Introduction
In recent years, there have been frequent reports of fires and even explosions caused by lithium-ion batteries. In this we will discuss about the analysis and summary of lithium battery. Lithium-ion batteries are mainly composed of negative electrode material, electrolyte and positive electrode material. The chemical activity of the negative electrode material graphite is close to that of metallic lithium in the charged state, and the SEI film on the surface decomposes at high temperature, and the lithium ions embedded in the graphite react with the electrolyte and the binder polyvinylidene fluoride to release a lot of heat.
The electrolyte generally uses an organic solution of alkyl carbonate, which has flammable properties. The positive electrode material is usually a transition metal oxide, which has a strong oxidizing property in the charged state. It is easy to decompose and release oxygen at high temperature. The released oxygen undergoes an oxidation reaction with the electrolyte, and then a large amount of heat is released. Therefore, from the point of view of materials, lithium-ion batteries have strong danger, especially in the case of abuse, the safety problem is more prominent.
1. Thermal stability analysis of lithium-ion battery materials
The fire hazard of lithium-ion batteries is mainly determined by the amount of heat generated by chemical reactions in various parts of the battery. The fire hazard of a lithium-ion battery ultimately depends on the thermal stability of the battery material, which in turn depends on the chemical reactions that take place between its internal parts. At present, people mainly rely on differential scanning calorimeter (DSC), (TGA), adiabatic acceleration calorimeter (ARC), etc. to study the thermal stability of battery-related materials.
(i) Factors Affecting the Thermal Stability of Negative Electrode Materials:
The onset temperature of the exotherm of the anode material increases with the particle size. The thermal stability of lithium intercalated natural graphite with different particle sizes was investigated by DSC. It was found that three exothermic peaks appeared in all samples. The first exothermic peak of the sample is located around 150 °C, while the positions of the latter two exothermic peaks are obviously different, and the onset temperature of the latter two exothermic peaks increases with the increase of particle size. This study shows that the first exothermic peak is the decomposition of the SEI film, and the last two exothermic peaks are the reaction of lithium intercalated graphite with PVDF and electrolyte.
The relationship between the specific surface area and thermal stability of graphite materials was investigated with ARC, and it was found that the reaction rate increased by two orders of magnitude when the specific surface area of graphite materials increased from 0.4 m2/g to 9.2 m2/g. Therefore, the reaction rate of carbon anode materials increases with the increase of specific surface area. Carbon materials with different structures produce different heats of reaction, and the graphite structure produces more heat than the amorphous carbon structure. The thermal stability of carbon fiber, hard carbon, soft carbon and MCMB was investigated by DSC. The study found that the first exothermic peaks of the four carbons all appeared at 100 ℃, and this exothermic peak was considered to be generated by the decomposition of the SEI film; as the temperature increased to 230 ℃, the carbon structure and specific surface area were thermally stable to the material The influence of the properties gradually emerge, and carbon electrode materials with graphite structure (carbon fiber, MCMB) generate more heat than carbon electrode materials with amorphous structure (soft carbon, hard carbon). XRD showed that the total loss of lithium intercalation was linearly related to the carbon specific surface area at around 230 °C.
(ii) Influencing Factors of Thermal Stability of Cathode Materials:
The onset temperature of the reaction between the cathode material and the electrolyte increases as the stoichiometric number decreases. The effect of the change of x on the reaction of the cathode materials LixCoO2, LixNiO2, LixMn2O4 and LixC6 with the electrolyte was investigated by DSC. Through the research, it is concluded that there is an exothermic reaction between the electrolyte and the positive electrode material. When the value of x decreases, the reaction temperature rises to the range of 200-230 °C, and the LixCoO2, LixNiO2, LixMn2O4 materials all react strongly with the electrolyte.
The thermal stability of LixCoO2 was studied with ARC. Above the critical temperature, LixCoO2 undergoes an oxygen-releasing reaction and a large amount of heat is released. When x = 0.25, the onset temperature of the exothermic reaction is about 230 °C. Li Yi et al. measured the natural reaction temperature of 18650 type LiCoO2 to be 170 ℃ in the heat resistance test., indicating that the onset temperature of the decomposition reaction is lower. Therefore, it can be seen that the initial temperature of the decomposition reaction of the positive electrode material increases with the decrease of X. The higher the Ni content in the cathode material, the more stable it is, and the higher the Mn content, the more stable it is.
The thermal stability of materials with different compositions of Li1 -xNi1-2xCoxMnxO2 was studied by DSC. It was found that with the decrease of Ni content, the exothermic onset temperature and peak temperature of Li1 – xNi1-2xCoxMnxO2 higher, and less heat is produced. Maneli et al. studied the exothermic heat of reaction of several cathode materials with 1mol LiPF6 EC/DEC, as shown in Table 1,
1. Thermal stability of common cathode materials
(iii) Factors Affecting the Thermal Stability of the Electrolyte:
The organic solvent DMC is an important factor causing the instability of the electrolyte, and the higher the DMC content, the more unstable the electrolyte. The electrolytes of EC+DEC, EC+DMC, PC+DEC and PC+DMC mixed solvent dissolved in 1mol/L LiPF6 were studied by DSC in a closed container, and it was found that the electrolyte containing DMC was better than the electrolyte containing DEC. more likely to react.
