Ten major issues in lithium battery production! Professional Engineer Experience Sharing

Ten major issues in lithium battery production! Professional Engineer Experience Sharing

1、 What is the reason for pinholes in negative electrode coating? Is it the reason why the material is not well dispersed? Is it possible that the poor particle size distribution of the material is the reason?

The appearance of pinholes should be caused by the following factors: 1. The foil is not clean; 2. The conductive agent is not dispersed; 3. The main material of the negative electrode is not dispersed; 4. Some ingredients in the formula contain impurities; 5. The conductive agent particles are uneven and difficult to disperse; 6. The negative electrode particles are uneven and difficult to disperse; 7. There are quality issues with the formula materials themselves; 8. The mixing pot was not cleaned thoroughly, resulting in residual dry powder inside the pot. Just go to process monitoring and analyze the specific reasons yourself.

Also, regarding the black spots on the diaphragm, I have encountered them many years ago. Let me briefly answer them first. Please correct any mistakes. According to analysis, it has been determined that the black spots are caused by the local high temperature of the separator caused by the polarization discharge of the battery, and the negative electrode powder adheres to the separator. Polarization discharge is caused by the presence of active substances attached to the powder in the battery coil due to material and process reasons, resulting in polarization discharge after the battery is formed and charged. To avoid the above problems, it is first necessary to use appropriate mixing processes to solve the bonding between active substances and metal collectives, and to avoid artificial powder removal during battery plate manufacturing and battery assembly.

Adding some additives that do not affect battery performance during the coating process can indeed improve certain performance of the electrode. Of course, adding these components to the electrolyte can achieve consolidation effect. The local high temperature of the diaphragm is caused by the non-uniformity of the electrode plates. Strictly speaking, it belongs to a micro short circuit, which can cause local high temperature and may cause the negative electrode to lose powder.

2、 What are the reasons for excessive battery internal resistance?

In terms of technology:

1. The positive electrode ingredient has too little conductive agent (the conductivity between materials is not good because the conductivity of lithium cobalt itself is very poor)

2. There is too much adhesive for the positive electrode ingredient. (Adhesives are generally polymer materials with strong insulation properties)

3. Excessive adhesive for negative electrode ingredients. (Adhesives are generally polymer materials with strong insulation properties)

4. Uneven distribution of ingredients.

5. Incomplete binder solvent during ingredient preparation. (Not completely soluble in NMP, water)

6. The density design of the coating slurry surface is too high. (Long ion migration distance)

7. The compaction density is too high, and the rolling is too compacted. (Excessive rolling may cause damage to the structure of active substances)

8. The positive electrode ear is not firmly welded, resulting in virtual welding.

9. The negative electrode ear is not firmly welded or riveted, resulting in false soldering or detachment.

10. The winding is not tight and the core is loose. (Increase the distance between positive and negative electrode plates)

11. The positive electrode ear is not firmly welded to the housing.

12. The negative electrode ear and pole are not firmly welded.

13. If the baking temperature of the battery is too high, the diaphragm will shrink. (Reduced diaphragm aperture)

14. Insufficient liquid injection amount (conductivity decreases, internal resistance increases quickly after circulation!)

15. The storage time after liquid injection is too short, and the electrolyte is not fully soaked

16. Not fully activated during formation.

17. Excessive leakage of electrolyte during the formation process.

18. Insufficient water control during the production process, resulting in battery expansion.

19. The battery charging voltage is set too high, causing overcharging.

20. Unreasonable battery storage environment.

In terms of materials:

21. The positive electrode material has high resistance. (Poor conductivity, such as lithium iron phosphate)

22. Impact of diaphragm material (diaphragm thickness, small porosity, small pore size)

23. Effects of electrolyte materials. (Low conductivity and high viscosity)

24. Positive electrode PVDF material influence. (high in weight or molecular weight)

25. The influence of positive electrode conductive material. (Poor conductivity, high resistance)

26. Effects of positive and negative electrode ear materials (thin thickness, poor conductivity, uneven thickness, and poor material purity)

27. Copper foil and aluminum foil materials have poor conductivity or surface oxides.

28. The riveting contact internal resistance of the cover plate pole is too high.

29. The negative electrode material has high resistance. other aspects

30. Deviation of internal resistance testing instruments.

31. Human operation.

3、 What are the issues that should be paid attention to when the electrode is not evenly coated?

This problem is quite common and was originally relatively easy to solve, but many coating workers are not good at summarizing, resulting in some existing problem points being defaulted to normal and unavoidable phenomena. Firstly, it is necessary to have a clear understanding of the factors that affect surface density and the factors that affect the stable value of surface density in order to solve the problem in a targeted manner.

