Lithium battery polarity is prone to powder

2022/04/08

  Author :Iflowpower – Portable Power Station Supplier

The lithium-ion battery pole peel strength refers to a strong degree of strongly adhering to the electrode active substance and the collector, is one of the tight indicators of the pole. For the detection of the polar peel strength, it can be used to assess the use of the binder reasonably, and the mass of the propellant, the coating process. The peel strength between the coating and the current fluid will affect the cycle performance and internal resistance of the lithium ion battery.

Stripping strength Evaluation method: (1) Qualitative evaluation - Capacity: Hold the cutting tool to make the knife perpendicular to the surface of the test plate, and it has formed a predetermined number of cuttings on the coating in the coating in the coating in the coating in the coating in the coating in the coating. All cuts should be scratched into the surface of the substrate until the substrate; remove the particles scraped by the surface, take the finger to flatten the portion of the viscose on the grid; put the tape for a period of time, hold one end of the tape, And with a 60 ° angle as close as possible, tear the tape in 0.5 seconds to 1 second.

(2) Quantitative evaluation - 180 ° stripping method The extreme side is free to 40 °, sandwiched on the side of the pole free end and the experimental plate, and under the clamp, in the same environment, tensile test machine is 50 mm / The tensile speed of MIN continuously peeling until the polarity and coating are completely separated, and it can be read straight to the electrode sheet. Detection method: The transverse tape in a certain width is passed through the bottom of the scaled steel sheet, and the end surface is flush; the double-sided adhesive is in the translucent tape, the length is the same as the width of the translucent tape, the position is middle; finally The detection sample is attached to the double-sided gel, the end surface is parallel, and the pressure wheel is free to return to the surface of the pole. When the negative electrode sheet of the test sample is not passed, after the upper holder of the tensioner is sandwiched, the negative electrode sheet is peeled off, and the detection of the peel strength when the pole collector and the coating are completely separated.

As a result, the homogeneity value of the stable stage BC segment is used as the peel strength value. Factors affecting peel strength: Detection factors: (1) Effect of polar width on peel strength When the pole width is 20 ~ 35mm, as the negative pole width has increased, the peel strength gradually increases, and the double is used. When the semicake is 25mm wide, when the polar width is 20mm, 25 mm, the peeling process causes the edge of the pole and double-sided adhesive, and the area of ​​the polar double-sided adhesive and the negative electrode coating area is small, the bond strength is rubbed with the adhesive Added by the viscoe-surface contact area, when the pole width is greater than 25 mm, the different width poles are different from the center of gravity of the peeling process, when the pole width is 35 mm, the center of gravity reaches the optimal position, the detection value Stability.

(2) The impact of the stretching distance data range on peel strength is known from the figure, and the stretch distance data range is very tight, and the effect on the peel strength is quite large. Because the detection curve begins with the peak, there is a wave value in the beginning of the detection curve, so the uniform value of a smooth piece of data should be taken as a polar peel strength. It has an abnormality for alert peeling process, and it can be gently peeled off 1 mm before peeling, and then detect, and the stretch distance data is still selected from 25 to 80 mm, and the range of the pole detection curve in this range is stable.

If the probe curve is still unstable, re-sampling detection. When the polar stretching speed is 50 ~ 200mm / min, as the stretching speed increases, the peeling strength decreases, and it tends to be smooth, but from the detected data stability, the stretching speed is 100 mm / min. The standard deviation of the detection result is the least, and the result of the results of the detection is the worst.

This is important because when the stretching speed is too slow, the dual-sided adhesive and pole contact time, the desired peel strength is large, and the stability of the probation is poor. In summary, from the aspects of data accuracy, time cost, and detection stability, select the pole tensile speed of 100 mm / min. (3) The effect of the number of rolls on the peel strength is within a certain range, and the peel strength is substantially changed as the number of rolls, the number of presses on the surface roll of the presses is affected by the peel strength of the negative pole sheet.

Not big, the important principle is the large pressure of the negative electrode plate after the roll pressure, which has caused the active substance and the current fluid to closely bind, and the pressure used in the detection is small, only to bond the pole Use in double-sided plastic, does not change the peel strength of the coating. From the data stability of the detection, when the number of rolls is 3 times, the relative standard deviation is minimized, only 0.96%, so the number of times the pressure wheel on the surface roll pressure is 3 times.

Process factors Effect: (1) After the roll speed is different, the difference is not clear, and the relative standard deviation is only 0.25%; however, the peel strength difference of the polar sheet is divided, and the relative standard deviation is up to 11.89%.

When the roller speed increased from 5 m / min to 70 m / min, the peeling strength was lowered from 10.89 N / m, lowered to 8.64 N / m, and evenly reduced by 20.

66%. This is important because the faster the roller pressure speed is consistent, the shorter the roller pressure speed, the shorter the contact time of the pole and the pressure roller, the larger the polarity rebound, resulting in reduced peel strength to a certain range of roller pressure (5 In ~ 70M / min), when the active material is still viscous, the roll speed is slow, extended the contact time of the pressure roller and the polar sheet, and the pole is extended, thereby improving the peel strength. The faster the roller pressure is, the lower the peel strength of the negative electrode sheet.

When the roll speed is 5 m / min, the intensity of the pole reaches the maximum. (2) Whether the storage time can be seen from Fig. 2, whether it is the negative electrode sheet after the roller pressure, the roller press, the peeling strength decreases with time, and then tends to be smooth, and finally.

After the roller pressure, the pole was stored 11d, and the peel strength was uniformly reduced by 18.99%. After the roller pressure rear pole is stored 11d, the peel strength is evenly reduced by 12.