The electrolyte allows the cathode to react at a lower temperature, and different solvents and lithium salts in the electrolyte are suitable for different cathode materials. The exothermic reactions between Li0.5CoO2 and LiMn2 O4 charged cathodes and electrolyte was investigated by ARC and XRD methods, respectively. The research shows that the decomposition reaction of Li 0.5 CoO2 powder occurs when the temperature is higher than 200 ℃, and oxygen is released, while the exothermic reaction with EC/DEC solvent occurs at 130 ℃, and the reaction is inhibited after adding LiPF6 to the solvent. For LiMn2O4 material, the crystal transformation occurs at 160 ℃ and exothermic, the presence of solvent has no effect on this reaction. After adding LiPF 6 to the electrolyte, the reaction between LiMn 2 O 4 and the electrolyte intensifies with the increase of LiPF 6 concentration.
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2. The safety analysis of lithium-ion battery abuse
The safety of lithium-ion batteries mainly depends on the thermal stability of battery materials, and is also closely related to abuse conditions such as battery overcharge, needle penetration, extrusion, and high temperature.
(i) Analysis of Overcharge Safety:
The overcharge test is to simulate the potential safety hazards of the battery when the charger voltage detection is wrong, the charger fails or the wrong charger is used. The thermal runaway caused by overcharge may come from two aspects: one is the Joule heat generated by the current, and the other is the reaction heat generated by the side reactions of the positive and negative electrodes. When the battery is overcharged, the voltage of the negative electrode gradually increases. When the amount of DE lithiation of the negative electrode is too large, the process of DE lithiation becomes more and more difficult, which leads to a sharp increase in the internal resistance of the battery, so a large amount of Joule heat is generated, which is in large scale. It is more obvious when charging at a rate. The high-voltage positive oxidant in the overcharged state releases a lot of heat, and the negative electrode will also undergo an exothermic reaction with the electrolyte after the temperature rises. Thermal runaway occurs when the heat release rate is greater than the heat dissipation rate of the battery and the temperature rises to a certain level.
Tobishim et al. comparatively studied the overcharge performance of aluminum shell square batteries using LiCoO2 and LiMn2 O4 as positive electrode materials respectively. The results show that LiCoO2 cells will explode when charged at a current of 2C to a voltage of 10V, while LiMn2 O4 cells are respectively When overcharged at 2C/10V and 3C/10V, there was no smoke, fire or explosion, but only bulging, which shows that Mn has better overcharge resistance than Co. Leasing et al. studied the effect of different proportions of graphite on the overcharge performance of LiCoO2 cells. The results show that the overcharge performance of the cell mainly depends on the cathode material and does not change with the increase of graphite content. This shows that the precipitation of metallic lithium in the negative electrode during the overcharge process is not the key to affecting the overcharge performance, but the thermal stability of the excessively DE lithiation LiCoO2 or the oxidation reaction of the electrolyte on its surface.
(ii) High Temperature Safety Analysis:
The simulated environment high temperature test can be carried out using the hot box test. The hot box test is to simulate the improper use of the battery at high temperature, such as placing a mobile phone in an exposed car, or placing a mobile phone or electronic product in a microwave oven, the temperature can reach 130 ℃ or even 150 ℃. In the case of thermal abuse, the heat source comes from the positive and negative electrode materials inside the battery and their reaction with the electrolyte. The separator melts and shrinks at high temperature, resulting in a short circuit of the positive and negative electrodes. The Joule heat generated by the short circuit is also an important heat source in the hot box test. Table 2 summarizes the thermal behavior of the lithium-ion battery system in a certain temperature range when the electrolyte system is 1 mol/L LiPF6/(PC+EC+DMC).
