The market capacity of lithium-ion batteries is affected by the battery’s thermochemical characteristics and safety issues. For commercial lithium-ion batteries, thermal safety must be guaranteed to around 60°C. However, the increase in temperature will accelerate the decomposition of active materials or the thermal decomposition caused by the interaction of the electrolyte with different components, and the performance of the battery will be greatly hindered due to the collapse of the structure. The thermochemical properties can be described by a variety of complex reaction pathways, such as the interface reaction of the negative electrode/electrolyte, the interface reaction of the positive electrode/electrolyte, and the decomposition reaction of the electrolyte and the electrode. These thermal decompositions lead to thermal runaway, which can cause serious safety problems, such as fire and explosion.
When understanding the thermochemical characteristics of the negative electrode and battery safety, the decomposition of the SEI film is very important. The SEI film is formed at the negative electrode/electrolyte interface during the first charge. As shown in Figure 1, the SEI film begins to decompose at around 80°C and is completely decomposed at 100-120°C.
The composition of the SEI film varies according to the composition of the electrolyte, and usually includes general metastable substances such as Li2CO3, LiF and (CH2OCO2Li)2. In order to control thermochemical properties and prevent thermal runaway, it is necessary to understand the reactions between battery components. Generally, the exothermic reactions that affect thermochemical properties on the negative electrode are similar to those that occur on the positive electrode. Here are some summary:
1) As the temperature rises, the metastable SEI film decomposes through an exothermic reaction at 90-120 ℃.
2) When the temperature is higher than 120°C, the exothermic reaction is mainly the reaction between the electrolyte and the lithium ions inserted in the negative electrode.
3) The temperature of the reaction between the fluorine-containing binder (such as PVdF) and the lithium ions embedded in the negative electrode is similar to the above-mentioned situation.
4) The thermal decomposition of the electrolyte occurs at a temperature above 200°C.
5) When overcharged, metallic lithium may deposit on the negative electrode and react with electrolyte and binder.
Spotnitz and Franklin obtained DSC (differential scanning calorimetry) results of lithium secondary battery components, which were based on kinetic parameters such as exothermic onset temperature, exothermic peak temperature, heat capacity, activation energy, and rate coefficient. As shown in Figure 2, the thermal characteristics of the negative electrode are related to the decomposition of the SEI film and the exothermic reaction of the carbon negative electrode/electrolyte and the carbon negative electrode/binder. Similar to the positive electrode, the thermochemical properties of the negative electrode show different heat values, which are related to the state of charge of lithium. Since the thermochemical characteristics vary with the types of electrodes and electrolytes, in order to produce a lithium battery with good thermal stability, it is very important to understand the path of the exothermic reaction and the optimization of the battery design. The reaction enthalpy should be minimized to avoid thermal runaway caused by the thermal reaction between the negative electrode and the electrolyte at a relatively low temperature (below 200°C).