Prevent Short Circuit of Battery

At present, the measures to improve the safety of lithium-ion batteries and prevent battery explosions mainly include improving the thermal stability of the battery,  improving the overcharge protection ability and improving the ability to prevent the battery from short circuit. This article will provide an overview of the causes and prevention of short circuits in Li-ion batteries.

The positive and negative active materials of the lithium-ion battery pass through the separator or the generated crystals pierce the separator, which will cause an internal short circuit in the battery. The porosity, pore size and distribution of the separator affect the internal resistance of the battery and the uniformity of the insertion, extraction and migration of lithium ions on the electrode surface. Generally speaking, a diaphragm with a porosity of about 40%, a uniform distribution and a pore size of 10 nm can prevent the movement of small particles of positive and negative electrodes, thereby improving the safety of lithium-ion batteries; the insulation voltage of the diaphragm is related to the contact between the positive and negative electrodes. Directly, the insulation voltage of the separator depends on the material and structure of the separator and the assembly conditions of the battery; the use of a composite separator (such as PP/PE/PP) with a relatively large difference between the thermal closure temperature and the melting temperature can prevent the battery from thermal runaway. PE (125°C) with low melting point is used to close the pores at lower temperature, and PP (155°C) can maintain the shape and mechanical strength of the separator, prevent the contact between the positive and negative electrodes, and ensure the safety of the battery.

The formation of lithium dendrites is one of the main causes of short circuits in Li-ion batteries. The carbon negative electrode is used to replace the metal lithium sheet negative electrode, so that the deposition and dissolution of lithium on the surface of the negative electrode during the charging and discharging process becomes the intercalation and extraction of lithium in the carbon particles, which prevents the formation of lithium dendrites. But this does not mean that the safety of lithium-ion batteries has been solved. During the charging process of lithium-ion batteries, if the positive electrode capacity is too large, metal lithium will be deposited on the surface of the negative electrode, and the negative electrode capacity will be too large, and the battery capacity loss will be serious. Therefore, an excess of 10% of the negative electrode is required in the assembly process. Second, the coating thickness and its uniformity also affect the intercalation and deintercalation of lithium ions in the active material. For example, if the negative electrode film is thick and non-uniform, due to the different polarizations in various places during the charging process, metal lithium may be locally deposited on the negative electrode surface. In addition, improper use conditions can also cause a short circuit of the battery. Under low temperature conditions, the deposition rate of lithium ions is greater than the intercalation rate, which leads to the deposition of metal lithium on the electrode surface and causes a short circuit. Therefore, controlling the ratio of positive and negative materials and enhancing the uniformity of coating are the keys to preventing the formation of lithium dendrites.

In addition, the crystallization of the binder and the formation of copper dendrites can also cause internal short circuits in the battery. In the coating process, it is desirable to remove all the solvent in the slurry by heating. If the heating temperature is too high, the binder may be crystallized, the active material may peel off, and the battery may be short-circuited.

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