The factors that affect the density of the coating surface include:

1. Material itself factors

2. Formula

3. Mixing materials

4. Coating environment

5. Knife edge

6. Slurry viscosity

7. Pole speed

8. Surface levelness

9. Coating machine accuracy

10. Oven Wind Force

11. Coating tension and so on

Factors affecting the uniformity of the electrode:

1. Slurry quality

2. Slurry viscosity

3. Traveling speed

4. Foil tension

5. Tension balance method

6. Coating traction length

7. Noise

8. Surface flatness

9. Blade flatness

10. Flatness of foil material, etc

The above is only a list of some factors, and you need to analyze the reasons yourself to specifically eliminate the factors that cause abnormal surface density.

4、 Excuse me, is there any special reason why the positive and negative current collectors are made of aluminum foil and copper foil respectively? Is there any problem with using it in reverse? Have you seen many literature that directly uses stainless steel mesh? Is there a difference?

1. Both are used as fluid collectors because they have good conductivity, soft texture (which may also be beneficial for bonding), and are relatively common and inexpensive. At the same time, both surfaces can form a layer of oxide protective film.

2. The oxide layer on the surface of copper belongs to semiconductors, with electron conduction. The oxide layer is too thick and has a high impedance; The oxide layer on the surface of aluminum is an insulator, and the oxide layer cannot conduct electricity. However, due to its thin thickness, electronic conductivity is achieved through tunneling effect. If the oxide layer is thick, the conductivity level of the aluminum foil is poor, and even insulation. Before use, it is best to clean the surface of the fluid collector to remove oil stains and thick oxide layers.

3. The positive electrode potential is high, and the aluminum thin oxide layer is very dense, which can prevent the oxidation of the collector. The oxide layer of copper foil is relatively loose, and to prevent its oxidation, it is better to have a lower potential. At the same time, it is difficult for Li to form a lithium intercalation alloy with Cu at a low potential. However, if the copper surface is heavily oxidized, Li will react with copper oxide at a slightly higher potential. AL foil cannot be used as a negative electrode, as LiAl alloying may occur at low potentials.

4. The fluid collection requires pure composition. The impure composition of AL will lead to the non compact surface facial mask and pitting corrosion, and even more, the destruction of the surface facial mask will lead to the formation of LiAl alloy. Copper mesh is cleaned with hydrogen sulfate and then baked with deionized water, while aluminum mesh is cleaned with ammonia salt and then baked with deionized water. The conductive effect of the spray mesh is good.

5、 I have a question to ask. We use a battery short circuit tester when testing coil cores for short circuits. When the voltage is high, it can accurately test the short circuit cells. Additionally, what is the high voltage breakdown principle of the short circuit tester? We look forward to your detailed explanation. Thank you!

How high a voltage is used to measure a short circuit in a battery cell is related to the following factors:

1. Your company's technological level;

2. Structural design of the battery itself

3. Diaphragm material of the battery

4. The purpose of the battery

Different companies use different voltages, but many companies use the same voltage regardless of model size or capacity. The above factors can be arranged in descending order: 1>4>3>2, which means that your company's process level determines the size of short-circuit voltage.

Simply put, the breakdown principle is due to the presence of potential short-circuit factors such as dust, particles, larger diaphragm holes, burrs, etc. between the electrode and the diaphragm, which can be referred to as weak links. At a fixed and high voltage, these weak links make the contact resistance between the positive and negative electrode plates smaller than elsewhere, making it easier to ionize air and generate arcs; Alternatively, the positive and negative poles have already been short circuited, and the contact points are small. Under high voltage conditions, these small contact points instantly have large currents passing through them, converting electrical energy into heat energy, causing the membrane to melt or break down instantly.

6、 What is the effect of material particle size on discharge current? Looking forward to a reply, thank you!

Simply put, the smaller the particle size, the better the conductivity. The larger the particle size, the worse the conductivity. Naturally, high rate materials are generally high in structure, small particles, and high conductivity.

Just from a theoretical analysis, how to achieve it in practice can only be explained by friends who make materials. Improving the conductivity of small particle materials is a very difficult task, especially for nanoscale materials, and materials with small particles will have relatively small compaction, i.e. small volume capacity.

7、 May I ask you a question? Our positive and negative electrode plates have rebounded by 10um in a day after being baked for 12 hours after being rolled. Why is there such a large rebound?

There are two fundamental influencing factors: materials and processes.

1. The performance of materials determines the rebound coefficient, which varies among different materials; The same material, different formulas, and different rebound coefficients; The same material, the same formula, the thickness of the tablet is different, and the rebound coefficient is different;

2. If the process control is not good, it can also cause rebound. Storage time, temperature, pressure, humidity, stacking method, internal stress, equipment, etc.

