Corrosion of current collectors in lithium-ion batteries is a critical issue that impacts performance, longevity, and safety. Copper and aluminum foils are the most commonly used materials due to their high electrical conductivity and mechanical stability. However, they are susceptible to degradation under electrochemical and environmental stressors. Understanding the corrosion mechanisms and mitigation strategies is essential for improving battery reliability.

Copper is typically used as the anode current collector due to its stability at low potentials, while aluminum serves as the cathode current collector because of its resistance to oxidation at high voltages. Despite these advantages, both metals corrode under specific conditions. In copper, corrosion occurs when the electrode potential exceeds its oxidation threshold, leading to the dissolution of copper ions into the electrolyte. This process is exacerbated in high-voltage environments or when the battery operates outside its intended voltage window. Dissolved copper can migrate to the cathode, deposit on the separator, and create internal short circuits. Aluminum corrosion, on the other hand, is primarily driven by electrochemical reactions with the electrolyte, particularly when fluoride-containing salts like LiPF6 decompose into hydrofluoric acid (HF). The breakdown of the native oxide layer on aluminum exposes the bare metal to further attack, increasing interfacial resistance and reducing cell efficiency.

Electrolyte composition plays a significant role in corrosion dynamics. Conventional electrolytes based on LiPF6 are prone to hydrolysis, generating acidic byproducts that accelerate metal dissolution. Additives such as vinylene carbonate (VC) and fluoroethylene carbonate (FEC) can stabilize the solid-electrolyte interphase (SEI) on anodes, indirectly protecting the copper foil. However, their effectiveness depends on concentration and compatibility with other electrolyte components. For aluminum, the presence of HF is a major concern. Some electrolytes incorporate scavengers like lithium hexafluorophosphate (LiPF6) stabilizers or alternative salts such as lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) to minimize acid formation. The choice of solvent also matters; carbonate-based blends are more aggressive toward aluminum than ether-based electrolytes, which offer better passivation.

Voltage windows are another critical factor. Copper corrodes when the anode potential drops below 3.5 V vs. Li/Li+, a scenario that can occur during overdischarge or cell reversal. Aluminum, while stable above 3 V, degrades if the cathode potential exceeds 4.5 V, especially in high-nickel or lithium-rich cathodes. Operating outside these limits accelerates degradation, necessitating precise voltage control in battery management systems.

Environmental humidity introduces additional challenges. Moisture infiltrating the cell reacts with LiPF6 to form HF, corroding both current collectors. Dry room manufacturing mitigates this, but residual moisture or seal failures can still compromise performance. The hygroscopic nature of some electrolytes exacerbates the problem, making humidity control during production and operation essential.

Protective strategies focus on enhancing corrosion resistance without compromising conductivity. Alloying copper with elements like nickel or chromium improves stability by forming a more robust passive layer. However, increased resistivity and cost are trade-offs. Aluminum alloys with magnesium or manganese exhibit better resistance to pitting corrosion but may require additional treatments for optimal performance.

Passivation layers are widely used to shield current collectors. Copper can be coated with carbon or graphene to prevent direct electrolyte contact. Aluminum benefits from artificial oxide layers grown via anodization or atomic layer deposition (ALD). These coatings must be thin enough to avoid significant resistance increases while providing sufficient barrier properties. Ceramic coatings like Al2O3 or TiO2 have shown promise in reducing aluminum dissolution, though adhesion and uniformity remain challenges.

Polymer coatings offer a flexible alternative. Conductive polymers such as polyaniline or polypyrrole can be applied to copper to inhibit oxidation. For aluminum, hydrophobic polymer films reduce electrolyte penetration and HF attack. The drawback is potential delamination during cycling, which necessitates rigorous adhesion testing.

Testing methods are crucial for evaluating corrosion resistance. Electrochemical impedance spectroscopy (EIS) measures interfacial resistance and detects early-stage corrosion by analyzing impedance spectra over a frequency range. A low-frequency impedance increase often indicates metal dissolution or passivation layer breakdown. Potentiodynamic polarization provides insights into corrosion rates by sweeping the potential and measuring current response. Open-circuit potential monitoring tracks voltage shifts that signal corrosion onset. Accelerated aging tests in controlled humidity and temperature conditions simulate long-term degradation, helping validate protective strategies.

Material characterization techniques complement electrochemical testing. X-ray photoelectron spectroscopy (XPS) identifies surface chemical states, revealing oxide layer composition and degradation products. Scanning electron microscopy (SEM) visualizes morphological changes like pitting or cracking, while energy-dispersive X-ray spectroscopy (EDS) maps elemental distribution to pinpoint corrosive attack zones.

The interplay between materials, electrolytes, and operating conditions makes corrosion a multifaceted problem. No single solution is universally effective; instead, a combination of alloying, coatings, and electrolyte optimization is often required. Future research may explore advanced coatings with self-healing properties or novel electrolyte formulations that inherently suppress corrosion. As battery systems push toward higher energy densities and wider voltage ranges, addressing current collector degradation will remain a priority for ensuring reliability and safety.

In summary, corrosion in battery current collectors arises from complex electrochemical and environmental factors. Copper and aluminum degradation mechanisms differ but share common drivers like electrolyte decomposition and voltage excursions. Protective strategies must balance conductivity and corrosion resistance, leveraging materials science and electrochemical insights. Rigorous testing ensures that mitigation approaches are effective under real-world conditions, paving the way for more durable and efficient energy storage systems.