Semco university – All about the Lithium-Ion Batteries

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Production problems of lithium batteries – Semco University

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Lithium Batteries

What is the cause of pinholes in negative electrode coating? Is it because the material is not well dispersed? Could it be the reason for the poor particle size distribution of the material?

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

  • The foil is not clean;
  • The conductive agent is not dispersed;
  • The negative electrode body material is not dispersed;
  • There are impurities in some ingredients in the formula;
  • The conductive agent particles Uneven, difficult to disperse;
  • The negative particles are uneven and difficult to disperse;
  • The formula material itself has quality problems;
  • The stirring pot is not cleaned, resulting in dry powder residues in the pot.
  • What are the specific reasons, you can go to the process monitoring and analysis.

In addition, I have encountered dark spots on the diaphragm many years ago. I will briefly answer it first. Please correct me if I am wrong.

According to the analysis, it is determined that the black spot is caused by the local high temperature of the separator caused by the polarized discharge of the battery, and the negative electrode powder adheres to the separator, and the polarized discharge is caused by the material and process reasons, and there is active material attached to the powder in the battery core., resulting in a polarized discharge after the battery is formed into a charge. To avoid the above problems, first of all, a suitable mixing process should be used to solve the bonding between the active material and the metal collective, and man-made powder removal should be avoided in the manufacture 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 properties of the pole piece. Of course, adding these ingredients 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 pieces. Strictly speaking, it is a micro-short circuit. The micro-short circuit will cause a local high temperature, which may cause the de-powder of the negative electrode.

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

Process:

  • Too little conductive agent for positive electrode ingredients (the conductivity between materials is not good, because the conductivity of lithium cobalt itself is very poor)
  • Too much binder for positive electrode ingredients. (The binder is generally a polymer material with strong insulating properties)
  • Too much binder for negative electrode ingredients. (The binder is generally a polymer material with strong insulating properties)
  • The ingredients are not uniformly dispersed.
  • The binder solvent is not complete during batching. (Not completely soluble in NMP, water)
  • The design of the surface density of the coating and pulling pulp is too large. (Ion migration distance is large)
  • The compaction density is too high and the roller is too compact. (The roller is too dead, and the active material structure is sometimes destroyed)
  • The positive lug is not welded firmly, and virtual welding occurs.
  • The negative electrode ear is not welded or riveted firmly, and there is virtual welding anode-soldering.
  • The winding is not tight and the core is loose. (To increase the distance between the positive and negative electrodes)
  • The welding of the positive lug and the shell is not firm.
  • The negative electrode tab and the pole post are not welded firmly.
  • The battery baking temperature is too high, and the diaphragm shrinks. (Diaphragm pore size reduction)
  • Too little liquid injection (conductivity decreases, internal resistance increases rapidly after cycling!)
  • The shelving time after injection is too short, and the electrolyte is not fully infiltrated
  • Not fully activated during formation.
  • There is too much leakage of electrolyte during the formation process.
  • The moisture control in the production process is not strict, and the battery swells.
  • The battery charging voltage is set too high, resulting in overcharging.
  • The battery storage environment is unreasonable.

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Material:

  • The positive electrode material has high resistance. (Poor conductivity, such as lithium-ion phosphate)
  • Influence of diaphragm material (diaphragm thickness, small porosity, small pore size)
  • The influence of electrolyte material. (Small conductivity, high viscosity)
  • Influence of cathode PVDF material. (Large amount or large molecular weight)
  • Influence of positive electrode conductor material. (Poor conductivity, high resistance)
  • Influence of positive and negative electrode tab materials (thin thickness, poor conductivity, uneven thickness, and poor material purity)
  • Copper foil, aluminum foil material has poor conductivity or oxides on the surface.
  • The internal resistance of the riveted contact of the cover plate pole is too large.
  • The negative electrode material has high resistance. other aspects
  • Deviation of internal resistance test instrument.
  • Human operation.

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The electrode sheet is unevenly coated, what problems should be paid attention to?

This problem is relatively common, and it was relatively easy to solve, but many coating workers are not good at summarizing, resulting in the default of some existing problems as normal and unavoidable. First of all, it is necessary to clearly know the factors that affect the areal density and the factors that affect the stable value of the areal density, so that the problem can be solved in a targeted manner.

