Cryogenic electron microscopy (cryo-EM) has emerged as a powerful tool for investigating the solid-electrolyte interphase (SEI) layer in failed lithium-ion batteries. The SEI layer is critical for battery performance and longevity, but its sensitivity to air, moisture, and electron beam damage makes traditional characterization methods inadequate. Cryo-EM techniques address these challenges by preserving the native structure of the SEI and enabling high-resolution analysis without introducing artifacts.

The SEI layer forms on the anode surface during the initial cycles of a lithium-ion battery, primarily through the reduction of electrolyte components. In failed cells, the SEI may exhibit structural and compositional irregularities, such as excessive thickness, inhomogeneity, or the presence of detrimental decomposition products. Understanding these features requires techniques that maintain the SEI in its pristine state. Cryo-EM achieves this by rapidly freezing samples to cryogenic temperatures, immobilizing the SEI and preventing further reactions or degradation.

Sample preparation is a critical step in cryo-EM analysis of SEI layers. The process begins with disassembling the failed cell in an inert environment, such as an argon-filled glovebox, to prevent exposure to air and moisture. The electrode of interest is carefully extracted and cleaned to remove residual electrolyte without disturbing the SEI. A thin section of the electrode is then plunge-frozen in a cryogen, typically liquid ethane or nitrogen, at temperatures below -150°C. This rapid vitrification avoids ice crystal formation, which could distort the SEI structure.

Once frozen, the sample is transferred to a cryo-electron microscope under cryogenic conditions using specialized holders that maintain temperatures below -170°C. This step is crucial to prevent sample warming and subsequent structural changes. Cryo-EM imaging is performed at low electron doses to minimize beam-induced damage, which is particularly important for beam-sensitive materials like organic SEI components. High-resolution transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) modes are commonly employed to visualize the SEI at atomic or near-atomic resolution.

Cryo-EM provides several advantages for SEI analysis. First, it preserves the amorphous and nanocrystalline phases within the SEI, which are often lost or altered in conventional room-temperature TEM. Second, it enables the detection of metastable intermediates and transient species that may contribute to SEI instability. Third, cryo-EM can be combined with energy-dispersive X-ray spectroscopy (EDS) or electron energy loss spectroscopy (EELS) to map elemental distributions and chemical states within the SEI without inducing beam damage.

In failed cells, cryo-EM has revealed key structural defects in the SEI, such as cracks, voids, and delamination from the electrode surface. These defects can lead to increased impedance, lithium dendrite growth, and capacity fade. Cryo-EM also identifies inhomogeneous SEI compositions, including localized accumulations of inorganic compounds like lithium fluoride or lithium carbonate, as well as organic polymers. Such heterogeneity can create uneven current distribution and accelerate degradation.

Another application of cryo-EM is the study of SEI evolution during cycling. By comparing SEI layers from cells at different stages of failure, researchers can track morphological and chemical changes that precede catastrophic failure. For example, repeated volume changes in silicon anodes can cause SEI fracture and reformation, leading to progressive thickening and loss of ionic conductivity. Cryo-EM captures these dynamic processes in a frozen state, providing insights into failure mechanisms.

Cryo-EM is also valuable for evaluating SEI modifications in next-generation batteries. For instance, in lithium metal batteries, the SEI plays a decisive role in suppressing dendrite growth. Cryo-EM can assess the effectiveness of artificial SEI layers or electrolyte additives in stabilizing the interface. Similarly, in solid-state batteries, cryo-EM helps characterize the interphase between solid electrolytes and electrodes, where undesirable reactions can lead to high interfacial resistance.

Despite its advantages, cryo-EM has limitations. The technique requires specialized equipment and expertise, making it less accessible than conventional microscopy methods. Sample preparation is time-consuming and demands stringent handling protocols to avoid contamination or warming. Additionally, cryo-EM data interpretation can be complex due to the low signal-to-noise ratio inherent in low-dose imaging.

Future developments in cryo-EM technology may address these challenges. Advances in detector sensitivity, automated data acquisition, and image processing algorithms are improving resolution and throughput. Correlative microscopy approaches, combining cryo-EM with other techniques like X-ray photoelectron spectroscopy (XPS) or Raman spectroscopy, could provide complementary chemical information.

In summary, cryo-EM is a transformative technique for analyzing SEI layers in failed battery cells. By preserving sensitive structures and enabling high-resolution imaging, it uncovers critical details about SEI morphology, composition, and failure modes. These insights guide the development of more robust battery materials and designs, ultimately enhancing performance and safety. As cryo-EM methodologies continue to evolve, their role in battery research and failure analysis will expand, offering deeper understanding of interfacial phenomena in energy storage systems.