Atomic-layer deposition (ALD) is a precision coating technique that has gained prominence in improving cathode performance for lithium-ion and next-generation batteries. By enabling conformal, ultrathin coatings at the atomic scale, ALD can enhance electrochemical stability, reduce interfacial degradation, and suppress undesirable side reactions. Other coating techniques, such as chemical vapor deposition (CVD), physical vapor deposition (PVD), and solution-based methods, also play a role but differ in uniformity, scalability, and material compatibility. This article examines ALD and alternative coating approaches, focusing on their application to cathode materials and the functional benefits of specific coating materials.

ALD operates through sequential, self-limiting surface reactions, allowing precise control over film thickness and composition. This method is particularly advantageous for coating high-surface-area or porous cathode materials, where uniformity is critical. For example, aluminum oxide (Al2O3) deposited via ALD on layered oxide cathodes (e.g., NMC or NCA) has been shown to mitigate transition metal dissolution and electrolyte decomposition. Studies indicate that even sub-nanometer Al2O3 coatings can reduce capacity fade by up to 20% after extended cycling, as they act as a physical barrier against HF attack from electrolyte decomposition. Similarly, lithium phosphate (Li3PO4) ALD coatings improve ionic conductivity while preventing direct cathode-electrolyte contact, enhancing rate capability and cycle life.

The choice of coating material depends on its chemical and electrochemical properties. Al2O3 is widely studied due to its stability in battery environments and ability to scavenge HF. Lithium-containing coatings, such as Li3PO4 or lithium aluminum oxide (LiAlO2), are advantageous because they do not impede Li+ transport. In contrast, non-lithiated coatings may introduce additional interfacial resistance if not optimized. For high-voltage cathodes, coatings like lithium niobate (LiNbO3) or lithium zirconate (Li2ZrO3) have demonstrated effectiveness in suppressing oxygen release and phase transitions, which are critical for nickel-rich cathodes.

Alternative coating techniques include CVD, which offers higher deposition rates than ALD but with less precise thickness control. PVD methods, such as sputtering, provide dense coatings but struggle with conformality on complex geometries. Wet-chemical coating, while cost-effective, often results in non-uniform layers and requires post-treatment. Each method has trade-offs in terms of scalability, cost, and performance impact. For instance, solution-coated Li2TiO3 on NMC cathodes has shown improved cycling stability but may introduce impurities if not carefully processed.

The mechanism by which coatings enhance cathode performance involves multiple factors. First, they physically isolate the cathode from the electrolyte, reducing parasitic reactions such as oxidative electrolyte decomposition. Second, certain coatings act as artificial cathode-electrolyte interphases (CEIs), promoting stable Li+ transport while blocking electron transfer that leads to side reactions. Third, coatings can stabilize the cathode surface against structural degradation, particularly in high-nickel or manganese-rich systems prone to surface reconstruction. For example, ALD-applied TiO2 on LiMn2O4 has been shown to suppress Mn dissolution by passivating surface defects.

Thickness optimization is critical for ALD and other coatings. Too thick a layer may impede Li+ diffusion, while insufficient coverage fails to protect the cathode. Research suggests an optimal range of 1-5 nm for most oxide coatings, balancing protection and ionic conductivity. Beyond oxides, ALD has been explored for sulfides, fluorides, and polymer-like layers, each offering unique advantages. For instance, AlF3 coatings combine HF scavenging with electrochemical inertness, while hybrid organic-inorganic layers may improve flexibility and adhesion.

In addition to ALD, emerging techniques like molecular layer deposition (MLD) enable organic or hybrid coatings tailored for specific interfacial challenges. MLD can create ultrathin polymer films that enhance mechanical stability while maintaining ionic conductivity. Such innovations are particularly relevant for solid-state batteries, where cathode-coating compatibility is crucial to prevent interfacial resistance.

The scalability of ALD remains a challenge due to its slow deposition rate and high equipment costs. However, spatial ALD and roll-to-roll systems are being developed to address these limitations for industrial adoption. Meanwhile, hybrid approaches combining ALD with solution processing or magnetron sputtering are being investigated to balance performance and cost.

In summary, ALD and advanced coating techniques provide a powerful toolkit for optimizing cathode performance. By selecting appropriate materials and deposition methods, researchers can tailor interfacial properties to suppress degradation mechanisms while maintaining high energy density and rate capability. Continued advancements in coating technology will play a pivotal role in enabling next-generation cathodes for higher-energy, longer-lasting batteries.