In industrial production, the lithium battery pole piece is generally compacted by continuous rolling of the roller machine, the process is shown in Figure 1.
Figure 1 Schematic diagram of the pole piece rolling process
After the pole piece is compacted, the porosity of the coating changes from the initial value εc,0 to εc. In a previous article, "Basic Analysis of Lithium Battery Pole Rolling Process": Lithium-ion battery pole piece The compaction process also follows the index formula (1) in the powder metallurgy field, which reveals the relationship between coating density or porosity and compaction load.
(1)
Where ρc,0 is the initial value of the coating density, ρc is the density of the coating after compaction. qL is the line load acting on the pole piece, which can be calculated by equation (2):
qL=FN/WC (2)
FN is the rolling force acting on the pole piece, WC is the width of the pole piece coating. ρc, max and γC can be obtained by fitting the experimental data, respectively indicating the maximum compaction density that the coating can reach under a certain process condition and coating Laminated real impedance.
Convert the compaction density into porosity, and the exponential formula (1) is transformed into the formula (3):
(3)
Reference '1' based on the above compaction process model, the effects of different active materials and different areal densities on the compacted porosity of the pole pieces were investigated. The particle size distribution and morphology of the raw materials are shown in Table 1. The parameters of the pole piece composition and the areal density are shown in Table 2. No. 1 is a mixture of two different particle sizes of NCA1 and NCA2, and No. 2-5 is NCA1, NCM811, NCM622, NCM111, respectively. Different materials, the same slurry composition and areal density, single-sided coating 223g/m2. No.6-12 is a one-pot slurry, coating different areal density. No.13-15 is another report in the literature.
Initial porosity and minimum porosity prediction
The theoretical porosity of a simple cubic incompressible hard particle simple cubic stack is 47.64%, and the actual lithium ion battery pole pieces No. 1–5 and 7–12 initial porosity are basically 42-48%, with theoretical values. Slightly biased, this is mainly because on the one hand the particles are not ideal spherical, on the other hand there are binders and conductive agents in the coating. The initial porosity of No. 6 and 13–15 is relatively high, No. 6 Because the surface density is relatively low, the initial porosity is high, and from the No. 6-12 pole piece, as the surface density of the pole piece increases, the initial porosity gradually decreases, but the reduction is smaller and smaller. The thick coating is dry. During the process, the upper layer exerts gravity on the lower layer to make the coating density higher. No.13-15 pole piece has high initial porosity because of higher binder and conductive agent content and higher coating porosity. In addition, the morphology of the active material will also affect the initial porosity.
Figure 2 Initial porosity and predicted minimum porosity
In Figure 2, the lowest porosity is also predicted, including:
(1) The minimum porosity obtained in the experiment with a minimum roll gap of 25 microns ε C, min_a,
(2) Fitting the predicted minimum porosity ε C, min_e according to formula (3)
(3) ε C, min_p = p∙ & epsilon; C, assuming p = 0.4 predicted minimum porosity.
The porosity of the simple cube stack is 47.64%, and the porosity of the dense cube stack is 25.95%. Assuming the compaction process, the particle stacking method is changed from a simple cube stack to a close packed cubic stack, at which point p=0.54. To the deviation between the actual situation and the theory, it is reasonable to take P=0.4.
Apply εC,min_p=p∙εC, assuming the minimum porosity predicted by p=0.4 to the compaction process model, and formula (3) becomes formula (4):
(4)
Effect of Active Substance Type on Compaction Impedance γ
Figure 3 is a graph showing the relationship between the porosity and the line load after compaction of No. 1-5 different active material pole pieces, wherein the data points are experimental values, and the line is a curve fitted according to formula (4). What is the effect of each variable and random error? Statistically, the difference between the data point and its corresponding position on the regression line is called the residual. The square of each residual is added as the sum of squared residuals, which means random. The effect of error. NCM111 and NCA in the compaction process, the polar sheet porosity changes regularly, under the same load, NCM111 has a lower porosity. Two different particle size distributions of NCA mixed particles, small particles in Filling between large particles, lower compaction density.
