Thermal stability of cathode material: LiCoO2 composite oxide/Ni-Co-Mn ternary oxide/spinel LiMn2O4/LiFePO4 active material
As mentioned earlier, the thermal stability of the positive electrode material plays an important role in battery safety, especially the thermal stability of the positive electrode in the charged state can be used as a measure of battery safety. The thermal stability of the cathode material is related to the structural stability. The newly synthesized and discharged active materials are in a stable state, but when lithium ions are released during the charging process, the positive electrode becomes thermodynamically unstable. In a metastable state. When the applied energy is greater than the activation energy, it will transform into a stable state and release a large amount of heat through an exothermic reaction. To ensure battery safety, reducing heat generation or increasing activation energy can prevent the positive electrode material from turning into an unstable state. There are various ways to obtain the safety of the battery, the most important of which is to reduce the heat release of the positive electrode material caused by structural changes. In other words, it is necessary to ensure the structural stability of the positive electrode material in the charged state. Recently, in order to increase the battery capacity, the positive electrode material is required to withstand higher charging voltage. Since a large amount of energy is concentrated in the positive electrode under high voltage, the change in the structure of the active material may release more heat.
The method of measuring the thermal safety of the positive electrode material is as follows: After 2~3 charge and discharge cycles, separate the positive electrode from the battery in the charged state. In this process, special care should be taken to avoid short circuit. At the same time, it should be carried out in a glove box. May reduce exposure to air. The separated positive electrode is thoroughly cleaned with electrolyte and then subjected to thermal analysis to observe any temperature changes caused by the exothermic reaction. It is worth noting that the reactivity of the positive electrode is affected by the washing solution and washing method. In the analysis of the positive electrode, considering the influence of different electrolyte types, quantities and binders, the thermal stability can be better understood. The thermal stability of various cathode materials is described below.
①LiCoO2 composite oxide
The thermal stability of LiCoO2 under different charging voltages: a is the discharged state; b~f shows the change of thermal stability as the charging progresses. In the discharge state, there is no exothermic peak, so it is thermally stable. As the voltage increases, the exothermic peak increases and the starting position becomes lower. As the charging progresses, the structural stability of the material decreases. When 55% of the lithium is removed, the onset of the exothermic peak and the peak intensity are basically similar. That is, when the content of lithium is lower than a certain value, the structural stability of LiCoO2 remains unchanged. To analyze the thermal stability, it is necessary to pay attention to the relationship between the intensity and shape of the exothermic peak and the type and composition of the electrolyte.
the thermal analysis results of charged LiCoO2 and active materials before and after washing with organic solvents. The electrode was washed with DEC for 36h, and then dried in a seven-vacuum oven at 65°C for 12-14 and then measured. the shape and size of the exothermic peak are different before and after washing, which indicates that the residual salt in the electrolyte contributes to the heat generated by the positive electrode. For the washed sample, there are two exothermic peaks, and the exothermic onset point is at 178°C. Corresponding LixCoO2 decomposes at 178~250°C and 250-400°C. The decomposition of LixCoO2 is carried out according to the following chemical equation:
LixCoO2→xLiCoO2+(1-x)/3Co3O4+(1-x)/3O2(g)↑
As mentioned earlier, the structural change of LixCoO2 is exothermic and oxygen evolution. The exothermic onset temperature of unwashed samples is lower than 160°C, and a large exothermic peak appears between 167 and 250°C. This is not entirely due to the decomposition of LixCoO2, but also related to the synergistic reaction with the electrolyte. No thermal reaction was found between the electrolyte and oxygen. Explain that these reactions do not release heat. Since the stability of the thermal stability cross-structure is related to the reaction between the surface of the material and the electrolyte, it is also affected by the specific surface area of the material.
② Ni-Co-Mn ternary oxide
The Ni, Mn and Co ternary systems have different compositions. Here we only discuss Lix[Ni1/3Mn1/3Co1/3]O2 and LixCoO2. The structure of this composite oxide is similar to that of LixCoO2, but because three elements with different valences form a superlattice structure, the material is more stable.
Compared with Lix[Ni1/3Mn1/3Co1/3]O2 and LixCoO2 has a higher exothermic peak onset temperature and a smaller exothermic peak, which can be achieved by the superlattice of this material. The resulting structural stability enhancement is explained. When the Ni content in the Ni-Co-Mn ternary system increases, the exothermic onset temperature is similar or increases, but the exothermic peak becomes larger and more increased.
Because LixCoO2 has a higher specific capacity than ternary materials, more lithium will be released under the same charging voltage, so it will be more unstable, and its exothermic reaction is more obvious than LixCoO2. However, the strength of the reaction can be weakened by washing the surface of the material with an organic solvent, because the lithium salt or other components in the electrolyte will participate in the exothermic reaction of the positive electrode.
③ Spinel LiMn2O4
The spinel LiMn2O4 in either the charged state or the discharged state is thermodynamically stable, and structural changes will not release heat.
Spinel LiMn2O4 has a much higher onset temperature of exothermic peak than LiCoO2, but its exothermic peak is also very large. Considering that the spinel LiMn2O4 has no structural changes, most of the heat should come from the reaction between the active material particles and the electrolyte. The thermal stability of spinel LiMn2O4 can be improved by adjusting the electrolyte composition or reducing the specific surface area of the active material.
④LiFePO4 active material
Since the structure of LiFePO4 is not affected by charging or heat, it can provide excellent battery stability. LiFePO4 active material can remain stable up to 230°C, and some endothermic reactions caused by electrolyte volatilization or binder decomposition occur at about 250°C. There is no observable exothermic reaction between the electrolyte and the cathode material, indicating that the surface structure of LiFePO4 is very stable, and it has a good application prospect in the fields that require high stability in the future.
