Current collectors, typically aluminum (Al) for cathodes and copper (Cu) for anodes, play a critical role in lithium-ion batteries by providing electrical connectivity between electrode materials and external circuits. However, these foils are susceptible to degradation during cycling, which can compromise battery performance and safety. The primary degradation modes include corrosion, delamination, and fatigue cracking, each with distinct mechanisms and detection methods. Understanding these failure modes is essential for improving battery longevity and reliability.

Corrosion is a significant degradation mechanism affecting current collectors, particularly aluminum in cathodes. Aluminum forms a native oxide layer that provides initial protection against electrochemical corrosion. However, under prolonged cycling, especially at high voltages or in the presence of trace moisture, this passive layer can break down. Localized pitting corrosion occurs when aggressive species, such as hydrofluoric acid from electrolyte decomposition, penetrate the oxide layer. This leads to increased contact resistance and uneven current distribution. Copper, though less prone to oxidation, can corrode in the presence of dissolved oxygen or acidic byproducts. Corrosion manifests as surface roughening, mass loss, and increased electrical resistance. In-situ resistance monitoring can detect corrosion by tracking changes in through-thickness or sheet resistance. Techniques like electrochemical impedance spectroscopy (EIS) provide additional insights into interfacial degradation.

Delamination refers to the loss of adhesion between the current collector and the electrode coating, often caused by mechanical stress or chemical incompatibility. During cycling, repeated volume changes in active materials induce shear forces at the interface, weakening the binder adhesion. Additionally, electrolyte penetration at the edges of the foil can dissolve binders or form insulating layers, further promoting detachment. Delamination reduces effective electrode area, increasing local current density and accelerating degradation. Detection methods include ultrasonic imaging, which identifies interfacial gaps, and cross-sectional microscopy to observe adhesive failure. Electrical techniques, such as measuring the increase in contact resistance or using four-point probe mapping, can also identify delaminated regions.

Fatigue cracking arises from cyclic mechanical stresses during battery operation. Current collectors experience repeated expansion and contraction due to lithiation and delithiation of adjacent active materials. Over time, this leads to microcrack initiation and propagation, particularly near defects or grain boundaries. Aluminum, being more ductile, tends to undergo plastic deformation before cracking, while copper is more prone to brittle fracture under high strain rates. Cracks reduce electrical conductivity and may propagate into the electrode layer, causing active material isolation. X-ray computed tomography (CT) and scanning electron microscopy (SEM) are effective for visualizing crack networks. Resistance monitoring can detect cracking through abrupt increases in resistance or intermittent electrical discontinuities during cycling.

Quantitative studies have demonstrated the impact of these degradation modes. For example, aluminum current collectors cycled at voltages above 4.2 V exhibit a 20-30% increase in sheet resistance after 500 cycles due to corrosion. Delamination can lead to a 15-25% loss in capacity retention when the interfacial contact area is reduced by more than 10%. Fatigue cracking in copper foils has been shown to cause a 40-50% rise in localized resistance when crack density exceeds a critical threshold. These values highlight the importance of early detection and mitigation.

Resistance monitoring is a practical approach for tracking degradation. By integrating voltage and current measurements during cycling, changes in the effective resistance of the current collector can be correlated with specific failure modes. For instance, a gradual resistance increase suggests corrosion, while sudden jumps indicate cracking or delamination. Advanced battery management systems (BMS) can incorporate such diagnostics to predict end-of-life or trigger maintenance protocols.

Imaging techniques provide complementary insights. Post-mortem analysis using SEM or atomic force microscopy (AFM) reveals surface morphology changes, while energy-dispersive X-ray spectroscopy (EDS) identifies chemical alterations. Non-destructive methods like infrared thermography detect hotspots caused by uneven current distribution from delamination or cracking. Synchrotron X-ray imaging offers real-time observation of defect propagation under operating conditions.

Material selection and processing improvements can mitigate these issues. Using corrosion-resistant alloys or coatings, such as carbon-coated aluminum, enhances durability. Optimizing foil thickness and grain structure reduces susceptibility to fatigue. Adhesion promoters and flexible binders improve interfacial stability against delamination. Accelerated aging tests combined with multimodal characterization help validate these solutions.

In summary, current collector degradation during cycling presents a multifaceted challenge in battery design. Corrosion, delamination, and fatigue cracking each contribute to performance decay through distinct mechanisms. Resistance monitoring and advanced imaging techniques enable early detection and diagnosis. Addressing these failure modes through material engineering and real-time diagnostics is crucial for advancing battery reliability and lifespan. Future research should focus on integrating these detection methods into operational battery systems for proactive management.