Summarization of Surface Coating on Cathode Materials
Dec 16, 2024
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...
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