Scanning electron microscopy (SEM) plays a critical role in the investigation of cathode material degradation, providing high-resolution imaging and analytical capabilities essential for understanding structural and compositional changes. Cathode materials such as lithium nickel manganese cobalt oxide (NMC), lithium iron phosphate (LFP), and high-nickel variants undergo complex degradation mechanisms, including cracking, phase separation, and transition metal dissolution. SEM enables direct visualization and elemental analysis of these phenomena, offering insights that complement other characterization techniques without overlapping with X-ray diffraction (XRD) or purely electrochemical degradation analysis.

One of the primary degradation mechanisms in cathode materials is particle cracking, which occurs due to repeated volume changes during lithium insertion and extraction. SEM allows for the direct observation of microcracks and fractures in cathode particles, revealing their propagation patterns and severity. In NMC cathodes, cycling-induced stresses lead to intergranular and intragranular cracks, which can be clearly distinguished using secondary electron imaging. High-nickel cathodes, such as NMC811 or NCA, are particularly prone to cracking due to their higher volumetric strain compared to lower-nickel compositions. SEM analysis shows that these cracks often originate at grain boundaries and propagate inward, increasing particle isolation and impedance. LFP cathodes, while more structurally stable, still exhibit minor cracking under extreme cycling conditions, though the extent is significantly less than in layered oxide cathodes.

Phase separation is another critical degradation mode that SEM helps characterize. In NMC materials, prolonged cycling can lead to local variations in lithium concentration, resulting in phase segregation. Backscattered electron imaging in SEM highlights these compositional differences due to atomic number contrast, revealing domains with varying nickel, manganese, and cobalt content. High-nickel cathodes are especially susceptible to phase separation due to their tendency toward cation mixing and surface reconstruction. SEM combined with energy-dispersive X-ray spectroscopy (EDS) can map transition metal distribution, showing regions where nickel-rich phases separate from manganese or cobalt-rich regions. In LFP cathodes, phase separation between lithium-rich and lithium-depleted phases is less pronounced due to the two-phase equilibrium nature of the material, but SEM can still detect inhomogeneities in carbon coating or iron clustering.

Transition metal dissolution is a major contributor to capacity fade and impedance growth, particularly in NMC and high-nickel cathodes. SEM-EDS analysis provides direct evidence of transition metal migration from the cathode to the anode or electrolyte. For instance, manganese dissolution in NMC materials can be tracked by identifying manganese deposits on separator surfaces or anode particles. High-nickel cathodes suffer from nickel dissolution, which SEM reveals through particle surface roughening and the presence of nickel-containing residues in adjacent components. LFP cathodes exhibit minimal transition metal dissolution due to the stability of iron in the phosphate matrix, but SEM can still detect iron-containing impurities or corrosion products in extreme cases.

The surface morphology changes of cathode materials are also effectively studied using SEM. In NMC cathodes, surface reconstruction layers form due to electrolyte interaction, appearing as amorphous or structurally degraded regions in high-resolution images. High-nickel cathodes develop thicker surface layers, often rich in nickel oxides or carbonate species, which SEM can distinguish from the bulk material. LFP particles generally maintain their surface integrity, but SEM can identify carbon coating degradation or iron oxide formation under harsh conditions.

Cross-sectional SEM analysis is particularly valuable for examining subsurface degradation features. Focused ion beam (FIB)-SEM enables precise sample preparation, revealing internal cracks, voids, or compositional gradients not visible in surface imaging. In NMC cathodes, cross-sections show how cracks propagate from the surface into the bulk, sometimes leading to particle fragmentation. High-nickel cathodes often exhibit a core-shell-like structure with nickel-enriched surfaces and manganese-rich interiors, detectable through EDS line scans. LFP cross-sections typically display uniform composition but may show delamination between active material and conductive additives after extensive cycling.

SEM also aids in correlating mechanical degradation with electrochemical performance. For example, particles with severe cracking show poor electronic connectivity, while those with intact structures maintain better cycling stability. High-nickel cathodes with extensive surface reconstruction exhibit higher impedance, visible in SEM as porous or non-uniform surface layers. LFP cathodes with degraded carbon coatings show increased particle isolation, directly observable in SEM images.

In summary, SEM provides indispensable insights into cathode material degradation by enabling direct visualization and elemental analysis of cracking, phase separation, and transition metal dissolution. NMC and high-nickel cathodes exhibit more pronounced degradation features compared to LFP, but SEM helps identify critical failure modes across all material systems. The technique’s ability to resolve microstructural and compositional changes at high resolution makes it a cornerstone of battery degradation analysis, complementing other methods without redundancy.