Abstract: Lithium-ion battery packs have the characteristics of large slurry viscosity, thick coating, thin substrate, and high precision requirements. Currently, slit extrusion coating technology is widely used. Finite element analysis using experiments and fluid dynamics Methods The initial stage flow field of lithium-ion battery anode slurry on the copper foil substrate was analyzed. The results showed that the thickness of the simulated coating was consistent with the experimental results, indicating that the calculation model was reliable. When the slurry inlet velocity was At 0.035 m/s, the slurry removed by the substrate in the external flow field area can be replenished in a timely manner, and both the upper flow channel and the lower flow channel can be stabilized in the shortest time. This is the optimum coating operation range.
The pole piece production process is the basic process for manufacturing lithium-ion power batteries. The requirements for equipment accuracy, level of intelligence, and reliability of production performance are very high. At present, the lithium ion power battery industry has widely adopted slit extrusion coating. Technology. Slit coating is an advanced, pre-measured coating technique. The fluid fed to the extrusion die is completely coated on the substrate. For a given loading rate, coating width, substrate speed The coating quantity of the coating can be more accurately estimated irrespective of the rheological properties of the slurry fluid. However, in the actual process, the uniformity of the coating solution, stability, edge and surface effects are affected by the flow of the coating solution. Change characteristics, and thus directly determine the quality of the coating. Lithium-ion power battery pole coating process has its own characteristics: Double-sided single-layer coating; Slurry wet coating is thick, generally 100 ~ 300μm; High coating accuracy; The coated substrate is aluminum foil or copper foil with a thickness of 6-30 μm. Relatively few research reports on the coating characteristics of lithium-ion battery poles. Schmit et al. studied the negative electrode slurry extrusion of lithium-ion batteries. Coating process during coating The stability of the coating was found in the intermittent coating and continuous coating process, and the effect of process parameters on the thick edge phenomenon was analyzed. Later, they established a set of experimental devices, in the extrusion type The pressure drop of the slurry fluid was measured during the coating process and the relationship between the fluid pressure drop and the wet thickness of the coating was studied.
In this paper, Lithium-ion power battery graphite negative electrode slurry as a research object, analysis of the basic quality of the negative pole piece production, observation of the coating at the beginning of the film shape, while using fluid dynamics software Fluent lithium ion battery slurry coating early The flow field is subjected to finite element simulation to analyze the flow of slurry from the coating start time to the coating stabilization time, thereby visually observing the coating state of the slurry, studying the influencing factors of the coating stability, and providing a theoretical basis for coating process optimization. stand by.
1 Experimental method and finite element model
1.1 Experimental Methods
Our company has established a lithium-ion battery production demonstration line with a daily output of 20,000 Ah. The negative electrode slurry mixer is a homemade G45-100-2D-DZ vacuum mixer with an effective volume of 100L. The negative electrode coating machine is a self-made M12-650B-4C-DZ type. Slot extrusion coating machine. The coating feeding system adopts 2NBL20F type screw pump of Japan Bingshen Company. It adopts a circular cavity single slot die for coating, the upper die head, 0.55mm thick slit The spacers were placed on the water platform after the lower die assembly was completed. The slit dimensions were measured using a KEYENCE VHX-1000 optical microscope 'Figure 1(a)'. The results are shown in Figure 1(b). The size w is (543.5 ± 7.5) μm. The middle size of the slit is small, and the two sides are slightly larger. This slit size distribution can obtain a uniform coating.
Mixing and stirring graphite, conductive agent, sodium carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR) and distilled water to prepare a negative electrode slurry for lithium ion batteries. The volume of slurry per batch is 68L, and the solid matter content is 52.0%. The slurry density is (1 450±22) kg/m 3. The coated substrate is a 10 μm thick copper foil with an areal density of 8.9 mg/cm 2. Before the coating is officially started, the screw pump feed is first opened to block the die head. At the exit of the slit, the die return valve was opened, and the slurry was circulated in the die for 20 min to ensure that the die cavity was filled with fluid. Figure 2(a) is a schematic diagram of the flow field between the die and the substrate after coating is stabilized. The main parameters include coating gap H, slit size w, coating speed v, feed flow Q, coating wet thickness h, and coating width B. In this experiment: H=0.20mm, w=0.55 mm, L= 0.275 mm, B = 250 mm, v = 0.15 m/s, Q = 4.8 × 10 - 4 m3/s. The pole pieces with a length of about 500 m are wound into one roll for both the A-side and the B-side. Next, cut the head and tail pole piece, take a circular pole piece with a diameter of d=60 mm, measure the sample mass M, and calculate the surface density of the coating according to equation (1).
