Summarization of Surface Coating on Cathode Materials

The long-range and fast-charging capabilities of electric vehicles rely on high-performance lithium-ion batteries, with cathode materials being one of the most crucial components. However, cathodes are prone to cracking during cycling and exhibit persistent side reactions with electrolytes, significantly compromising the battery's cycle life and rate performance. Surface coating can mitigate stress, enhance the wettability of liquid electrolytes, reduce interfacial charge transfer resistance, and decrease side reactions, thereby effectively optimizing cathode materials. Nevertheless, the influence of the physicochemical properties of surface coatings on electrochemical performance, as well as their evolution during cycling, still requires further understanding. Additionally, the optimal surface coating materials and methods have not been systematically summarized and concluded.

1. Requirements for Cathode Surface Coating

The requirements for surface coating include: 1) being thin and uniform; 2) possessing ionic and electronic conductivity; 3) having high mechanical properties and remaining stable after charging/discharging cycles; 4) the coating process being simple and scalable.

2. The Roles of Surface Coating on Cathode Materials

The roles of surface coating on cathode materials include: 1) serving as a physical barrier to inhibit side reactions; 2) scavenging HF to prevent chemical attack by the electrolyte and mitigate the dissolution of transition metals; 3) enhancing electronic and ionic conductivity; 4) modifying surface chemistry to facilitate interfacial ion charge transfer; 5) stabilizing the structure and reducing phase transition stress.

3 Coating Structure/Morphology

3.1 Uniform and Thin Coating
The coating layer should be uniform and thin. Complete coverage of the cathode particles will protect the cathode from electrolyte attack and inhibit side reactions. Additionally, a thin coating layer enhances the kinetics at the interface, improving battery performance.

3.2 Thick Coating
A thick coating provides a good physical barrier between the cathode and the electrolyte. However, thicker coatings can hinder the diffusion of lithium during intercalation and deintercalation processes, potentially performing well under high-temperature operations.

3.3 Island-Like/Rough Coating Layer
Achieving a uniform and thin coating across the entire material using dry and wet coating processes is challenging. The coating layers formed by these processes are rough and uneven.

4. Coating Processes/Strategies

4.1 Wet Processes
4.1.1 Sol-Gel Coating
The sol-gel coating process is commonly used for synthesizing cathode materials and surface coating. However, the use of water or other solvents increases costs. Additionally, solvents like water can cause lithium leaching and alter the stoichiometry of the cathode surface.

4.1.2 Hydrothermal/Solvothermal Coating
The coating layers developed through hydrothermal/solvothermal processes are nanoscale and uniform, allowing for the control of the stoichiometry of the coating layer. However, they are difficult to process, with expensive precursor salts and low yields.

4.2 Dry Coating Processes
Dry coating methods may be the most feasible and suitable, but achieving a uniform coating is challenging.

4.3 Vapor-Phase Chemical Processes
4.3.1 Chemical Vapor Deposition (CVD)
Chemical Vapor Deposition (CVD): At a certain temperature, reactants decompose on the substrate material, causing the material to deposit from the vapor phase. The main advantage of CVD is the ability to produce low-porosity, uniform, and thin coating layers.

4.3.2 Atomic Layer Deposition (ALD)
The coating layer formed by Atomic Layer Deposition (ALD) is of atomic-scale thickness. Its greatest advantage lies in the ability to form uniform, high-quality coating layers with precise control. However, it suffers from low yield, slow processing times, high precursor costs, toxicity, and complex processes.

5. Types of Coating Materials

5.1 Metal Oxides
Metal oxide coatings serve as a physical barrier between the cathode material and the electrolyte, without participating in electrochemical reactions. The drawback is their poor lithium-ion conductivity. In some cases, the rate performance of cathode materials coated with metal oxides decreases, caused by increased impedance (Rct). However, there are few reports that such inert metal oxide coatings can improve charge transfer.

5.2 Phosphates

Phosphate coatings can improve the ion transport properties of cathode materials. The poor cycling and safety issues of nickel-rich layered oxides hinder their large-scale use. Surface coating is an effective method to mitigate the challenges of nickel-rich cathodes. The Li3PO4 coating on the NCM surface prevents direct contact between the NCM cathode surface and the electrolyte, thereby inhibiting side reactions and the formation of resistive surface films.

5.3 Cathode Materials as Coatings

Cathode materials have been used as coating materials for cathodes. Generally, more stable materials should be coated onto less stable materials to improve the overall stability and performance of the material. The advantage is that they provide a physical barrier between the cathode and the electrolyte, inhibiting side reactions and improving charge transfer kinetics, resulting in better electrochemical performance of the cathode material. However, it is difficult to achieve uniform and thin coatings of cathode materials. Moreover, high heat treatment temperatures are required to form good coatings, which may lead to decomposition of the cathode material. For this type of coating, optimal coating materials and conditions need to be selected. For example, ultra-thin spinel (LiMn2O4) coating on lithium-rich Li1.2Mn0.6Ni0.2O2 layered oxide (USMLLR) improves electrochemical and thermal performance. The benefit is that it not only ensures the high capacity of lithium-rich layered oxide materials but also provides high rate performance, while improving charge transfer at the surface due to the excellent Li+ conductivity of LMO.

5.4 Solid Electrolytes and Other Ionic Conductors as Coatings

Solid electrolytes have high ionic conductivity at room temperature and are suitable as cathode coating layers, but their electronic conductivity is low. Due to their high ionic conductivity, they are expected to improve charge transfer at the cathode/electrolyte interface. Additionally, solid electrolyte coatings provide a physical barrier, inhibiting side reactions. Coating lithium lanthanum titanate (LLTO) on LiNi0.6Co0.2Mn0.2O2 (NCM) can improve rate performance, attributed to the high ionic conductivity of the LLTO coating layer and the inhibition of side reactions. However, increased coating thickness can inhibit the electron transfer process during charging/discharging.

5.5 Conductive Polymers

Conductive polymer coatings can form uniform thin films with high electronic conductivity, improving charge transfer at the cathode/electrolyte interface. These polymers can accommodate volume changes, reducing crack formation.

5.6 Surface Doping

The surface coating method forms a physical barrier on the cathode surface, which is generally less reactive to the electrolyte, thus improving the structural and thermal stability of the material. Since the crystal structure and composition changes at the interface are similar, surface doping does not hinder Li+ diffusion, reduces Rct and mechanical stress at the interface, and reduces the likelihood of cracking.

6. Structure-Property Correlation: Coating Thickness and Lithium-Ion Diffusion
Certain surface coatings can hinder ion diffusion while providing other advantages, whereas some coatings can enhance ion diffusion but compromise other properties. Considering these effects as a compromise has always been a focus of battery research when adopting coatings. The structural property correlation between coating thickness and the lithium-ion diffusion rate of the coating layer is an effective method to measure this compromise criterion.