The factors that affect the surface density of the coating are:
  • The material itself
  • Recipe
  • Stir the mixture
  • Coating environment
  • Knife edge
  • Slurry viscosity
  • Pole piece walking speed
  • Surface levelness
  • Coating machine accuracy
  • Oven wind
  • Coating tension, etc…
Factors affecting the uniformity of the pole piece:
  • Slurry quality
  • Slurry viscosity
  • Walking speed
  • Foil tension
  • Tension balance method
  • Coating Traction Length
  • Noise
  • Surface flatness
  • Blade flatness
  • Foil flatness, etc…

The above is just a list of some factors, and it is necessary to analyzes the reasons and eliminate the factors that cause the abnormal surface density in a targeted manner.

Ask, is there any special reason for the positive and negative current collectors to use aluminum foil and copper foil respectively? Is there any problem with the reverse? I have seen a lot of literature that directly use stainless steel mesh. Is there any difference?

  • Both of them are used as current collectors because they have good electrical conductivity, soft texture (maybe this will also be conducive to bonding), and are relatively common and cheap. At the same time, a layer of oxide protective film can be formed on the surface of both.
  • The oxide layer on the copper surface is a semiconductor, which is electronically conductive. The oxide layer is too thick and the impedance is large; while the oxide layer on the aluminum surface is an aluminum oxide insulator, and the oxide layer cannot conduct electricity. However, due to its thinness, electronic conductivity is achieved through the tunnel effect. If the oxide layer is thick, the conductivity of the aluminum foil is poor, and even insulation. Generally, the current collector should be cleaned on the surface before use. On the one hand, the oil stains can be washed off, and the thick oxide layer can be removed at the same time.
  • The positive electrode potential is high, and the aluminum thin oxide layer is very dense, which can prevent the current collector from oxidizing. The copper foil oxide layer is looser. To prevent its oxidation, it is better to have a lower potential. At the same time, it is difficult for Li and Cu to form a lithium intercalation alloy at low potential. Lithium intercalation occurs in copper. Al foil cannot be used as a negative electrode, and Li Al alloying occurs at low potentials.
  • The current collector requires pure components. The impure composition of Al will cause the surface film not to be dense and cause pitting corrosion, and even more Li Al alloy will be formed due to the damage of the surface film. The copper mesh is washed with bi-sulphate, then washed with deionized water, and then baked. The aluminum mesh is washed with ammonia salt, washed with deionized water, and then baked, and then the spray mesh has a good conductive effect.

Fifth, there is a question to ask. When we measure the short circuit of the coil core, the battery short-circuit tester we use can accurately test the short-circuit cell when the voltage is high. Also, what is the high-voltage breakdown principle of the short-circuit tester? I look forward to your detailed explanation. Thanks!

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

  • The technological level of your company;
  • The structural design of the battery itself
  • Battery separator material
  • The purpose of the battery

Different companies use different voltages, but many companies use the same voltage regardless of model size and capacity. The above factors can be arranged in the order from heavy to light: 1>4>3>2, that is to say, your company’s technological level determines the short-circuit voltage. The breakdown principle is simply because between the pole piece and the diaphragm, if there are some potentials 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 plates smaller than other places, and it is easy to ionize the air to generate arcs; or the positive and negative electrodes have been short-circuited, the contact point is small, and the high voltage Under certain conditions, a large current flows through these small contact points instantaneously, and the electrical energy is instantly converted into heat energy, causing the diaphragm to melt or break down instantaneously.

How does the particle size of the material affect the discharge current? Looking forward to reply, thank you!

To put it simply, the smaller the particle size, the better the conductivity, and the larger the particle size, the worse the conductivity. Naturally, high-rate materials are generally small particles with high structure and high conductivity. It is only a theoretical analysis, and how to achieve it in practice can only be explained by friends who are materials will be relatively small, that is, the volume capacity is small.

Seven, I would like to ask you a question, our positive and negative pole pieces rebounded by 10um after being baked for 12 hours and stored for one day after being rolled on the opposite side. Why is there such a big rebound?

