Using EC-AFM for Nanoscale Battery Testing

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Battery testing is vital for maintaining battery performance and stability throughout its usable life. Several diagnostic methods exist to characterize batteries, including both physical testing and theoretical modeling (such as physics-based models). As batteries become more advanced and have higher levels of charge, understanding the fundamental mechanisms that underpin safety and performance is more important than ever before.

Understanding these mechanisms often requires observing the battery materials at the atomic or nanoscale level, as small changes at those levels—multiplied many times over the entire battery—cause macroscale changes that affect battery performance and longevity. Probing the battery’s properties at the nanoscale is only possible through a select number of physical characterization instruments, one of which is electrochemical atomic force microscopy (EC-AFM).

What Is AFM?

Atomic force microscopy (AFM) is an analytical characterization technique from the wider family of scanning probe microscopy (SPM) techniques. In AFM, a cantilever with a nanoscale pyramidal-shaped tip (known as a probe) is scanned above the sample and measures the features of a surface at a nanoscale resolution.

AFM instruments measure the force between the probe and surface to generate data about the surface topology and its properties—electrical, mechanical, or thermal. In AFM, the probe physically taps the surface of the material as the instrument performs a scan. When the tip is close to the surface of the material, intermolecular attractions between the tip and the surface cause the cantilever beam to move toward the surface and tap it. When the cantilever moves toward the surface, a laser beam deflects off it onto a position-sensitive photodiode (PSPD), which recognizes the positional change. A feedback loop generates a topographic map (and any specific properties if measuring for them) by piecing together the lateral and vertical measurements across the whole material.

What Is EC-AFM?

EC-AFM is a specialized form of AFM that provides a more detailed analysis of the morphological properties of battery materials and offers insights into the evolution of a battery’s mechanical, chemical, and physical properties during operation.

EC-AFM measurements are performed in a liquid electrolyte environment that contains a reference and a counter electrode. This environment simulates an electrochemical cell so potential battery materials can be better analyzed to represent real-world operations. The instrument is also placed in an argon-filled housing during the test to ensure that no moisture or oxygen affects the measurement.

While EC-AFM is a standalone AFM technique, unlike conventional AFM, it can also be operated in three different modes:

• Ex situ: Characterizes the material and electrochemical properties of battery materials outside the electrochemical cell.
• In situ: Analyzes battery materials in the cell but not when the battery is running, because it measures the properties of the electrodes before and after charging and discharging.
• Operando: Images the material and provides electrochemical measurements during battery operation to provide direct, real-time insights into battery performance.

How EC-AFM Is Used to Test Batteries

In addition to deducing the general material (e.g., cracking potential) and electrical property information, EC-AFM can deduce various mechanistic properties to see how different battery operations and molecular-scale mechanisms are performing within the battery. These mechanisms include the formation of solid-electrolyte interphase (SEI) and cathode-electrolyte interphase (CEI) layers, the volumetric expansion and degradation of some anode materials, and the formation of dendrites.

EC-AFM in operando mode can analyze the properties of the cell itself and provide key information on the properties and mechanisms of the anode, cathode, and electrolyte:

• Anode: Characterizing the SEI layer, detecting volume changes, characterizing dendrite formation, analyzing conductive phases, and analyzing the capability for a material to host and intercalate metal ions.
• Cathode: Understanding surface degradation, ion diffusion, and CEI mechanisms, and detecting intermediate molecules with nanoscale resolution.
• Electrolyte: Characterizing the electric double layer (EDL) for ionic liquid electrolytes and characterizing solid-solid interfaces in solid-state batteries.

Characterizing SEI and CEI

During the first cycle of a battery’s life, an SEI passivation layer forms on the anode. The formation of this layer is driven by the decomposition of different molecular species (both organic and inorganic) in the battery.

The SEI layer is seen as both good and bad in a battery. The formation of the layer causes an irreversible capacity loss in the cell. However, this layer is also critical for ensuring long-term cyclability, rate capability, and safety inside the cell. Understanding the SEI layer and controlling its properties at the anode–electrolyte boundary are key for maximizing the performance and long-term stability of a battery. EC-AFM has the resolution to probe the nanoscale and molecular characteristics of SEI layers to tailor the properties of the SEI and improve battery performance.

On the cathode, EC-AFM can also probe the properties of the CEI layer, as well as the properties of the cathode, which is important because the cathode tends to be the limiting electrode in many batteries. Many cathode materials can form a CEI layer at a high potential, where electrolyte components decompose and deposit on the cathode. Understanding the structure and properties of the CEI is vital for ensuring the high-voltage performance of cathode materials. Additionally, EC-AFM can be used to look at the morphological and dynamic evolution processes of the CEI layer to provide mechanistic insights into the formation of the layer.

Calculating Volume Change and Deformation in the Anode

Certain anode materials—such as metal alloy, silicon, and graphite—have a high specific capacity. Graphite is commonly used as an anode because it is stable even if other materials have higher capacities. This durability is because silicon and metal alloy anodes undergo volumetric expansion during operational use, which causes the anodes to crack and pulverize. This ultimately leads to irreversible capacity loss and lower coulombic efficiency, which significantly affects battery performance.

EC-AFM can visualize the structural and property changes of the anode during operation and provide a much more realistic representation of the electrochemical environments in real-world battery systems. It can also study interactions at the electrolyte–electrode interface during these volume changes. EC-AFM can look at the structural evolution of the anode, as well as what happens to the SEI layer under volumetric changes, to build a clear picture of the electrode expansion and capacity fading while cycling.

Studying Dendrite Formation

Dendrite formation affects a battery's safety, performance, and long-term durability. Dendrites result from non-uniform metal stripping and deposition during cycling—arising from discontinuous SEI layers—which reduces coulombic efficiency and the number of available metal ions. It can also cause internal shorting, thermal runaway, and cell fires.

Studying the interfacial process at the anode–electrolyte interface during plating and stripping is critical for understanding how dendrites form. These insights can help avoid dendrite formation in the future and ultimately improve battery performance. Still, dendrites tend to grow large and fast, making it difficult to analyze them. By controlling the current density, the nanoscale resolution of EC-AFM can be harnessed to reveal any nanoscale changes in dendrite formation. EC-AFM is particularly useful for characterizing the morphological and mechanical properties at the anode–electrolyte interface to provide information on the early growth stages of dendrites (such as the nucleation of metal ions) or for studying fine structures in the SEI and dendrites. Insights at this scale can help optimize the interface materials and prevent dendrite formation.

Conclusion

EC-AFM is an analytical characterization technique that offers a nanoscale resolution. By probing the surface, this technique can provide material, surface, and electrochemical insights into both battery materials and how they perform in an electrochemical cell. In addition to providing key property information, EC-AFM can provide insights into various fundamental battery mechanisms that govern how a battery performs and remains stable over long cycle periods. By imaging and deducing properties at such a small scale, EC-AFM can provide new analysis that is not easily available, improving the performance of the next-generation of advanced batteries.