Lithium-ion battery thermal stability and safety analysis of overcharge, high temperature and short circuit

　　In recent years, reports of fires and even explosions caused by lithium-ion batteries are not uncommon.

　　Lithium-ion batteries have strong dangers, especially in the case of abuse, safety issues are 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. In the final analysis, the fire hazard of lithium-ion batteries depends on the thermal stability of the battery material, and the thermal stability of the battery material depends on the chemical reactions that occur between its internal parts.

　　1) Influencing factors of the thermal stability of anode materials

　　The onset temperature of the negative electrode material's heat generation increases with the increase of the particle size.

　　2) Factors affecting the thermal stability of cathode materials

　　The initial temperature of the reaction between the positive electrode material and the electrolyte increases as the stoichiometric number decreases.

　　The higher the content of Ni in the cathode material, the more unstable it is, and the higher the content of Mn, the more stable it is.

　　3) Influencing factors of electrolyte thermal stability

　　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 electrolyte can make the positive electrode react at a lower temperature, and different solvents and lithium salts in the electrolyte are suitable for different positive electrode materials.

　　2. 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 stick, extrusion, and high temperature.

　　1) Overcharge

　　The overcharge test simulates the potential safety hazards of the battery when there is an error in the charger voltage detection, the charger fails or the wrong charger is used.

　　The thermal runaway caused by overcharging may come from two aspects: on the one hand, the Joule heat generated by the current, and on the other hand, the reaction heat generated by the side reactions of the positive and negative electrodes. When the battery is overcharged, the negative electrode voltage gradually increases, and the process of delithiation becomes more and more difficult. This causes the internal resistance of the battery to increase sharply, and therefore generates a large amount of Joule heat, which is more obvious when charging at a high rate. The positive electrode oxidant in the overcharged state releases a lot of heat, and the negative electrode also reacts exothermically with the electrolyte. When the rate of heat release is greater than the rate of heat dissipation of the battery, and the temperature rises to a certain level, thermal runaway will occur.

　　2) High temperature

　　The simulated environment high temperature test can be carried out by using the hot box test. The hot box test simulates the improper use of the battery at a high temperature, such as placing a mobile phone in an exposed car, or putting a mobile phone or electronic product in a microwave oven, and the temperature can reach 130°C or even 150°C. In the case of thermal abuse, the heat source is not only from the positive and negative electrode materials inside the battery and its reaction with the electrolyte, the separator film melts and shrinks at high temperatures, resulting in a short circuit between the positive and negative electrodes. The Joule heat generated by the short circuit is also an important heat source during the hot box test. .

　　3) Short circuit

　　The short circuit of the battery is divided into external short circuit and internal short circuit.

　　External short circuit generally refers to the short circuit caused by the direct contact between 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 impacted or squeezed.

　　The safety research of internal short circuit generally adopts methods such as acupuncture, extrusion, etc. The purpose is to simulate the situation of the battery being punctured, collided, and squeezed by foreign objects. Acupuncture causes a short circuit of the battery at the acupuncture point. 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, it will cause thermal runaway, causing smoke, fire or even explosion hazards. Extrusion is similar to acupuncture, both of which cause local 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 flammable electrolyte will not leak from the hot zone, and the heat dissipation effect from the hot zone is worse.

　　It is often much more difficult to test the local internal short circuit of the battery caused by squeezing and needle sticking than the external short circuit test. This is because the internal heat of the battery tends to be uniform when the battery is externally short-circuited, and the Joule heat generated by the external short-circuit battery will not Directly trigger the thermal runaway reaction of the battery.

　　Lithium-ion battery (LIB) has become the main energy storage solution in modern social life. Among them, lithium iron phosphate battery is a perfect replacement for lead-acid batteries, and it is the first choice for grid-connected peak shaving, off-grid energy storage, photovoltaic energy storage, UPS, data center and other industries.