Table 2 Main thermal behaviors in Li-ion battery system
| Temperature Range ___Chemical reaction | Heat / J-g21 |
| 90~120 SEI membrane decomposition | 186 |
| 110 ~ 150 LirC6 reacts with electrolyte | 350 |
| 130~180 PE diaphragm melting | 90 |
| 180~500 Li0, 1Ni0, electrolyte decomposition | 600 |
| 220~500 Li045 nio4 and electrolyte decomposition | 450 |
| 130 ~220 solvent and LiPF. reaction | 250 |
| 240~350 Li, C6 reacts with PVDF | 1500 |
| 660 Melting of AI collector fluid | President of the No. 39 Electricity |
When the temperature is between 90 and 120 °C, the metastable layer of the solid electrolyte interface film (SEI) formed on the surface of the carbon negative electrode by multiple charging and discharging first decomposes and exotherms; as the temperature increases, the separator absorbs heat and melts successively; When the temperature is between 180 and 500 °C, the positive electrode and the electrolyte undergo a strong exothermic reaction and generate gas; the SEI film can prevent the interaction between the lithium-intercalated carbon and the organic electrolyte. When the temperature is higher than 120 °C, the rupture of the SEI film cannot protect the negative electrode. , the anode material may start an exothermic reaction with the solvent and generate gas, when the temperature rises to At 240 to 350°C, the fluorine-containing binder begins to undergo a violent chain growth reaction with the lithium-intercalating carbon, releasing a lot of heat. The reaction between the negative electrode and the electrolyte may exhaust the lithium, so this reaction will not occur; if the temperature continues to rise to at 660°C, the Al current collector will undergo endothermic melting. These conditions are very dangerous for large lithium-ion power batteries, affecting the life and safety of the battery.
3. Short-circuit safety analysis:
The short circuit of the battery is divided into an external short circuit and an internal short circuit. External short circuit generally refers to the short circuit caused by the direct contact of the positive and negative electrodes; internal short circuit refers to the short circuit in the area where the battery is affected by foreign objects when the battery is punctured by a sharp object or is collided or squeezed.
External Short-Circuit Safety Analysis:
The external short-circuit safety research is tested by the method of directly connecting the positive and negative electrodes to the outside. Li Yi and others conducted research on the external short circuit of the battery. They will study the lithium cobalt oxide 18650 type lithium-ion battery and 6-cell notebook battery (6 18650 type batteries, 3 batteries in series into 1 group, 2 groups in parallel, remove the protection circuit) The positive and negative electrodes are short-circuited with wires, and a thermocouple is attached to the surface of the battery to detect the temperature change of the battery surface. The temperature curve of the battery surface was recorded with a paperless recorder. The temperature curves of the two groups of tests are shown in Figure 1.
As can be seen from Figure 1, the maximum temperatures of the two groups of batteries are 73.3 °C and 65.1 °C respectively. Although such a temperature will not cause the battery to burn and explode, because of its continuous heat release, for large-capacity battery packs, If the heat cannot be dissipated in time, it may cause a fire or even an explosion.
Internal Short-Circuit Safety Analysis:
The safety research of the internal short circuit of the battery is generally tested by acupuncture, extrusion and other methods, and the purpose is to simulate the situation of the battery being punctured, collided, and squeezed by foreign objects. Acupuncture causes the battery to short-circuit at the acupuncture point, and the short-circuit area forms a local hot zone due to a large amount of Joule heat. When the temperature of the hot zone exceeds the critical point, thermal runaway will occur, resulting in the danger of smoke, fire or even explosion. Extrusion is similar to acupuncture in that both cause localized internal short circuits and may cause thermal runaway. The difference is that squeezing does not necessarily cause damage to the battery casing. If the casing is not damaged, it means that the flammable electrolyte will not leak from the hot zone, and the heat dissipation effect of the hot zone is poorer.
It is often much more difficult to test the local internal short-circuit of the battery through extrusion and acupuncture than to pass the external short-circuit test. This is because when the battery is short-circuited outside the battery, the internal heat of the battery tends to be uniform, and the Joule heat generated by the externally short- circuited battery will not Directly trigger the thermal runaway reaction of the battery.
The test conditions such as acupuncture and extrusion have a great influence on the test results, because the internal short-circuit conditions caused by the acupuncture and extrusion tests under different conditions are different, and the size of the internal short-circuit resistance has a greater effect on the heat generation power in the short- circuit area. Impact. There are 4 forms of internal short circuit in the battery: (1) between the Al current collector and the negative electrode material (LiC6, C6); (2) between the Al current collector and the Cu current collector; (3) between the positive electrode material and LiC6 (4) Between the positive electrode material and the Cu current collector.
Santhanagopalan et al. established an electrochemical finite element thermal model of the battery to systematically simulate and analyzes the exothermic power and battery temperature inside the battery under these four short-circuit conditions, and designed corresponding experiments to verify. The results show that the short circuit between the Al current collector and the charged graphite is the most dangerous, because in this case, the short circuit resistance is small, the current is large, the thermal power is high, the heat conduction and heat dissipation are relatively slow, and the activity of the carbon negative electrode is high, so it is easy to Cause a series of subsequent electrical and chemical reactions, resulting in accidents.
CONCLUSION
When a battery is over-charged, the battery’s surface temperature increases first because of ohmic heat, and after that, internal short-circuit occurs, resulting in the battery experiencing thermal runaway. However, in the case of over-discharge, the battery’s surface temperature rise is limited. If a battery is over-charged, it will affect its thermal stability, and even trigger thermal runaway at worst. Therefore, during actual use, attention needs to be paid so as to prevent a battery from over-charge.
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