10 major problems in lithium battery production

What is the reason for the pinholes in the anode coating?

The appearance of pinholes should be caused by the following factors:

1. The foil material is not clean;
2. The conductive agent is not dispersed;
3. The main material of the lithium ion battery anode is not dispersed;
4. There are impurities in some ingredients in the formula;
5. The conductive agent particles are uneven and difficult to disperse;
6. The anode particles are uneven and difficult to disperse;
7. The formula material itself has quality problems;
8. The mixing pot is not cleaned, resulting in dry powder residue in the pot.

To avoid the above problems, it is necessary to first use a suitable mixing process to solve the bonding between the active material and the metal collective, and to avoid artificial de-powdering in the production of battery plates and battery assembly. Adding some additives that do not affect the performance of the battery during the coating process can indeed improve some performances of the pole piece.

Of course, adding these ingredients into the electrolyte can achieve the effect of consolidation. The local high temperature of the diaphragm is caused by the inhomogeneity of the pole piece. Strictly speaking, it is a micro-short circuit. The micro-short circuit will cause local high temperature and may cause the powder removal of the anode.

What are the reasons for the excessive internal resistance of the battery?

Process:

1. Too little conductive agent in the cathode ingredients (the conductivity between materials is not good, because the conductivity of lithium cobalt itself is very poor)
2. Too much binder in cathode ingredients. (Binders are generally li polymer battery materials with strong insulation properties)
3. There is too much binder in the anode ingredients. (Binders are generally polymer materials with strong insulation properties)
4. The ingredients are unevenly dispersed.
5. The binder solvent is not complete during batching. (Not completely soluble in NMP, water)
6. The density design of the coating drawing surface is too large. (Large ion migration distance)
7. The compaction density is too large, and the roller compaction is too tight. (Rolling is too dead, and some active material structures are destroyed)
8. Cathode tabs are not welded firmly, and false welds appear.
9. The welding or riveting of the anode tab is not strong, and there is a weak welding and desoldering.
10. The winding is not tight and the winding core is loose. (Increase the distance between the cathode and anode plates)
11. The welding between the cathode ear and the shell is not firm.
12. The welding of the anode lug and the pole is not strong.
13. The battery baking temperature is too high and the diaphragm shrinks. (diaphragm pore size shrinks)
14. The amount of liquid injection is too small (the conductivity decreases, and the internal resistance increases rapidly after cycling!)
15. The shelf time after liquid injection is too short, and the electrolyte is not fully infiltrated
16. Not fully activated during formation.
17. Too much electrolyte leakage during the formation process.
18. The moisture control in the production process is not strict, and the battery expands.
19. The charging voltage of the battery is set too high, resulting in overcharging.
20. The battery storage environment is unreasonable.

Materials:

1. The cathode materials have high resistance. (Poor conductivity, such as lithium iron phosphate)
2. Effect of diaphragm material (diaphragm thickness, small porosity, small pore size)
3. Electrolyte material influence. (small conductivity, high viscosity)
4. The influence of cathode PVDF material. (a large amount or a large molecular weight)
5. The influence of cathode conductive material. (poor conductivity, high resistance)
6. The influence of cathode and anode tab materials (thin thickness, poor conductivity, uneven thickness, poor material purity)
7. Copper foil, aluminum foil material has poor conductivity or oxides on the surface.
8. The internal resistance of the riveted contact of the pole of the cover plate is relatively large.
9. The anode material has high resistance. other aspects
10. The deviation of the internal resistance test instrument.
11. Human manipulation.

What should be paid attention to when the electrode coating is uneven?

First of all, it is necessary to clearly understand the factors that affect the surface density and the factors that affect the stable value of the surface density, so that the problem can be solved in a targeted manner.

