Polymer lithium-ion battery is widely used in modern electronic devices and new energy fields, but its thermal runaway problem seriously threatens its safety. It is of great significance to clarify the causes and develop effective flame-retardant electrolytes.
The occurrence of thermal runaway is due to the interweaving of multiple causes. From the perspective of the battery, the electrode material plays a key role in the charging and discharging process. In the long-term charging and discharging cycle, the negative electrode may gradually grow lithium dendrites due to uneven lithium ion deposition. These lithium dendrites are like "hidden dangers" lurking inside the battery. As they grow, they will continue to penetrate the diaphragm, causing the positive and negative electrodes to directly contact and short-circuit. At the moment of short circuit, a large amount of heat is released rapidly like a "volcanic eruption", igniting the "fuse" of thermal runaway. At the same time, the positive electrode material is no longer stable at high temperature, and a decomposition reaction occurs, which not only releases oxygen, but also destroys its own structure, further exacerbating the unstable state inside the battery.
As an indispensable part of the battery, the electrolyte may also become an "accomplice" of thermal runaway. Commonly used organic electrolytes are flammable. Once decomposed by heat, a large amount of flammable gas will be produced. These gases accumulate in the limited space inside the battery, like a "time bomb", greatly increasing the risk of thermal runaway. Moreover, the solid electrolyte interface film (SEI film) inside the battery can protect the electrode under normal circumstances, but under abnormal conditions such as high temperature, the SEI film will decompose and lose its protective effect on the electrode, triggering a series of side reactions, further promoting the process of thermal runaway.
External factors should not be underestimated either. Mechanical abuse is one of the common external causes. For example, when the polymer lithium-ion battery is hit, squeezed or punctured, the internal structure will be severely damaged. Diaphragm rupture, electrode deformation and other conditions follow one after another, causing the positive and negative electrodes to short-circuit, instantly generating a lot of heat, creating conditions for thermal runaway. In terms of electrical abuse, overcharging, over-discharging and short-circuiting occur from time to time. When overcharging, the battery is overcharged, a large amount of energy accumulates, and the internal pressure rises rapidly. The battery is like an over-inflated balloon that may "explode" at any time; over-discharging will cause the chemical reaction inside the battery to be unbalanced and generate additional heat; and short-circuiting will cause the current to increase sharply in an instant, and the heat will accumulate rapidly, greatly increasing the possibility of thermal runaway. Thermal abuse cannot be ignored. When the battery is in a high temperature environment or the heat dissipation system fails, the internal temperature of the battery will continue to rise, triggering the above series of internal reactions, and eventually leading to thermal runaway.
In order to effectively curb thermal runaway, the development of flame-retardant electrolytes has become a key breakthrough. In recent years, researchers have made many achievements in this regard. One is to develop flame-retardant additives, which can significantly improve the flame retardant properties of electrolytes by adding them to traditional electrolytes. For example, some phosphorus-containing compounds will decompose and produce free radicals when heated. These free radicals can capture active groups in the combustion process and interrupt the chain reaction of combustion, thereby effectively preventing the combustion of electrolytes. There are also some halogenated additives that reduce the flammability of electrolytes and improve the thermal stability of batteries by changing the physical and chemical properties of electrolytes.
Another type is the development of new polymer electrolytes. All-solid polymer electrolytes fundamentally avoid the problems of electrolyte leakage and flammability by virtue of their solid-state characteristics. It has high mechanical strength and thermal stability, and can inhibit the growth of lithium dendrites to a certain extent and reduce the risk of short circuits. Moreover, some specially designed polymer electrolytes will undergo structural changes at high temperatures, forming a more stable phase state, further enhancing the safety of the battery. In addition, gel polymer electrolytes also show good application prospects. They have both the high ionic conductivity of liquid electrolytes and some advantages of solid polymer electrolytes. Through reasonable formula design, they can effectively improve the flame retardant properties while ensuring battery performance.
However, the current research and development of flame-retardant electrolytes still faces challenges. Some flame-retardant additives will reduce the ionic conductivity of the electrolyte and affect the battery charging and discharging efficiency; the preparation process of new polymer electrolytes is complex and the cost is high, which limits large-scale applications. In the future, it is necessary to further optimize material design, deeply explore the flame retardant mechanism, and reduce costs at the same time, so as to achieve the widespread application of flame-retardant electrolytes and provide solid guarantees for the safe use of polymer lithium-ion batteries.
