Causes and control methods of lithium battery self-discharge

Lithium batteries often experience voltage drops during use or storage due to reasons such as electrolyte compatibility, graphite negative electrode characteristics, and assembly inconsistencies. A large part of the voltage drop is caused by the self-discharge of the battery core itself.

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The size of battery self-discharge can be expressed in two ways: one is measured by how many mV the voltage drops every day, and the unit is mV/day. A good battery will not drop more than 2mV in one day; the other is also the commonly used K value. Notation, that is, the voltage drop per unit time, that is, mV/h, how many mV the voltage drops in one hour. The K value of a good battery is generally within 0.08mV/h.

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1. Causes of self-discharge

There are two reasons for excessive self-discharge of lithium batteries: physical micro short circuit and chemical reaction. Two reasons will be analyzed below:

1. Physical micro short circuit

Physical micro-short circuit is the direct cause of low voltage of lithium batteries. Its direct manifestation is that after the battery is stored at room temperature or high temperature for a period of time, the battery voltage is lower than the normal cut-off voltage. Compared with self-discharge caused by chemical reactions, self-discharge caused by physical micro-short circuit will not cause irreversible loss of lithium battery capacity. There are many situations that cause physical micro-short circuits, which are divided into the following categories:

1. Dust and burrs

When we disassemble a slightly short-circuited battery, we often find that black spots appear on the separator of the battery. If the black spot is in the middle of the diaphragm, there is a high probability of dust breakdown. If the majority of black spots are at the edge, it is caused by the burrs produced during the pole piece cutting process . These two points are easier to distinguish.

1. Metal impurities in positive and negative electrodes

In the battery, metal impurities undergo chemical and electrochemical corrosion reactions and dissolve into the electrolyte:

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After this, Mn + migrates to the negative electrode, and metal deposition occurs:

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As time goes by, metal dendrites continue to grow, and finally penetrate the separator, causing micro-short circuits in the positive and negative electrodes, continuously consuming power, and causing the voltage to decrease.

&#;Cathode metal impurities

After the charging reaction, the metal impurities in the positive electrode also break down the separator, forming black spots on the separator, causing a physical micro-short circuit. Generally speaking, as long as it is a metal impurity, it will have a greater impact on the self-discharge of the battery, and generally the metal element has the greatest impact. According to some literature, the order of influence is as follows: Cu>Zn>Fe>Fe2O3. For example, many cathode lithium iron materials will face the problem of excessive self-discharge, which is caused by excessive iron impurities.

&#; Negative metal impurities

Due to the formation of the primary battery, the metal impurities in the negative electrode will be freed and deposited on the separator, causing the separator to conduct, forming a physical micro-short circuit. This is often encountered with some low-end negative electrode materials in China. Metal impurities in the negative electrode slurry have less influence on self-discharge than metal impurities in the positive electrode, among which Cu and Zn have a greater impact on self-discharge.

1. Metal impurities in auxiliary materials

For example, metal impurities in CMC and tape

1. Chemical reaction
2. Moisture

The moisture causes the electrolyte to decompose and releases a large number of electrons, which are then embedded in the oxidation structure of the positive electrode, causing the positive electrode potential to drop and causing low voltage.

In addition, when there is H2O in the battery, it will react with LiPF6 to produce corrosive gases such as HF; at the same time, it will react with solvents and other gases to produce CO2 and other gases, causing the battery to expand; HF will react with many substances in the battery, such as the main components of SEI, and destroy SEI film; generates CO2, H2O, etc.; CO2 causes the battery to expand, and the regenerated H2O participates in the reactions of LiPF6, solvents, etc., forming a vicious chain reaction.

SEI film : 1) The solvent enters the graphite layer and reacts with LixC6, causing irreversible capacity loss; 2) Repairing the damaged SEI consumes Li+ and solvent, which further causes irreversible capacity loss.

1. Electrolyte solvent

The addition of certain electrolyte solvents will cause the battery voltage to drop too quickly. I tried a solvent before. After adding it, the ion conductivity increased significantly, but the self-discharge rate was 3 times faster than that of normal solvents.

Possible mechanism: These solvents are not resistant to oxidation and undergo slow chemical reactions during storage, consuming capacity and causing the voltage to drop.

