Battery Forum Original: Self-discharge (lithium-ion battery) analysis

Battery Forum Original: Self-discharge (lithium-ion battery) analysis

Classification of discharge:

From the effect of self-discharge on the battery, self-discharge can be divided into two types: self-discharge where the loss capacity can be reversibly compensated; and self-discharge where the loss capacity cannot be reversibly compensated. According to these two classifications, we can give some self-discharge reasons approximately contoured.

Reasons for self-discharge:

1. Causes of reversible capacity loss: The reason for the reversible capacity loss is that a reversible discharge reaction occurs, and the principle is consistent with the normal discharge reaction of the battery. The difference is that the normal discharge electron path is an external circuit and the reaction speed is fast; the self-discharge electron path is an electrolyte, and the reaction speed is very slow.

2. Causes of irreversible capacity loss: When an irreversible reaction occurs inside the battery, the resulting capacity loss is irreversible capacity loss. The types of irreversible reactions that occur mainly include:

A: Irreversible reaction between the positive electrode and the electrolyte (relatively mainly occurs in two materials which are prone to structural defects such as lithium manganate and lithium nickelate, such as the reaction of lithium manganate with lithium ions in the electrolyte: LiyMn2O4+xLi+ +xe-→Liy+xMn2O4, etc.);

B: Irreversible reaction between the anode material and the electrolyte (the SEI film formed during the formation is to protect the anode from corrosion by the electrolyte, and the reaction between the anode and the electrolyte may be: LiyC6→Liy-xC6+xLi++xe, etc. );

C: Irreversible reaction caused by impurities in the electrolyte itself (for example, a reaction in which CO2 may occur in the solvent: 2CO2+2e+2Li+→Li2CO3+CO; reaction in O2 in the solvent: 1/2O2+2e+2Li+→Li2O). A similar reaction irreversibly consumes lithium ions in the electrolyte, which in turn loses battery capacity.

D: Irreversible reaction caused by micro short circuit caused by impurities during production. This phenomenon is the most important cause of the large self-discharge of individual batteries. The dust in the air or the metal powder on the pole piece and the diaphragm when it is made will cause an internal micro short circuit. Absolute dust-free production can not be done, when the dust is not enough to penetrate the diaphragm and then make the positive and negative poles short-circuit contact, its impact on the battery is not great; but when the dust is serious enough to pierce the diaphragm "degree" The impact on the battery will be very obvious. Due to the existence of the "degree" of piercing the diaphragm, when testing the self-discharge rate of a large number of batteries, it is often found that the self-discharge rate of most batteries is concentrated in a small range, and only a small part of the battery is self-discharged. The discharge is obviously high and the distribution is discrete. These should be the batteries that the diaphragm is pierced.

Finally, it should be noted that the side reactions occurring inside the lithium-ion battery are very complicated. Although Wenwu has checked some information, due to the limited energy of the level, the degree can only be analyzed for the time being. Let's take a look.

Self-discharge test method:

1. Measure the capacity loss after the battery has been put on for a period of time: The original purpose of the self-discharge study was to study the capacity loss after the battery was put on hold. However, the following reasons cause the test capacity loss to be difficult to implement: A. The irreversible degree during the charging process is too large, and even if the discharge is performed immediately after charging, the discharge capacity/charge capacity value is hardly guaranteed to be within 100% ± 0.5%. Such a large error requires that the hold time between tests must be very long. And this obviously does not meet the needs of daily production. B. A large amount of power and manpower are required to test the capacity, and the process is complicated and increases the cost. Based on the above two considerations, "the loss of the discharge capacity after the measurement is compared with the previous charge capacity" is generally not taken as the self-discharge standard of the battery.

2. Measure the K value over a period of time: a very important indicator of the degree of self-discharge, K value = ΔOCV / Δt. The common unit of K value is mV/d. Of course, this is related to the factory's own standard (or the personal preference of the factory boss), the performance of the battery itself, and the measurement conditions. The method of measuring the K value of the two voltages is simpler and the error is smaller, so the K value is a conventional method for measuring the self-discharge of the battery. The following text may mix the K value with self-discharge, please pay attention.

Factors affecting self-discharge and K-value:

1. Positive and negative materials, electrolyte types, and diaphragm thickness types: Since self-discharge occurs largely between materials, the properties of the material have a great influence on self-discharge. But what are the effects of various specific parameters of the material (such as the particle size of the positive and negative electrodes, the conductivity of the electrolyte, the porosity of the separator, etc.) on self-discharge, and what are the reasons for this? This issue is not the focus of research. . First, the problem itself is too complicated. Second, it does not make much sense for mass production or research. However, it is good for colleagues in Wenwu who have done experiments and found that the self-discharge rate of ternary batteries is higher than that of lithium cobalt oxide batteries. But no matter how much, I don’t know (Zi Yu: knowing it is knowing, I don’t know if I don’t know, it’s wisdom).

2. Storage time: The storage time becomes longer, on the one hand, the absolute value of the pressure drop is increased (nonsense), and on the other hand, the “instrument absolute error/pressure drop value” is reduced in phase, thereby making the result more accurate. . Wenwu discovered through experiments that the self-discharge was tested using an instrument with an accuracy of 0.1 mV. When the test time exceeded 14 days, the problem cell (what is the problem cell will be answered in the following text) can be distinguished from the normal cell ( Of course, the value of K in the batch of Wenwu is very small, about 0.13mV/d).

