Battery manufacturing can be divided into three stages. The first stage is electrode production (including slurry mixing, electrode coating, drying, rolling, slitting and making electrode), the second stage is battery cell assembly (including winding/stacking, shelling, liquid injection and sealing), and the third stage is battery cell activation (including formation, capacity division, detection and sorting). As one of the key components of the battery, the battery electrode design, material selection and preparation process directly affect the comprehensive performance of the battery.
In the battery manufacturing process, the coating process plays a key role. The quality of pole piece coating, such as coating thickness uniformity, surface density distribution and defects, has a great impact on the consistency, cycle life, energy density, safety performance and other aspects of the battery.
In order to improve the process quality of pole piece coating and improve coating efficiency, we must first understand the development of coating and select a suitable coating method. Secondly, we must reduce the cost of experimental trial and error through process simulation, explore the factors affecting coating quality, and achieve the purpose of guiding production by optimizing and improving various parameters. Finally, based on online detection technology, the quality of coating is monitored online to avoid production defects caused by uncontrollable factors such as human and environmental factors.
This article will discuss the research status of coating process in battery manufacturing from three aspects: coating method, coating process simulation and coating detection, so as to promote the improvement of electrode coating process quality, coating efficiency and production quality control.
Electrode coating process simulation
The purpose of coating process simulation is to study the influence of structure, process and slurry parameters on the coating process, and to guide production by optimizing these parameters. At present, the simulation object related to the coating process is slit extrusion coating, because this coating method is currently the mainstream coating method on the market.
In order to deeply analyze the influence of various factors on the coating results, the coating simulation research is generally divided into two stages: internal flow field simulation and external flow field simulation. The internal flow field refers to the process in which the stirred slurry enters from the feed port through compression or other means and reaches the die slit for extrusion; while the external flow field refers to the process in which the slurry is extruded from the die slit onto the moving copper foil or aluminum foil.
2.1 Internal flow field simulation
The internal flow field of slot extrusion coating refers specifically to the dynamic area between the slurry flowing from the feed port to the die gap. The core purpose of the simulation is to deeply explore the key factors that affect the flow field distribution of the slurry inside the die, and to achieve uniform velocity distribution of the slurry at the die outlet through fine parameter adjustment and optimization. These key factors mainly include the position of the feed port, the structural design of the internal cavity of the die, and the structural configuration of the gasket. By comprehensively considering these factors, the flow characteristics of the slurry can be more accurately controlled and the coating quality can be improved.
2.1.1 Feed port position
The position of the feed port has a decisive influence on the flow path and flow velocity distribution of the slurry after entering the die head, which is directly related to the uniformity of the slurry at the outlet. Bai Ziyan conducted a simulation analysis to explore in detail the flow field characteristics inside the die head when the feed port is located at the bottom end face and the side end face of the die head, and found that under the same feeding conditions, bottom feeding can achieve a more uniform distribution of slurry outlet velocity and pressure than side feeding. Jung et al. analyzed the influence of the feed port position at the center and end face of the cavity, as well as the periodic pulsating flow of the feed on the slurry flow and lateral outlet uniformity, and found that different feed ports will change the outlet flow rate and the transient response of the flow rate under steady-state conditions. When feeding at the center of the cavity, the uniformity of the slurry at the outlet is higher, as shown in the following figures (b) and (d).
2.1.2 Cavity structure
The cavity structure parameters have a great influence on the flow of slurry inside the die and its uniformity at the outlet. Starting from the cavity structure parameters, the researchers used simulation software to explore its influence on the uniformity of the fluid at the outlet.
Based on computational fluid dynamics (CFD), the flow field inside a double-cavity extrusion die was simulated, and the influence of different cavity sizes on the uniformity of the slurry velocity at the outlet was explored. When the ratio of the diameters of the two cavities remains unchanged, increasing the cavity diameter can effectively improve the uniform distribution of the slurry velocity at the outlet, as shown in Figure (g) below, and a simplified model of the relationship between the inlet pressure and the cavity structure parameters was established. Some people simulated and compared the influence of three different cavities [double cavity, gradient cavity and hanger cavity, as shown in Figures (a) to (c) below, on the uniformity of the slurry at the structural slit, and found that the transitional and hanger cavity structures have higher homogenization capabilities for the slurry, as shown in Figures 8 (d) to (f) below. Different cavity structures affect the pressure distribution of the slurry inside the die head. Through simulation modeling, the structural parameters are optimized to achieve uniform distribution of the slurry inside the cavity, thereby improving the uniformity of the slurry at the outlet and further improving the coating quality.
2.1.3 Gasket structure
The thickness and shape of the gasket directly affect the thickness of the coating and the uniformity of the slurry at the outlet. By analyzing the distribution of the slurry at the outlet under different structural parameters, the optimal gasket structure can be determined to achieve the desired coating effect. Han et al. explored the double-stripe coating process, as shown in Figure (a) below. By designing the structure of the gasket, the corresponding coating type can be achieved. The analysis, simulation and experimental verification of different gasket structures (uniform, shrinkage, and divergence) were compared. It was found that different gasket structures can control the velocity distribution of the slurry at the die outlet [(b)], and then control the edge shrinkage of the coating in the lateral direction. At the same time, it was found that the shrinkage and expansion gasket structures have a greater impact on the coating quality than uniformity.
Jin et al. studied the flow characteristics of slurries at different shear rates in the die. By changing the geometric shape of the gasket [(c)], it was found that optimizing the structural design of the gasket can reduce the uneven distribution of the slurry at the die outlet, as shown in Figure (d) below. By optimizing the die head gasket structure design, different types of coating effects can be achieved and the distribution of the slurry at the outlet can be adjusted to achieve the desired coating thickness and uniformity, thereby improving the coating quality.
