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Oct 25 2019

Self Discharge Problem of Lithium Battery

Ⅰ. Introduction         

 The secondary battery has the self-discharge phenomenon, but the voltage of lithium battery is relatively high and the self-discharge rate is relatively low, which is a big selling point of it, so  what is self-discharge?


9V lithium battery

Ⅱ. Analyses

1) After the battery is stored for a long time, the sulfuric acid sinks, which cause a potential difference between the upper and lower plates. Because of it, self-discharge is resulting in. The electrolyte overflowing from the battery accumulates on the surface of the battery cover, and the positive and negative polarities form a passage.

2) The electrolyte and battery plate materials are impure, a potential difference is formed between the impurities and the plates and between the different impurities deposited on the plates, and through the electrolyte generates partial discharge.

3) The active material of the battery plate is detached, and the lower part of the deposit is too many to cause the plate short-circuit, and the battery electrolyte is layered up and down to cause self-discharge.

 

Ⅲ. What is Self-discharge Rate?

A battery that initially has a certain amount of power, when it left open circuit for a period of time in a prescribed environment, and the power will be lost for a variety of reasons. The ability of the battery to retain as much power as possible is the ability to maintain the battery’s charge, and the ratio of the remaining charge to the original charge is the self-discharge rate.

1 Innate Factor

Why does the battery placed in the open circuit lose its charge? The innate factors mainly come from the loss of electrochemical material inside the battery and the internal short circuit. The loss of the battery material is irreversible, causing the loss of the capacity of the battery, and the loss is the embodiment of the capacity recovery performance; the power loss caused by the short circuit consumes the current power, and the capacity will not affected by this part of the reaction.

The sum of the power loss (irreversible) caused by the capacity loss and the simple power loss (reversible) is the self-discharge amount.

 

1.1 Side reactions of electrochemical materials

Material side reactions mainly occur in three parts: positive electrode material, negative electrode material and electrolyte.

The positive electrode material is mainly a compound of various types of lithium, which always has a slight reaction with the electrolyte, and the environmental conditions are different, and the degree of reaction is also different. The positive electrode material reacts with the electrolyte to produce an insoluble product, making the reaction irreversible. The positive electrode material involved in the reaction lost its original structure and the battery lost its corresponding power and permanent capacity.

In the negative electrode material, the graphite negative electrode originally has the ability to react with the electrolyte. During the combination process, the reaction product SEI film adheres to the surface of the electrode, so that the electrode and the electrolyte stop the intense reaction. However, this reaction has also been carried out in small amounts because the defects of the SEI film. The reaction of the electrolyte with the negative electrode consumes both the lithium ion and the negative electrode material in the electrolyte, in other words, the loss of electricity caused by the reaction also brings about the loss of the maximum available capacity of the battery.

The electrolyte, except reacting with the positive and negative electrodes, reacts with impurities in the positive and negative materials, even the impurities in the material itself. These reactions all produce irreversible products, resulting in a reduction of lithium ions, which is the reason for the loss of available capacity.

1.2 Internal short circuit

In the production process of the battery, some dust impurities are inevitably mixed. The properties of these impurities are complicated, and some impurities may cause slight conduction of the positive and negative electrodes, so that the charge is neutralized and the power is damaged.

The dimensional deviation of the current collector and the processing burrs may also turn on the positive and negative electrodes. In the early life cycle of the battery, it only shows less self-discharge, but with the time went by, it has the greater possibility to cause large-scale short-circuit of the positive and negative poles, which is a main factor of the thermal runaway of the battery.

1.3 SEI film defect

The original function of the SEI film is to isolate the positive and negative electrodes so that only lithium ions can pass through and electrons cannot pass. If there is a problem with the quality of the film, the effect of the it will not work out properly. Even a small defect will also have a significant impact on the self-discharge rate.

lithium-ion packs

2 Acquired Factors

The self-discharge rate of the battery will vary depending on the application environment, service stage, and application state.

2.1 Temperature

The higher the ambient temperature, the higher the activity of the electrochemical material. The reactions involving the positive electrode material, the negative electrode material, and the electrolyte mentioned are more intense, resulting in more capacity loss in the same period of time.

2.2 External short circuit

The external short circuit of the battery placed in the open circuit is mainly affected by the degree of air pollution and the humidity of the air. Regular battery self-discharge characteristics test experiments will require strict laboratory environment and humidity level. Because high air humidity can cause the conductivity to rise, and air pollution mainly means that conductive particles may be contained in the air, so that the conductivity of the air will increase.

2.3 Charges

The researchers specifically compared the effect of the charge on the self-discharge rate. The overall trend is that the higher the charge, the higher the self-discharge rate. In a word, it means that the higher the charge, the higher the positive potential and the lower the negative potential. Thus, the stronger the positive electrode oxidability, the stronger the negative polarity of the negative electrode and the more intense the side reaction. 

2.4 Time

With the same power and capacity loss, the longer the time, the more power and capacity are lost. However, the self-discharge rate is generally used as an indicator for comparison of different batteries. That is, at the same precondition, the effect of time can only be said to affect the self-discharge amount.

2.5 Aging of SEI film

As battery recycling times continues to increase, the uniformity and compactness of the SEI film will change. The aging SEI film gradually expose shortcomings when protects the negative electrode, causing more contact between the negative electrode and the electrolyte, which increases side reactions. In addition, different quality SEI films will also bring different self-discharge rates in the early life of the battery.

Therefore, the self-discharge rate as a characterization of the SEI film quality is often considered in production, therefore, one of the methods to decrease the self-discharge rate is to increase the additive and improve the SEI film quality.

 Lithium-ion Battery

3 Self-discharge Rate Test

3.1 The purpose of testing self-discharge rate

The self-discharge rate test has some reference value.

One is to regard the self-discharge rate as the inspection and testing index of the battery quality. Applying it in the national standard to compare the product level of different manufacturers horizontally and check the quality of the industry.

The other is used for cell sorting. The consistency of the cells is an important parameter for the quality of the battery packs after grouping. Various methods have been studied to group the cells, and it is expected that the cells with the same consistency are used in the same battery pack. The self-discharge rate is one of the commonly used indicators for static screening.

Another use is, as an indicator of product quality control. Testing the same batch of batteries, if some batteries have a high self-discharge rate, indicating that their quality is defective, they must be selected and disposed of separately.

Finally, the self-discharge rate is regarded as an indicator to measure the degree of aging of the battery and is used to evaluate the life cycle of the battery.

3.2 Test method 

The common test method for self-discharge rate is to measure the battery charge in the course of time, and obtain a ratio as the self-discharge rate. This method is time consuming and costly, and is often used in a few occasions, such as product certification testing, product sampling inspection, and so on.

In the general production process, people will look for alternatives. It found that the slope of the curve is relatively large when the battery is low, at the curve of the open circuit voltage and the charge capacity in the lower state of charge. And a small voltage drop will result in a large voltage drop. As shown in the figure below, the horizontal axis is the charge amount and the vertical axis is the open circuit voltage. It can be seen that the phase is very steep when the charge is less than 10%.

SOC-OCV Curve of Lithium Battery 

Figure 1. SOC-OCV Curve of Lithium Battery

The self-discharge rate observed in the low-power state was verified by the test based on the defined manner, and the relative self-discharge rate was consistent. In the case of core sorting and factory quality control, which need to test the self-discharge rate in large quantities, this method shows its advantages.

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