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

  • The performance of the material determines the coefficient of rebound, different materials have different coefficients of rebound; the same material, different formulas, different coefficients of rebound; the same material, the same formula, the thickness of the tablet is different, the coefficient of rebound is different;
  • If the process is not well controlled, it will also cause rebound. Storage time, temperature, pressure, humidity, stacking method, internal stress, equipment, etc.
How to solve the problem of cylindrical battery leakage?

The cylinder is closed and sealed after the liquid is injected. Therefore, the sealing naturally becomes the difficulty of cylindrical sealing. At present, there are several ways to seal the cylindrical battery:

  • Laser welding sealing
  • Sealing ring
  • Glue sealing
  • Ultrasonic vibration sealing
  • A combination of two or more of the above types of sealing
  • Other sealing methods

Several causes of fluid leakage:

  • The seal is not strong enough to cause liquid leakage. Usually, the seal is deformed and the seal is contaminated, which is a poor seal.
  • The stability of the seal is also a factor, that is, the inspection is qualified during the seal, but the seal is easily damaged, resulting in liquid leakage.
  • Gas is produced during formation or testing, reaching the maximum stress that the seal can withstand, impacting the seal, causing liquid leakage. The difference from the second point is that the second point belongs to the leakage of defective products, and the third point belongs to the destructive leakage, that is, the sealing is qualified, but the internal pressure is too high and the sealing is damaged.
  • Other leakage methods.

How to solve it depends on the cause of the leakage. As long as the cause is found, it is easy to solve. 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 damage types, which are used for random inspection.

When we do the experiment, the electrolyte is too much. Will the excess electrolyte affect the performance of the battery without overflowing? Not overflow?

There are several situations:

  • The electrolyte is just right
  • The electrolyte is slightly excessive
  • There is a large excess of electrolyte, but the limit is not reached
  • There is a large excess of electrolyte, which is close to the limit
  • It has reached the limit and can be sealed
Conclusion

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 a design problem, and it is generally designed to be 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 is in use or tested, due to various reasons: the electrolyte is decomposed and some gas is generated; the battery heats up, resulting in thermal expansion; the above two situations can easily cause the battery’s drum shell (also called deformation) or leakage. liquid, 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 be more exaggerated, the liquid can also become a battery. That is to insert the positive and negative electrodes into a container with a large amount of electrolyte at the same time (for example, a 500ML beaker), at this time, the positive and negative electrodes can be charged and discharged, and it is also a battery, so the excess electrolyte here is not a little bit. The electrolyte is just a conductive medium. However, the volume of the battery is limited, and within the limited volume, space utilization and deformation must be considered.

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The amount of liquid injection is too small. Will it cause a drum shell after the battery is divided?

It can only be said that it is not necessarily, it depends on how little the injection volume is.

  • If the battery cell is completely infiltrated by the electrolyte, but there is no residue, the battery will not bulge after the capacity is divided;
  • If the battery cell is completely infiltrated by the electrolyte, there is a small amount of residue left, but it is less than your company’s requirement (of course, this requirement is not necessarily the best value, there is a slight deviation). drum shell;
  • If the battery cell is completely infiltrated by the electrolyte, there will be a large amount of electrolyte remaining, but your company’s requirements for the amount of liquid injection are higher than the actual amount. At this time, the so-called insufficient liquid injection amount is only a concept of the company, and cannot be a real response. The appropriate degree of the actual liquid injection volume of the battery, and the divided battery does not bulge;
  • Substantial infusion volume is insufficient. It also depends on the degree. If the electrolyte is barely able to infiltrate the battery cells, it may or may not be bulging after the capacity separation, but the probability of the separation capacity battery bulging is higher;

If the injection volume of the battery cell is seriously insufficient, the electrical energy during the formation of the battery cannot be converted into chemical energy.

Then, the following summary can be made: Assuming that the actual optimal liquid injection volume of the battery is Mg, the liquid injection volume is small, and the following situations are divided:

  • Injection volume = M: the battery is normal
  • The liquid injection volume is slightly less than M: the battery capacity is not bulging, the capacity may be normal, or it may be slightly lower than the design value, the probability of circulating bulging increases, and the cycle performance becomes poor;
  • The liquid injection volume is much less than M: the battery volume distribution drum rate is quite high, the battery has low capacity, and the cycle stability is extremely poor. Generally, the capacity is less than 80% for dozens of weeks. 4.M=0, the battery is not bulging and has no capacity.

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