Lithium batteries are widely used in various power products, and battery safety has always been a focus of attention. The cause of battery safety problems is mainly caused by thermal runaway. According to current research, the main causes of battery thermal runaway are:
　　1. Thermal decomposition of electrolyte; 2. Reduction of electrolyte by lithium-intercalated substances; 3. Oxidation of electrolyte by positive electrode materials ; 4. Decomposition of positive and negative materials; 5. Short circuit caused by diaphragm damage. Of course, there are other reasons, which I won’t focus on in this article. Today we mainly introduce the application of PTC materials in preventing thermal runaway of lithium-ion batteries. PTC (Positive Temperature Coefficient), which means a positive temperature coefficient, generally refers to semiconductor materials or components with a large positive temperature coefficient. PTC materials have the characteristic that the resistivity rises with the rise of temperature. When a large current passes through the PTC device, the Joule heat generated by the PTC will cause the temperature to rise, and then the resistance of the element will increase rapidly, thereby limiting the passage of current and so on.
　　There are three main ways to apply PTC materials to lithium-ion batteries, all of which can improve the safety of lithium-ion batteries to a certain extent: (1) Make
　　PTC materials into electrode coatings XMFeng et al. from Wuhan University Use epoxy resin, carbon black and curing agent polyamide to make PTC material, coat the PTC material on the surface of aluminum foil to form a PTC coating, and then coat the active material on the surface of the PTC coating: A/B/C are respectively
　　cobalt Lithium acid active material, PTC coating, aluminum foil. The advantage of this structure is that the PTC material can improve the safety of lithium-ion batteries, and at the same time prevent lithium ions from intercalating into the PTC material from destroying the structure of the conductive agent, thereby improving the rate performance and cycle life of the battery. (2) Mix the PTC material into the electrode material Makiko Kise et al. of Japan introduced the PTC material into the battery, and mixed the PTC material and the electrode material together to form a mixed electrode. The PTC material is a mixture of high-density polyethylene (HDPE) and carbon black. Its protective mechanism is: under normal conditions, the conductive agent in the material participates in electron transport; under abuse conditions, when the battery temperature rises and exceeds the melting point of HDPE in the PTC material, the polymer rapidly diffuses to inhibit the conductive agent from conducting. channel transmission.
　　The disadvantage of this method is that the PTC material itself has a large impedance and poor conductivity, which is unfavorable to performance such as battery capacity and rate. In order to improve this problem, other conductive agents can be added to the PTC material for improvement.
　　(3) Coating the surface of the electrode material with PTC material In addition to the above two methods, Lan Xia et al. of Wuhan University coated a layer of PTC material on the surface of the active material lithium cobaltate, and the PTC material used was poly-3decylthiophene , the electrode made of this coated active material can not only sensitively sense the temperature rise of the battery caused by external abuse such as overcharge and internal short circuit, but also suppress the thermal reaction caused by the highly active electrode material. The protection mechanism is that lithium ions and electrons can be transmitted normally at normal temperature, and the coating layer becomes an insulating layer at high temperature, and the transmission of ions and electrons is blocked, which acts as a high-temperature blocking effect. While playing a safety role, the coated electrode can effectively protect the electrode material from being corrupted by the electrolyte, and improve the chemical stability and thermal stability of the electrode material.
　　The commonly used PTC materials are mainly divided into organic materials and ceramic materials, and different types of PTC materials have different conductive mechanisms. However, its characteristic of increasing resistivity with temperature greatly improves the safety of lithium-ion batteries, and the application technology and process of PTC materials need further breakthroughs.