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Lithium ion battery; Measures to improve safety performance

Lithium ion battery; Measures to improve safety performance


1. Improve the thermal stability of anode and anode materials

The cathode materials can be synthesized with good thermal stability by optimizing the synthesis conditions and improving the synthesis methods. Or use composite technology (such as doping technology), surface coating technology (such as coating technology) to improve the thermal stability of cathode materials.

The thermal stability of anode material is related to the type of anode material, the size of material particles and the stability of SEI film formed by the anode. If the particles are made into negative electrode at a certain ratio, the contact area between particles can be expanded, the electrode impedance can be reduced, the electrode capacity can be increased, and the possibility of precipitation of active metal lithium can be reduced.

The quality of the SEI film directly affects the performance and safety of the charge and discharge of lithium-ion batteries. It is helpful to improve the quality of the SEI film by using the carbon materials with weak oxidation, reduction, doping and surface modification as well as the spherical or fibrous carbon materials.

The stability of electrolyte is related to the type of lithium salt and solvent. The thermal stability of the battery can be improved by using lithium salt with good thermal stability and solvent with wide potential stable window. The safety of the battery can be improved by adding some solvents with high boiling point, high flash point and non-flammable to the electrolyte.

The type and quantity of conductive agent and binder also affect the thermal stability of the battery. The binder and lithium react at high temperature to produce a large amount of heat. Different binder has different calories.The caloriesof PVDF is almost 2 times that of the non-fluorine binder.

2. Improve the battery overcharge protection ability

In order to prevent lithium ion battery from overcharging, a special charging circuit is usually used to control the charge and discharge process of the battery, or a safety valve is installed on a single battery to provide a greater degree of overcharging protection. Secondly, positive temperature coefficient resistor (PTC) can also be used. Its use mechanism is to increase the internal resistance of the battery when the battery is heated up due to overcharge, so as to limit the overcharge current. Special diaphragm can also be used. When the battery is abnormal and the diaphragm temperature is too high, the pore of the diaphragm will shrink and block to prevent the migration of lithium ions and prevent the battery from overcharging.

3. Prevent battery short circuit

As for the diaphragm, the pore rate is about 40%, and the distribution is uniform. The diaphragm with a pore diameter of 10nm can prevent the movement of positive and negative minimal particles, thus improving the safety of lithium ion battery.

The insulation voltage of the diaphragm is directly related to its prevention of the contact between positive and negative electrodes. The insulation voltage of the diaphragm depends on the material and structure of the diaphragm as well as the assembly conditions of the battery.

Composite diaphragm (such as PP/PE/PP) with large difference between thermal closure temperature and melting temperature can prevent thermal runaway of the battery. The membrane surface is coated with ceramic layer to improve the temperature resistance of the membrane. The use of low melting point PE(125℃) in the lower temperature conditions play a closed cell purpose,PP(155℃) but also to maintain the shape of the diaphragm and mechanical strength, to prevent positive and negative contact, to ensure the safety of the battery.

It is well known that lithium anode is replaced by graphite anode, which changes the deposition and dissolution of lithium on the surface of anode into the embedding and exiting of lithium in carbon particles in the process of charge and discharge, preventing the formation of lithium dendrite. However, this does not mean that the safety of lithium-ion batteries has been solved. In the charging process of lithium-ion batteries, if the capacity of the positive electrode is too large, lithium metal will be deposited on the surface of the negative electrode, and the negative electrode capacity is too large, and the battery capacity loss is more serious.

The thickness and uniformity of the coating also affect the embedding and exiting of lithium ions in the active material. For example, the density of the negative surface is thick and uneven, so the size of the polarization is different in the charging process, and it is possible to have local deposition of lithium metal on the negative surface.

In addition, improper use conditions will also cause short circuit of the battery. Under low temperature conditions, because the deposition speed of lithium ions is greater than the embedding speed, lithium metal deposition on the surface of the electrode will cause short circuit. Therefore, the key to prevent the formation of lithium dendrite is to control the proportion of anode and anode materials and to enhance the uniformity of coating.

In addition, the crystallization of binder and the formation of copper dendrite will also cause the battery internal short circuit. In the coating process, the solvent in the paste will be completely removed by baking and heating the coating. If the heating temperature is too high, the binder may also crystallize, which will peel off the active substance and make the battery internal short circuit.

Under the condition of overdischarge, when the battery is overdischarged to 1-2V, the copper foil as the negative electrode collector will begin to dissolve and precipitate on the positive electrode. When the battery is less than 1V, copper dendrite will begin to appear on the surface of the positive electrode, causing the internal short circuit of the lithium ion battery.

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