Current collector corrosion and interfacial degradation between active materials and collectors represent critical failure modes in battery systems, particularly in lithium-ion batteries where aluminum (Al) and copper (Cu) foils serve as the primary current collectors. These failure mechanisms can significantly impact battery performance, safety, and longevity. The degradation processes include pitting corrosion, passive layer formation, contact resistance increase, moisture-induced corrosion, and potential-driven dissolution. Understanding these phenomena is essential for improving battery reliability.

Pitting corrosion is a localized form of corrosion that occurs when the protective oxide layer on aluminum or copper current collectors is compromised. In aluminum foils, the native oxide layer (Al₂O₃) provides initial protection against corrosion. However, in the presence of aggressive species such as chloride ions or acidic environments, this passive film can break down, leading to the formation of small pits. These pits act as initiation sites for further corrosion, ultimately increasing the electrical resistance and reducing the mechanical integrity of the foil. Copper foils, though less prone to pitting than aluminum, can still suffer from localized attack in highly oxidizing or acidic conditions. The formation of copper oxides (Cu₂O, CuO) or copper hydroxides can exacerbate the issue, leading to uneven current distribution and capacity fade.

Passive layer formation is another critical degradation mechanism. While a thin, stable passive layer is beneficial for preventing bulk corrosion, excessive growth of these layers can increase interfacial resistance. Aluminum naturally forms a thin Al₂O₃ layer upon exposure to air, which is generally stable in neutral or mildly alkaline conditions. However, in lithium-ion batteries, the high operating potentials of cathodes (often above 3.5 V vs. Li/Li⁺) can drive further oxidation of aluminum, thickening the passive layer. This increased thickness raises the contact resistance between the active material and the current collector, leading to power loss and inefficient charge transfer. Copper, on the other hand, is typically used on the anode side, where it remains relatively stable due to lower operating potentials. However, if the anode potential drops too low (e.g., during over-discharge), copper can dissolve and redeposit, leading to internal short circuits.

Contact resistance increase is a direct consequence of interfacial degradation. Poor adhesion between the active material coating and the current collector can result from corrosion-induced surface roughening or delamination due to mechanical stresses during cycling. The formation of resistive interfacial layers, such as aluminum fluoride (AlF₃) in the presence of fluoride-containing electrolytes, further exacerbates the problem. This increased resistance leads to higher heat generation, localized hot spots, and accelerated degradation. In extreme cases, it can contribute to thermal runaway.

Moisture-induced corrosion is a significant concern, particularly during battery manufacturing and storage. Trace amounts of water in the electrolyte or residual moisture in electrode materials can react with lithium salts (e.g., LiPF₆) to form hydrofluoric acid (HF). HF is highly corrosive to aluminum, accelerating the breakdown of the passive layer and promoting pitting. Even small concentrations of water (below 50 ppm) can initiate these reactions, emphasizing the need for stringent moisture control in production environments. Copper is less susceptible to HF attack but can still suffer from surface oxidation in humid conditions, leading to increased interfacial resistance.

Potential-driven dissolution is particularly relevant for aluminum current collectors in high-voltage applications. At potentials above 4.2 V vs. Li/Li⁺, aluminum can undergo anodic dissolution, especially in electrolytes containing reactive species like PF₆⁻ or BF₄⁻. This dissolution leads to thinning of the foil and contamination of the electrolyte with aluminum ions, which can migrate and deposit on the anode, further degrading performance. Copper dissolution occurs at low potentials (below 1.5 V vs. Li/Li⁺), which can happen during over-discharge or cell reversal. Dissolved copper ions can migrate to the cathode and deposit, creating metallic copper particles that catalyze electrolyte decomposition and promote dendrite growth.

Several factors influence the severity of these failure modes. Electrolyte composition plays a crucial role; additives that stabilize the passive layer or scavenge corrosive species can mitigate degradation. For example, lithium bis(oxalato)borate (LiBOB) has been shown to reduce aluminum corrosion by forming a protective film. Temperature also affects corrosion kinetics; higher temperatures accelerate both pitting and passive layer growth. Mechanical factors such as electrode calendaring pressure and bending stresses during cell assembly can disrupt the passive layer, exposing fresh metal to corrosive attack.

Detection and mitigation strategies are essential for addressing these issues. Electrochemical impedance spectroscopy (EIS) is a valuable tool for monitoring interfacial resistance changes due to passive layer growth or delamination. Post-mortem analysis using techniques like scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) can reveal the chemical composition and morphology of corroded surfaces. Mitigation approaches include surface treatments such as carbon coating or conductive polymer layers on current collectors to enhance corrosion resistance. Optimizing electrolyte formulations to minimize HF generation and incorporating corrosion inhibitors are also effective strategies.

The impact of current collector corrosion extends beyond individual cell performance. In large-scale battery systems, such as electric vehicle packs or grid storage installations, uneven degradation can lead to cell-to-cell variations, reducing overall system efficiency and lifespan. Robust battery management systems (BMS) must account for these degradation mechanisms by implementing adaptive balancing algorithms and corrosion-aware state-of-health (SoH) estimation models.

In summary, current collector corrosion and interfacial degradation are multifaceted failure modes that require careful consideration in battery design and operation. Pitting corrosion, passive layer formation, contact resistance increase, moisture-induced corrosion, and potential-driven dissolution collectively contribute to performance decay and safety risks. Addressing these challenges through material selection, electrolyte engineering, and advanced diagnostics is critical for developing next-generation batteries with improved reliability and longevity.