Thermal stability of lithium ion battery
Update:9/27/2022 2:53:55 PM View : 1271
The safety problem of lithium-ion battery is the internal heat of the battery. If the heat generated by the failure cannot be released quickly, the battery temperature will continue to rise, and the rising temperature will intensify the chemical and electrochemical reactions between active substances, electrolyte and other components in the battery, resulting in a large amount of heat and gas, which will eventually lead to the thermal runaway of the battery, until smoke, fire, explosion and other safety accidents occur.
The internal reaction heat sources of the electric core mainly include the following aspects:
1) Decomposition reaction of SEI membrane
During the first charging and discharging process of lithium ion battery, the cathode material and electrolyte react on the solid phase interface to form a layer of interface protection film - SEI (solid electrolyte interface) covering the cathode surface, which prevents the interaction between the electrolyte and the carbon anode. But when the temperature rises, the reaction activity increases, the SEI membrane decomposes, and the reaction is exothermic.
2). Reaction of lithium intercalated carbon with solvent
When the temperature rises, the SEI film cannot protect the negative electrode, and the solvent may react with lithium metal or embedded lithium.
3). Reaction of lithium intercalated carbon with fluorinated binder
It is found that when the temperature exceeds 260 ℃, the adhesive PVDF reacts with LixC6.
4). Decomposition reaction of electrolyte
The thermal decomposition reaction of lithium ion battery electrolyte is mainly the reaction between solvent and lithium salt when the temperature rises. When the charging voltage of the lithium ion battery exceeds the decomposition voltage of the electrolyte, the electrolyte will also undergo decomposition reaction, release heat and generate gas.
5) . Decomposition reaction of positive pole
6) Layered LiCoO2, LiNiO2, spinel like LiMn204 and olivine like LiFePO4 are cathode materials that have been studied extensively, and they are stable at temperatures below 650 ℃. The above materials are in a metastable state when charged, and will decompose when the temperature rises.
6). Lithium metal reaction
When the lithium ion battery is overcharged, the lithium metal is deposited on the surface of the negative electrode, and the reaction between the metal lithium and the electrolyte may occur.
7). Enthalpy change of positive and negative active substances
The enthalpy of lithium embedded in the cathode material changes when lithium ion batteries are charged and discharged.
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, sometimes called polarization internal resistance heat generation. When the external circuit of the battery is short circuited, the heat generation of the internal resistance of the battery is dominant.
Three sources are summarized:
A. The reversible heat generated by chemical reaction comes from the electrochemical reaction, and its size is related to the entropy change of the material;
B. Irreversible heat is mainly caused by ohmic internal resistance and polarization;
C. The heat generated by the side reaction is mainly from the chemical reaction between the electrolyte and the electrode material.
For the battery, even under normal charging and discharging conditions, the heat of part A and part B always exists. If the heat of part C occurs, the battery may have safety problems. The heat of Part C mainly comes from the following five parts:
Thermal decomposition of cathode materials; Oxidation of electrolyte at positive electrode; Thermal decomposition of electrolyte; Thermal decomposition of cathode materials; Reduction of electrolyte at the negative electrode.
The internal reaction heat sources of the electric core mainly include the following aspects:
1) Decomposition reaction of SEI membrane
During the first charging and discharging process of lithium ion battery, the cathode material and electrolyte react on the solid phase interface to form a layer of interface protection film - SEI (solid electrolyte interface) covering the cathode surface, which prevents the interaction between the electrolyte and the carbon anode. But when the temperature rises, the reaction activity increases, the SEI membrane decomposes, and the reaction is exothermic.
2). Reaction of lithium intercalated carbon with solvent
When the temperature rises, the SEI film cannot protect the negative electrode, and the solvent may react with lithium metal or embedded lithium.
3). Reaction of lithium intercalated carbon with fluorinated binder
It is found that when the temperature exceeds 260 ℃, the adhesive PVDF reacts with LixC6.
4). Decomposition reaction of electrolyte
The thermal decomposition reaction of lithium ion battery electrolyte is mainly the reaction between solvent and lithium salt when the temperature rises. When the charging voltage of the lithium ion battery exceeds the decomposition voltage of the electrolyte, the electrolyte will also undergo decomposition reaction, release heat and generate gas.
5) . Decomposition reaction of positive pole
6) Layered LiCoO2, LiNiO2, spinel like LiMn204 and olivine like LiFePO4 are cathode materials that have been studied extensively, and they are stable at temperatures below 650 ℃. The above materials are in a metastable state when charged, and will decompose when the temperature rises.
6). Lithium metal reaction
When the lithium ion battery is overcharged, the lithium metal is deposited on the surface of the negative electrode, and the reaction between the metal lithium and the electrolyte may occur.
7). Enthalpy change of positive and negative active substances
The enthalpy of lithium embedded in the cathode material changes when lithium ion batteries are charged and discharged.
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, sometimes called polarization internal resistance heat generation. When the external circuit of the battery is short circuited, the heat generation of the internal resistance of the battery is dominant.
Three sources are summarized:
A. The reversible heat generated by chemical reaction comes from the electrochemical reaction, and its size is related to the entropy change of the material;
B. Irreversible heat is mainly caused by ohmic internal resistance and polarization;
C. The heat generated by the side reaction is mainly from the chemical reaction between the electrolyte and the electrode material.
For the battery, even under normal charging and discharging conditions, the heat of part A and part B always exists. If the heat of part C occurs, the battery may have safety problems. The heat of Part C mainly comes from the following five parts:
Thermal decomposition of cathode materials; Oxidation of electrolyte at positive electrode; Thermal decomposition of electrolyte; Thermal decomposition of cathode materials; Reduction of electrolyte at the negative electrode.