(1)
Where: Scoat is the areal density of the coating; Scopper is the areal density of the substrate copper foil.
1.2 Finite Element Model
The fluid flow state between the extrusion die and the coating roller was simulated using Fluent6.3.26 fluid mechanics finite element software. The coating flow field was as shown in Figure 2(a). The inside of the extrusion die slit was In calculation area 1, the external area between the slit exit and the substrate is computation domain 2, as shown in Fig. 2(b). A two-dimensional planar model is used. The calculation domain entrance is set as the velocity entrance, and the exit is set as the pressure exit. The pressure value is 101325 Pa. The substrate is set as the moving boundary, the moving speed is the coating speed v, and the other boundary of the die head sets the static boundary conditions. The calculation domain meshes are shown in Figure 2(c). The average size is 0.01mm.
The coating flow field state is an incompressible two-phase unsteady flow of air and slurry, regardless of the heat transfer process. The VOF model is used to track the free flow interface of the slurry '7'. CICSAM is selected due to the large difference in viscosity between the slurry and the air. Interface capture technology. Assume that the static contact angle between the negative electrode slurry and the substrate copper foil is 50°, and the contact angle with the outer wall of the extrusion die is 60°. The initial slurry liquid fills the extrusion die gap' (Figure 2) ( b) in the middle of surface 1 ', but there is no spill outside the slit. After the coating flow field is calculated, the slurry flows out of the slit at a steady rate.
2 Results and Discussion
2.1 Experimental results
Fig. 3 shows the distribution of coating surface densities on the A side and the AB side of the prepared negative electrode plate. The surface density of the A side coating is (9.67±0.067) mg/cm2. The areal density of the coating on both sides of the AB is (19.32±0.084) mg/cm2, the amount of pole piece coating is uniform and meets the requirements of pole piece quality, which shows that the coating process is stable and reliable.
Figure 4 shows the topography of the pole piece coating at the initial stage of coating. The pole piece is at the coating start position at 0cm. At the start of coating, the slurry delivered does not form a stable supply. The die slits flow out of the slurry on the pole piece. The intermittent coating was formed. As the coating progressed, the slurry supply was gradually stabilized, the coatings were continuously connected to each other, and the uncoated area was gradually reduced. A stable coating was formed on the pole piece at 90 cm. The coating speed was 0.15 m. /s, The total time from the start of coating to the coating is 6 s. This process consists of two phases: (1) The slurry forms a stable slurry flow state in the cavity of the pipe and extrusion die, in the slit The outlet forms a stable slurry outflow rate, which is the stable flow process of the internal flow field of the extrusion die; (2) The slurry flows out of the die slit and interacts with the substrate, and the slurry generates viscous force due to the movement of the substrate. Spread on the surface of the substrate, and finally form a stable coating, that is, the steady flow process of the external flow field of the extrusion die.
2.2 Preliminary Analysis of Flow Field
During the flow of the slurry in the outer flow field of the slit, it is affected by forces that affect each other, including 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 from the extrusion die to the moving substrate. Inertia force formed by the process, the gravity of the fluid. In the actual coating process, the shear rate γ can be estimated by equation (2):
2.3 Simulation results
In the simulation process, the laminar flow model is adopted for the viscosity. The simulation assumes that the viscosity of the negative electrode slurry does not change. The material parameters of the negative electrode slurry used, the geometric parameters of the die, and the process parameters are shown in Table 1. The slurry inlet velocity is selected as 0.030, 0.035 and 0.050. m/s three values to study the effect of process parameters on coating results.