8、 How to solve the leakage problem of cylindrical batteries?

The cylinder is closed and sealed after liquid injection, so sealing naturally becomes the difficulty of cylinder sealing. Currently, there are probably several ways to seal cylindrical batteries:

1. Laser welding sealing

2. Sealing ring sealing

3. Glue sealing

4. Ultrasonic vibration sealing

5. Combination of two or more sealing types mentioned above

6. Other sealing methods

Several causes of leakage:

1. Poor sealing can cause liquid leakage, usually resulting in deformation and contamination of the sealing area, indicating poor sealing.

2. The stability of sealing is also a factor, that is, it passes the inspection during sealing, but the sealing area is easily damaged, causing liquid leakage.

3. During formation or testing, gas is produced to reach the maximum stress that the seal can withstand, which can impact the seal and cause liquid leakage. The difference from point 2 is that point 2 belongs to defective product leakage, while point 3 belongs to destructive leakage, meaning that the sealing is qualified, but excessive internal pressure can cause damage to the sealing.

4. Other leakage methods.

The specific solution depends on the cause of the leakage. As long as the cause is identified, it is easy to solve, but the difficulty lies in the difficulty of finding the cause, as the sealing effect of the cylinder is relatively difficult to inspect and mostly belongs to the type of damage used for spot checks.

9、 When we conducted experiments, the electrolyte was always in excess. Can I ask if excessive electrolyte has an impact on battery performance without spillage?

No overflow? There are several situations:

1. The electrolyte is just right

2. Slightly excessive electrolyte

3. Excessive amount of electrolyte, but not reaching the limit

4. A large amount of electrolyte is excessive, approaching the limit

5. It has reached its limit and can be sealed

The first scenario is an ideal one, with no problems.

The second situation is that a slight excess is sometimes a precision issue, sometimes a design issue, and usually a little over design.

The third scenario is not a problem, it's just a waste of cost.

The fourth situation is a bit dangerous. Because during the use or testing process of batteries, various reasons can cause electrolyte to decompose and produce some gases; The battery heats up, causing thermal expansion; The above two situations can easily cause bulging (also known as deformation) or leakage of the battery, increasing the safety hazards of the battery.

The fifth scenario is actually an enhanced version of the fourth scenario, which poses even greater danger.

To exaggerate, liquid can also become a battery. That is to insert both the positive and negative electrodes into a container containing a large amount of electrolyte (such as a 500ML beaker) at the same time. At this time, the positive and negative electrodes can be charged and discharged, which is also a battery. Therefore, the excess electrolyte here is not a little. Electrolyte is just a conductive medium. However, the volume of the battery is limited, and within this limited volume, it is natural to consider space utilization and deformation issues.

10、 Will the amount of liquid injected be too small, and will it cause bulging after the battery is divided?

It can only be said that it may not be necessary, it depends on how little liquid is injected.

1. If the battery cell is completely soaked in electrolyte but there is no residue, the battery will not bulge after capacity division;

2. If the battery cell is completely soaked in the electrolyte and there is a small amount of residue, but the amount of liquid injected is less than your company's requirement (of course, this requirement is not necessarily the optimal value, with a slight deviation), the split capacity battery will not bulge at this time;

3. If the battery cell is completely soaked in electrolyte and there is a large amount of residual electrolyte, but your company's requirements for the injection amount are higher than actual, the so-called insufficient injection amount is only a company concept, and it cannot truly reflect the suitability of the actual injection amount of the battery, and the split capacity battery does not bulge;

4. Substantial insufficient liquid injection volume. This also depends on the degree. If the electrolyte is barely able to soak the battery cell, it may or may not bulge after partial capacitance, but the probability of the battery bulge is higher;

If there is a serious shortage of liquid injection in the battery cell, the electrical energy during the formation of the battery cannot be converted into chemical energy. At this time, the probability of the bulge of the capacitance cell is almost 100%.

So, it can be summarized as follows: Assuming that the actual optimal liquid injection amount of the battery is Mg, there are several situations where the liquid injection amount is relatively small:

1. Liquid injection volume=M: Battery normal

2. The liquid injection amount is slightly less than M: the battery does not have a bulging capacity, and the capacity may be normal or slightly lower than the design value. The probability of cycling bulging increases, and the cycling performance deteriorates;

3. The liquid injection amount is much less than M: the battery has a relatively high capacity and bulging rate, resulting in low capacity and poor cycling stability. Generally, the capacity is less than 80% after several weeks

4. M=0, the battery does not bulge and has no capacity.