NCM111, NCM622, NCM811 three materials comparison, NCM811 pole piece with the increase of the load, the porosity began to decrease rapidly, due to their larger particle diameter, the initial porosity is also larger.
Fig. 3 Relationship between porosity and line load of different active materials: The experimental value and the fitted line of formula (4), χ2 represents the sum of squared residuals.
The compaction data of these five materials are fitted by the formula (4), and the compaction impedance γ is obtained as shown in Fig. 4. The coating compaction resistance γC represents the resistance against the compaction process, and the larger the value, the harder it is to compact the pole piece. If the pole piece is to be compacted to a certain porosity, the larger the γC is, the larger the line load is required. As can be seen from Fig. 4, the two NCA mixed particles, the small particles are filled between the large particles, and the pole piece compaction is easier. The NCM811 is larger and easier to compact.
Figure 4 Compaction impedance of several materials
Effect of areal density on compaction impedance γ
No.6–12 pole piece, the coating surface density gradually increased from 80g/m2 to 285g/m2, the corresponding coating porosity and loading compaction line load relationship are shown in Figure 5, the data point is the experimental test value. The curve is a curve obtained by fitting according to formula (4). For No. 6–8, the pole piece coating has a low areal density, the initial porosity is relatively high, and the compaction process, as the load increases, the compaction impedance decreases slope. Large, while No.9–12 surface density increases, the initial porosity of the coating decreases, and the slope of the compaction impedance decreases when the load increases.
Fig. 5 Porosity-line load relationship of different compacted density pole pieces: experimental data points and fitting curves
The curve fitting can obtain the compaction impedance of various pole pieces, the compaction impedance γ and the coating areal density MC, and analyze the relationship between them, as shown in Fig. 6. The compaction impedance γ and the areal density are linear. Relationship: γ=μ*MC, in the series of experiments No. 6–12, μ=1.31kN·m/g. As the areal density increases, the coating compaction becomes more and more difficult. For different active substances, pressure The surface density influence factor μ of the real process model is listed in Table 3.
Figure 6: Linear relationship between compacted impedance and areal density
Table 3 Surface Density Influence Factor μ of Compacted Impedance of Different Active Substances
Pole piece compaction process model
According to the above analysis, taking into account the types of active substances, morphology and particle size distribution, and the surface density of the coating, the lithium-ion battery pole piece compaction process model is:
(5)
Where p=εC,min/εC,0 represents the minimum porosity of the pole piece ε the ratio of C,min to the initial porosity εC,0, related to the type and morphology of the particle, for spherical particles , generally p=0.4. γ=μ*MC indicates the compaction impedance of the pole piece, which characterizes the degree of compaction of the pole piece and is related to the areal density MC of the coating. The surface density influence factor of the compaction impedance of different active materials The value of μ is shown in Table 3.
In the article "Principles and Processes of Lithium Battery Pole Roller", three commonly used lithium ion battery Pole Rollers and their process characteristics are introduced: Manual spiral pressure Pole piece mill, gas-liquid booster pump Pressure type pole piece rolling mill, hydraulic servo pressure type pole piece rolling mill. Among them, when the gas-liquid booster pump pressurized pole piece rolling mill compacts the pole piece, the hydraulic cylinder pressure F set by the equipment parameter is not completely applied to the pole piece. During pole piece rolling, the hydraulic cylinder pressure F is decomposed into the force acting on the wedge iron between the upper and lower rolls and the effective rolling force acting on the pole piece. Special attention is required when applying the compaction process model.
This paper summarizes and summarizes the compaction process experiments of several common lithium ion battery cathode materials, and proposes process model parameters, prediction and optimization of process parameters, but the actual process is often more complicated. This paper is for reference only. .