Fig. 5, Fig. 6, and Fig. 7 show the flow of slurry at various times when the inlet velocity is 0.030, 0.035, and 0.050 m/s, respectively, from the start of coating to the stabilization of the coating flow field. After the flow field is stabilized, the slurry at the outlet is The distribution of volume fraction along the x-axis (VOF) is shown in Fig. 8. From Fig. 8, the thickness of the coating at VOF=1.0 and VOF=0.5~0.6 is known. The results are listed in Table 2, and Reynolds flows at different speeds. The number Re, the flow field stabilization time t are listed in Table 2. In this production practice, the slurry flow rate Q is 4.8×10 - 4m3/s, the width B of the slit and coating is 0.25m, and the actual slurry slot flow out The speed U = Q/(Bw) is 0.035m/s. The wet thickness h of the coating can then be calculated according to equation (5):
When the inlet velocity is 0.035m/s, the time from the start of calculation to the stabilization of the flow field is the minimum, which is 37.54 ms. The time required to form a uniform coating in the experiment is 6 s (Figure 4), much larger than the settling time of the simulation flow field. This is due to the fact that during the actual coating, the stabilization stage includes the stability of the flow field within the die and the stability of the external flow field of the die, but this calculation mainly simulates the stability of the external flow field. No matter whether the inlet velocity increases or decreases, the coating flow The field settling time has increased. When the inlet velocity is 0.030 m/s, the flow field settling time is 48.75 ms. When the inlet velocity is 0.050 m/s, the flow field settling time is 63.46 ms.
At an inlet speed of 0.030 m/s, the slurry from the slit fills between the die and the substrate at 10 ms after the start of coating 'Fig. 5(a)', while the substrate moves in the positive direction along the y axis. The resulting viscous force causes the slurry to follow the substrate. As the slurry moved away from the substrate cannot be replenished in time, a large amount of air is trapped in the coating 'Fig. 5(b)', and the air entrained slurry is finally at the base. The coating shown in Fig. 5(c) is formed on the material and its morphology is similar to that of the coating shown in Fig. 4. With the continuous supply of slurry, the flow field (y>0) on the flow field is basically stable, and the flow field Downstream channel area (y<0)也由复杂状态逐步趋于稳定, 如图5(d)所示, 最后形成比较稳定的涂布流场[图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 carried by the substrate can be sufficiently replenished in time, and the coating will not be entangled in a large amount. In the air, the downflow flow field quickly reaches a steady state 'Fig. 6(b)', and the flow of the upper flow path creates an unstable state under the gravitational disturbance 'Fig. 6(b) and (c)', but with the coating The cloth is continuously carried out, and the upper flow channel also reaches a steady state quickly. Fig. 6(d) and (e)'. Therefore, under this condition, the coating flow field has a short stabilization time, which is the optimum coating process operation range.
When the inlet velocity is 0.050 m/s, the slurry supply is sufficient, and a large amount of air will not be entrained from the down flow field (Figure 7(a) and (b)', and the flow field of the downflow channel can reach a steady state quickly”. (b)'. However, due to the large inlet velocity, a relatively thick coating is formed (Table 2). The flow field in the upper flow path is easily affected by gravity and takes a long time to reach the ''Fig. 7(c)'' thick coating formation. The gap results in a rapid collapse of the flow field in the upper flow channel (Fig. 7(d)'), which takes a longer time, about 63.46 ms, and the coating flow field reaches a steady state. Fig. 7(e)'.
3 conclusions
Through the above experiments and finite element analysis results, the following conclusions are drawn:
(1) The lithium ion battery anode slurry was coated on the copper foil with a squeeze coater. The surface density of the A-side coating was (9.67±0.067) mg/cm2, and the areal density of the two-side AB coating was (19.32±0.084). ) mg/cm2, the amount of pole piece coating uniform, stable and reliable coating process.
(2) The finite element software Fluent of fluid mechanics was used to simulate the flow state of the slurry in the coating field. The thickness of the coating obtained by the simulation was in good agreement with the experimental results, which showed that the calculation model was reliable.
(3) The initial flow field conditions at the inlet velocity of 0.030, 0.035, and 0.050 m/s were simulated. When the inlet velocity is 0.030 m/s, the slurry in the initial stage is too late for the substrate to move. A large amount of air is entrained in the coating of the lower runner area, resulting in complicated fluid conditions in the upper and lower runners. The coating flow field takes a long time to stabilize. When the inlet speed is 0.050 m/s, the slurry supply is sufficient, and the down runner can Faster to reach stability, but the upper runner needs a longer time to reach a steady state due to the thick coating. When the inlet speed is 0.035 m/s, the coating flow field reaches a steady state faster, and the required time is the shortest. This is the best. Coating process operating range.