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1. SEI film is unstable

During the storage process, the warehouse has a certain temperature, which causes the SEI film to fall off and react again, causing battery bloating, low pressure, etc.

1. Poor packaging

the pole position is over-sealed, it may cause corrosion of the pole and consume the low voltage of the lithium source . If other positions are over-sealed, the electrolyte may penetrate the CPP layer and corrode the aluminum foil, causing perforation of the aluminum-plastic film, and moisture entering causing low-pressure flatulence.

Many times, flatulence and low pressure appear together. At this time, it is often more serious, and the battery will eventually be scrapped.

1. Control of self-discharge

Self-discharge of lithium batteries will reduce product quality and customer satisfaction. So how to deal with battery self-discharge? This can be done from the following aspects:

1. Strictly control the introduction of dust

The most common cause of battery low voltage is dust and burrs. How to control the introduction of dust is a very important and difficult task. The management of many manufacturers attaches great importance to the control of workshop dust, but it is often ignored at the practical level.

On the one hand, the factory building design must be reasonable. In the area where the pole piece manufacturing process is located, dust control is not particularly strict, especially in the slurry preparation area. However, during the lithium battery assembly process, dust impurities must be strictly prevented from entering. In the design of the factory building, different areas must be strictly separated and isolated and protected.

The second is to do a good job in 5S work in the operating area. Having good habits and high 5S literacy can improve the product yield rate. For example, clean up before work to ensure that there is no dust, clean the equipment after operation to ensure that no impurities remain, etc.

1. Improve the pole piece manufacturing process

Burrs are also the culprit of self-discharge in lithium batteries. Burrs are mainly formed during pole piece slitting. There are many reasons for the formation of burrs:

(1) The raw materials of the positive and negative electrode slurries are made of materials with larger BET, and a lot of conductive agent is added, which causes the active material, conductive agent and other particles to be weakly bonded and burrs appear. Then it is necessary to choose appropriate materials and improve slurry preparation, coating and other processes to ensure that the pole pieces will not lose powder or produce burrs.

(2) The slicing tool is not replaced in time, causing burrs. Be familiar with the life of the cutter and replace it in time according to usage.

1. Raw material quality management and control

As mentioned above, metal impurities in the positive and negative electrode materials are also one of the causes of self-discharge of lithium batteries. Therefore, the company needs to strengthen the inspection of incoming materials to ensure that the incoming materials meet the standards, otherwise it will cause great losses. At the same time, we must also strengthen the quality control of raw materials to prevent failure caused by the introduction of impurities and moisture during storage.

1. Environmental control

In terms of environmental control, it is not only necessary to control the number of dust particles in the environment, but also to control the moisture content in key steps to prevent excessive moisture content from adversely affecting the quality of lithium batteries.

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In this article, we will explore the many benefits and diverse applications of lithium batteries for inverters.

Advantages of using lithium batteries in inverters

Lithium batteries have changed the tide of the energy storage industry because of their superior performance and numerous other advantages. Here are some of the key advantages of using lithium batteries for inverters:

1. Higher energy density: Lithium batteries have a significantly higher energy density compared to traditional lead-acid batteries. This means they can store more energy in a smaller and lighter package, making them ideal for portable inverters and systems with limited space. With higher energy density, lithium batteries can power devices for longer durations.
2. Longer lifespan: Lithium batteries have a longer lifespan compared to other battery types. They can withstand a greater number of charge cycles, resulting in reduced maintenance and replacement costs. This longevity makes lithium batteries an economical choice in the long run.
3. Higher charge/discharge efficiency: Lithium batteries offer higher charge and discharge efficiency, meaning they can convert a higher percentage of stored energy back into usable power. This makes them more efficient in delivering power to inverters and helps optimize the overall energy consumption.
4. Faster charging: Lithium batteries have a faster charging time compared to other battery types. This allows for quicker recharging of the batteries, minimizing downtime and ensuring a continuous power supply.
5. Low self-discharge rate: Lithium batteries have a low self-discharge rate, which means they retain their charge for longer periods when not in use. This is particularly advantageous for inverters in emergency backup systems, as they can maintain their charge for extended periods without the need for frequent recharging.

The advantages of lithium batteries for inverters extend beyond the points mentioned above. Now, let&#;s compare lithium batteries with other types of batteries commonly used in inverters.

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Conclusion

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