The long driving range and fast charging of electric vehicles require high-performance lithium-ion batteries, and the cathode material is one of the most important components. However, the cathode is prone to rupture during the cycle and has continuous side reactions with the electrolyte, which seriously damages the cycle life and rate performance of the battery. Surface coating can reduce stress, increase the wettability of the liquid electrolyte, reduce the interfacial charge transfer resistance, and reduce side reactions, thereby effectively optimizing the cathode material. However, the influence of the physicochemical properties of the surface coating on the electrochemical performance and the evolution of the surface coating during the cycle still need to be further understood. In addition, the optimal surface coating materials and coating methods have not been systematically summarized and summarized.
1. Requirements for positive electrode surface coating
The requirements for surface coating include: 1) thin and uniform; 2) ionic and electronic conductivity; 3) high mechanical properties and stability after charge/discharge cycles; 4) the coating process is simple and scalable.
2. The role of surface coating
The role of surface coating on positive electrode materials: 1) Physical barrier, inhibiting side reactions; 2) Removing HF, preventing chemical erosion of electrolytes, and reducing dissolution of transition metals; 3) Improving electronic and ionic conductivity; 4) Surface chemical modification, promoting interfacial ion charge transfer; 5) Stabilizing the structure and reducing phase change stress.
Coating material type
1. Metal oxide
Metal oxide coating acts as a physical barrier between the cathode material and the electrolyte and does not participate in the electrochemical reaction. The disadvantage is poor lithium ion conductivity. In some cases, the rate performance of the cathode material coated with metal oxide decreases. This is caused by increased impedance (Rct). However, there are few reports that this type of inert metal oxide coating can improve charge transfer.
2. Phosphate
Phosphate coating 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 way to alleviate the challenges of nickel-rich cathodes. Li3PO4 coating on the NCM surface prevents the NCM cathode surface from directly contacting the electrolyte, thereby inhibiting side reactions and the formation of resistive surface films.
3. Electrode materials as coatings
Electrode materials have been used as cathode coating materials. Generally, more stable materials should be coated on less stable materials to improve the overall stability and performance of the materials. The benefit is that they provide a physical barrier between the cathode and the electrolyte, inhibit side reactions, improve charge transfer kinetics, and give the cathode material better electrochemical performance. However, it is difficult to achieve uniform and thin electrode material coating. Moreover, higher heat treatment temperatures are required to form a good coating, which may cause the cathode material to decompose. For this type of coating, it is necessary to select the best coating material and coating conditions.
4. Solid electrolytes and other ionic conductors as coatings
Solid electrolytes have high ionic conductivity at room temperature and are suitable as cathode coatings, but have low electronic conductivity. Due to their high ionic conductivity, they are expected to improve charge transfer at the cathode/electrolyte interface. In addition, solid electrolyte coatings provide a physical barrier that inhibits side reactions.
5. Conductive polymers
Conductive polymer coating can form a uniform film with high electronic conductivity and improve charge transfer at the cathode/electrolyte interface.
6. Surface doping
The surface coating method is to form a physical barrier layer on the surface of the positive electrode, which is usually less reactive to the electrolyte, thus improving the structure and thermal stability of the material.