Lithium battery science | Ten production problems and solutions (Part 1)
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2023/10/23 14:46:09
As an efficient and environmentally friendly energy storage device, lithium batteries are widely used in electric vehicles, mobile devices and energy storage systems. There are some problems in the production process of lithium batteries, such as high material costs, complex production processes, safety risks, etc., which restrict the development of the lithium battery industry. This paper will summarize the problems existing in the production process of lithium batteries, and put forward the corresponding solutions.
What are the causes of pinholes in negative coating? Is the material dispersion bad reason? Is it possible that the material size distribution is not good?
The appearance of pinholes may be caused by the following factors
1. The foil is unclean
2. The conductive agent is not dispersed
3. The main material of the negative electrode is not dispersed
4. There are impurities in some ingredients of the formula
5. Conductive particles are uneven and difficult to disperse
6. The negative particles are uneven and difficult to disperse
7. The formula material itself has quality problems
8. The stirring pot is not cleaned, resulting in dry powder residue in the pot
Specific reasons can go to the process monitoring analysis
Dark spot of diaphragm
According to the analysis, it is determined that the black spot is caused by the local high temperature of the diaphragm caused by the polarization discharge of the battery, and the negative powder is bonded to the diaphragm, and the polarization discharge is caused by the material and process reasons, and the active substance attached powder exists in the battery roll core, resulting in the polarization discharge after the battery formation and charging. In order to avoid the occurrence of problems, it is first necessary to adopt a suitable slurry process to solve the problem of bonding between the active substance and the metal collective. In the process of battery plate manufacturing and battery assembly, it is necessary to avoid the phenomenon of artificial depowder.
During the coating process, adding some additives that do not affect the performance of the battery can improve some properties of the pole sheet. Adding additives to the electrolyte can achieve the effect of consolidation. The local high temperature of the diaphragm is caused by the non-uniformity of the pole sheet, which strictly belongs to the micro short circuit, which will cause local high temperature and may lead to the depowder of the negative electrode.
What are the reasons for the excessive internal resistance of the battery?
Technical aspect
1. Too few conductive agents for positive ingredients (poor conductivity between materials, because lithium cobalt itself is very poor conductivity)
2. Too much positive binder (binder is generally polymer material, strong insulation performance)
3. Too much negative binder (binder is generally polymer material, strong insulation performance)
4. Uneven dispersion of ingredients
5. The binder solvent is not complete when mixing (can not completely dissolve in NMP, water)
6. The density of the coating surface is too large (large ion migration distance)
7. Compaction density is too large, roll compaction is too solid (roll compaction is too dead, the structure of the active substance is damaged)
8. The positive ear is not welded properly, resulting in virtual welding
9. Negative ear welding or riveting is not strong, there is virtual welding, welding
10. Winding is not tight, and the core is relaxed (increasing the distance between the positive and negative electrodes)
11. The positive ear is not solidly welded to the shell
12. The negative ear and pole are not well welded
13. The battery baking temperature is too high, and the diaphragm shrinks (the diaphragm aperture shrinks)
14. Too little liquid injection (conductivity decreased, internal resistance increased quickly after circulation)
15. The shelving time after 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 battery charging voltage is set too high, resulting in overcharge
20. The battery storage environment is unreasonable
Material aspect
21. Large resistance of positive electrode material (poor conductivity, such as lithium iron phosphate)
22. Diaphragm material influence (diaphragm thickness, small porosity, small pore size)
23. Electrolyte material influence (small conductivity, large viscosity)
24. Positive PVDF material influence (large amount or large molecular weight)
25. Positive conductor material influence (poor conductivity, high resistance)
26. Positive and negative ear material influence (thin thickness poor conductivity, uneven thickness, poor material purity)
27. Copper foil, aluminum foil material poor conductivity or surface oxides
28. The internal resistance of the riveted contact of the cover pole is too large
29. Negative material resistance is large, other aspects
30. Internal resistance test instrument deviation
31. Human operation
What are the problems of uneven electrode coating?
Factors affecting the density of coating surface
1. Material itself
Step 2: Recipe
3. Stir the mixture
4. Coating environment
5. Knife edge
6. Slurry viscosity
7. Pole speed
8. Surface water flatness
9. Coating machine accuracy
10. Oven wind
11. Coating tension, etc...
Factors affecting the uniformity of polar plates
1. Quality of slurry
2. Slurry viscosity
3. Walk fast
4. Foil tension
5. Tension balance method
6. Coating traction length
7. Noise
8. Surface roughness
9. Flatness of knife edge
10. Foil flatness and so on...
The above only lists some factors, and the specific reasons should be analyzed, and the factors that lead to the abnormal surface density should be targeted.
Is there any special reason for using aluminum foil and copper foil respectively for the positive and negative fluid collector? What's wrong with using it the other way around? See a lot of literature directly with stainless steel mesh, is there a difference?
1. Both are used as fluid collectors because they have good electrical conductivity and soft texture (which may also facilitate bonding), and they are also relatively common and inexpensive. At the same time, both can form an oxide protective film on the surface.
2. Copper surface oxide layer belongs to semiconductor, electron conduction, oxidation layer is too thick, large impedance; The alumina oxide layer on the surface of aluminum is an insulator, the oxide layer cannot conduct electricity, but because it is very thin, electronic conductance is achieved through the tunnel effect, if the oxide layer is thick, the conductivity level of aluminum foil is poor, and even insulation. General fluid collection before use is best to go through the surface cleaning, on the one hand to wash away the oil, while removing the thick oxide layer.
3. The positive electrode potential is high, and the thin aluminum oxide layer is very dense, which can prevent the oxidation of the fluid collection. The copper foil oxide layer is relatively loose, in order to prevent its oxidation, it is better to have a lower potential, and Li is difficult to form a lithium inlay alloy with Cu at a low potential, but if the copper surface is oxidized in large amounts, Li will react with copper oxide at a slightly higher potential. AL foil cannot be used as a negative electrode, and LiAl alloying occurs at low potentials.
4. The collecting fluid requires pure composition. The impurity of the composition of AL will lead to the surface film is not dense and point corrosion occurs, and even more due to the destruction of the surface film leads to the formation of LiAl alloy. The copper mesh is cleaned with hydrogen sulfate and then baked with deionized water, and the aluminum mesh is cleaned with ammonia salt and then baked with deionized water.
When measuring coil core short circuit, the battery short circuit tester used can accurately test the short circuit battery when the voltage is high, and 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 your company
2. The structural design of the battery itself
3. Battery diaphragm material
4. Use of batteries
Different companies choose voltages that are not quite the same, but many companies use the same voltage regardless of model size and capacity. The above factors can be listed in order from most important to least: 1> 4> 3> 2, that is, the technical level of your company determines the size of the short circuit voltage.
In simple terms, the breakdown principle is because between the pole plate and the diaphragm, if there are some potential short-circuit factors, such as dust, particles, larger diaphragm holes, burrs, etc., we can call it a weak link. At a fixed, high voltage, these weak links make the contact resistance between the positive and negative electrodes less than other places, easy to ionize the air to produce electric isolation; Or positive and negative has been short-circuited, the contact point is small, under high pressure conditions, these small contact points instantaneously through the large current, electrical energy is instantly converted into heat energy, resulting in the diaphragm melting or instant breakdown.
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