05%. This is an important part that the sodium carboxymethylcellulose (CMC) moisture absorption in the negative electrode sheet is caused. The CMC has a strong moisture absorbing property, the degree of moisture absorption is related to the atmospheric temperature and relative humidity.

When the balance is reached, it is no longer absorbing. When the pole is placed in a 20% environment of the humidity, the active material in the polar sheet will denature over time, so the negative electrode sheet after the roll is pressed, and to be stored in a vacuum drying box Immediately after the rolling pressure, transfer to the next step, reduce the risk of negative absorbent fits. In summary, the polar peel strength is stored in 1D to the maximum.

(3) The SBR content can be seen from Fig. 4, as the SBR content is increased from 1.25% to 2.

99%, and the roller precipulum peeling strength is increased from 11.07 N / m to 97.33 N / m, and evenly increased by 779.

22. %; Rear tensile peel strength rose from 9.20 N / m to 41.

77 N / m, and evenly increased by 354.02%. But the flexibility of the polarity is also deteriorated and is easy to break.

This is because: As the SBR content in the slurry increases, the density of the bonded agent directly touched directly, the polar peeling strength is increased; 2 With the new SBR content in the slurry, extreme After the film is coated, the binder content has increased in the polar unit volume, making the active material and the conductive agent more tightly, the pore bending capacity is deteriorated, resulting in deterioration of polar flexibility. (4) Skille Process Adhesive In the slurry, it is not a need for averaging the peel strength of the polar sheet. The peel strength of the polar sheet is beneficial to improve the peel strength of the polar sheet by reasonable process.

(5) The high-speed drying of the coating baking system causes the binder to float, and the electrode has a very significant concentration gradient. Regarding this improvement, it can be considered from the drying system. The baking system is divided into three, constant drying rates, increment drying rate, decreased drying rate.

Related to other two baking modes, the decrementing dry rate can make the binder distribution more uniform, and better quality can be obtained while increasing efficiency. The electrode peel strength of the lithium-ion battery has a significant effect on performance, so it is necessary to explore the influencing factors of the peeled strength, this paper is important to discuss the coating process and the impact of slurry. The initial stage of the change of the internal structure of the electrode during drying process, each component is averaged in the electrode.

During the drying process, since the solvent evaporation, the electrode film is contracted, there is a solubility gradient at the gas-liquid interface, and the surface is formed on the surface of the solidified particulate layer. The solvent migrates the surface migration between the graphite particles by capillary force, and is accompanied by particle layer expansion. The SBR adhesive and carbon black particle size are at least 100 times higher than graphite, and the graphite particle spacing is sufficient to allow SBR and carbon black free flow, and the solvent flow will be migrated with the polymer and small particles, so that they are enriched in the upper layer of the electrode , When the solvent is completely evaporated, the electrode film shrinks stop.

The drying process is important to the following three stages: (1) The solvent content of the air-membrane interface is continuously dropped until the critical concentration is reached. (2) From this critical point, the solvent concentration at the air-membrane interface remains the same, and (3) When the solvent is further evaporated, the solvent solvent solvent is lowered at the air-membrane interface until the film formation. The effect of baking rate on the peel strength is known from the drying process.

During the dried process, the SBR binder and carbon black migrates the surface as the solvent evaporates, then the baking rate has the impact of the internal component distribution of the electrode. how about it? The figure below shows the different baking rates, from the concentration of the binder concentration of the air-liquid interface, from the figure, in the case of a low baking rate (LDR), the binder distribution of different positions is relatively average. In the case of a high baking rate (HDR), the concentration of the binder from the collector to the gas-liquid interface is constantly large, and the binder distribution is small in the collector and the dressing area, which will lead the stripping strength to decrease Further, the electrode impedance will also increase accordingly.

Improvement of coating processes When designing high energy density batteries, the electrode thickness is generally large. According to the electrode characteristics of the baking process, Darjenliu et al. Proposed a new type of bilateral coating that separates two different binder content sluries, and the adhesive content of the upper coating is lower than the lower coating.

The increasing coating adhesive content is to make up for the impact of the adhesive to surface migration during the drying process. The figure below shows a different coating method, wherein the baking temperature, 3.0% by weight, 3.

5 wt%, 4.0% by weight, 4.5 wt%, 5.

0% by weight, respectively, indicating a slurry coating of different binder content, respectively. Floor. The impact of the coating surface density on the peel strength, such as the fixed speed of the particles in the electrode structure faster than the compensation flow rate of the solvent to the surface of the electrode, the migration of the binder will greatly decrease, the smaller the solvent in the wet coating, the temperature High, the faster the evaporation speed of the solvent.

It can be seen from the figure below that the low-facing density electrode, the electrode impedance is substantially unaffected by temperature, because the total amount of solvent is low and the electrode structure is cured, the binder does not have a distinct migration. The high-facing density electrode is completely different, and the constant speed evaporation phase is very long, and the migration of the binder is sufficient. As the temperature increases, the adhesive and carbon black migration are intensified, resulting in peel strength Difference, and electrode impedance will also increase.

At 80 ° C, the peel strength of different densities is not large. When reaching 110 ° C, the migration speed will increase, the surface density is increased to 5.4 mg / cm2, then the drying time will increase, which will supply a sufficient migration time to the adhesive, and the surface density is further increased to 12 mg / When CM2, the peel strength will be reduced by 52%.

When the temperature increases to 130 ° C, the migration phenomenon will further exacerbate, resulting in deterioration of peel strength. The reduction in solid content of the slurry to peel strength will result in a decrease in peel strength, because after the solid content is lowered, the temperature to be dried, which will result in the migration of the binder. Interand, resulting in deterioration of peel strength.

Adhesive proportion can also increase peel strength. .

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