Introduction
In electrochemical energy storage systems, the performance of the separator is a key component, and its performance directly affects the safety and efficiency of the system. Composite separator coating technology can effectively improve the mechanical strength, thermal stability and ion transport characteristics of the separator by introducing functional coatings. Among them, the controllability of the coating process has a decisive impact on the uniformity, thickness and final performance of the coating. In this paper, we will discuss the controllable coating technology of composite separator coatings, and analyze their process parameters, characterization methods and related standards.
Coating process principle
The controlled coating process is designed to evenly coat the slurry on the surface of the base film, creating a functional layer of consistent thickness. The process mainly involves three stages: fluid delivery, coating formation, and drying and curing. Coating uniformity can be achieved by adjusting parameters such as slurry rheological characteristics, coating speed and clearance. The coating thickness d can be approximated by the formula:
d = (Q × η) / (v × w)
where Q is the slurry flow, η is the slurry viscosity, v is the coating speed, and w is the coating width. This formula is an ideal model, and factors such as slurry sedimentation and base film surface energy need to be considered.
Key process parameters
Achieving controllable coating requires precise control of a number of process parameters. The main parameters include slurry solid content, viscosity, coating speed, drying temperature and ambient humidity. When the slurry was prepared, the solid content affected the porosity and density of the coating. Viscosity is directly related to coating leveling and defect formation. The coating speed needs to match the slurry delivery rate to avoid fluctuations in coating thickness. The drying process needs to be controlled in sections to prevent cracking or curling of the coating due to rapid volatilization of solvents.
| Process parameters | Impact |
| Solid content of slurry | Coating porosity and density |
| Slurry viscosity | Leveling and defect control |
| Coating speed | Uniformity of coating thickness |
| Drying temperature | Solvent volatilization and internal stress |
| Ambient humidity | Coating surface topography |
Coating characterization methods
Coating quality needs to be evaluated by a variety of characterizations. Thickness measurement can be done using contact thickness gauge or optical profiler; The surface morphology and pore structure can be observed by electron microscopy. The bonding strength of the coating to the base film can be quantified by peel tests. In addition, ionic conductivity, thermal shrinkage and mechanical puncture strength are the key indicators to evaluate the functionality of the coating. This characterization data needs to be correlated with process parameters to optimize the coating process window.
Standards and specifications
The coating process and coating performance evaluation should refer to relevant domestic and foreign standards. In terms of mechanical properties, the requirements for tensile strength and elongation of the separator can be referred to in the standard. Thermal stability testing is usually based on the heat shrinkage test method; The electrochemical properties involve test specifications such as ion mobility number and interface impedance. During the process development process, it is necessary to ensure that the coating thickness tolerance and uniformity meet the requirements of product specifications.
| Performance category | Related test items |
| Mechanical properties | Tensile strength, puncture strength |
| Thermal performance | Thermal shrinkage, closed cell temperature |
| Electrochemical properties | Ionic conductivity, interface impedance |
| Structural characteristics | Porosity, average pore size |
Technical challenges and prospects
Current controlled coating technology still faces some challenges. For example, when coating high-solids slurry, defects such as stripes or orange peel patterns are prone to occur; The thickness control of ultra-thin coatings (such as less than 2 microns) is difficult; Edge effects in wide coatings can affect coating uniformity. Future development trends include the development of online monitoring and closed-loop control systems to realize real-time adjustment of coating parameters; explore new coating methods such as slit coating and microgravure coating to improve coating consistency; At the same time, the application of environmentally friendly water-based slurry is also an important direction.
References
1. Fundamentals of Coating Technology, Transactions of the Chinese Society of Materials Science and Engineering, 2020.
2. Performance Test Method for Composite Separator Coatings, Electrochemical Energy Storage Technology, 2021.
3. Research on the relationship between slurry rheology and coating quality, Journal of Chemical Engineering, 2019.
4. International standard: diaphragm mechanical property test standard, issued by the International Electrotechnical Commission.
5. Progress in Controllable Coating Technology, Energy Materials Review, 2022.