the application of artificial or potential optimization of natural Solid-Electrolyte Interphases (SEI) presents itself as a viable strategy to further stability within the full cells of Ca-S batteries. Envisioning the manufacturing, galvanic processes may offer significant merits for battery production, regarding its ability to build natural SEI layers.

This work leans on a detailed exploration of battery systems sharing similar electrochemical foundations with galvanic systems, asserting that an investigation into galvano systems can furnish vital insights not only into the fabrication process but also unravel the undercurrent processes active during battery charging. A focal point of this research is the internal surfaces, which potentially delivers data to the understanding the electrochemical reactions, which produces the particle-like structures.

In our pursuit to further unravel the intricacies of the internal particle surfaces, we used Focused Ion Beam (FIB) as preparation method to craft lamellas (see Fig.) from galvanic plating samples generated using both pulsed and constant currents. Despite the soft nature of the material posing challenges in FIB preparation, refined techniques were utilized to mitigate these issues. The analysis extended to Scanning Transmission Electron Microscopy (STEM) observations, intending to offer spectroscopic insights. Energy-Dispersive X-ray Spectroscopy (EDX) coupled with Electron Energy Loss Spectroscopy (EELS) for a detailed chemical characterization. This approach aimed to unravel bonding configurations and potential electronic transitions during the battery's operational life and to identify the potential variations in material phases or the presence of different chemical components. This approach afforded a closer look at the atomic arrangements potentially influencing the electrochemical properties.

The project was financially supported by the Bundesministerium für Bildung und Forschung (BMBF), under project CaSino, Grant number (Förderkennzeichen) 03XP0487D.

[1] Li, Z., Fuhr, O., Fichtner, M. & Zhao-Karger, Z. Towards stable and efficient electrolytes for room-temperature rechargeable calcium batteries. Energy Environ. Sci.
12, 3496–3501 (2019).

Figure 1