Lithium batteries are widely used in various power products, and battery safety has always been the focus of attention. The battery safety problem is mainly caused by thermal runaway. According to the current research, the main causes of battery thermal runaway are:

1. Thermal decomposition of electrolyte; 2. The electrolyte is reduced by lithium intercalation material; 3. Electrolyte is oxidized by positive material; 4. Decomposition of positive and negative pole materials; 5. Short circuit caused by diaphragm damage. Of course, there are other reasons. This article will not focus on them. Today, we mainly introduce the application of PTC materials in preventing thermal runaway of lithium ion batteries. PTC (Positive Temperature Coefficient), which means positive temperature coefficient, generally refers to semiconductor materials or components with large positive temperature coefficient. PTC materials have the characteristics that the resistivity increases with the temperature. When a large current passes through the PTC device, the Joule heat generated by PTC will cause the temperature to increase, and then the resistance of the element will increase rapidly, thereby limiting the current flow.

There are three main ways for PTC materials to be used in lithium-ion batteries, which can improve the safety of lithium-ion batteries to a certain extent:

(1) The PTC material was made into electrode coating. X.M.Feng of Wuhan University and others made PTC material by using epoxy resin, carbon black and curing agent polyamide. The PTC material was coated on the aluminum foil surface to form a PTC coating, and then active substances were coated on the PTC coating surface:

A/B/C are respectively lithium cobalate active substance, PTC coating and aluminum foil. The advantage of this structure is that PTC material can improve the safety of lithium ion battery, and prevent lithium ion from being embedded in PTC material to damage the structure of conductive agent, thus improving the rate performance and cycle life of battery. (2) The PTC material was mixed into the electrode material. Makiko Kise et al. of Japan introduced the PTC material into the battery to mix the PTC material with the electrode material to make a mixed electrode. PTC material is made of high-density polyethylene (HDPE) and carbon black. Its protective mechanism is: under normal conditions, the material as a conductive agent to participate in electronic transmission; Under abuse conditions, when the battery temperature rises and exceeds the melting point of HDPE in the PTC material, the polymer rapidly diffuses and inhibits the transmission of conductive channel of the conductive agent.

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