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Oct 25 2019

What is the Self-discharge of Batteries?

Ⅰ Abstract

In theory, the chemicals within the battery does not react with each other when the battery is not used, and redox reactions only occur when an electrical appliance is connected to the battery.

In real life however, ordinary batteries, including dry batteries (alkaline), will generate electricity due to a chemical reaction inside the battery. In other words, a minuscule amount of the chemical substances inside the batteries reacts even without any connections between the electrodes. Many people regard the circuit as only one of the possible paths that can channel an electron flow to the desired location. There are also things such as degrading electrolyte, leaks and whatsoever, but the most prominent is due to chemical reactions within the cell itself.

Those internal reactions reduce the stored charge of the battery and thus decrease the capacity of the battery little by little. This phenomenon is called self discharge.

This Video Explains the Lithium Ion Self-discharge

Catalog

Ⅰ Abstract

Ⅱ Why There is Self Discharge?

2.1 Three Important Points

2.2 Physical Factor

2.3 Object Factors

Ⅲ Self Discharge Test

3.1 The Purpose of Testing Self-discharge Rate

3.2 Test Methods

3.3 The Role of Testing Self-discharge

Ⅳ Typical Batteries Self-discharge Analysis

4.1 Li-ion Batteries

4.2 Lead-acid Batteries

4.3 Ni-Cd Batteries

4.4 Ni-MH Batteries

Ⅴ One Question Related to Self Discharge and Going Further

5.1 Question

5.2 Answer


Ⅱ Why There is Self Discharge?

2.1 Three Important Points

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.

 

2.2 Physical Factor

Why does the battery placed in the open circuit lose its charge? The physical 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.

 

2.2.1 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.

 

2.2.2 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 batteries, thus regular battery self-discharge characteristics test experiments should be required in a strict laboratory environment and proper humidity level.

 

2.2.3 SEI Film Defect

The original function of the SEI film is to isolate the positive and negative electrodes electrons cannot pass. If there is a problem with the quality of the film, the effect of the it will not work out properly, for example, battery flatulence and low pressure may be caused. Even a small defect will also have a significant impact on the self-discharge rate.

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. One of the methods to decrease the self-discharge  is to increase the additive and improve the SEI film quality.

18650 Li-ion battery

2.3 Object Factors   

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

2.3.1 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.

 

2.3.2 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.

 

2.3.3 Time

With the same power and capacity loss, the longer the time, the lost is large. However, the self-discharge rate is generally used as an indicator for comparison of different batteries. That is, at the same precondition, the time can be seem a factor to affect the self-discharge degree.

9V lithium battery

Self Discharge 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 Methods

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 Batteries (SOC: state of charge, OCV: open circuit voltage)

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.

 

3.3 The Role of Testing Self-discharge

1) Predict the problem cell

In the same batch of batteries, the materials and manufacturing process are basically the same. When the self-discharge of individual batteries is obviously too large, the reason is likely to be a serious micro short circuit due to impurities and burrs piercing the separator. The performance of this type of battery will not be much different from that of a normal battery in a short term, but as the internal irreversible reaction gradually deepens after long-term storage, the performance of the battery will be much lower than its factory performance and other normal battery performance.

The result is: the irreversible loss of the maximum capacity is significantly higher (for example, the irreversible capacity loss in three months reaches 5%, and the normal battery will reach this value in one year), the rate capacity retention rate (0.5C / 0.2C, 1C / 0.2C ) decrease, the cycle becomes worse and have lithium precipitation. Therefore, to ensure the quality of the factory batteries, batteries with large self-discharge must be eliminated. However, due to the large loss of irreversible capacity of the self-discharged battery, the battery can be re-capacited after being left for at least one quarter. If the capacity does not significantly decay, batteries can be used.

2) Battery pack

For batteries that need to be assembled, the K value (in the lithium battery industry, it refers to the voltage drop of the battery per unit time) is one of the important indicator. When batteries set are assembled into a battery pack, the difference in the self-discharge of the battery will cause the batteries inside the battery pack to appear unbalanced. If there is a leakage current path inside the battery, it may also cause self-discharge. Particulate pollutants and dendrite growth will create a "micro short circuit" inside the battery, forming a leakage current path, which may cause battery failure. Therefore, a battery with excessive self-discharge indicates that there may be a failure. In the process of measuring and calculating the K value, due to the obvious difference in the self-discharge level at different initial voltages, it is necessary to ensure that the primary voltage of the battery is within a small range, and a better primary voltage range is the battery factory's own ex-factory voltage.

3) Help set the battery ex-factory voltage and capacity.

Some customers have such requirements: it is required that the battery to be shipped to the customer with a capacity of 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, due to different processes, materials, and energy storage stages, a separate experiment is needed for this problem and the data of other experiments cannot be simply applied.

