Systematic post-mortem analysis of lithium-ion batteries following accelerated aging tests provides critical insights into degradation mechanisms, material changes, and failure modes. Accelerated aging tests simulate years of battery use in a compressed timeframe through elevated temperatures, high charge-discharge rates, or voltage extremes. The post-mortem process involves careful disassembly, material characterization, and comparative analysis between chemistries such as nickel-manganese-cobalt (NMC) and lithium iron phosphate (LFP).

The disassembly procedure begins with discharging the cell to a safe voltage to minimize risks of short-circuiting or thermal events. Cells are opened in an argon-filled glovebox to prevent air exposure, which can alter electrode surfaces or electrolyte composition. The casing is carefully removed, and components are separated: electrodes, separator, current collectors, and electrolyte. Each component is documented for physical deformities, such as electrode cracking, separator shrinkage, or electrolyte discoloration.

Scanning electron microscopy (SEM) paired with energy-dispersive X-ray spectroscopy (EDS) is used to examine electrode morphology and elemental composition. NMC cathodes often show particle cracking and transition metal dissolution after aging, visible as microstructural fractures in SEM images. EDS mapping reveals manganese and cobalt migration into the anode, accelerating capacity fade. In contrast, LFP electrodes exhibit minimal particle fracturing due to their olivine structure’s mechanical stability, though iron dissolution may still occur at high voltages.

Anode analysis typically focuses on solid electrolyte interphase (SEI) growth and lithium plating. Graphite anodes in NMC cells develop thicker SEI layers with aging, containing lithium fluoride and carbonate species detected via EDS. Lithium plating appears as dendritic deposits under SEM, particularly in fast-charged or low-temperature aged cells. LFP systems, operating at lower voltages, show less SEI growth and minimal plating due to their flatter voltage profile.

Fourier-transform infrared spectroscopy (FTIR) identifies organic and inorganic breakdown products in the electrolyte. NMC electrolytes degrade into compounds like lithium alkyl carbonates, polycarbonates, and phosphates, stemming from solvent oxidation at high voltages. LFP electrolytes show fewer oxidative byproducts but may form lithium fluoride from salt decomposition. FTIR peaks between 1600-1800 cm⁻¹ indicate carbonyl groups from solvent breakdown, while peaks near 1000 cm⁻¹ suggest PF₆⁻ decomposition.

Mechanical degradation differs notably between NMC and LFP systems. NMC electrodes experience higher volumetric changes during cycling, leading to binder fatigue and particle isolation. Calendering pressure during manufacturing influences crack propagation; over-compressed electrodes are more prone to aging-related damage. LFP electrodes, with minimal volume expansion, maintain better structural integrity but may suffer conductive carbon network degradation.

Cross-section analysis via SEM reveals delamination at the electrode-current collector interface, a common failure mode in aged cells. NMC cathodes often show aluminum current collector corrosion, especially under high-voltage hold conditions. LFP cells exhibit less corrosion but may develop resistive layers from phosphate deposition.

Electrolyte depletion is quantified by comparing fresh and aged electrolyte volumes. NMC cells lose more electrolyte due to continuous SEI growth and gas evolution, whereas LFP systems retain higher electrolyte volumes. Gas chromatography-mass spectrometry (GC-MS) identifies gaseous byproducts like ethylene, CO₂, and hydrogen, with NMC cells producing higher concentrations under thermal stress.

Contrasting NMC and LFP post-mortem data highlights trade-offs between energy density and longevity. NMC degradation is dominated by cathode instability and electrolyte oxidation, while LFP degradation centers on anode SEI evolution and conductive additive breakdown. These findings inform material selection for applications prioritizing cycle life (LFP) or energy density (NMC).

Post-mortem protocols must account for artifacts introduced during disassembly. Mechanical cutting can induce cracks, and air exposure may oxidize sensitive materials. Control samples and consistent handling procedures minimize such errors.

The systematic approach outlined—disassembly, SEM/EDS, FTIR, and comparative analysis—provides a framework for evaluating battery aging across chemistries. Future work may integrate additional techniques like X-ray photoelectron spectroscopy (XPS) for SEI analysis or tomography for 3D structural mapping. Standardizing these methods ensures reproducible insights into battery degradation pathways.

Quantitative data from post-mortem studies feed into degradation models, improving lifetime predictions. For instance, NMC capacity fade correlates with cathode particle cracking severity, while LFP fade aligns with SEI resistance growth. Such correlations enable targeted improvements, such as binder optimization for NMC or electrolyte additives for LFP.

In summary, post-mortem analysis after accelerated aging reveals chemistry-specific degradation patterns. NMC systems suffer from cathode-electrolyte interactions, while LFP systems face anode-related limitations. These insights guide material development, manufacturing processes, and operational strategies to enhance battery performance and safety.