In electrochemical systems, passivation layers play a critical role in determining the stability and longevity of metal components, particularly in battery electrolytes. Aluminum, commonly used as a current collector in lithium-ion batteries, exhibits passivation behavior when exposed to electrolytes containing LiPF6. The formation and properties of these layers can be systematically analyzed using potential-pH (Pourbaix) diagrams and film growth kinetics, providing insights into corrosion resistance and electrochemical performance.

Potential-pH diagrams offer a thermodynamic framework for predicting the stability of aluminum in aqueous and non-aqueous environments. These diagrams map the regions of immunity, corrosion, and passivation as a function of electrode potential and pH. For aluminum in LiPF6-based electrolytes, the diagram reveals that passivation occurs within a specific potential range where aluminum oxide (Al2O3) or aluminum fluoride (AlF3) becomes thermodynamically stable. The exact composition of the passivation layer depends on the local chemical environment, including the presence of trace water and HF impurities. In LiPF6 electrolytes, hydrolysis reactions produce HF, which reacts with aluminum to form AlF3. The stability of this layer is crucial because it prevents further dissolution of aluminum, which could lead to increased resistance or cell failure.

The kinetics of passivation layer growth on aluminum involves several stages. Initially, a thin oxide layer forms spontaneously upon exposure to air or electrolyte. This native oxide is typically 2-5 nm thick and consists primarily of amorphous Al2O3. When the aluminum is polarized within the passive region, the layer grows further through a combination of ionic transport and interfacial reactions. The growth rate follows a logarithmic or inverse logarithmic relationship with time, indicating that the process is self-limiting due to the increasing barrier for ion diffusion through the thickening film. The rate-determining step is often the migration of Al3+ ions or O2- ions through the oxide lattice, with activation energies typically ranging between 70-100 kJ/mol for Al2O3 formation in non-aqueous electrolytes.

The presence of fluoride ions in LiPF6-based electrolytes complicates the passivation mechanism. Fluoride competes with oxide in the formation of the passive layer, leading to mixed compositions such as AlOxFy. The incorporation of fluoride can alter the electronic and ionic conductivity of the layer, as AlF3 is more insulating than Al2O3 but may also introduce defects that affect long-term stability. The ratio of oxide to fluoride in the layer depends on the concentration of HF, which is influenced by the purity of the LiPF6 salt and the water content in the electrolyte. Studies have shown that HF concentrations as low as 50 ppm can significantly increase the fluoride content in the passivation layer.

The protective quality of the passivation layer is evaluated through its breakdown potential and pitting resistance. Breakdown occurs when the applied potential exceeds a critical value, causing localized dissolution and pit formation. For aluminum in LiPF6 electrolytes, the breakdown potential typically ranges between 3.5-4.5 V vs. Li/Li+, depending on the electrolyte composition and temperature. Pitting susceptibility is influenced by chloride impurities, which disrupt the passive layer and initiate localized corrosion. The critical chloride concentration for pitting initiation in LiPF6 electrolytes is approximately 10 ppm, highlighting the importance of high-purity materials in battery manufacturing.

The thickness and morphology of the passivation layer are characterized using techniques such as X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). XPS reveals the chemical composition and bonding states of the elements in the top few nanometers of the layer, while EIS provides information about the ionic resistance and capacitance of the film. A well-formed passivation layer exhibits high impedance values, often exceeding 10^5 ohm·cm², indicating effective barrier properties. The capacitance of the layer, typically in the range of 1-10 μF/cm², is inversely proportional to its thickness and can be used to estimate the film growth rate.

Temperature has a pronounced effect on passivation layer stability. Elevated temperatures accelerate both the growth and dissolution of the layer, with Arrhenius-type behavior observed for these processes. At temperatures above 60°C, the passive layer may undergo phase transformations or partial dissolution, reducing its protective capability. This is particularly relevant for batteries operating under high-load or high-temperature conditions, where aluminum current collectors may experience increased corrosion rates.

The role of electrolyte additives in modifying passivation layers has been extensively studied. Additives such as vinylene carbonate or lithium bis(oxalato)borate (LiBOB) can interact with the aluminum surface, forming complexes that enhance passivation. These additives may reduce the solubility of aluminum species or promote the formation of more compact and adherent layers. The effectiveness of an additive is often quantified by its ability to increase the breakdown potential or reduce the passive current density.

In practical battery systems, the stability of the aluminum passivation layer is critical for long-term cycling performance. Degradation of the layer can lead to increased contact resistance, voltage hysteresis, and capacity fade. Strategies to improve passivation include optimizing the electrolyte composition, using ultra-dry materials, and applying surface treatments to the aluminum foil. Advanced current collector designs may incorporate thin coatings of conductive ceramics or polymers to supplement the native passivation layer.

Understanding the interplay between thermodynamics and kinetics in passivation layer formation enables better control over aluminum corrosion in battery applications. Potential-pH diagrams provide a roadmap for identifying stable operating conditions, while film growth kinetics inform strategies for enhancing layer quality. The continued development of high-performance passivation layers will contribute to the reliability and efficiency of energy storage systems.