After the lithium-ion battery is produced, a series of tests must be carried out before reaching the hands of consumers to ensure the safety of the battery as much as possible and reduce potential safety hazards.
Ⅰ. Lithium-ion battery safety testing indicators
1. Extrusion test
Place the fully charged battery on a flat surface, apply a 131KN squeezing force by the hydraulic cylinder, and squeeze the battery flat with a steel rod with a diameter of 32mm. Once the squeezing pressure reaches the maximum, stop squeezing, and the battery will not catch fire or explode.
2. Impact test
After the battery is fully charged, place it on a flat surface, place a steel column with a diameter of 15.8mm vertically in the center of the battery, and freely drop a weight of 9.1kg onto the steel column above the battery from a height of 610mm. The battery does not catch fire or explode.
3. Overcharge test
The battery is fully charged with 1C, and the overcharge test is carried out according to the 3C overcharge of 10V. When the battery is overcharged, the voltage rises to a certain voltage and stabilizes for a period of time. When approaching a period of time, the battery voltage rises rapidly. When it rises to a certain limit, the battery is high. The cap is pulled off, the voltage drops to 0V, and the battery does not catch fire or explode.
4. Short circuit test
After the battery is fully charged, short-circuit the positive and negative electrodes of the battery with a wire with a resistance of not more than 50m, and test the surface temperature change of the lithium-ion battery pack.
5. Acupuncture test
Place the fully charged battery on a flat surface and pierce the battery radially with a 3mm diameter steel needle. The test battery does not catch fire or explode.
6. Temperature cycle test
The lithium-ion battery temperature cycle test is used to simulate the safety of lithium-ion batteries when they are repeatedly exposed to low and high temperature environments during transportation or storage. The test is carried out by using rapid and extreme temperature changes. After the test, the sample should be free of fire, explosion and liquid leakage.
Ⅱ. Lithium-ion battery safety solutions
In view of the many hidden safety hazards of lithium-ion batteries in the process of materials, manufacturing and use, how to improve the parts that are prone to safety problems is a problem for lithium-ion battery manufacturers to solve.
1. Improve the safety of electrolyte
There is a high reactivity between the electrolyte and the positive and negative electrodes, especially at high temperature. In order to improve the safety of the battery, improving the safety of the electrolyte is one of the more effective methods. The safety hazards of electrolytes can be effectively solved by adding functional additives, using new lithium salts and using new solvents. According to the different functions of additives, they can be mainly divided into the following categories: safety protection additives, film-forming additives, positive electrode protection additives, stabilizing lithium salt additives, lithium precipitation-promoting additives, current collector anti-corrosion additives, and wettability-enhancing additives.
In order to improve the properties of commercial lithium salts, researchers have carried out atomic substitutions on them and obtained many derivatives. Among them, the compounds obtained by replacing atoms with perfluoroalkyl groups have many advantages, such as high flash point, similar electrical conductivity, and enhanced water resistance, which is a class of promising lithium salt compounds. In addition, an anionic lithium salt obtained by chelating a boron atom as a central atom with an oxygen ligand has high thermal stability.
2. Improve the safety of electrode materials
Lithium iron phosphate and ternary composite materials are considered as cathode materials with low cost and excellent safety, and may be widely used in the electric vehicle industry. Regarding the positive electrode material, the common method to improve its safety is coating modification. For example, the surface coating of the positive electrode material with metal oxide can prevent the direct contact between the positive electrode material and the electrolyte, inhibit the phase transition of the positive electrode material, and improve the Its structural stability reduces the disorder of cations in the lattice to reduce the heat generation of side reactions.
Regarding the negative electrode material, since the surface of the lithium ion battery is often the most prone to thermochemical decomposition and exothermic part, improving the thermal stability of the SEI film is a key method to improve the safety of the negative electrode material. The thermal stability of anode materials can be improved by weak oxidation, metal and metal oxide deposition, polymer or carbon coating.
3. Improve the safety protection design of the battery
In addition to improving the safety of battery materials, commercial lithium-ion batteries use many safety protection measures, such as setting battery safety valves, thermal fuses, connecting components with positive temperature coefficients in series, using thermally sealed diaphragms, loading dedicated protection circuits, and dedicated battery management. systems, etc., are also means of enhancing security.
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