Self-discharge phenomenon of the battery is one of the factors affecting the consistency, self-discharge will lead to a large difference in the battery SOC after a period of time, affecting the battery capacity and stability, although the self-discharge phenomenon of lithium batteries is not obvious, but still exists, self-discharge exists on all types of batteries, this article mainly explains the phenomenon of self-discharge of the battery, I hope it can give you some help.
Classification of self-discharge.
From the impact of self-discharge on the battery, self-discharge can be divided into two types: self-discharge in which the loss of capacity can be reversibly compensated; self-discharge in which the loss of capacity cannot be reversibly compensated. According to these two classifications, we can give some causes of self-discharge in an approximate outline.
Causes of self-discharge.
1. Causes of reversible capacity loss: The cause of reversible capacity loss is the occurrence of a reversible discharge reaction, the principle is the same as the normal discharge reaction of the battery. The difference is that the normal discharge electron path is the external circuit, the reaction speed is very fast; self-discharge electron path is the electrolyte, the reaction speed is very slow.
2. Causes of irreversible capacity loss: When irreversible reactions occur inside the battery, the resulting capacity loss is irreversible capacity loss. The types of irreversible reactions occurring mainly include.
A: Irreversible reaction between cathode and electrolyte (relatively mainly occurs in lithium manganate and lithium nickelate, which are two materials prone to structural defects, for example, the reaction between lithium manganate cathode and lithium ion in electrolyte.
LiyMn2O4+xLi++xe-→Liy+xMn2O4, etc.).
B:Irreversible reaction between cathode material and electrolyte (the SEI film formed during chemistry is to protect the cathode from corrosion of electrolyte, the possible reactions between cathode and electrolyte are
LiyC6→Liy-xC6+xLi++x, etc.).
C:Irreversible reaction caused by impurities carried by the electrolyte itself
(e.g. possible reaction of CO2 in solvent: 2CO2 + 2e- + 2Li+ → Li2CO3 + CO.
The reaction that occurs with O2 in the solvent: 1/2O2 + 2e-+2Li+→ Li2O).
Similar reactions irreversibly consume the lithium ions in the electrolyte, which in turn loses battery capacity.
D:Irreversible reaction caused by micro-short circuit caused by impurities when made. This phenomenon is the most important cause of high self-discharge of individual batteries. Dust in the air or metal powder on the poles and diaphragm during manufacture can cause internal micro-short circuit. Absolute dust-free production is not possible, and when the dust is not enough to pierce the diaphragm and thus short-circuit the positive and negative electrodes, its impact on the battery is not significant; however, when the dust is so serious that it pierces the diaphragm "degree", the impact on the battery will be very obvious. Because of the existence of whether to pierce the diaphragm "degree", so in testing a large number of battery self-discharge rate, often found that most of the battery self-discharge rate is concentrated in a small range, while only a small part of the battery self-discharge is significantly high and discrete distribution, these should be the diaphragm is pierced battery.
Finally, it should be noted that the lithium-ion battery internal side reactions are very complex, although Wen Wu checked some information, but due to the limited level of energy, for the time being can only analyze the extent of this, you can make do with it.
Self-discharge test method.
Measure the capacity loss of the battery after a period of time: the original purpose of the self-discharge study is to study the capacity loss of the battery after being set aside. However, the following reasons make testing capacity loss difficult to implement: A. The degree of irreversibility in the charging process is so large that even if the discharge is carried out immediately after charging, it is difficult to ensure that the discharge capacity/charge capacity value is within 100% ± 0.5%. Such a large error requires that the shelf time between tests must be very long. B. Testing capacity requires a lot of electricity and human resources, which makes the process complicated and increases the cost. Based on the above two considerations, "measuring the loss of discharge capacity after shelving compared to the previous charge capacity" is generally not used as the self-discharge standard for batteries.
2. Measure the K value over a period of time: a very important indicator to measure the degree of self-discharge K value = △OCV / △t. K value is commonly measured in mV / d, of course, this is related to the factory's own standards (or the factory boss's personal preference), the performance of the battery itself, the measurement conditions. The method of measuring voltage twice to calculate K value is easier and has less error, so K value is the conventional method to measure battery self-discharge. Please note that the following text may mix K value with self-discharge.
Factors affecting self-discharge and K value.
1. positive and negative electrode materials, electrolyte type, diaphragm thickness type: Since self-discharge occurs largely between materials, the performance of materials has a great impact on self-discharge. But how much does each specific parameter of the material (such as particle size of cathode and anode, conductivity of electrolyte, porosity of diaphragm, etc.) affect the self-discharge and what are the reasons for the effect? This question is not the focus of the study. One is that the problem itself is too complicated, and the other is that it is not very meaningful for mass production and research. But the good thing is that Wen Wu's colleagues have done experiments and found that the self-discharge rate of ternary batteries is higher than that of lithium cobalt-acid batteries. But then more, it is not known (Zi said: know it is known, do not know is not known, is also wise).
