Lithium-ion batteries (LIBs) are crucial for portable electronics and increasingly important for electric vehicles. However, long-term operation leads to electrode corrosion, particularly of the aluminum (Al) current collectors, which significantly impacts battery lifespan and performance. While Al's inherent oxide film provides initial protection, this is insufficient against aggressive liquid organic electrolytes commonly used in LIBs. These electrolytes, often ester-based, can facilitate Al corrosion through various mechanisms, including reactions with HF (generated from LiPF6 hydrolysis) and other electrolyte components. Previous studies have explored Al passivation and corrosion, with some attributing it mainly to electrochemical reactions rather than HF hydrolysis. However, a comprehensive understanding of the coupled effects of anions and solvents on Al corrosion at commercially relevant voltages (>4.5 V) is lacking. This study aims to address this gap by investigating the corrosion behavior of Al current collectors in commercial LIB electrolytes, linking it directly to battery performance degradation, and elucidating the underlying mechanisms. This will involve comparing the performance of reclaimed Al current collectors from spent LIBs with fresh Al foils, examining the influence of applied potentials, and utilizing Density Functional Theory (DFT) simulations to understand the adsorption of electrolyte components on the Al surface.

Existing research has explored the degradation of LIB components, including separators, current collectors, electrolytes, and electrodes. Studies have focused on the corrosion of Al and Cu current collectors, noting that Al's inherent oxide layer (comprising crystalline hydro-soft aluminite and an amorphous alumina barrier layer) is not entirely effective against electrolyte attack. The corrosion mechanisms are different in organic (ester-based) electrolytes compared to aqueous environments. HF, a common contaminant in LIB electrolytes, plays a role in Al corrosion. Previous work has highlighted the passivation of Al, often attributing it to electrochemical reactions rather than HF hydrolysis. However, there is limited research on the coupled impacts of electrolyte anions and solvents on Al corrosion and its relationship to battery performance, especially at high voltages. This paper aims to fill this knowledge gap by investigating Al corrosion in commercially relevant LIBs.

The study employed reclaimed Al current collectors (RA) from spent LiCoO2 batteries and fresh Al (FA) foils. RA was characterized using AFM, XRD, and XPS to examine its morphology, surface roughness, and elemental composition. Electrochemical performance was assessed using LiCoO2||Li half-cells with FA and RA as current collectors. Cyclic voltammetry (CV), amperometric i-t curves, and electrochemical impedance spectroscopy (EIS) were used to analyze the electrochemical behavior of Al in the electrolyte (1 M LiPF6 in EC/DEC/FEC). The impact of different applied potentials (4.0 V, 4.2 V, 4.3 V, 4.4 V, 4.5 V, 4.6 V, 4.8 V, and 5.0 V) on Al corrosion was investigated using chronoamperometry and surface characterization. DFT calculations were used to investigate the adsorption of electrolyte components (HF and EC) on different Al crystallographic planes ((111), (220), and (311)) to clarify the adsorption and subsequent oxidation mechanisms. This involved determining the binding energies of these molecules on the Al surface.

Reclaimed Al (RA) current collectors exhibited inhomogeneous morphology with cracks, pits, and Al/O/F complexes on their surfaces, indicating corrosion. XPS analysis revealed differences in passivation layer composition between RA and FA, with RA showing a thicker, inhomogeneous layer and less evidence of Al metal. Electrochemical testing showed a higher anodic dissolution current for RA compared to FA, confirming increased corrosion. Amperometric i-t curves and EIS demonstrated significantly lower corrosion resistance for RA. In LiCoO2||Li half-cells, RA exhibited a capacity loss >20% compared to FA, highlighting the negative impact of Al corrosion on battery performance. Coulombic efficiency was also lower for RA (98.70%) than FA (99.84%), indicating irreversible capacity loss due to Al corrosion. AFM-KPFM analysis revealed that at potentials above 4.4 V, the Al surface undergoes significant morphological changes, including the formation of nanograins and cracks, suggesting a transition from passivation to active corrosion. DFT calculations revealed that EC molecules have a strong interaction with the Al surface, particularly on the (220) plane, possibly facilitating the corrosion process by forming a corrosion bridge. At higher voltages (above 4.4 V), outward diffusion of Al³⁺ dominates, leading to the breakdown of the passivation layer. The dissolved Al³⁺ ions deposit on the Li anode, damaging the SEI and hindering Li⁺ transfer, further contributing to battery performance degradation.

The findings directly link Al current collector corrosion to significant performance degradation in LIBs. The observed capacity fade (>20%) and reduced Coulombic efficiency demonstrate the substantial impact of corrosion. The mechanism involves the formation of a passivation layer, its breakdown at higher voltages due to ethylene carbonate adsorption and the coupled effects of electrolyte components, and subsequent localized corrosion. The outward diffusion of Al³⁺ and its deposition on the anode further worsens performance by damaging the SEI. The DFT results illuminate the role of solvent-Al interactions in initiating corrosion. This research underscores the need for improved Al current collector design to enhance the long-term stability and performance of LIBs, particularly in high-voltage applications.

This study comprehensively investigates Al current collector corrosion in LIBs, linking its degradation to substantial performance loss. The findings reveal a complex interplay between passivation layer formation, breakdown under high voltages due to solvent interaction, and subsequent localized corrosion. These results highlight the importance of designing robust Al current collectors to prevent corrosion, thereby extending battery lifespan and improving overall performance. Future research could focus on developing novel protective coatings or modifying electrolyte formulations to mitigate Al corrosion in high-voltage LIBs.

The study focused on LiCoO2 cathodes and a specific electrolyte composition. The results may not be directly generalizable to other cathode materials or electrolyte formulations. The use of reclaimed Al collectors introduces potential variations due to the complex history of the spent batteries. While efforts were made to standardize the pre-treatment of the reclaimed Al, inherent variations could slightly affect the outcome. Further research is required to determine the extent of these variations.