Coating process is a process based on the study of fluid properties, which involves coating one or more layers of liquid onto a substrate. The substrate is usually a flexible film or backing paper, and then the coated liquid coating is dried in an oven or cured to form a special functional film layer.

The coating process of lithium-ion power battery electrodes has its own characteristics: double-sided single-layer coating is carried out sequentially, and most of the double-sideds coating machine on the market are also carried out sequentially on both sides; The wet coating of the slurry is relatively thick, generally ranging from 100 to 300 μ M; The slurry is a non Newtonian high viscosity fluid; Compared to general coating products, the precision requirement for polar film coating is high, which is similar to the precision of film coating; Coating substrate with a thickness of 4-30 μ M's aluminum or copper foil. At present, the lithium-ion power battery industry has widely adopted narrow slot extrusion coating technology to manufacture battery electrodes. Extrusion coating technology can obtain high-precision coatings and can also be used for high viscosity fluid coating. It is widely used in many fields such as flexible electronics, functional films, flat panel displays, micro nano manufacturing, printing, etc. In the actual process, the uniformity, stability, edge and surface effects of the coating solution are affected by the rheological properties of the coating solution, which directly determines the quality of the coating. Numerous researchers have devoted themselves to the study of coating windows through theoretical analysis, coating experimental techniques, fluid dynamics finite element techniques, and other research methods. Coating windows refer to the range of process operations that can achieve stable coating and uniform coating. They are influenced by three factors: (1) fluid properties, such as viscosity μ、 surface tension σ、 density ρ； (2) Geometric parameters of the extrusion die head, such as coating spacing H and die head slit size w; (3) Coating process parameters, such as coating speed v, slurry feeding flow rate Q. For extrusion coating, there is an upper limit and a lower limit of coating speed at a fixed flow rate, and the range between the upper and lower limits of coating speed is the coating window. The upper limit of the coating window is mainly affected by the stability of the coating liquid. For example, when the flow rate is insufficient or the coating speed is too fast, the coating liquid droplets begin to become unstable, which is prone to defects such as air infiltration and lateral waves. When the lower limit of the coating window occurs, if the flow rate is too high or the coating speed is too slow, the fluid cannot be carried away in time, and a large number of coating droplets accumulate, which can easily form water blockage or vertical flow.

The schematic diagram of narrow slit extrusion coating is shown in Figure 1. A certain flow rate of slurry enters the internal cavity of the die from the feeding port of the extrusion head and forms a stable pressure. The slurry is finally sprayed out at the narrow slit outlet of the die head and coated on the foil. The coating roller rotates and moves forward with the foil. The coating is dried in an oven.

Picture 1: Schematic diagram of slit extrusion coating

Generally, the coating machine uses a servo motor with a precision planetary reducer to directly drive the coating roller. A servo motor converts the input signal into angular displacement or angular velocity output of the shaft, and the direction and speed of the shaft change with the voltage direction and magnitude of the signal (in pulse form). Therefore, the power fluctuation of the servo motor can also affect the coating accuracy. The ordinary single-phase thyristor rectifier power supply has a very high fluctuation capacity during low-speed coating, and this fluctuation will greatly reduce the coating quality.

At the same time, the battery slurry supply system can also cause unstable coating volume, because the low precision feeding valve, pneumatic diaphragm pump, and unstable pump pressure can cause the flow rate and pressure of the slurry at the mold inlet to be unstable, resulting in changes in the slurry flow rate at the narrow slit outlet of the mold.

In short, the main purpose of coating is to produce a uniform and consistent polar coating. The conditions for uniform and stable coating must meet the following conditions:

(1) The properties of the slurry are stable, without settling, and the viscosity and solid content remain unchanged;

(2) The slurry feeding supply is stable, forming a uniform and stable flow state inside the mold head or on the coating and transfer rollers;

(3) The coating process forms a stable flow field between the mold head and the coating roller within the coating window range;

(4) Stable foil walking, no belt sliding, severe shaking and wrinkles: tension and correction control.

Therefore, the electrode slurry coating equipment has become one of the key equipment for the successful development and production of lithium-ion batteries. The coating accuracy of the coating machine is inevitably related to the viscosity of the slurry, equipment, and coating process. The quality of the coating directly affects the quality of the battery. From the perspective of process flow, coating is a key process in the preparation of battery cells, and from the perspective of equipment value, it is also a key process in the preparation of battery cells (high-end series priced over ten million). From the perspective of nonlinear control, it is also a key process in the preparation of battery cells; The uniformity, consistency, alignment, baking stability, adhesive diffusion, and surface density stability of the coating are closely related to this; The performance of simultaneous coating is directly related to practical indicators such as cost and qualification rate.

In the manufacturing process of lithium batteries, key indicators such as battery capacity, rate, safety, and cycle life are closely related to the coating process.