3. Storage conditions: The increase in temperature and humidity will increase the degree of self-discharge. This is well understood and has been seen in the literature downloaded from the forum and will not be repeated.

4. The initial voltage of the test: the initial voltage (or the primary voltage) is different, and the resulting K value is significantly different. Wenwu once divided a batch of batteries into three groups. The initial voltage was 3.92V in Group A (our factory voltage), 3.85V in Group B, and 3.8V in Group C. Then the K value was measured (the batch of batteries has been carried out before the experiment). Screening, self-discharge level is similar and storage and test conditions are completely consistent). It was found that the K value of group A was X, and the K value of group B was about 1.8X, while the group C was also X, but the voltage had a first rise and then fall. Similar conclusions are also reflected in other self-discharge tests. However, the battery's self-discharge study is ultimately a loss of capacity, so although the K values ​​differ a lot under different initial voltage conditions, the difference in capacity loss is not known. Considering that the test capacity error is too large (the charge/discharge control can be controlled at 100% ± 1% during the cycle), so no such experiment has been done. Interested friends can try it.

Measuring the role of self-discharge:

1. Predict the problem cell. In the same batch of batteries, the materials used and the control of the production are basically the same. When the self-discharge of individual batteries is obviously large, the reason is probably that the internal micro-short circuit is caused by impurities and burrs piercing the diaphragm. Because the effect of the micro short circuit on the battery is slow and irreversible. Therefore, in the short term, the performance of such batteries will not be much different from that of normal batteries, but with the gradual deepening of the internal irreversible reaction after long-term suspension, the performance of the battery will be far lower than its factory performance and other normal battery performance. The performance is: the irreversible loss of the maximum capacity is obviously high (for example, the irreversible capacity loss of three months reaches 5%, and the normal battery reaches this value for one year) and the rate capacity retention rate (0.5C/0.2C, 1C/0.2C). ) Reduction, cycle deterioration, and prone to lithium deposition after cycling (this is obtained from the results of the Wenwu experiment). Therefore, in order to ensure the quality of the factory battery, the battery with large self-discharge must be removed.

Then the next question is how to determine a self-discharge of a battery? As mentioned above, there are many factors affecting self-discharge, so it is unrealistic to give an empirical K value to all batteries as a unified standard. Wenwu only did an experiment (110pcs battery test for 3 months self-discharge, then pick out the problem battery), I can give a reference: pick a battery with a K value about twice the average K value of the entire batch of batteries as Bad product. If there is a serious micro-short circuit inside the battery, this is equivalent to a "quality" change compared to a normal battery, and its K value level will be significantly different from that of a normal battery. The consistency of the K value of the battery without problems is significantly stronger than the K value of the problematic battery, so it is not difficult to pick out the problem battery. After picking out the problem battery, how to deal with it is a consideration. If you want to know if these K values ​​are too large, whether the battery can be used as a product, Wenwu also has a suggestion (but this kind of experiment has not been done): in view of the irreversible self-discharge battery The capacity loss is very large, so the battery can be re-divided after being left for at least one quarter, and the capacity is not significantly attenuated, so it is considered to be no problem.

The above is an experiment + your own understanding, the error is inevitable, for reference only.

2. Match the battery. For batteries that need to be paired, the K value is one of the important criteria. In the process of measuring and calculating the K value, it should be noted that since the self-discharge level is significantly different under different initial voltages, it is necessary to ensure that the primary voltage of the battery is within a small range. I think the better standard of the voltage range is the battery factory's own factory voltage. If the problem battery has been picked out, then the remaining battery self-discharge rate should not be very different. At this time, the value of using K value as one of the matching standards is very large. Wenwu has not done similar experiments, and the matching problem It has always been a headache (see a document that after 1200 cycles of battery matching, the number of theoretical cycles is less than 200!), so I will not comment too much.

3. Help to develop battery factory voltage and factory capacity. Some customers have such requirements: regardless of the battery factory voltage, the factory capacity, just require the battery to be shipped to the customer, the capacity is 60%. At this time, it is necessary to evaluate the degree of self-discharge that the battery will generate during transportation to determine the factory voltage or capacity of the battery. In addition, since the self-discharge difference of the battery in different processes, different materials and different energy storage stages is obvious, a separate experiment is needed for this problem, and the data of other experiments cannot be simply applied.

Several misunderstandings of self-discharge:

1. Self-discharge after charging: Some friends said that the battery voltage drop is very fast after charging, saying that this is self-discharge too fast. The reason for this is that the battery is polarized during charging, causing the charging voltage to be higher than the actual battery voltage. The process of voltage drop after charging is the process in which the battery voltage returns from the charging voltage to its own voltage. The result of the charging voltage - the actual voltage of the battery, called the overpotential, is not what the so-called "virtual power", and there is no virtual power in the electrochemical terminology. Therefore, the voltage drop after charging is mainly the disappearance of the overpotential, and the proportion of self-discharge in it is very small and completely negligible. In addition, from Wenwu's own data, the voltage after charging is basically stable for at least 4h, and the static time is not the same regardless of whether the charging ends with constant current or constant voltage.

Misunderstandings only think of this one for the time being, and the follow-up will continue to be added.

This article is taken from: Battery Forum (http://club.battery.com.cn) For details, please refer to: http://club.battery.com.cn/thread-118348-1-1.html

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