In the R&D and production process of lithium batteries, the preparation of pole pieces is the basic link that determines the performance of the battery. As a key equipment to achieve the uniform coating of active material slurry on the current collector, the application of laboratory film coating machine is directly related to the consistency of areal density, thickness uniformity and microstructure of the electrode piece, which in turn affects the energy density, cycle life and safety of the battery. This paper will systematically discuss the technical principles, key parameter control and its impact on the quality of lithium battery electrode pieces in the preparation of lithium battery electrode pieces.
Coating process principle
The laboratory film coating machine mainly transfers the mixed electrode slurry to the copper foil or aluminum foil current collector evenly through scrapers, slit extrusion or micro-gravure coating methods. Its core process can be briefly described as: slurry conveying, quantitative coating, preliminary drying and winding. The design accuracy and motion stability of the coating head are the physical basis for ensuring the uniformity of coating. The coating thickness (wet film) is usually determined by the coating gap, the rheological characteristics of the slurry and the coating speed, and its basic relationship can be approximately expressed as:
h ≈ k · (G, η, v)
Among them,his the thickness of the wet film,Gfor coating gaps,ηis the viscosity of the slurry,vfor the coating speed,kis the coefficient associated with the type of coating head. This process requires the rapid removal of most of the solvent in the drying section to form a wet electrode sheet with a certain bond strength to prevent the slurry from flowing or cracking.
Process parameters
Quality control of the coating process relies on several interrelated parameters. The rheological properties of the slurry are intrinsic factors, while the coating machine parameters are external control conditions.
| Slurry viscosity | It affects the amount of transfer and spreading uniformity, and too high is easy to cause streaks, and too low is easy to cause flow. |
| Coating speed | Matches with drying rate, affecting production efficiency and wet film leveling time. |
| Coating gaps | The basic setpoint that directly determines the thickness of the wet film. |
| Drying temperature and wind speed | Affect the evaporation rate of solvents, and improper results in surface crusting, internal stress cracks, or adhesive migration. |
| Substrate tension | Control the flatness of the foil to prevent wrinkling or stretch deformation. |
These parameters need to be optimized through systematic experiments to achieve the target areal density and thickness while ensuring that the electrode is defect-free. Areal density uniformity is a key indicator, and its deviation needs to be controlled within a narrow range to ensure consistency between cells.
Defects in the electrode pieces generated by the coating process can cause problems in subsequent battery assembly and cycling. Uneven coating can lead to local areal density differences, resulting in uneven current distribution within the battery, accelerating local aging or lithium precipitation. Streaks or pinholes on the surface of the coating may reduce active substance utilization or cause a local short circuit risk of the diaphragm. Uneven adhesive distribution due to improper control of the drying process can weaken the structural integrity of the electrodes and affect cycle life. Therefore, the optimization of the coating process at the laboratory stage is aimed at establishing a reliable parameter window for scale-up production and an in-depth understanding of the intrinsic connection between process-structure-performance.
With the increasing requirements for battery energy density and power density, coating technology is facing new challenges. Coating slurries with high solids content and viscosity requires higher shear dispersion and conveying accuracy. The preparation of ultra-thick electrodes or three-dimensional structured electrodes puts forward new requirements for the design and drying strategy of coating heads. In addition, in order to reduce the use of solvents, solvent-free coating technologies such as dry electrode preparation are being explored, which complements and challenges the traditional coating machine technology path. Laboratory coating machines are evolving towards higher precision, more intelligent (such as online thickness measurement and closed-loop control), and broader process adaptability to support the development of new materials and new system batteries.
Laboratory coating machines are indispensable tools in the preparation of lithium battery electrodes, and their process optimization is an important bridge between material formulation and battery performance. By precisely controlling the coating parameters, uniform, defect-free electrode coatings can be obtained, providing a reliable basis for evaluating material properties and optimizing battery designs. An in-depth understanding of the interaction of various parameters in the coating process and its impact on the microstructure and macroscopic performance of the electrode pieces is of practical significance for promoting the advancement of lithium battery technology.