Lithium dendrite growth is a critical failure mechanism in lithium-ion and lithium-metal batteries, leading to short circuits, capacity loss, and thermal runaway. Identifying dendrite formation as a root cause requires advanced microscopy techniques, including transmission electron microscopy (TEM) and atomic force microscopy (AFM). These tools provide high-resolution imaging and real-time observation of dendrite nucleation and propagation, enabling researchers to correlate structural changes with electrochemical performance. In-situ methods further enhance understanding by capturing dynamic processes under operational conditions. The insights gained from these techniques are pivotal in developing mitigation strategies, particularly in solid-state electrolyte systems, where dendrite suppression is essential for commercial viability.

Transmission electron microscopy offers atomic-scale resolution, making it indispensable for studying lithium dendrite formation. TEM imaging reveals the crystallographic structure of lithium deposits, distinguishing between needle-like dendrites and mossy or granular lithium. Electron diffraction patterns confirm the metallic nature of dendrites, while energy-dispersive X-ray spectroscopy (EDS) maps elemental distribution, highlighting lithium accumulation at interfaces. High-resolution TEM (HRTEM) captures lattice fringes, exposing defects and grain boundaries that serve as nucleation sites for dendrites. By coupling TEM with electrochemical cells, researchers observe dendrite growth in real time, correlating voltage hysteresis with morphological changes. For example, during cycling, TEM has shown that dendrites initiate at electrode imperfections and propagate along preferential crystallographic planes, penetrating separators and causing internal shorts.

Atomic force microscopy complements TEM by providing topographical and mechanical data in ambient or controlled atmospheres. AFM operates in multiple modes, including contact, tapping, and conductive AFM (C-AFM), each offering unique insights. Tapping mode maps surface roughness, identifying early-stage dendrite nucleation as localized height variations. C-AFM measures current distribution, pinpointing conductive dendrites amid insulating solid electrolyte interphase (SEI) layers. Force spectroscopy quantifies mechanical properties, revealing dendrite hardness and adhesion forces, which influence penetration through electrolytes. In-situ AFM setups integrate electrochemical cells, enabling simultaneous cycling and imaging. Studies using this approach have demonstrated that dendrite growth accelerates during high-current charging, with dendrites exhibiting fractal geometries that exacerbate mechanical stress on solid electrolytes.

In-situ observation methods bridge the gap between ex-post analysis and real-world battery operation. Environmental TEM (ETEM) allows imaging under gaseous environments, mimicking battery conditions without compromising vacuum requirements. Liquid-cell TEM introduces electrolytes into the microscope, capturing dendrite evolution at nanometer resolution. These techniques reveal that dendrites grow anisotropically, with tip-driven propagation being a dominant mechanism. Synchrotron X-ray tomography provides three-dimensional dendrite visualization, showing how dendrites branch and interconnect within electrodes. Quartz crystal microbalance (QCM) measurements detect mass changes during lithium plating, correlating dendrite formation with charge transfer kinetics. These methods collectively establish that dendrite growth is stochastic but influenced by current density, electrolyte composition, and mechanical confinement.

Solid-state electrolytes (SSEs) are a promising solution to dendrite-related failures, but their implementation requires rigorous microscopy validation. TEM and AFM studies highlight key challenges, such as grain boundary diffusion and interfacial instability. Polycrystalline SSEs exhibit preferential dendrite propagation along grain boundaries, where ionic conductivity is higher but mechanical strength is lower. TEM cross-sections of cycled SSEs show dendrites penetrating through voids or cracks, undermining the electrolyte's structural integrity. AFM force curves measure interfacial stiffness, identifying soft spots where dendrites are likely to nucleate. In-situ experiments demonstrate that even in SSEs, lithium can form filamentary deposits if the applied pressure is insufficient to maintain electrode-electrolyte contact.

The transition from liquid to solid electrolytes shifts the dendrite growth paradigm but does not eliminate it. While SSEs have higher shear moduli, theoretical thresholds for dendrite suppression are rarely met in practice. Microscopy reveals that inhomogeneous current distribution and interfacial reactions create localized hotspots for lithium deposition. For instance, TEM imaging of lithium-metal anodes paired with SSEs shows that surface oxides or contaminants act as nucleation sites, bypassing the electrolyte's mechanical resistance. AFM-based nanoindentation quantifies the critical stress required for dendrite penetration, guiding the design of multilayer or composite electrolytes. These findings underscore the need for integrated approaches combining material optimization, interface engineering, and operational protocols.

Advanced microscopy also informs failure analysis protocols for battery manufacturers. Post-mortem TEM of failed cells identifies dendrite-induced short circuits through characteristic melt zones or re-deposited lithium. AFM topography scans of cycled electrodes distinguish between reversible lithium plating and irreversible dendrite formation, aiding in cycle life predictions. Standardizing these techniques enables root cause analysis across production batches, linking process variations to dendrite-related failures. For example, electrode coatings with suboptimal porosity exhibit higher dendrite densities, as visualized by TEM tomography.

In summary, TEM and AFM are indispensable tools for diagnosing lithium dendrite growth as a failure root cause. Their ability to resolve nanoscale features and dynamic processes provides actionable insights for battery design, particularly in solid-state systems. In-situ methods further enhance understanding by replicating operational conditions, revealing mechanistic details that ex-situ techniques cannot capture. While solid-state electrolytes offer inherent advantages, microscopy evidence confirms that dendrite mitigation requires addressing interfacial and microstructural heterogeneities. By leveraging these advanced characterization tools, researchers and manufacturers can develop more reliable and safer battery technologies.