Introduction
In the process of energy storage and conversion material preparation, the coating process of high-load electrode slurry is a key link that affects the performance of the final product. Slurries are usually composed of active substances, conductive agents and binders, etc., and it is challenging to achieve uniform dispersion and stable coating of each component under the conditions of high solids content and high viscosity. As a common laboratory coating equipment, the operating parameters of the scraper coating machine have a significant impact on the uniformity of slurry dispersion. The purpose of this paper is to explore how to optimize the dispersion uniformity of the coating by adjusting the process parameters when processing high-load electrode slurry, and to provide a reference for the development of related processes.
Brief description of the coating principle
Scraper coating is a method of evenly coating the slurry on the substrate through a squeegee with adjustable gaps. The basic process is that the excess slurry is placed at the front end of the substrate, and the scraper is translated at a set height to scrape off the excess slurry, leaving a uniform wet film. The coating thickness (h) can theoretically be determined by factors such as scraper clearance (G), slurry viscosity (η), coating speed (v), etc., and the relationship can be approximately expressed as:
h ≈ k · G · f(η, v)
where k is the coefficient related to the rheological properties of the slurry. For high-load slurries, the non-Newtonian fluid properties are significant, and the actual coating thickness and uniformity will deviate from the ideal model.
Key influencing factors
The main factors affecting the dispersion uniformity of high-load slurry coating can be divided into three categories: slurry characteristics, equipment parameters and operating environment. The characteristics of the slurry include solid content, viscosity, rheological behavior and component compatibility. The equipment parameters include scraper clearance, tool head type, coating speed and substrate material; The operating environment involves temperature, humidity and slurry pretreatment methods. These factors are interrelated and together determine the distribution of active substances and conductive networks in the coating.
Experimental parameter design
In order to systematically study the influence of various parameters, a series of control experiments were designed. The basic formula of the slurry is fixed, and the scraper gap, coating speed and slurry pretreatment conditions are adjusted in turn, and the sample is prepared. The uniformity, surface morphology and internal structure of the coating were characterized by areal density measurement, optical microscopy and electrochemical impedance spectroscopy. The key experimental variables are shown in the table below:
| Variables were studied | Value range or status |
| Scraper gap | 100 microns to 500 microns |
| Coating speed | 5 mm/s to 50 mm/s |
| Slurry pretreatment | Stand-still, defoam, stir at low speed |
| Substrate type | Metal foils, polymer films |
Results and discussion
The experimental results show that the matching of scraper gap and coating speed is crucial for uniformity. At lower speeds, increasing the gap helps to obtain a thicker coating, but it is easy to cause slurry to accumulate at the tool head, resulting in transverse streaks. At higher speeds, the gap needs to be appropriately reduced to maintain film thickness, but may exacerbate the edge effect of shear thinning slurry. For high-viscosity slurries, when the coating speed is 10-20 mm/s and the gap is 200-300 microns, the coating surface density is relatively small compared to the standard deviation, showing good uniformity.
Slurry pretreatment should also not be ignored. Slurry that has not been adequately defoamed or dispersed is prone to pores or aggregates in the coating after coating, which affects the continuity of the conductive network. The rheological curve of the slurry after pretreatment is more stable, and the reproducibility of the coating process is improved. In addition, differences in substrate surface energy can also affect slurry spreading, and hydrophilic substrates often result in a more uniform initial wetting layer.
Optimization recommendations
Based on the above findings, in order to improve the dispersion uniformity of high-load slurry, the following comprehensive measures are recommended: first, the slurry should be fully mechanically dispersed and vacuum defoamed to ensure its rheological performance is stable; Secondly, according to the target wet film thickness and the rheological characteristics of the slurry, the appropriate matching interval between the scraper gap and the coating speed is determined through pre-experiments. Finally, keep the ambient temperature and humidity stable, and choose a substrate material that is compatible with the slurry. After coating, the parameters can be adjusted in time through rapid characterization methods such as infrared drying monitoring.
Conclusion
The application of scraper coating machine in the coating of high-load electrode slurry, its dispersion uniformity is affected by the coupling of multiple factors. By systematically adjusting the slurry pretreatment, scraper clearance, coating speed and substrate selection, the coating uniformity can be effectively improved. This study provides a process optimization idea for the preparation of high-performance electrode coatings on a laboratory scale, and the related methods can also be extended to the preparation process of other high-solids functional coatings. In the future, online monitoring technology can be further combined to realize real-time regulation of the coating process.
References
1. Slurry Rheology and Coating Process Basics, Journal of Material Preparation Technology, 2020.
2. Review of Uniformity Characterization Methods for High Solids Coatings, Laboratory Instruments and Applications, 2021.
3. Analysis of flow behavior of non-Newtonian fluids in slits, Transactions of the Chinese Society of Chemical Engineering, 2019.