Ten Major Problems and Analysis of Lithium Battery Production
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1. What is the reason for pinholes in the negative electrode coating? Is it because the materials are not dispersed well? Is it possible that the particle size distribution of the material is not good?
The occurrence of pinholes in negative electrode coating can be attributed to several factors:
1.1 Impurities on the foil surface.
1.2 Poor dispersion of the conductive agent.
1.3 Inadequate dispersion of the negative electrode active material.
1.4 Impurities in certain components of the formulation.
1.5 Non-uniform distribution and difficult dispersion of the conductive agent particles.
1.6 Non-uniform distribution and difficult dispersion of the negative electrode particles.
1.7 Quality issues with the formulation materials themselves.
1.8 Residual dry powder in the mixing tank due to incomplete cleaning.
To determine the specific causes, it is necessary to conduct process monitoring and analysis. Regarding the issue of black spots on the separator, I encountered it many years ago. I will give a brief answer first. Please correct me if I am wrong. It is indeed caused by localized high temperature in the separator due to polarization discharge during battery operation. This polarization discharge is a result of materials and process-related factors, where active material particles attach to the jelly roll and cause polarization discharge during battery charging. To avoid these problems, it is important to adopt appropriate slurry processes to address the adhesion between the active material and the current collector. Additionally, care should be taken during electrode manufacturing and battery assembly to prevent detachment of the powder. Adding certain additives during the coating process, which do not affect battery performance, can improve specific properties of the electrode. These additives can also be included in the electrolyte to achieve consolidation effects. The localized high temperature in the separator is indeed caused by the non-uniformity of the electrode, which can be considered a form of micro-short circuit. Micro-short circuits can lead to localized high temperature and potential detachment of the negative electrode powder.
2. What are the causes of excessive battery internal resistance?
In terms of process factors:
2.1 Insufficient conductive agent in positive electrode formulation (poor conductivity between materials, as lithium cobalt itself has very poor conductivity).
2.2 Excessive binder in positive electrode formulation (binders are generally polymer materials with strong insulation properties).
2.3 Excessive binder in negative electrode formulation (binders are generally polymer materials with strong insulation properties).
2.4 Non-uniform dispersion of ingredients.
2.5 Incomplete solvent dissolution of binders during ingredient mixing (cannot completely dissolve in NMP or water).
2.6 Excessive coating and slurry surface density (increased ion migration distance).
2.7 Excessive compaction density, over-pressing during calendering (excessive calendering pressure may damage the structure of active materials).
2.8 Weak welding of positive electrode tabs, resulting in virtual welding.
2.9 Weak welding or riveting of negative electrode tabs, resulting in virtual welding or detachment.
2.10 Loose winding, relaxed winding cell (increased distance between positive and negative electrode sheets).
2.11 Weak welding of positive electrode tabs to the casing.
2.12 Weak welding of negative electrode tabs to the electrode column.
2.13 Excessive baking temperature for batteries, causing shrinkage of the separator (reducing pore size).
2.14 Insufficient electrolyte injection (reduced conductivity, increased internal resistance after cycling!).
2.15 Insufficient standing time after electrolyte injection, incomplete electrolyte infiltration.
2.16 Incomplete activation during formation.
2.17 Excessive electrolyte leakage during the formation process.
2.18 Inadequate control of moisture during production, causing battery swelling.
2.19 Excessive charging voltage setting, leading to overcharging.
2.20 Improper storage environment for batteries.
Regarding material factors:
2.21 High resistance of positive electrode material (poor conductivity, such as lithium iron phosphate).
2.22 Influence of separator material (thickness, low porosity, small pore size).
2.23 Influence of electrolyte material (low conductivity, high viscosity).
2.24 Influence of PVDF binder material in the positive electrode (excessive quantity or high molecular weight).
2.25 Influence of conductive agent material in the positive electrode (poor conductivity, high resistance).
2.26 Influence of positive and negative electrode tab materials (thin thickness, poor conductivity, uneven thickness, poor material purity).
2.27 Poor conductivity of copper foil or aluminum foil material, or presence of surface oxides.
2.28 High contact resistance in the riveting of cover plate and electrode column.
2.29 High resistance of negative electrode material.
2.30 Deviation in internal resistance testing equipment.
2.31. Human error in operations.
3. The electrode sheets are unevenly coated. What issues should we pay attention to?
This issue is quite common, and it is relatively easy to solve. However, many coating operators are not good at summarizing, which leads to considering certain existing problems as normal and unavoidable phenomena. First, it is important to understand the factors that affect the coating surface density and the factors that influence the stable value of the surface density in order to solve the problem effectively. The factors influencing the coating surface density include:
3.1 Material factors
3.3 Mixing and blending
3.4 Coating environment
3.5 Doctor blade
3.6 Slurry viscosity
3.7 Speed of the electrode sheet
3.8 Surface flatness
3.9 Coating machine precision
3.10 Oven airflow
3.11 Coating tension, etc.
The factors affecting the uniformity of the electrode sheet include:
3.12 Quality of the slurry
3.13 Slurry viscosity
3.14 Speed of the electrode sheet
3.15 Foil tension
3.16 Tension balance method
3.17 Coating pull length
3.19 Surface flatness
3.20 Doctor blade evenness
3.21 Foil evenness, etc.
These are just some of the factors listed, and it is necessary to analyze the specific situation and systematically eliminate the factors that cause abnormal surface density.
