The safety problem of lithium-ion batteries is the problem of heat inside the battery. If the heat generated by the failure cannot be released quickly, the temperature of the battery will continue to rise, and the rising temperature will intensify the chemical and electrochemical reactions between the components such as the active material and the electrolyte inside the battery. A large amount of heat and gas is generated, which eventually leads to thermal runaway of the battery, until safety accidents such as smoke, fire, and explosion occur.
The main sources of reaction heat inside the cell are as follows:
1. Decomposition reaction of SEI film
During the first charge and discharge of a lithium-ion battery, the negative electrode material reacts with the electrolyte at the solid-phase interface to form an interface protective film - SEI (solid electrolyte interface) covering the surface of the negative electrode, preventing the electrolyte from interacting with the carbon negative electrode. interaction between. But when the temperature increases, the reactivity increases, the SEI film decomposes, and the reaction is exothermic.
2. The reaction of lithium intercalated carbon with solvent
When the temperature increases, the SEI film cannot protect the anode, and the solvent may react with metallic lithium or intercalated lithium.
3. Reaction of lithium intercalated carbon with fluorinated binder
It is found that the binder PVDF reacts with LixC6 when the temperature exceeds 260℃.
4. Decomposition reaction of electrolyte
The thermal decomposition reaction of lithium-ion battery electrolyte is mainly the reaction of solvent and lithium salt when the temperature increases. When the charging voltage of the lithium-ion battery exceeds the decomposition voltage of the electrolyte, the electrolyte will also undergo a decomposition reaction, release heat, and generate gas.
5. Decomposition reaction at the positive electrode
Layered LiCoO2, LiNiO2, spinel-like LiMn2O4 and olivine-like LiFePO4 are the most studied cathode materials, and they are stable at temperatures below 650 °C. The above materials are in a metastable state when charged and decompose when the temperature is increased.
6. Lithium metal reactions
When the lithium-ion battery is overcharged, lithium metal is deposited on the surface of the negative electrode, and the reaction between metal lithium and the electrolyte may occur.
7. Enthalpy change of positive and negative active materials
When a lithium-ion battery is charged and discharged, the enthalpy of lithium intercalation into the cathode material changes.
8. Heat generated by current passing through internal resistance
The battery has an internal resistance (Rc). When the current passes through the battery, the heat generated by the internal resistance is I2RCt, which is sometimes called the heat generated by the polarization internal resistance. When the battery is short-circuited externally, the heat generation from the internal resistance of the battery dominates.
The three sources can be summarized as:
1. The reversible heat generated by the chemical reaction comes from the electrochemical reaction, and the size is related to the entropy change of the substance;
2. Irreversible heat, mainly due to ohmic resistance and polarization;
3. The heat generated by the side reaction mainly comes from the chemical reaction between the electrolyte and the electrode material.
For the battery, even in the case of normal charge and discharge, the heat of the A and B parts always exists. If the heat of the C part occurs, it may lead to safety problems in the battery. The heat of Part C mainly comes from the following five parts:
Thermal decomposition of positive electrode material; oxidation of electrolyte in positive electrode; thermal decomposition of electrolyte; thermal decomposition of negative electrode material; reduction of electrolyte in negative electrode.
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