Unlocking the future of energy storage: Research in calcium-sulfur batteries

As the world shifts towards renewable energy sources, the demand for efficient energy storage technologies has never been more critical. Electrochemical energy storage, particularly through batteries, has emerged as a reliable solution. However, the reliance on lithium for large-scale battery production poses significant challenges, including environmental concerns associated with lithium mining and the finite availability of this essential resource.

In electrochemical energy storage systems, such as calcium-sulfur (CaS) and well-established lithium-ion batteries, charging and discharging are driven by critical electrochemical reactions at the interfaces between electrodes and electrolytes. The research focuses on understanding these interfacial reactions, particularly in calcium-sulfur batteries, which currently face rapid capacity loss after just a few charge-discharge cycles.

The degradation mechanisms likely stem from side reactions occurring at the surface of the calcium anode, making it a crucial area for optimization. These reactions involve interactions with the known polysulfide shuttle and degradation products from the electrolyte. Such interactions can lead to passivation of the anode, which interrupts the flow of ions between the electrodes.

By identifying and optimizing these degradation mechanisms, the NMI researchers aim to enhance the stability and lifespan of calcium-sulfur batteries, paving the way for their broader adoption in energy storage applications.

A CaS battery cell was provided by DLR for post-mortem analysis. During charging, particle-like calcium deposits formed on the anode surface as a result of electrochemical plating. At the NMI, a cross section was prepared by a focused ion beam (FIB) milling. Energy-dispersive X-ray spectroscopy (EDX) scans revealed a solid electrolyte interphase (SEI) layer formed on top of the particles.

The SEI is formed by parasitic reactions between the calcium surface and electrolyte degradation products. This passivating layer progressively blocks ionic transport at the anode–electrolyte interface, potentially interrupting ion exchange between the electrodes and limiting cell performance.

Thin calcium anodes were developed via controlled electrochemical deposition at the fem Research Institute.

The morphology of the plated calcium layer was characterized using scanning electron microscopy (SEM) at the NMI.

To investigate the crystalline structure at high spatial resolution, a focused ion beam (FIB) lamella was extracted from an individual deposited particle. This specimen was analyzed using high-resolution transmission electron microscopy (TEM), which yielded atomically resolved structural information.

The analysis of battery materials presents unique challenges. Electrochemical reactions occur in complex, multi-component systems, making it difficult to interpret their origins and mechanisms. For instance, reactions at the anode surface are not homogeneous. Initial investigations at the NMI revealed that material growth is localized and primarily forms particle-like structures. However, elemental analysis of their cross-sections cannot always clarify the sources of the materials or the SEI's detailed composition.

Additionally, sample preparation is critical; handling materials under inert conditions is essential to identify and avoid artifacts that could distort results. Inert conditions are vital for preserving the original electrochemical state of battery materials. Calcium anodes, for example, oxidize rapidly upon exposure to air, which can lead to misleading results.

The ZEISS Uniport protect inert shuttle solution now allows for sample transfer under an argon or vacuum atmosphere instead of the former nitrogen inert gas. Nitrogen is also known to slowly react with reactive metals. This capability is crucial for obtaining accurate and meaningful results from the analyses.

By maintaining strict inert handling protocols, the team ensures that their findings are reliable and reflective of the true chemistry at play. 

A key focus of the future work is the development of an in-situ model battery system for electron microscopy. This innovative approach would enable real-time observation of interfacial reactions during battery operation, providing insights that are currently unattainable through traditional post-mortem analysis.

To overcome existing obstacles, the field requires true operando electron microscopy platforms that allow for electrochemical cycling under realistic conditions. By enabling direct observation of mechanisms at the micro-, meso-, and nanoscale, they can capture critical early-stage phenomena, such as SEI formation, that are essential for improving battery performance.

The SEI has a very complex chemical composition and structure. Advanced correlative microscopy combines structural, chemical, and electrochemical information to provide deeper insights by leveraging their synergy.

An artificial SEI can transform interfaces into functional components that stabilize electrochemical reactions and prevent parasitic side reactions. These artificial layers could enable reproducible cycling behavior, longer lifetimes, and higher coulombic efficiencies for calcium–sulfur systems and other multivalent metal batteries. 

The NMI's research into calcium-sulfur batteries represents a significant step towards sustainable energy storage solutions. By addressing the challenges of battery degradation and exploring innovative technologies, they are paving the way for a cleaner, more efficient energy future. As they continue to push the boundaries of scientific knowledge, the potential for transformative advancements in energy storage is within reach.