Generally, thermal runaway of batteries under extreme conditions can lead to a chain reaction. When an internal battery is subjected to thermal, mechanical, or electrical abuse, it may lead to temperature runaway of a single battery. When a single battery runs away, it causes the temperature around the battery to rise rapidly. The thermal runaway battery may further burn or explode, which will lead to the thermal runaway of the surrounding batteries. The spread of thermal runaway of batteries may lead to the thermal runaway of the battery pack.
To design safer lithium batteries, it is necessary not only to understand the process of thermal runaway from a macro perspective but also to understand the internal changes when the battery runs away. The thermal runaway process commonly used in lithium-ion batteries is also a chain reaction. The thermal decomposition temperature of different battery components is different. Compared with lithium-ion batteries, the thermal runaway of polymer solid-state batteries is different. The thermal decomposition of the positive electrode and the SEI layer is similar to that of lithium-ion batteries. The thermal runaway of polymer solid-state batteries can be divided into three stages.
Thermal Runaway Stage
During the initial stage of the process, which involves the gradual increase of temperature, the battery begins to heat up. As the battery heats up, it undergoes various changes that are essential to its performance and longevity. For instance, the increase in temperature causes the electrolyte within the battery to become more fluid, allowing for the efficient transfer of ions between the electrodes. Additionally, the heating process can cause certain chemical reactions to occur within the battery that ultimately lead to an improvement in its overall performance. It is important to note that this initial stage plays a crucial role in determining the battery’s long-term performance and should be carefully monitored and controlled to ensure optimal results.
During the second stage of battery operation, the temperature of the battery increases at a steady rate, reaching even higher temperatures than in the first stage. As this occurs, the anode and cathode within the battery undergo thermal decomposition, a process in which the chemical compounds comprising them break down due to the increased heat. This in turn causes a significant and rapid increase in the overall temperature of the battery. It is important to note that this increase in temperature can have negative effects on the battery’s performance and lifespan, making proper monitoring and maintenance of battery temperature a crucial aspect of battery management.
In the third stage, the battery temperature sharply rises to its maximum. For ordinary PEO polymer electrolyte, the thermal decomposition temperature of PEO is about 400℃. The oxygen released by the anode decomposition will promote PEO thermal decomposition and combustion. Different positive electrodes have different effects on the safety of polymer solid-state batteries. The commonly used LFP positive electrode has higher stability and is more conducive to improving the safety of polymer solid-state batteries than NCM positive electrode. The NCM high-voltage positive electrode has higher energy density and poorer stability, which also increases the risk of polymer solid-state batteries.
Compared with other negative electrodes, metallic lithium has a higher theoretical specific capacity (3860 mAh g−1). At the same time, it has the lowest reduction potential (-3.04 V compared to the standard hydrogen electrode). Therefore, replacing traditional negative electrodes with metallic lithium can significantly improve the energy density of lithium batteries.
However, the efficiency of current lithium negative electrode batteries is not high enough. The metallic lithium battery can form an SEI film with the electrolyte currently used. Unoxidized lithium metal has a silver metallic luster, soft texture, and a melting point of 189°C. Lithium metal is very stable in inert gas, but the negative electrode of lithium is unstable in the air. Lithium metal reacts with nitrogen, carbon dioxide, and oxygen to form lithium nitride, lithium carbonate, and lithium oxide.
Metallic lithium releases hydrogen gas and produces a lot of heat when put into water. Metallic lithium has good stability under sealed conditions. However, when the battery ruptures, the exposed lithium metal becomes extremely unstable in the air, greatly increasing the risk of the battery. At the same time, the negative electrode of lithium metal will increase the firefighting cost of battery accidents.
How to Deal with Fire?
Water & CO2
Currently, water and carbon dioxide cannot be used to extinguish lithium metal fires. However, the safety of lithium metal negative electrodes mainly depends on the inhibition of lithium dendrites. There is little research on how to solve the safety of lithium metal negative electrodes in air and fires. In order to industrialize the negative electrode of metallic lithium, solving the safety problem of lithium becomes more and more important.
Adding flame retardants is cost-effective and easy to use. Although adding flame retardants is easy to operate, they have a significant impact on the electrochemical performance of SPE. At the same time, adding flame retardants has a great influence on the physical and chemical properties of polymers, and flame retardants are easy to precipitate. Reactive flame retardants react with polymers to obtain inherently non-flammable polymer electrolytes. On the one hand, the mechanical and electrochemical properties of inherently non-flammable polymer electrolytes are relatively stable, and the flame-retardant effect is more lasting. In terms of structure, compared with liquid electrolytes, the gelation of electrolytes can solve the problem of leakage. At the same time, gel electrolytes are not so flammable.
LIBs have made significant contributions to the development of human society, but safety issues are becoming increasingly prominent. The use of solid electrolytes instead of liquid electrolytes still cannot meet people’s requirements for battery safety. As energy density continues to increase, capacity grows, and power battery packs are used, their safety hazards become increasingly serious.
For polymer solid-state lithium batteries, their electrodes and electrolytes are dangerous in extreme environments. Figure 5 summarizes the components of lithium batteries and safety improvement strategies. SPE can significantly improve the safety of metal lithium batteries, but safety issues still exist. Designing safer polymer solid-state batteries is challenging because they need to address the safety issues of electrolytes. Efficient flame retardants can improve the safety of solid fuels, but flame retardant additives have a certain effect on the electrochemical performance of electrolytes. In addition, the preparation of multifunctional additives that have flame retardancy and other properties is also a key issue, such as improving conductivity and suppressing lithium dendrites.
In addition to electrolytes, the safety of electrodes is also crucial. For metal lithium batteries, the metal lithium negative electrode undergoes a violent reaction when exposed to air, releasing a large amount of heat, and when exposed to water, it produces flammable hydrogen gas. It may cause combustion and explosion. Therefore, a lithium metal negative electrode with high safety, no dendrites, and good interface stability can greatly improve the safety of the battery.