4. Is there any special reason why the current collectors of the positive and negative electrodes are made of aluminum foil and copper foil respectively? Is there any problem with using it the other way around? Many documents using stainless steel mesh directly. Is there any difference?
4.1 The choice of using copper and aluminum as current collectors is primarily due to their good conductivity, soft texture (which can facilitate bonding), relative commonness, and cost-effectiveness. Additionally, both copper and aluminum can form a protective oxide layer on their surfaces.
4.2 The oxide layer on the surface of copper is a semiconductor and allows for electron conduction. If the oxide layer is too thick, it increases impedance. On the other hand, the oxide layer on aluminum, known as aluminum oxide, is an insulator and does not conduct electricity. However, due to its thinness, electron conduction can occur through tunneling effects. If the oxide layer on aluminum is too thick, it can result in uneven conductivity or even insulation. It is generally recommended to clean the current collectors’ surfaces before use to remove oil and thick oxide layers.
4.3 The positive electrode has a higher potential, and aluminum’s thin oxide layer is highly dense, which helps prevent oxidation of the current collector. In contrast, the oxide layer on copper is relatively porous. To prevent oxidation, it is better to keep the potential lower. Additionally, lithium has difficulty forming lithium-copper alloys at low potentials. However, if there is a significant amount of oxidation on the copper surface, lithium can react with copper oxide to form lithium-copper alloys at slightly higher potentials. Aluminum foils should not be used as negative electrodes since they can undergo alloying with lithium to form LiAl alloys at low potentials.
4.4 It is important for the current collectors to have pure compositions. Impurities in aluminum can lead to the formation of non-dense surface films and localized corrosion, and in severe cases, the formation of LiAl alloys due to the damage of the surface film. Copper meshes should be cleaned with bisulfate and rinsed with deionized water before baking, while aluminum meshes should be cleaned with ammonium salts and rinsed with deionized water before baking. This preparation helps achieve good conductivity upon mesh coating.
5. When measuring the short circuit of the winding jelly roll, the battery short circuit tester used can accurately test the short circuit cell at what voltage. Also, what is the high voltage breakdown principle of the short circuit tester?
The voltage used to test short circuits in battery cells depends on several factors, including:
5.1 The manufacturing process and expertise of the company: Different companies may have different standards and practices for voltage testing based on their level of expertise.
5.2 The structural design of the battery itself: The internal components and construction of the battery can influence the voltage required to test for short circuits.
5.3 The membrane (separator) material used in the battery: The properties of the membrane material can affect the voltage needed for short circuit testing.
5.4 The intended use of the battery: Different applications may require different voltage levels for short circuit testing.
These factors can be ranked in order of importance as follows: 1 > 4 > 3 > 2. This means that the manufacturing process and expertise of the company have the greatest influence on determining the voltage for short circuit testing.
In simple terms, the breakdown principle is that if there are potential weak points such as dust, particles, larger membrane pores, or burrs between the electrode sheets and the separator, these weak points can create a lower internal resistance between the positive and negative electrodes. Under a fixed, higher voltage, these weak points can facilitate the ionization of air, leading to the generation of an electric arc. Alternatively, if the positive and negative electrodes are already short-circuited and there is a small contact point, a high current can pass through these small contact points instantly, converting electrical energy into heat energy, resulting in membrane melting or instantaneous breakdown.
6. What is the impact of material particle size on discharge current?
In simple terms, smaller particle size generally leads to better conductivity, while larger particle size results in poorer conductivity. Consequently, high-rate materials often consist of small particles with high conductivity. However, while this analysis holds true in theory, the practical implementation of achieving high conductivity in small particle size materials is a challenging task, especially for nanoscale materials. Additionally, materials with small particle sizes tend to have lower compaction density, which translates to smaller volume capacities.
7. After the positive and negative electrodes were rolled, they rebounded by 10um after being baked for 12 hours and stored for one day. Why is there such a big rebound?
There are two fundamental influencing factors: material and process.
7.1 Material: The properties of the material determine the rebound coefficient. Different materials have different rebound coefficients. For the same material, different formulations can result in different rebound coefficients. Additionally, for the same material and formulation, varying the thickness of the pressed sheet can also lead to different rebound coefficients.
7.2 Process: Poor process control can also contribute to rebound. Factors such as storage time, temperature, pressure, humidity, stacking method, internal stress, equipment, and more can influence the rebound.