Factors that affect the density of the coating area are:

1. Factors of the material itself
2. Recipe
3. Stir the mixture
4. Coating environment
5. Knife edge
6. Slurry viscosity
7. Electrode travel speed
8. Surface levelness
9. Coating machine precision
10. Oven power
11. Coating tension, etc…

Factors affecting the uniformity of pole pieces:

1. Slurry quality
2. Slurry viscosity
3. Walking speed
4. Foil tension
5. Tension balance method
6. Coating traction length
7. Noise
8. Surface flatness
9. Knife edge flatness
10. Foil flatness, etc…

The above is just a list of some factors, and the specific reasons must be analyzed by themselves, and the factors that cause the abnormal surface density should be eliminated in a targeted manner.

Why use aluminum foil and copper foil for the current collectors

1. Both are used as current collectors because they have good conductivity and soft texture (maybe this is also conducive to bonding), and they are relatively common and cheap. At the same time, a layer of oxide protective film can be formed on the surface of both. .

2. The oxide layer on the copper surface is a semiconductor, and the electrons are conductive. The oxide layer is too thick and the impedance is large;

The aluminum oxide layer on the surface of aluminum is an insulator, and the oxide layer cannot conduct electricity, but because it is very thin, it realizes electronic conduction through the tunnel effect. If the oxide layer is thicker, the conductivity of the aluminum foil is poor, or even insulated.

Generally, it is best to clean the surface of the current collector before use. On the one hand, it can remove oil stains and at the same time remove the thick oxide layer.

3. The cathode potential is high, and the thin aluminum oxide layer is very dense, which can prevent the oxidation of the current collector. The oxide layer of copper foil is looser. In order 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 intercalate lithium with copper oxide at a slightly higher potential. Al foils cannot be used as anodes, and LiAl alloying will occur at low potentials.

4. The current collector requires pure components. The impure composition of Al will lead to the incompact surface film and pitting corrosion, and even the damage of the surface film will lead to the formation of LiAl alloy. The copper mesh is cleaned with bisulfate, then washed with deionized water and then baked. The aluminum mesh is cleaned with ammonia salt, then washed with deionized water, and then baked. The conductive effect of spraying the mesh is good.

What is the high voltage breakdown principle of the short circuit tester?

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

1. The technical level of manufacturer
2. The structural design of the battery itself
3. Battery diaphragm material
4. The purpose of the battery

Different companies use different voltages, but many companies use the same voltage regardless of the model size and capacity. The above factors can be arranged in order from heavy to light: 1>4>3>2, that is to say, the company’s technological level determines the short-circuit voltage.

Simply speaking, the principle of breakdown is that if there are some potential short-circuit factors between the pole piece and the diaphragm, such as dust, particles, large diaphragm holes, burrs, etc., we can call it a weak link.

At a fixed, higher voltage, these weak links make the contact internal resistance between the cathode and anodes smaller than other places, and it is easy to ionize the air to generate arcs;

Or the cathode and anode poles have been short-circuited, and the contact points are small. Under high-voltage conditions, these small contact points have a large current passing through them instantly, and the electric energy is instantly converted into heat energy, causing the diaphragm to melt or break down instantly.

How does the material particle size affect the discharge current?

Simply put, the smaller the particle size, the better the conductivity, and the larger the particle size, the worse the conductivity. Naturally, high-magnification materials are generally high-structure small particles with high conductivity. It is only theoretically analyzed that it is very difficult to improve the conductivity of small particle size materials, especially nanoscale materials, and the compaction of small particle materials will be relatively small, that is, the volume capacity is small.

Why do the cathode and anode pole pieces have such a big rebound?

Why do the cathode and anode pole pieces have such a big rebound after the rollers are aligned?

There are two most essential influencing factors: material and process.

1. The performance of the material determines the rebound coefficient, and different materials have different rebound coefficients; The same material, different formulations, different rebound coefficients; the same material, the same formula, different tablet thickness, different rebound coefficients;

2.If the process procedure is not well controlled, it will also cause rebound. Storage time, temperature, pressure, humidity, accumulation method, internal stress, equipment and so on.

How to solve the leakage problem of cylindrical battery?

