Atomic force microscopy (AFM) has emerged as a powerful tool for studying battery materials and interfaces under operational conditions. Environmental and operando AFM techniques enable real-time observation of dynamic processes such as solid electrolyte interphase (SEI) formation, dendrite growth, and electrode degradation. These methods provide nanoscale resolution while maintaining electrochemical control, offering insights that are inaccessible through post-mortem analysis or ex-situ characterization.
The integration of AFM with controlled atmospheres and electrochemical cells is critical for simulating real battery conditions. Environmental AFM allows researchers to control humidity, temperature, and gas composition, which are essential factors in battery performance and degradation. For example, oxygen and moisture levels significantly influence SEI formation on lithium metal anodes. Operando AFM setups incorporate working electrodes, counter electrodes, and reference electrodes within the AFM system, enabling simultaneous electrochemical measurements and topographical imaging. This configuration permits direct observation of morphological changes during charge-discharge cycles.
One of the most studied processes using operando AFM is SEI formation. The SEI layer plays a crucial role in battery performance, influencing cycle life, safety, and efficiency. Operando AFM has revealed that SEI formation occurs in distinct stages, beginning with nucleation sites that grow into a heterogeneous film. The dynamics of this process depend on electrolyte composition, applied potential, and current density. For instance, in lithium-ion batteries, the addition of fluoroethylene carbonate (FEC) to the electrolyte has been shown to produce a more uniform and stable SEI layer, as observed through real-time AFM imaging.
Another key application of operando AFM is the study of dendrite formation in metal electrodes. Lithium dendrites can cause short circuits and thermal runaway, posing significant safety risks. Operando AFM has demonstrated that dendrite initiation is influenced by surface defects, local current density, and mechanical properties of the electrode. By applying mechanical pressure through the AFM tip, researchers have quantified the modulus of growing dendrites, providing data critical for designing suppression strategies. These observations have led to innovations such as artificial SEI layers and structured electrolytes that mitigate dendrite growth.
Challenges in environmental and operando AFM for battery studies are numerous. Maintaining electrochemical control while achieving high-resolution imaging is difficult due to the dynamic nature of battery interfaces. The AFM tip can interfere with local electrochemical processes, and the presence of liquid electrolytes complicates imaging due to meniscus forces and tip-sample interactions. Additionally, the temporal resolution of AFM is often insufficient to capture rapid processes such as nucleation events, requiring sophisticated synchronization with electrochemical stimuli.
Breakthroughs in instrumentation have addressed some of these challenges. High-speed AFM systems now allow frame rates sufficient to track SEI growth and dendrite propagation in real time. Advanced cantilever designs reduce tip-sample interactions in liquid environments, improving imaging stability. Furthermore, the development of multifunctional probes enables simultaneous mapping of topography, mechanical properties, and electrical characteristics. For example, conductive AFM tips can measure local ionic conductivity while imaging morphological changes.
The integration of spectroscopic techniques with operando AFM has expanded its capabilities. Combining AFM with Raman spectroscopy or infrared spectroscopy provides chemical information alongside topographical data. This multimodal approach has been used to correlate SEI composition with its mechanical properties, revealing how additives influence interfacial stability. Such insights are invaluable for optimizing electrolyte formulations and electrode materials.
Environmental AFM has also been applied to study the effects of gas atmospheres on battery interfaces. For instance, the presence of CO2 in the atmosphere can alter SEI composition and morphology on lithium metal anodes. By controlling gas composition during imaging, researchers have identified reaction pathways that lead to improved or degraded performance. These findings inform the design of batteries for specific environmental conditions, such as those encountered in aerospace applications.
Thermal effects on battery interfaces can also be investigated using environmental AFM. Heating stages integrated with AFM systems allow observation of morphological changes at elevated temperatures, simulating thermal abuse conditions. Studies have shown that SEI layers undergo structural transitions at specific temperatures, which correlate with increased impedance and capacity fade. This information is critical for developing thermal management strategies and improving battery safety.
Despite these advances, limitations remain. The small scan areas typical of AFM may not capture representative processes across entire electrodes. Statistical analysis of multiple imaging regions is often necessary to draw general conclusions. Additionally, the complexity of operando setups can introduce artifacts, requiring careful validation of results. Future developments in large-area scanning and automated data analysis will enhance the reliability and throughput of these techniques.
Operando AFM has also been instrumental in studying next-generation battery systems. For solid-state batteries, AFM has elucidated the role of interfacial voids and mechanical stress in performance degradation. In lithium-sulfur batteries, operando imaging has tracked the precipitation and dissolution of lithium polysulfides, informing strategies to mitigate shuttle effects. These studies highlight the versatility of AFM in addressing diverse challenges across battery chemistries.
The continued refinement of environmental and operando AFM techniques will deepen understanding of battery interfaces and enable rational design of improved materials. By bridging the gap between nanoscale phenomena and macroscopic performance, these methods play a pivotal role in advancing battery technology. As instrumentation becomes more sophisticated and accessible, operando AFM is poised to become a standard tool in battery research and development.