Post-aging analysis of battery cells is critical for understanding degradation mechanisms and validating accelerated aging models. A combination of advanced analytical techniques provides comprehensive insights into material-level changes after aging. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are employed to examine electrode morphology evolution. SEM reveals macroscopic changes in particle cracking, electrode delamination, and porosity reduction, while TEM provides atomic-scale observations of crystallographic changes and nanoscale degradation. Particle fracture in silicon anodes or graphite exfoliation becomes clearly visible through these methods, with quantitative measurements showing particle size distribution shifts exceeding 20% in some high-energy cells after 1000 cycles.

X-ray photoelectron spectroscopy (XPS) delivers critical data about solid electrolyte interphase (SEI) composition changes. Depth profiling through Argon sputtering enables layer-by-layer analysis of SEI chemical evolution. Post-aging XPS typically shows increased lithium fluoride content in carbonate-based electrolytes, with fluoride atomic concentration rising from 5-8% in fresh cells to 15-22% in aged cells. Oxygen-containing species like lithium carbonates show relative decreases, indicating SEI reorganization during cycling. Sulfur signatures from electrolyte additives appear at the electrode surface, with concentrations correlating with capacity fade rates.

Differential scanning calorimetry (DSC) measures thermal stability changes in aged materials. Exothermic reactions between degraded electrodes and electrolytes show onset temperature reductions of 10-30°C compared to fresh cells. The heat flow profile changes reveal new reaction pathways, with aged lithium-ion cells exhibiting additional exothermic peaks between 180-220°C corresponding to metastable SEI components. Total heat generation increases by 30-50% in cells cycled beyond 80% state of health, directly linking material degradation to thermal runaway risks.

Failure analysis protocols following SAE J2464 standards provide systematic approaches for destructive teardown. The process begins with residual performance testing under controlled conditions, measuring final impedance, capacity, and open-circuit voltage. Cells undergo controlled discharge to 0% state of charge before disassembly in argon-filled glove boxes with oxygen and moisture levels below 1 ppm. Mechanical opening procedures preserve electrode integrity for analysis, with special attention to locating internal short circuits or metallic dendrites.

Layer separation techniques isolate individual components for analysis. Electrodes are rinsed with dimethyl carbonate to remove residual electrolytes without dissolving SEI components. Separators undergo pore structure analysis using porosimetry, showing 15-30% reduction in effective porosity after extensive cycling. Current collectors are examined for corrosion pits or coating delamination, with aluminum cathode foils showing increased fluorine content at pitting sites.

Accelerated aging model validation requires correlating analytical findings with degradation patterns from real-world usage. Models predicting capacity fade based on charge rates and temperature exposure are verified through material observations. Silicon anode particle cracking patterns observed in SEM align with models predicting mechanical stress-induced degradation at high cycling rates. Similarly, nickel-rich cathode surface reconstructions visible in TEM match predictions from oxidation-driven degradation models.

Cross-section analysis reveals through-thickness inhomogeneities in aged electrodes. Focused ion beam (FIB) milling creates clean cross-sections showing gradient porosity changes from current collector to separator interface. Energy-dispersive X-ray spectroscopy (EDS) line scans quantify transition metal dissolution gradients, with manganese content decreasing by 40-60% near separator interfaces in aged NMC cathodes.

Gas chromatography-mass spectrometry (GC-MS) of extracted electrolytes identifies decomposition products correlating with aging conditions. Ethylene carbonate breakdown generates ethylene gas quantities measurable at 50-200 ppm in aged cells, with concentrations scaling with capacity loss. Fluorinated decomposition products from electrolyte additives appear at 5-15 ppm levels and show strong correlations with impedance growth.

Mechanical property changes are quantified through nanoindentation of aged electrodes. Silicon composite anodes show hardness reductions from 0.8 GPa to 0.3-0.5 GPa after extensive cycling, indicating binder degradation. Cathode coatings exhibit increased brittleness, with fracture toughness decreasing by 35-50% after high-temperature aging.

Destructive physical analysis includes measurements of electrode thickness swelling. Graphite anodes typically expand by 15-25% after full aging, while nickel-rich cathodes contract by 3-8%. These dimensional changes correlate with model predictions of structural degradation and active material loss. Swelling pressure measurements show increases from initial 0.5-1 MPa to 3-5 MPa in aged prismatic cells, explaining observed case deformation.

Post-mortem electrochemical impedance spectroscopy (EIS) on extracted electrodes separates degradation contributions. Half-cell measurements using lithium reference electrodes quantify charge transfer resistance increases of 200-400% in aged cathodes compared to 50-150% in anodes. The impedance growth patterns validate accelerated test protocols when matching distributions are observed in field-retired cells.

Material recycling analysis provides additional degradation insights. Hydro-metallurgical leaching efficiency drops by 20-40% for aged cathode materials due to phase changes and surface passivation. This recycling behavior provides secondary validation of material transformations predicted by aging models.

Safety validation involves testing aged materials under abuse conditions. Nail penetration tests on aged cells show thermal runaway onset temperatures decreasing by 30-50°C compared to fresh cells. The heat release rates increase by factors of 2-3, directly linking material degradation to safety performance reduction. These measurements confirm accelerated aging models predicting safety margin reductions.

Statistical analysis of multiple cell teardowns establishes degradation mode distributions. Primary failure modes are categorized as anode SEI growth (40-60% of cases), cathode structural degradation (20-35%), or mechanical changes (10-25%), depending on cycling conditions. The distributions match those found in field-aged cells when acceleration factors are properly accounted for in testing protocols.

The comprehensive analytical approach provides multi-dimensional validation of aging models. Material observations ground truth the electrochemical predictions, creating closed-loop validation of acceleration methodologies. This enables reliable prediction of battery lifetime and degradation patterns from accelerated testing protocols.