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
The physical micro-short circuit is the direct cause of the low voltage of the lithium battery. Its direct manifestation is that after the battery is stored at room temperature and 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 reaction, 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:
a. Dust and burrs
When we disassemble the battery with micro-short circuit, we often find that black spots appear on the separator of the battery. If the position of the black spot is in the middle of the diaphragm, then there is a high probability of dust breakdown. If the black spots are in the majority at the edge, it is caused by the burrs generated during the pole piece cutting process, and these two points are easier to distinguish.
b. Metal impurities in positive and negative electrodes
In batteries, metallic 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 goes by, the metal dendrites continue to grow, and finally penetrate the separator, resulting in a micro-short circuit between the positive and negative electrodes, which consumes electricity continuously, resulting in a decrease in voltage.
① Positive electrode metal impurities
After the charging reaction, the metal impurities on the positive electrode also break down the separator and form 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 single metal has the greatest impact. According to some documents, the order of influence is as follows: Cu>Zn>Fe>Fe2O3. For example, many cathode iron-lithium materials will face the problem of excessive self-discharge, which is caused by excessive iron impurities.
②Anode metal impurities
Due to the formation of the primary battery, metal impurities in the negative electrode will dissociate and deposit on the diaphragm to cause the diaphragm to conduct and form a physical micro-short circuit. Some low-end negative electrode materials in China often encounter such a situation. The metal impurities in the negative electrode slurry have less influence on self-discharge than the metal impurities in the positive electrode, among which Cu and Zn have a greater influence on self-discharge.
c. Metal impurities in auxiliary materials
For example, metal impurities in CMC and tape
2. Chemical reaction
a. Moisture
Moisture causes the electrolyte to decompose, releasing a large number of electrons, which are then embedded in the positive electrode oxidation structure, causing the positive electrode potential to drop, resulting in low pressure;
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 to produce CO2 and other gases to cause battery expansion; HF will react with many substances in the battery such as the main components of SEI, destroying SEI film; generate CO2 and H2O, etc.; CO2 causes battery expansion, and the regenerated H2O participates in reactions such as LiPF6 and solvents, forming a vicious chain reaction.
The consequences of SEI film damage: 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.
b. Electrolyte solvent
Adding some electrolyte solvents will cause the voltage of the battery to drop too quickly. I tried a solvent before, and the ion conductance was significantly improved after adding it, but the self-discharge rate was 3 times faster than normal solvents.
Possible mechanism: These solvents are not resistant to oxidation, and a slow chemical reaction occurs during storage, which consumes capacity and causes a voltage drop.
c. SEI film is unstable
During the storage process, because the warehouse has a certain temperature, it causes the SEI film to fall off and re-react, resulting in battery flatulence, low pressure, etc.
d. Poor packaging
The position of the tab is over-sealed, which may cause corrosion of the tab and consume low voltage of the lithium source. If other positions are over-sealed, the electrolyte may corrode the aluminum foil through the CPP layer, causing the aluminum-plastic film to perforate, and water entering to cause low-pressure flatulence.
Many times, flatulence and low voltage appear together, and at this time they are often more serious, and the battery will eventually be scrapped.