Analysis of safety problems of lithium ion batteries
1. Use safe lithium ion battery electrolyte
At present, lithium ion battery electrolyte uses carbonate as a solvent, wherein linear carbonate can improve the charge and discharge capacity and cycle life of the battery, but their flash point is low, flashing at a lower temperature, and fluorination The solvent usually has a high flash point or even no flash point, so the use of a fluorinated solvent is advantageous for suppressing the combustion of the electrolyte. The fluorinated solvents currently studied include fluoroesters and fluoroethers.
The flame retardant electrolyte is a functional electrolyte, and the flame retardant function of such electrolytes is usually obtained by adding a flame retardant additive to a conventional electrolyte. The flame retardant electrolyte is the most economical and effective measure to solve the safety of lithium ion batteries, so it is especially valued by the industry.
The use of a solid electrolyte instead of an organic liquid electrolyte can effectively improve the safety of a lithium ion battery. The solid electrolyte includes a polymer solid electrolyte and an inorganic solid electrolyte. Polymer electrolytes, especially gel-type polymer electrolytes, have made great progress and have been successfully used in commercial lithium-ion batteries, but gel-type polymer electrolytes are actually dry polymer electrolytes and liquid electrolytes. As a result, its improvement in battery safety is very limited. Since the dry polymer electrolyte does not contain a liquid flammable organic plasticizer like a gel-type polymer electrolyte, it has better safety in terms of liquid leakage, vapor pressure, and combustion. The current dry polymer electrolytes are still unable to meet the application requirements of polymer lithium ion batteries, and further research is still needed to be widely used in polymer lithium ion batteries. Compared with the polymer electrolyte, the inorganic solid electrolyte has better safety, is non-volatile, does not burn, and has no leakage problem. In addition, the inorganic solid electrolyte has high mechanical strength, and the heat-resistant temperature is significantly higher than that of the liquid electrolyte and the organic polymer, so that the operating temperature range of the battery is expanded; the inorganic material is made into a film, and the lithium ion battery is more easily miniaturized, and such a battery With an extremely long storage life, it can greatly expand the application field of existing lithium-ion batteries.
Conventional electrolytes containing flame retardant additives have a flame retardant effect, but the solvent is still a volatile component, and there is still a high vapor pressure. For a sealed battery system, there is still a certain safety hazard. With a completely non-volatile, non-flammable room temperature ionic liquid as the solvent, it is hopeful to obtain an ideal high-safety electrolyte. Ionic liquid is an organic liquid substance composed entirely of ions at room temperature and adjacent temperature. It has high conductivity, wide liquid range, non-volatile and non-combustible. It is expected to solve lithium ion in ionic liquid used in lithium ion battery electrolyte. Battery safety issues.
2, improve the thermal stability of the electrode material
The safety problem of lithium-ion batteries is directly caused by unsafe electrolytes, but from the root cause, it is caused by the instability of the battery itself and the occurrence of thermal runaway. The occurrence of thermal runaway, in addition to the thermal stability of the electrolyte, the thermal stability of the electrode material is also one of the most important reasons, so improving the thermal stability of the electrode material is also an important part of improving the safety of the battery, but the electrode here The thermal stability of a material includes not only its own thermal stability, but also its thermal stability in interaction with the electrolyte material.
Generally, the thermal stability of the negative electrode material is determined by its material structure and the activity of the charged negative electrode. For carbon materials, spherical carbon materials, such as mesocarbon microbeads (MCMB), have a lower specific surface area and a higher charge and discharge platform than scaly graphite, so their charge state activity is smaller and thermal stability is relatively higher. Good, high security. The spinel structure of Li4Ti5O12 has better structural stability than layered graphite, and its charge and discharge platform is much higher, so the thermal stability is better and the safety is higher. Therefore, MCMB or Li4Ti5O12 is generally used as a negative electrode in power batteries that are currently required for higher safety. Generally, the thermal stability of the negative electrode material is not only the material itself, but also the thermal stability of the solid electrolyte interface film (SEI) at the interface between the negative electrode and the electrolyte is more concerned for the same material, especially graphite, and this is usually It is considered to be the first step in the occurrence of thermal runaway. There are two main ways to improve the thermal stability of SEI film: one is the surface coating of the negative electrode material, such as coating the amorphous carbon or metal layer on the graphite surface; the other is adding the film forming additive to the electrolyte in the battery. During the activation process, they form a highly stable SEI film on the surface of the electrode material, which is beneficial to obtain better thermal stability.
The thermal reaction between the positive electrode material and the electrolyte is considered to be the main cause of thermal runaway. It is especially important to improve the thermal stability of the positive electrode material. The development of the positive electrode material in the industry is also more concerned, except for its higher price and higher profit. In addition, its important position in battery safety is also an important reason for its concern. Like the negative electrode material, the essential characteristics of the positive electrode material determine its safety characteristics. LiFePO4 has a polyanion structure, in which the oxygen atom is very stable, it is not easily released by heat, so it does not cause violent reaction or combustion of the electrolyte; while other transition metal oxide cathode materials are easy to release oxygen when heated or overcharged, safe. Poor sex. Among the transition metal oxides, LiMn2O4 exists in the form of λ-MnO2 in the charged state. Because of its good thermal stability, the positive electrode material is relatively safe. In addition, the thermal stability of the positive electrode material can also be improved by bulk phase doping, surface treatment, and the like.
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