The general coating process simulation is decomposed into two steps: (1) The internal flow field simulation of the die uses a laminar flow model and a steady-state solution (click to read Lithium battery pole piece coating process simulation: flow field in the extrusion die); (2) The laminar flow transient solver is used for the flow field between the die and the foil. The multiphase flow method tracks the interface between the coating and the air.
This article introduces the second step, which is the work done in 2016. Taking graphite anode slurry as the research object, using fluid mechanics software Fluent to perform finite element simulation on the initial flow field of lithium ion battery slurry coating, analyze the process of slurry flowing from the die outlet to coating stability, and study coating stability Influencing factors of state.
1 The finite element model is determined through experiments. The solid matter content of the graphite anode slurry is 52.0%, and the slurry density is (1450±22) kg/m3. Figure 2(a) is a schematic diagram of the flow field between the die and the substrate. The main parameters include coating gap H, slit size w, coating speed v, feeding flow Q, coating wet thickness h, and coating width B . In this simulation: H=0.20mm, w=0.55 mm, L=0.275 mm, B=250 mm, v=0.15 m/s, Q=4.8×10^-4m3/s.
The fluid dynamics finite element software Fluent6.3.26 is used to simulate the flow state of the external flow field between the extrusion die and the coating roller. The coating flow field is shown in Figure 2(a). Take the inside of the extrusion die slit as the calculation area 1, and the outer area between the slit outlet and the substrate as the calculation area 2, as shown in Figure 2(b), using a two-dimensional plane model, and the calculation area entrance is set as the speed The inlet and outlet are set as pressure outlets, the pressure value is 101325 Pa, the substrate is set as a moving wall, the moving speed is the coating speed v, and other boundaries such as the die outer wall are set to static boundary conditions. The grid division of the computational domain is shown in Figure 2(c), and the average grid size is 0.01mm.
The state of the coating flow field is a two-phase unsteady flow process of incompressible air and slurry, regardless of the heat transfer process. The VOF model is used to track the free-flowing interface of the slurry. Due to the large difference in viscosity between the slurry and the air, the CICSAM interface capture technology is selected. It is assumed that the static contact angle between the negative electrode slurry and the base copper foil is 50°, and the contact angle with the outer wall of the extrusion die is 60°. At the initial moment, the slurry liquid filled the slit of the extrusion die [surface1 area in Figure 2(b)], but did not overflow the outside of the slit. After the coating flow field was calculated, the slurry flowed out from the slit at a stable speed. ,
2 The modeling process uses gambit to draw the geometric model and set the boundary conditions and divide the mesh, as shown in Figure 1. Import the geometric model into the FLUENT solver, and select the two-dimensional implicit transient solver. Select the VOF model to simulate the two-phase flow of air and slurry and capture the two-phase interface
The Reynolds number is relatively low, and the laminar flow model is selected.
Set the density and viscosity of the slurry
3 Results and discussion
3.1 Preliminary analysis of flow field During the flow of the slurry in the flow field outside the slit, the forces that affect each other include the viscous force generated inside the fluid due to the movement of the substrate, the surface force of the fluid, and the impact of the fluid flowing out of the extrusion die. The inertial force formed by the deceleration process of the moving substrate and the gravity of the fluid. In the actual coating process, the shear rate γ can be estimated by the formula (1): where: v is the coating speed, with a value of 0.15 m/s; H is the coating distance, with a value of 200×10^-6m; then γ =750 s^-1. In the finite element calculation, it is assumed that the viscosity of the negative electrode slurry does not change. It is reported in the literature that at this shear rate, the viscosity of the lithium ion anode slurry is 1 Pa·s.
In the coating flow field, the Reynolds number Re and the capillary number Ca can be respectively defined by equations (2) and (3): where: Reynolds number Re represents the ratio of fluid inertial force to viscous force. In this paper, when the computational domain entrance velocity v=0.035m/s, the Reynolds number Re = 0.002 4, and its value is much smaller than 1, which indicates that the inertial force formed by the impact of the slurry on the substrate has little disturbance to the flow field, and the slurry flows The state is a laminar flow process. The number of capillaries Ca represents the ratio of fluid viscosity to surface force. In this paper, Ca=3.597. Due to the high viscosity of lithium ion anode slurry, the effect of viscosity on the flow process is greater than the effect of fluid surface force during the coating process.