8. How to solve the leakage problem of cylindrical batteries?
Sealing cylindrical batteries after electrolyte injection is a challenging aspect of the sealing process. Currently, there are several methods used for sealing cylindrical batteries:
8.1 Laser welding sealing
8.2 Sealing with a gasket or O-ring
8.3 Adhesive sealing
8.4 Ultrasonic vibration sealing
8.5 Combinations of two or more of the above sealing methods
8.6 Other sealing methods
There are several factors that can cause liquid leakage in sealed cylindrical batteries:
8.7 Insufficient sealing: This can occur due to deformation or contamination at the sealing area, resulting in poor sealing.
8.8 Sealing stability: Even if the initial seal is satisfactory, if the sealing area is easily damaged afterward, it can lead to liquid leakage.
8.9 Gas generation during formation or testing: If gas is produced during the formation process and reaches the maximum stress that the seal can withstand, it can cause an impact on the seal, leading to liquid leakage. This is different from the second point because it is not a defect in the seal but rather excessive internal pressure causing seal damage.
8.10 Other leakage pathways: There can be additional factors or pathways that contribute to liquid leakage.
To address liquid leakage, it is crucial to identify the underlying causes. Once the root causes are identified, it becomes easier to find appropriate solutions. However, the challenge lies in identifying the causes since evaluating the sealing effectiveness of cylindrical batteries is difficult, often requiring destructive testing or random sampling.
9.During the experiment, the electrolyte was excessive. Without overflowing, will the excess electrolyte affect the battery performance?
There are several situations when the electrolyte does not overflow: 1. The electrolyte is just right 2. The electrolyte is slightly excessive 3. The electrolyte is excessively large, but it does not reach the limit 4. The electrolyte is excessively large and is close to the limit 5. The electrolyte is full to the limit, OK The first situation of sealing is the ideal situation and there is no problem. In the second case, a slight excess is sometimes a precision issue, and sometimes it is a design issue. Generally, the design is excessive. In the third case, there is no problem, it is just a waste of cost. The fourth situation is a bit dangerous. Because during the use or testing of the battery, there will be various reasons: the electrolyte will decompose and produce some gas; the battery will heat up and produce thermal expansion; the above two situations can easily cause the battery shell to bulge (also called deformation) or leak. fluid, which increases the safety hazard of the battery. The fifth situation is actually an enhanced version of the fourth situation and is even more dangerous. To be more exaggerated, liquids can also become batteries. That is, the positive and negative electrodes are inserted into a container containing a large amount of electrolyte at the same time (such as a 500ML beaker). At this time, the positive and negative electrodes can be charged and discharged. It is also a battery, so the excess electrolyte here is not just a little bit. Electrolyte is just a medium that conducts electricity. However, the volume of the battery is limited. Within the limited volume, it is natural to consider the issues of space utilization and deformation.
10. If the amount of liquid injected is too small, will it cause a drum shell after the battery is divided?
I can only say that it is not certain, it depends on how small the injection volume is. 1. If the battery cell is completely infiltrated by the electrolyte, but there is no residue, the battery will not bulge after the volume is divided; 2. If the battery cell is completely infiltrated by the electrolyte, there is a small amount of residue, but it is more liquid than your company’s requirements. The amount should be less (of course, this requirement is not necessarily the optimal value, there is a slight deviation), and the capacity-divided battery will not bulge at this time; 3. If the battery cell is completely soaked by the electrolyte, there will be a large amount of electrolyte residue, but your company The requirement for the liquid injection amount is higher than the actual amount. At this time, the so-called insufficient liquid injection amount is just a company’s concept, and it does not truly reflect the appropriateness of the actual liquid injection amount of the battery. The battery with different capacity does not have a bulging shell;4. The substantial injection volume is insufficient. This also depends on the degree. If the electrolyte can barely infiltrate the battery cell, the shell may or may not bulge after the volume is divided, but the probability of the battery shell bulging after the volume is divided is greater; if the amount of liquid injected into the battery cell is seriously insufficient, then the battery will be formed The electrical energy at this time cannot be converted into chemical energy. At this time, the probability of the drum shell of the split-capacity battery cell is almost 100%. Then, we can make the following summary: Assume that the actual optimal liquid injection volume of the battery is Mg. If the liquid injection volume is too small, it can be divided into the following situations: 1. Liquid injection volume = M: the battery is normal 2. The liquid injection volume is slightly less than M: The battery capacity is not bulging. The capacity may be normal or slightly lower than the design value. The probability of circulatory bulging increases and the cycle performance becomes worse. 3. The liquid injection volume is much less than M: the battery capacity bulging rate is quite high. , the battery has low capacity and extremely poor cycle stability. Generally, the capacity is less than 80% in dozens of weeks. 4.M=0, the battery has no bulging shell and no capacity.