This section systematically discusses the key factors that affect the slurry flow and outlet uniformity during slot extrusion coating, including the feed port position, die cavity structure, and gasket design. First, the feed port position has a direct impact on the flow path and velocity distribution of the slurry. Secondly, the design of the die cavity structure is also crucial to the slurry flow and outlet uniformity. Different structural parameters will have a significant impact on the slurry velocity distribution. Through simulation and optimization simulation, the uniform distribution of the slurry at the outlet can be achieved. Finally, the design of the gasket has also been proven to play an important role in the coating effect. By adjusting the thickness and shape of the gasket, the desired coating effect can be achieved, and the thickness and uniformity of the coating at the outlet can be controlled. In the coating process, the internal flow field is the first and primary stage, which can affect the subsequent coating effect. Only by ensuring that the slurry is evenly distributed at the outlet can the optimization and improvement of the second stage be effectively carried out. Therefore, optimizing the distribution of the internal flow field is crucial to improving the coating effect.
2.2 External flow field simulation
The external flow field is the second stage of coating. The die structure and process parameters (coating speed, die gap and slurry viscosity) directly affect the coating quality. Through numerical simulation, we can quantitatively explore the mechanism and effect of these factors on the coating effect. Through simulation, we can optimize the die structure parameters, find and determine the best process parameters (i.e. coating window), and achieve high efficiency of the coating process.
2.2.1 Die structure
The die shape and angle directly affect the coating quality and speed by affecting the formation of coating beads and the internal flow structure. Optimizing the die lip parameters is also the key to improving coating quality. Huang et al. proposed a bite edge die structure that expands the stable coating window. They analyzed three different die structures [(a)] and found that the maximum coating speed of the underbite concave die was 25.6% higher than that of the normal structure, as shown in Figure (b) below. In order to further analyze the effect of the die lip parameters on the coating, the effect of the die angle on the coating quality was explored without changing the die structure parameters.
Lee et al. analyzed the effect of the die head on the coating at different angles based on the viscous capillary model, and used the finite element method to calculate and analyze that the inclination angle of the die lip in transient and steady states [(c)] can control the pressure gradient inside the fluid. The analysis results are shown in Figure (d) above. Under periodic disturbances, the uniformity of the coating can be affected by controlling the shape and position of the upstream and downstream meniscus. Under the premise of determining the structural parameters, it is necessary to seek a stable operating area, that is, a stable coating window without defects.
2.2.2 Coating Window
The stable coating window mainly includes the coating speed range that can be achieved under different flow rates, as shown in Figure (a) below. Lee et al. studied the coating process of Newtonian and non-Newtonian fluids under different operating conditions through a simplified visco-capillary model and proposed an operational stable coating window. They proved that the simplified visco-capillary model can be used to predict the flow state of the coating system. An unstable coating window will lead to coating defects, as shown in Figure 11 (b) below. The causes of the defects can be explored through numerical simulation methods.
Chang et al. studied the minimum coating thickness of fluids with different viscosities and found that there is a critical Reynolds number in three regions. The study found that the coating area is mainly determined by the position of the downstream meniscus, and the type of coating defects is determined by the position and shape of the upstream meniscus. Huang et al. studied the formation mechanism of coating defects through numerical calculation and experimental comparative analysis, and simulated three types of defects: stripes, spots, and irregular defects. As shown in Figure (c) above, it was found that stripe defects are related to the contraction of the downstream meniscus, and point and irregular defects are related to the unstable development of the jagged dynamic contact line. How to improve coating efficiency under a stable coating window is also a direction that the current manufacturing industry continues to explore.
2.2.3 Double-sided coating
Traditional single-sided slit extrusion coating is a discrete process. Each side of the foil must undergo a separate and complete coating process. The first coated side must go through two layers of drying. In comparison, double-sided coating is more efficient than single-sided coating, and it is less likely to produce drying defects. Tan et al. studied a new type of double-sided simultaneous coating method with a contact slit die, as shown in the figure below. The support roller was replaced by a contact die. Through the comparison of simulation and experimental results, it was found that the contact die can effectively suppress the gap fluctuation caused by the movement of the foil during the second side coating, and an effective numerical model was established to analyze the influence of coating thickness and speed on coating uniformity. This coating method analyzes the influence of various factors on coating quality through simulation, reduces production costs and improves coating efficiency.
As the key stage of coating, the external flow field has many factors that affect the quality of the coating. The first is the shape and angle of the die lip, because it affects the formation and flow state of the coating beads. The second is that the coating process parameters must be within a stable coating window, including coating gap, coating speed and feed speed. If they are not within the stable coating window, coating defects will occur. Finally, double-sided coating is introduced to improve the efficiency of coating.
Through the means of numerical simulation, the influence of various factors on the coating effect can be explored and analyzed, which can guide the current battery pole coating production process to a large extent, and can effectively optimize the process parameters and improve the coating quality and efficiency. By establishing a multi-scale, multi-physical field coupled simulation model, the simulation results are closer to actual production. A simulation system is established to combine the internal and external flow fields for simulation. After inputting the corresponding production requirement parameters, the optimal process parameters can be provided, thereby achieving the purpose of improving coating efficiency and reducing production costs.
In the actual coating process, it is difficult to completely avoid defects and other problems. In order to ensure the quality of pole coating, in addition to optimizing the coating process, it is also necessary to use detection methods to ensure the coating quality.