2. Storage time: longer storage time, on the one hand, the absolute value of the pressure drop increases (nonsense), on the other hand, it is disguised to reduce the "absolute error of the instrument / pressure drop value", so that the results are more accurate. Wenwu found through experiments, using the accuracy of 0.1mV instrument testing self-discharge, when the test time of more than 14 days, to be able to distinguish the problem cells (what is the problem cells will be answered in the following text) and normal cells (of course, Wenwu that batch of battery K value is very small, 0.13mV / d or so).
3. storage conditions: the increase in temperature and humidity, will increase the degree of self-discharge. This is well understood and the literature downloaded from the forum has seen this kind of data, not to repeat.
4. The initial voltage of the test: the initial voltage (or primary voltage) is different, the resulting K value varies significantly. Wenwu had a batch of batteries into three groups, the initial voltage were Group A 3.92V (our factory voltage), Group B 3.85V, Group C 3.8V, and then measured the K value (the batch of batteries in the experiment has been screened before the self-discharge level is similar and storage, testing conditions are identical). It was found that the K value of group A was X, group B had a K value of about 1.8X, while group C would also be X, but the voltage had a phase of first rise and then fall. Similar findings were found in other self-discharge tests. However, the self-discharge study of the battery is ultimately the 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 (to do the cycle when the charge/discharge can be controlled at 100% ± 1% is good), so did not do such experiments. Interested parties can try.
The role of measuring self-discharge.
1. Predict the problem of battery cells. The same batch of cells, the materials used and made to control basically the same, when there is an individual battery self-discharge significantly large, the reason is likely to be internal due to impurities, burrs piercing the diaphragm and a serious micro-short circuit. Because the impact of micro-short circuit on the battery is slow and irreversible. Therefore, in the short term, the performance of such batteries will not differ much from normal batteries, but after long-term shelving with the gradual deepening of the internal irreversible reaction, the performance of the battery will be far below its factory performance and other normal battery performance. Performance: irreversible loss of maximum capacity is obviously high (such as three months irreversible capacity loss of 5%, while the normal battery to achieve this value to a year), multiplier capacity retention rate (0.5C/0.2C, 1C/0.2C) reduced, cycle deterioration and easy to appear after the cycle of lithium precipitation (this are the results of Wenwu experiments), etc.. Therefore, in order to ensure the quality of the factory battery, self-discharge large battery must be eliminated.
Then the next question is how to determine a battery self-discharge large? As mentioned earlier, there are many factors affecting self-discharge, so it is unrealistic to give an empirical K value as a unified standard for all batteries. Wenwu has only done one experiment systematically (110pcs batteries measured 3 months self-discharge, and then pick out the problem battery), I can give the reference is: the K value of about 2 times the average K value of the whole batch of batteries to pick out as defective products. If the battery has a serious internal micro-short circuit, then compared to the normal battery, this is equivalent to a "quality" change, the K value level will be significantly different from the normal battery. The consistency of the K-value of the battery without problems is significantly stronger than the K-value of the battery with problems, so it is not difficult to pick out the problem battery. If you want to know whether these K-value batteries can be shipped as A products, Wenwu also has a suggestion (but such experiments have not been done): Given the large irreversible capacity loss of self-discharge batteries, so the battery can be set aside for at least a quarter after re-allocation, the capacity is not significantly decayed, it is not considered a problem.
2. Grouping of batteries. For batteries that need to be grouped, 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 varies significantly at different initial voltages, it is necessary to try to ensure that the primary voltage of the battery is within a modest range. I think a better primary voltage range standard is the battery factory's own factory voltage. If the problem battery has been picked out, then the rest of the battery self-discharge rate should not be very different, at this point the K value as one of the grouping criteria in the end how meaningful, Wen Wu has not done similar experiments, and the grouping problem has been a great headache (read a literature that 1200 cycles after the battery grouping, the theoretical number of cycles less than 200!) So do not comment too much for the time being.
3. To help develop the battery factory voltage, factory capacity. Some customers have such requirements: regardless of the battery factory voltage, factory capacity of how much, just ask the battery shipped to the customer, the capacity of 60%. At this point, it is necessary to assess the extent of self-discharge of the battery in the transportation process, so as to determine the factory voltage or capacity of the battery. In addition, because of different processes, different materials, different energy storage stage of the battery self-discharge difference is obvious, so this issue requires a separate experiment rather than simply apply the data from other experiments.
Self-discharge misconceptions.
Self-discharge after charging: Some friends say that the battery voltage drop is very fast after charging, saying that this is self-discharge too fast. The occurrence of the situation is due to the polarization of the battery in the charging process, resulting in a charging voltage higher than the actual voltage of the battery. The process of voltage drop after charging, is the battery voltage from the charging voltage drop back to its own voltage process. The result of charging voltage - actual battery voltage, called super potential, is not what is called "virtual electricity", and electrochemical terminology does not have the name of virtual electricity. Therefore, the voltage drop after charging is mainly the disappearance of the super potential, self-discharge in which the proportion of very, very small can be completely ignored. In addition, from Wenwu's own data, the basic stability of the voltage after charging needs at least 4h, and regardless of whether the charge to constant current or constant voltage as the end, the difference in resting time is not very large.
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