 

Ⅳ Typical Batteries Self-discharge Analysis

The self-discharge is related to the solubility of the positive electrode material in the electrolyte and its instability (easy self-decomposition) after being heated. The self-discharge of rechargeable batteries is much higher than that of primary batteries. Moreover, different battery types have different monthly self-discharge rates. The self-discharge of primary batteries is significantly lower, not exceeding 2% per year at room temperature.

During storage, self-discharge is accompanied by an increase in the internal resistance of the battery, which will cause a reduction in the battery's load capacity. In the case of a large discharge current, the energy loss is obvious.

li-ion battery

Typical Batteries Self-discharge Rates

Table 1: Percentage of Self-discharge in Years and Months

Battery System

Estimated Self-discharge or Self Life

Zinc-carbon

2-3 years shelf life

Lithium-metal

10% in 5 years

Lead-acid

4-6% per month

Nickel-cadmium

15-20% per month

Alkaline

2–3% per year (7-10 years shelf life)

Lithium-ion

5% in 24h, then 1–2% per month (plus 3% for safety circuit)


 

4.1 Li-ion Batteries

Self Discharge Rates for a Lithium-ion Battery

Figure 2. Self Discharge Rates for a Lithium-ion Battery

The self-discharge reaction occurring inside a lithium-ion battery is very complicated. The self-discharge rate of lithium-ion batteries is generally 2% to 5% per month, and 5% -8% at normal temperature. When an irreversible reaction occurs inside the battery, the resulting capacity loss is irreversible, mainly including:

1) Irreversible reaction between positive electrode material and electrolyte

It mainly occurs in two materials that are prone to structural defects, such as lithium manganate and lithium nickelate. For example, the reaction of lithium manganate cathode and lithium ions in the electrolyte: LiyMn2O4+xLi++xe-→Liy+xMn2O4

2) Irreversible reaction between anode material and electrolyte

The SEI film of the Li-ion battery is to protect the negative electrode from the corrosion of the electrolyte. The possible reaction between the negative electrode and the electrolyte is: LiyC6→Liy-xC6+xLi++xe-

3) Irreversible reactions caused by impurities in the electrolyte itself

For example, the possible reaction of CO2 in the solvent: 2CO2 + 2e-+ 2Li + → Li2CO3 + CO; the reaction of O2 in the solvent: 1/2O2+2e+2Li+→Li2O

These reactions irreversibly consume lithium ions in the electrolyte, resulting in battery capacity loss.

 

4.2 Lead-acid Batteries

The self-discharge of the lead electrode comes from oxygen evolution and oxygen absorption corrosion, due to the small solubility of oxygen in the sulfuric acid and can be removed, and the high concentration of hydrogen ions in the electrolyte solution, the self discharge caused by oxygen evolution is obvious. The potential of the balanced electrode of lead is smaller than that of hydrogen. Since the oxygen evolution overpotential of hydrogen is high, the oxygen evolution reaction is not obvious. If the purity of lead is high and there are few impurities, the oxygen evolution corrosion will be lighter and the self-discharge will naturally be smaller.

 

4.3 Ni-Cd Batteries

For fully charged nickel oxide electrodes, due to the presence of unstable manganese dioxide, oxygen evolution reactions(OER) are likely to occur during storage, resulting in self-discharge. In one hand, the negative electrode of cadmium is very stable in the electrolyte solution, so the self-discharge rate of the nickel-cadmium battery is small; in the other hand, the high-rate discharge nickel-cadmium battery has a large electrode surface area, so the self-discharge rate is also large.

 

4.4 Ni-MH Batteries

Like other batteries, nickel-metal hydride batteries also have self-discharge phenomena. The device of the high-voltage NiMH battery fills the entire battery case with hydrogen gas, and the negative electrode electroactive material directly contacts with the positive electrode electroactive material nickel oxide, result in self-discharges during storage.

batteries pack

Batteries are all affected by self-discharge. It is not a production line issue but a battery characteristic; although improper fabrication practices and handling will increase the problem. What we should know is that self-discharge is permanent and cannot be reversed. In order to reduce self-discharge, it is recommended to store cells and batteries at lower temperatures.

 

5.1 Question

Why do batteries self-discharge?

5.2 Answer

Giving an example, the usual commercial zinc-carbon cell that is used to power something like a torch. Even without the designated circuit, the zinc shell will still react with the ammonium electrolyte and proceed to complete its reaction. That’s why cells will slowly lose charge and die out even when unused, as the amount of reactants within the cell decreased bit-by-bit. More modern rechargables are more carefully designed such that the possible self discharge is decreased, but it can never be eliminated as they are also the basis for a battery to function properly.

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