Technical Principles, Application Characteristics, and Performance Optimization Strategies of Four-Side Film Applicators for Lithium-Ion Battery

In the R&D and performance testing system of lithium-ion batteries, quality control during the electrode preparation process directly determines the accuracy and reliability of subsequent electrochemical performance evaluations. The uniformity and consistency of electrode coating thickness are among the core parameters influencing a battery’s charge-discharge capacity, cycle life, and rate capability. Traditional manual film coating relies on the operator’s experience to control scraping force and speed, which easily leads to uneven loading of electrode active materials, causing local polarization differences and subsequent deviations in test data—failings that cannot meet the demands of high-precision R&D. As a standardized, high-precision film preparation device, the four-side film applicator has become a critical tool for preparing lithium-ion battery electrode samples, particularly in laboratory R&D and small-batch sample testing scenarios. 

From the perspective of the structural characteristics of lithium-ion battery electrodes, both positive and negative electrode coatings require strict thickness control. Taking common ternary positive electrode materials as an example, the coating thickness typically ranges from 20μm to 200μm, while the thickness of graphite negative electrode coatings mostly falls between 15μm and 150μm. A thickness deviation exceeding 5% can cause fluctuations in the loading of active materials, directly affecting the battery’s energy density and charge-discharge efficiency. The core advantage of the four-side film applicator lies in its ability to achieve micron-level control of coating thickness through a precision mechanical structure, with its hardware design fully accounting for the specific needs of lithium-ion battery electrode preparation. The main body of the device is made of high-quality stainless steel, which not only has a high hardness of HRC55±2—resisting physical wear during long-term scraping and maintaining the precision stability of the groove structure—but also exhibits excellent corrosion resistance.

The working principle of the four-side film applicator is based on a direct correspondence between "groove depth and coating thickness," and flexible adjustment of electrode coating thickness is achieved through groove designs of different specifications. This principle is highly compatible with the thickness gradient requirements of lithium-ion battery electrode preparation. The device usually offers 9 models of thickness configurations, each containing 4 differentiated thickness specifications, covering a thickness range from 5μm to 1000μm—capable of accurately matching the thickness requirements of positive and negative electrode coatings for lithium-ion batteries. For example, models with specifications of 5μm and 10μm can be used for research on solid electrolyte interphase films requiring ultra-thin coatings; models with specifications of 25μm, 50μm, 75μm, and 1000μm can meet most R&D scenarios for conventional ternary positive electrode preparation; and for thick-coating electrodes (such as high-loading positive electrodes used in energy storage batteries), models with specifications ranging from 200μm to 1000μm can be selected. 

In the actual operation of lithium-ion battery electrode preparation, the standardized operating procedure of the four-side film applicator is key to exerting its precision advantages, and parameter optimization must be carried out in combination with the rheological properties of the electrode slurry and the characteristics of the substrate. The first step is substrate pretreatment, where the electrode substrate (aluminum foil or copper foil) must be placed flat and fixed on the experimental platform to avoid gaps between the film applicator and the substrate due to substrate warping. Lithium-ion battery electrode substrates typically have high ductility; if not fixed firmly, they are prone to displacement during the scraping process, causing coating wrinkles or edge thickness deviations. Therefore, it is recommended to use special fixtures or high-temperature resistant tape for fixation, while ensuring the substrate surface is free of oil and impurities to prevent affecting the bonding force between the slurry and the substrate. The next step is device positioning, where the film applicator should be placed close to the short side of the substrate and parallel to it.

In the slurry application step, the dosage must be adjusted according to the viscosity characteristics of the electrode slurry. It is generally recommended to apply approximately 5ml of slurry in front of the film applicator. The viscosity range of lithium-ion battery electrode slurries is relatively wide (the viscosity of water-based negative electrode slurries is mostly 1000mPa·s-5000mPa·s, and the viscosity of oil-based positive electrode slurries is mostly 3000mPa·s-10000mPa·s). For slurries with higher viscosity (such as high-solid-content positive electrode slurries), the dosage should be appropriately increased to avoid insufficient filling due to poor slurry fluidity, which could result in missing coatings or thin areas. For slurries with lower viscosity, the dosage must be strictly controlled to prevent edge overflow caused by excessive fluidity, which would affect the effective coating area. The control of scraping speed is particularly critical to coating quality. The constant speed of 150mm/s recommended by the device has been verified through numerous experiments and can achieve ideal spreading effects in most electrode slurry systems. 

After completing the scraping process, the wet film needs to undergo drying and curing. The uniform wet film prepared by the four-side film applicator can significantly improve the consistency of the drying process. If the wet film thickness is uneven, differences in shrinkage stress are likely to occur during drying, leading to substrate warping or coating cracking and affecting the mechanical stability of the electrode. In lithium-ion battery electrode R&D, this consistency not only ensures the precision of the coating thickness after drying but also guarantees the uniform distribution of active materials, conductive agents, and binders—providing high-quality electrode substrates for subsequent rolling, slicing, and battery assembly processes, and reducing battery performance fluctuations caused by preparation processes.

Equipment maintenance and precision calibration are important links to ensure its long-term adaptability to the needs of lithium-ion battery electrode preparation, and targeted strategies must be formulated based on the experimental frequency of lithium-ion battery R&D and slurry characteristics. The post-use cleaning step is particularly critical: lithium-ion battery electrode slurries contain active material particles (such as ternary materials and graphite) and binders. If these residues dry and solidify in the grooves of the film applicator, they will not only block the grooves and affect subsequent precision but also may mix into new slurries during the next use, causing sample contamination. Therefore, it is necessary to select a suitable cleaning solvent based on the slurry type: deionized water can be used for ultrasonic cleaning of water-based slurries, supplemented by a soft brush to remove residual particles in the grooves; for oil-based slurries, soaking and cleaning with N-methylpyrrolidone or acetone is required to ensure complete dissolution of the binder. After cleaning, the device surface and groove interior must be thoroughly wiped dry with a lint-free cloth to prevent rust on the stainless steel surface caused by residual moisture or solvent, which would affect groove precision.

Anti-rust treatment and storage environment control are equally important. After cleaning, a thin layer of anti-rust oil or neutral petroleum jelly should be applied to the device surface and groove interior to form a protective film that isolates air and moisture. This can effectively delay oxidative corrosion of the device, especially in humid environments or during long-term non-use. During storage, the device should be placed in a dry, well-ventilated environment with a temperature of 15°C-25°C and a relative humidity below 60%, avoiding direct sunlight or proximity to high-temperature equipment (such as vacuum drying ovens) to prevent groove deformation caused by thermal expansion and contraction of the device. 

In conclusion, as an indispensable film preparation device in lithium-ion battery R&D, the four-side film applicator has a high degree of compatibility between its technical principles and the preparation requirements of lithium-ion battery electrodes. Through its precision structural design, flexible thickness adjustment capabilities, and standardized operating procedures, it effectively solves the problem of insufficient precision in manual film coating and provides a reliable sample foundation for evaluating the performance of electrode materials. In practical applications, by optimizing operating parameters, strengthening equipment maintenance, and conducting precision calibration, the advantages of four-side film applicators in lithium-ion battery electrode preparation can be fully exerted—assisting researchers in accurately screening materials, optimizing processes, and innovating structures. With the continuous advancement of lithium-ion battery technology, the performance of four-side film applicators will be further upgraded. Their development toward high precision, multi-functionality, and intelligence will provide important guarantees for the continuous breakthrough of lithium-ion battery R&D and drive technological innovation and industrial upgrading in the new energy field.