3.2 Simulation results During the simulation process, the viscosity adopts a laminar flow model. It is assumed that the viscosity of the anode slurry does not change. The material parameters of the anode slurry used, the geometric parameters of the die and the process parameters are shown in Table 1. The slurry inlet speed is selected as 0.030, The three values of 0.035 and 0.050m/s are used to study the influence of process parameters on the coating results.
There is a process window in the coating process: the coating window is the process operation range that can be applied stably to obtain a uniform coating, which is affected by three types of factors: (1) fluid characteristics, such as viscosity μ, surface tension σ, Density ρ; (2) Extrusion die geometric parameters, such as coating spacing H, die slit size w; (3) Coating process parameters, such as coating speed v, slurry feed flow rate Q, etc.
For extrusion coating, at a fixed coating speed, there are upper and lower limits for the feed flow rate, and the range between the upper and lower limits 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 beads become unstable, and defects such as air infiltration and transverse waves are prone to occur. When the lower limit of the coating window occurs, if the flow rate is too large or the coating speed is too slow, the fluid cannot be taken away in time, and a large amount of coating liquid beads accumulate, which is easy to form water suffocation or vertical flow.
Fig. 5, Fig. 6, Fig. 7 are respectively the flow state of the slurry at different moments from the start of coating to the stabilization of the coating flow field when the inlet velocity is 0.030, 0.035 and 0.050m/s. After the flow field is stable, the volume fraction (VOF) distribution of the slurry along the x-axis at the outlet is shown in Figure 8. From Figure 5, it can be seen that the thickness of the coating is calculated when VOF=1.0 and VOF=0.5~0.6, and the results are listed in the table 2. At the same time, the flow field Reynolds number Re and the flow field stabilization time t under different speed conditions are listed in Table 2.
When the slurry inlet velocity is reduced from 0.035 m/s to 0.030m/s, the resulting coating thickness is reduced by 10×10^-6m, and when the inlet velocity is increased to 0.050 m/s, the coating thickness increases by 60× 10^-6m.
When the inlet velocity is 0.035m/s, the time from the beginning of the calculation to the stabilization of the flow field is the smallest, which is 37.54 ms. Regardless of whether the inlet velocity increases or decreases, the coating flow field stabilization time increases. When the inlet velocity is 0.030m/s, the flow field stabilization time is 48.75 ms, and when the inlet velocity is 0.050 m/s, the flow field stabilization time It is 63.46ms.
When the inlet speed is 0.030m/s, 10 ms after the start of coating, the slurry flowing out of the slit is filled between the die and the substrate [Figure 5(a)], and the substrate moves in the positive direction along the y-axis. The resulting viscous force causes the slurry to move with the substrate. Since the slurry taken away by the substrate cannot be replenished in time, a large amount of air is drawn into the coating [Figure 5(b)]. The coating as shown in Figure 5(c) is formed on the material. With the continuous supply of slurry, the upper flow channel area (y>0) of the flow field is basically stable, and the lower flow channel area (y <0) of the flow field also gradually stabilizes from a complex state, as shown in Figure 5(d). A relatively stable coating flow field is formed [Figure 5(e)].
When the inlet velocity is 0.035m/s, after the slurry fills the area between the die and the substrate [Figure 6(a)], the slurry taken away by the substrate can be replenished in time and a large amount will not be involved in the coating The flow field of the lower flow channel quickly reaches a stable state [Figure 6(b)], and the flow field of the upper flow channel will produce an unstable state under the interference of gravity [Figure 6(b) and (c)], but with the coating The cloth continued, and the upper runner soon reached a stable state [Figure 6(d) and (e)]. Therefore, under this condition, the coating flow field has a short stabilization time, which is the optimal coating process operating range.
When the inlet velocity is 0.050m/s, the slurry supply is sufficient, and a large amount of air will not be drawn into the flow field of the lower runner [Figure 7 (a) and (b)], and the flow field of the lower runner can quickly reach a stable state [Figure 7 (b)]. However, due to the high inlet velocity, a thicker coating is formed (Table 2). The flow field of the upper runner is easily affected by gravity, and it takes a long time to stabilize [Figure 7(c)]. The thick coating forms a gap and causes the upper runner to flow. The field collapsed quickly [Figure 7(d)], after a long time, about 63.46ms, the coating flow field reached a steady state [Figure 7(e)].
