The physical micro-short circuit is the direct cause of the low voltage of the lithium battery. The direct performance is that the battery voltage is lower than the normal cut-off voltage after the battery is stored at normal temperature and high temperature for a period of time. Compared with self-discharge caused by chemical reaction, self-discharge caused by physical micro-short circuit does not cause irreversible loss of lithium battery capacity.
Lithium batteries often suffer from voltage drop during use or storage due to electrolyte compatibility, graphite negative characteristics, and inconsistent assembly. A large part of the voltage drop is caused by the self-discharge of the cell itself.
The self-discharge size of the battery can be expressed in two forms: one is measured by how many mV the voltage drops per day, the unit is mV/day, and the good battery voltage drop does not exceed 2mV per day; the other is also the commonly used K value. Representation, that is, the pressure drop per unit time, that is, mV/h, how many mV is dropped in one hour, and the good battery K value is generally within 0.08mV/h.
K=OCV2-OCV1/â–³T
First, the cause of self-dischargeThere are two reasons for causing excessive self-discharge of lithium batteries: physical micro-shorts and chemical reactions. The following two reasons will be analyzed:
1, physical micro short circuit
The physical micro-short circuit is the direct cause of the low voltage of the lithium battery. The direct performance is that the battery voltage is lower than the normal cut-off voltage after the battery is stored at normal temperature and high temperature for a period of time. Compared with self-discharge caused by chemical reaction, self-discharge caused by physical micro-short circuit does not cause irreversible loss of lithium battery capacity. There are many situations that cause physical micro-short circuits, which are divided into the following types:
a, dust and burrs
We disassembled the micro-short-circuited battery and often found black spots on the battery's diaphragm. If the position of the black spot is in the middle of the diaphragm, then there is a high probability that the dust will break down. If the black point is at the edge position, it is caused by the burr generated during the slitting process. These two points are better distinguished.
b. Metal impurities of positive and negative electrodes
In the battery, the metal impurities undergo chemical and electrochemical corrosion reactions and dissolve into the electrolyte:
M → Mn+ + ne-;
Thereafter, Mn+ migrates to the negative electrode and metal deposition occurs:
Mn+ + ne-→ M;
As time increases, metal dendrites continue to grow and eventually penetrate the membrane, causing micro-short circuits between the positive and negative electrodes, consuming constant power, resulting in a voltage drop.
1 positive metal impurity
After the metal impurity of the positive electrode undergoes a charging reaction, it also breaks through the separator, forming a black spot on the separator, causing a physical micro short circuit. Generally speaking, as long as it is a metal impurity, it will have a great influence on the self-discharge of the battery, and generally the metal element has the greatest influence. According to some documents, the order of influence is as follows: Cu>Zn>Fe>Fe2O3. For example, many positive iron-lithium materials will face the problem of excessive self-discharge, which is caused by excessive iron impurities.
2 negative metal impurities
Due to the formation of the primary battery, the negative metal impurities will be freed and deposited at the diaphragm to cause the diaphragm to conduct, forming a physical micro-short circuit, which is often encountered in some low-end anode materials in China. The influence of metal impurities in the negative electrode slurry on self-discharge is less than that in the positive electrode, and Cu and Zn have a great influence on self-discharge.
c. Metal impurities of auxiliary materials
Such as CMC, metal impurities in the tape
2, chemical reaction
a, moisture
The water causes the electrolyte to decompose, releasing a large amount of electrons, and the electrons are re-inserted into the positive electrode oxidation structure, thereby causing the positive electrode potential to drop, resulting in a low pressure;
In addition, when H2O is present in the battery, it reacts with LiPF6 to produce a corrosive gas such as HF; at the same time, it reacts with a solvent or the like to generate a gas such as CO2 to cause the battery to expand; HF reacts with many substances in the battery such as SEI, and destroys SEI membrane; CO2 and H2O are formed; CO2 causes the battery to swell, and the regenerated H2O participates in the reaction of LiPF6 and solvent to form a malignant chain reaction.
The consequences of SEI membrane destruction: 1), the solvent enters the graphite layer to react with LixC6, causing irreversible capacity loss; 2), the damaged SEI repair consumes Li+ and solvent, etc., further causing irreversible capacity loss.
b, electrolyte solvent
Some electrolyte solvents will cause the battery voltage to drop too fast. I tried a solvent before, and the ion conductivity improvement effect is obvious after the addition, but the self-discharge rate is 3 times faster than the normal solvent.
Possible Mechanism: These solvents are not resistant to oxidation, a slow chemical reaction occurs during storage, and capacity is consumed to cause a voltage drop.
c, SEI film instability
During the storage process, since the warehouse has a certain temperature, the SEI film is detached and re-reacted, resulting in flatulence, low pressure, and the like of the battery.
d, poor packaging
The position of the tabs is over-sealed, which may cause extreme ear corrosion and consume low voltage from the lithium source. If the other position is over-sealed, the electrolyte may corrode the aluminum foil through the CPP layer, causing the aluminum plastic film to be perforated and entering the moisture to cause low-pressure flatulence.
Many times, flatulence and low pressure come together. This time is often more serious, and the battery will eventually be scrapped.
Lithium battery self-discharge will reduce product quality and reduce customer satisfaction, so how to deal with battery self-discharge? You can do it from the following aspects:
1. Strictly control the introduction of dust
The low voltage of the battery with the highest proportion is caused by dust and burrs. How to control the introduction of dust is a very important and difficult task. The control of workshop dust is highly valued by many manufacturers' management, but it is often overlooked at the actual level.
On the one hand, the design of the plant should be reasonable. In the area where the pole piece manufacturing process is located, the control of dust is not particularly strict, especially in the slurry preparation area, but it is necessary to prevent the entry of dust impurities during the assembly process of the lithium battery. In the design of the plant, different areas must be strictly separated and isolated.
The second is to do a good job in the 5S work area. Good habits and high 5S quality can improve the yield of the product. For example, clean it before work to ensure that there is no dust, and that there is no residue in the cleaning equipment after the operation.
2, improve the pole piece manufacturing process
The burr is also the main culprit in the self-discharge of the lithium battery. The burr is mainly formed in the pole piece cutting. There are several reasons for the formation of the burr:
(1) The raw material of the positive and negative electrode slurry is selected from a material with a large BET, and a large amount of a conductive agent is added, so that particles such as a living material and a conductive agent are not firmly bonded, and burrs are formed. Then choose the right materials, improve the process of slurry preparation, coating, etc., to ensure that the pole pieces will not drop powder and produce burrs.
(2) The slicing tool was not replaced in time, and the resulting burr was generated. Familiar with the life of the cutter, and replace it according to the use situation.
3. Raw material quality management and control
It is also mentioned above that the metal impurities in the positive and negative materials are also one of the reasons for the self-discharge of the lithium battery. Therefore, it is necessary to strengthen the company's inspection of incoming materials to ensure that the incoming materials meet the standards, otherwise it will cause great losses. At the same time, it is necessary to strengthen the quality control of raw materials to prevent the introduction of impurities and moisture during storage and cause failure.
4. Environmental control
In environmental control, it is necessary not only to control the amount of dust particles in the environment, but also to control the moisture content of key steps to prevent the excessive moisture content from adversely affecting the quality of lithium batteries.
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