The cylinder is closed and formed, and it is sealed after liquid injection. Therefore, the sealing naturally becomes the difficulty of cylindrical sealing. At present, there are probably the following methods for cylindrical lithium ion battery sealing:

1. Laser welding seal
2. Sealing ring
3. Glue seal
4. Ultrasonic vibration sealing
5. A combination of two or more of the above sealing types
6. Other sealing methods

Several causes of leakage:

1. Inadequate sealing causes liquid leakage, usually the sealing is deformed and the sealing is contaminated, which belongs to poor sealing.
2. The stability of the seal is also a factor, that is, the inspection is qualified when sealing, but the seal is easily damaged, resulting in liquid leakage.
3. Gas is produced during formation or testing, reaching the maximum stress that the seal can withstand, impacting the seal and causing liquid leakage. The difference with the second point is that the second point belongs to the leakage of defective products, and the third point belongs to the destructive type of leakage, that is, the seal is qualified, but the internal pressure is too high to damage the seal.
4. Other ways of leakage. The specific solution depends on the cause of the leakage. As long as the cause is found, it is easy to solve it. The difficulty is that it is difficult to find the cause, because the sealing effect of the cylinder is difficult to test, and most of them are destructive types, which are used for random inspection.

Does excess electrolyte affect battery performance?

There are several situations when the electrolyte does not overflow:

1. The electrolyte is just right
2. There is a slight excess of electrolyte
3. There is a large excess of electrolyte, but it has not reached the limit
4. There is a large excess of electrolyte, which is close to the limit
5. It is full to the limit and can be sealed

The first case is ideal and there is nothing wrong with it. In the second case, a slight excess is sometimes a precision problem, and sometimes it is a design problem, and generally the design is too much. In the third case, there is no problem, just a waste of cost.

The fourth situation is a little more dangerous. Because the battery will be used or tested for various reasons: cause the electrolyte to decompose and generate some gas; the battery will heat up and generate thermal expansion;

The above two situations can easily cause the bulge (also called deformation) or liquid leakage of the battery, which increases the safety hazard of the battery.

The fifth situation is actually an enhanced version of the fourth situation, and the danger is even greater. To exaggerate some more, the liquid can also become the battery. That is, insert the cathode and anodes into a container containing a large amount of electrolyte (for example, a 500ML beaker).

At this time, the cathode and anodes can be charged and discharged, and it is also a battery, so the excess electrolyte here is not a little bit . Electrolyte is only a conductive medium. However, the volume of the battery is limited. Within the limited volume, it is natural to consider the issues of space utilization and deformation.

Will underfilling electrolyte cause drum shells?

Depends on how little electrolyte is injected.

1. If the battery cell is completely soaked by the electrolyte, but there is no residue, the battery will not bulge after the capacity is divided;
2. If the battery cell is completely soaked by the electrolyte, there is a small part remaining, but the amount of liquid injected is less than the manufacturer’s requirement, and the shell of the divided battery will not bulge at this time;
3. If the cell is completely soaked by the electrolyte, there is a large amount of electrolyte remaining, but the manufacturer’s requirements for the amount of liquid injection are higher than the actual one. At this time, the so-called insufficient liquid injection is just a concept of the manufacturer, and it cannot be true. Respond to the appropriateness of the actual liquid injection volume of the battery, and the sub-capacity battery does not bulge;
4. Substantial insufficient fluid injection. It also depends on the degree. If the electrolyte can barely infiltrate the battery cell, the shell may or may not bulge after the capacity is divided, but the probability of the shell of the battery being divided is higher. If the liquid injection volume of the battery cell is seriously insufficient, then the electrical energy of the battery cannot be converted into chemical energy during formation. At this time, the probability of the shell of the sub-capacity battery cell is almost 100%.

Then, the following summary can be made: Assuming that the actual optimal liquid injection volume of the battery is Mg, and 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 smaller than M: the battery capacity is not enough to bulge the shell, the capacity may be normal, or slightly lower than the design value, the probability of circulating the bulging shell will increase, and the cycle performance will deteriorate;
3. The amount of liquid injection is much smaller than M: the battery shell rate is quite high, the battery has low capacity, and the cycle stability is extremely poor. Generally, the capacity is less than 80% in dozens of weeks.
4. M=0, the battery has no shell and no capacity.