1. Heat generation behavior of Li-ion battery
The improvement of the power performance of electric vehicles requires high-energy, high-power or large-sized battery packs to adapt to it, and the safety of power batteries is the key to the application and popularization of power batteries in electric vehicles. During the charging and discharging process of the power battery, the internal chemical reaction of the battery is complex. Taking a Li-ion battery as an example, the internal chemical reaction of the battery can be expressed as follows.
LiMO2+nC→Li1-xMO2+ LixCn (1-4)
In the formula, M is Co, Ni, Fe, Mn, etc.; positive compounds include LiCoO2, LiNiO2, LiMn2O4, LiFePO4, etc.; negative compounds include LiCx, TiS2, WO3, NbS2, V2O5, etc.
Most complex chemical reactions are accompanied by the generation of heat. Among them, the main heat-generating reactions of Li-ion power batteries include the decomposition of the solid electrolyte interface (SEI), the decomposition of the electrolyte, the decomposition of the positive electrode, the reaction between the negative electrode and the electrolyte, and the reaction between the negative electrode and the binder. In addition, due to the existence of the internal resistance of the battery, some heat will also be generated when the current passes through.
2. Decomposition of SEI
The negative electrode of the Li-ion power battery has a layer of SEI. The SEI is composed of a stable layer and a meta-stable layer. Its insulating structure mainly plays a protective role to prevent the negative electrode material from reacting with the electrolyte. When the temperature is 90~120℃, SEI will decompose due to instability. At this time, an exothermic reaction may occur in the metastable layer. Maleki et al. [used differential scanning calorimetry (DSC) to study the reaction of lithium carbide and electrolyte, and observed the decomposition of SEI around 100℃; using DSC, Zhang et al. found that SEI began to decompose and release heat at 130℃, and was not affected by the degree of lithium intercalation. Richard et al. used acceleration calorimetry (accelerating rate calorimetry, ARC) to measure the exothermic peak generated by the decomposition of SEI, and the measured peak was related to the amount of lithium intercalation in the carbon electrode. In LBF4 electrolyte, there is no exothermic peak in the self-heating curve, in LPF. In the electrolyte, the shape and characteristics of the exothermic peak depended on the type of solvent, which indicated that the decomposition characteristics of SEI were related to the composition of the electrolyte. Therefore, by controlling the battery temperature, it is possible to avoid battery failure or even combustion and explosion due to excessive battery temperature.
3. Electrolyte decomposition
Generally, the following five main reactions exist between the various components inside the battery at gradually increasing temperatures.
(1) Thermal decomposition of the electrolyte.
(2) Chemical reduction of the electrolyte on the surface of the negative electrode.
(3) Chemical oxidation of the electrolyte on the surface of the positive electrode.
(4) Thermal decomposition of positive and negative electrodes.
(5) The dissolution of the diaphragm and the resulting internal short circuit.
Among them, the first three reactions are directly related to the electrolyte, so the thermal safety of the electrolyte directly affects the safety performance of the entire Li-ion battery power system. Studies have shown that the exothermic heat of the reaction of LixMn2O4 with the electrolyte increases with the increase of the electrolyte. Kawamura et al. studied different electrolytes by DSC and found that no matter in propylene carbonate (PC) or ethylene carbonate (EC), no matter LiPF6 or LiCIO, diethyl carbonate (DEC) is more active than dimethyl carbonate (DMC), and the reaction exotherms at 230~280℃ are 375J·g-1 and 515J·g-1, respectively.
4. Cathode decomposition
In the oxidized state, the positive electrode active material is thermally decomposed and oxygen is released, the oxygen reacts with the electrolyte to release heat, or the positive electrode active material directly reacts with the electrolyte. This is because, as a Li-ion battery material that can be charged and discharged, the high charging voltage will cause the positive electrode material to exceed 200℃ and cause chemical reactions to occur. During the cycle of the battery, the positive electrode surface layer is formed due to the decomposition of the electrolyte on the surface of the positive electrode, which in turn promotes the decomposition of the positive electrode, reduces the thermal decomposition temperature, and generates more heat. Through research, MacNeil et al. concluded that the heat of reaction between LiCoO2 and the electrolyte is 265J·g-1. Through research, Venkatachalapathy et al found that the reaction heat of LixNi0.8Co0.2O2 is 642J·g-1, and the reaction heat of LixCoO2 is 381J·g-1. In the actual operating environment, the power system requires the Li-ion battery to have the characteristics of large capacity and high rate discharge, but the high temperature generated increases the risk of operation.
5. Reaction of negative electrode and electrolyte
Due to the instability of SEI, when the temperature is higher than 120℃, the SEI cannot protect the negative electrode, and the lithium embedded in the negative electrode reacts with the electrolyte to release heat. Through research, Sacken et al. found that in addition to the lithium intercalation state of the negative electrode, the composition of the electrolyte also has a critical influence on the onset temperature of thermal runaway, in which relative potency, solubility of SEI, and reactivity of organic solvents may play a large role. Richard et al. used DSC and ARC to study the thermal stability of lithium-intercalated graphite in electrolytes. The reduction reaction of the electrolyte caused by the lithium-intercalated carbon anode occurs in the range of 210~230℃, and the reaction heat is comparable to the heat generated by the SEI conversion process. The difference in the organic solvent and potassium salt composition of the organic electrolyte will also directly affect the thermal reaction on the carbon anode, resulting in a large difference in the onset temperature of thermal runaway. It is generally believed that if a lithium intercalated carbon electrode MCMB containing LiPF6EC/DEC (33:67) electrolyte is directly heated to 150℃ (the temperature is generally recognized as the initial value of thermal runaway for most commercial Li-ion batteries), the negative electrode will self-heat at a rate of 100℃·min-1. Therefore, knowing the general self-heating behavior of Li-intercalated carbon anodes in the presence of electrolyte is very useful for predicting the initial self-heating process of a practical Li-ion battery.
6. Reaction of anode and binder
A typical anode contains a binder with a mass ratio of 8% to 12%, and the reaction heat of LixC6 and the binder increases linearly with the degree of lithiation of the anode. The reaction products were analyzed by X-ray, and it was found that LiF was the main inorganic product. LiC6 has a lower onset temperature than metallic lithium because of the larger specific surface area of carbon. Maleki et al. reported that the heat of reaction between LiC6 and PVDF was 1.32×103J·g-1, the temperature at the beginning of the reaction was 200℃, and the maximum value was reached at 287℃. Biensan et al. reported that the heat of reaction between LiC6 and PVDF was 1.50×103J·g-1, and the temperature at the beginning of the reaction was 240℃. At 290℃, the reaction heat reached a peak and the reaction ended at 350℃.