In-situ transmission electron microscopy (TEM) has emerged as a powerful tool for investigating the nanoscale structural dynamics of battery materials under operational conditions. By enabling real-time observation of electrochemical processes, this technique provides critical insights into phenomena such as dendrite growth, crack propagation, and solid-electrolyte interphase (SEI) formation. The ability to visualize these processes at atomic or near-atomic resolution has significantly advanced the understanding of degradation mechanisms and informed the design of more durable battery materials.

Specialized TEM holders are essential for conducting in-situ battery experiments. These holders integrate electrochemical cells within the TEM, allowing researchers to apply electrical stimuli while simultaneously imaging the material’s response. Common designs include open-cell and closed-cell configurations. Open-cell holders expose the sample to the microscope’s vacuum environment, which simplifies imaging but limits the use of liquid electrolytes. Closed-cell holders, on the other hand, encapsulate the electrolyte between electron-transparent windows, enabling the study of liquid or solid electrolytes under near-realistic conditions. However, the windows can reduce image resolution due to electron scattering. Recent advancements have improved window materials, such as silicon nitride or graphene, to minimize these effects while maintaining electrochemical functionality.

Imaging modes in TEM play a crucial role in capturing dynamic processes. High-resolution TEM (HRTEM) reveals atomic-scale structural changes, such as phase transformations or lattice distortions during cycling. Scanning TEM (STEM), particularly high-angle annular dark-field (HAADF) imaging, provides contrast based on atomic number, making it useful for tracking the distribution of heavy elements like transition metals in cathodes. Electron energy-loss spectroscopy (EELS) and energy-dispersive X-ray spectroscopy (EDS) complement these techniques by offering chemical mapping capabilities, which are vital for studying element-specific reactions, such as lithium plating or transition metal dissolution.

One of the most significant contributions of in-situ TEM has been the observation of lithium dendrite growth. Dendrites, which form when lithium ions deposit unevenly on the anode during charging, can penetrate the separator and cause short circuits. In-situ studies have shown that dendrites initiate at surface defects and propagate along preferential crystallographic directions. The growth rate and morphology depend on factors such as current density and electrolyte composition. For instance, under high current densities, dendritic structures tend to be needle-like, whereas lower currents produce mossy deposits. These observations have guided strategies to suppress dendrites, including the use of artificial SEI layers or electrolyte additives that promote uniform plating.

Crack propagation in electrode materials, particularly in high-capacity anodes like silicon or lithium metal, has also been studied extensively. Silicon anodes undergo large volume expansions during lithiation, leading to mechanical stress and fracture. In-situ TEM has revealed that cracks nucleate at grain boundaries or pre-existing defects and propagate in a brittle manner. The real-time imaging of these processes has highlighted the importance of nanostructuring or buffer phases to mitigate cracking. Similarly, in cathodes like nickel-manganese-cobalt (NMC) oxides, repeated cycling can induce microcracks due to anisotropic lattice changes, which degrade performance by isolating active material particles. In-situ observations have shown that crack formation accelerates with higher charging voltages, supporting the development of voltage-limiting protocols.

The formation and evolution of the SEI layer, a passivating film that forms on anode surfaces, is another area where in-situ TEM has provided valuable insights. The SEI’s composition and stability are critical for battery longevity, as uncontrolled growth consumes lithium and increases impedance. In-situ studies have demonstrated that the SEI forms heterogeneously, with inorganic components like lithium fluoride or lithium carbonate appearing first, followed by organic species. The dynamics of SEI growth depend on electrolyte chemistry; for example, fluoroethylene carbonate additives promote thinner, more stable films. By correlating SEI structure with electrochemical performance, researchers have identified strategies to engineer more robust interfaces.

Despite its advantages, in-situ TEM faces several technical challenges. Sample preparation is particularly demanding, as battery materials must be thinned to electron transparency while preserving their electrochemical properties. Focused ion beam (FIB) milling is commonly used, but it can introduce artifacts like surface amorphization or gallium implantation. Alternative methods, such as mechanical polishing or cryo-ultramicrotomy, have been explored to minimize damage. Another challenge is electron beam effects, which can alter the material’s behavior. For instance, the beam can locally heat the sample or induce radiolysis in liquid electrolytes, complicating the interpretation of results. Strategies to mitigate these effects include reducing beam dose or using low-dose imaging techniques.

Comparing in-situ TEM with other microscopy techniques highlights its unique capabilities and limitations. Scanning electron microscopy (SEM) offers larger field-of-view imaging but lacks the resolution to observe atomic-scale processes. X-ray tomography provides 3D structural information but cannot resolve fine details or dynamic changes in real time. Optical microscopy is simpler but lacks the resolution to study nanoscale phenomena. In-situ TEM thus occupies a niche where high spatial and temporal resolution are paramount, albeit with trade-offs in sample environment complexity and potential beam effects.

The insights gained from in-situ TEM have direct implications for battery design. For example, observing dendrite growth has led to the development of textured current collectors that guide uniform lithium deposition. Similarly, visualizing SEI formation has informed the selection of electrolyte additives that enhance interface stability. As battery technologies evolve, particularly with the advent of solid-state or lithium-metal systems, in-situ TEM will continue to play a pivotal role in unraveling their fundamental behaviors. Future advancements in holder design, detector sensitivity, and correlative microscopy techniques promise to further expand the capabilities of this approach, enabling even deeper understanding of battery materials under operational conditions.

In summary, in-situ TEM has become an indispensable tool for probing the nanoscale dynamics of battery materials. By revealing the mechanistic details of dendrite growth, crack propagation, and SEI formation, it provides a foundation for improving battery performance and safety. While technical challenges remain, ongoing innovations in instrumentation and methodology are addressing these limitations, ensuring that in-situ TEM will remain at the forefront of battery research. The technique’s ability to bridge the gap between fundamental science and practical applications underscores its value in advancing energy storage technologies.