Reason analysis and prevention of lithium battery thermal runaway？
1. Overheating triggers thermal runaway of lithium-ion batteries.
The cause of the overheating of the power lithium battery comes from the unreasonable selection of the battery and the thermal design, or the temperature rise of the battery caused by the external short circuit, and the cable connector is loose, etc., which should be solved from two aspects: battery design and battery management; From the perspective of battery material design, materials can be developed to prevent thermal runaway and block the reaction of thermal runaway; from the perspective of battery management, different temperature ranges can be predicted to imply different safety levels, so as to perform hierarchical alarms.
2. Overcharging triggers thermal runaway of lithium-ion batteries.
The thermal runaway triggered by overcharging refers to the loss of the safety function of the battery management system itself for the overcharging circuit, resulting in the BMS of the battery being out of control but still charging. For this kind of overcharging reasons, the solution is first to find the fault of the charger, which can be solved by the complete redundancy of the charger; secondly, check whether the battery management is reasonable, for example, the voltage of each battery is not monitored.
As the battery ages, the consistency between the individual batteries will become worse and worse, and overcharging is more likely to occur at this time. This requires equalization of the entire battery pack to maintain battery pack consistency. For example, a series battery pack using the most common battery pack combination method of first parallel and then series, after solving the problem of monomer consistency, the best case is to have the same large capacity as the smallest capacity monomer. With this consistency, the capacity has risen and overcharging can also be prevented. In order to achieve consistency, there must be a way to estimate the capacity of each cell. An estimation of the state of the overall battery pack can be made based on the similarity of the charging curves. That is to say, as long as you know the charging curve of one of the single cells, the other curves should be similar to it. After the curve changes, they can be approximately coincident, and these differences in the middle of the curve change process are easy to calculate. Other monomers can be deduced from one monomer. With such a method, the consistency balance mentioned above can be carried out. Of course, this algorithm takes too long and needs to be simplified.
3. The internal short circuit triggers the thermal runaway of the lithium-ion battery.
Impurities in battery manufacturing, metal particles, expansion and contraction during charge and discharge, and lithium precipitation may all cause internal short circuits. This internal short occurs slowly, for a very long time, and it is not known when it will thermally run away. If the test is carried out, the verification cannot be repeated. At present, experts all over the world have not found a process that can repeat the internal short circuit caused by impurities, and they are all under research.
To solve the problem of internal short circuit, first of all, we must find a battery manufacturer with good product quality, and select the capacity of the battery and battery cells; secondly, conduct safety predictions for internal short circuits, and find cells with internal short circuits before thermal runaway occurs. This means that the characteristic parameters of the monomer must be found, and we can start with consistency. The batteries are inconsistent, and the internal resistance is also inconsistent. As long as you find the monomer with variation in the middle, you can identify it. Specifically, lithium-ion battery manufacturers believe that the equivalent circuit of a normal battery and the equivalent circuit with a micro-short circuit are actually the same in the form of the equation, except that the parameters of the normal cell and the cell with a micro-short circuit have changed. Variety. These parameters can be studied to see some characteristics of the internal short circuit changes.
One of the characteristics is the potential difference of the internal short-circuit monomer, and the difference between its internal resistance and other monomers is compared. R&D personnel should use the model to identify monomers. After measuring the voltage and current of each monomer, the internal resistance of each monomer can be estimated by using these data and combining with the model. After all the parameters of the monomer are estimated, according to the change of the parameters, it can be judged whether the consistency has changed significantly.
4. Mechanical triggering of lithium-ion battery thermal runaway.
Crashing is typically one way mechanically triggers thermal runaway. If a simulation of collision is performed in the laboratory, the closest thing is acupuncture. Through acupuncture tests on ternary lithium-ion batteries and lithium iron phosphate batteries to study the process of thermal runaway, it was found that lithium iron phosphate batteries did not release heat as violently as ternary lithium-ion batteries during this thermal runaway process. Experiments have shown that different materials will have different reactions when needled, and lithium iron phosphate is relatively safe; the solution to thermal runaway triggered by collisions is to design battery safety protection.
Generally speaking, after thermal runaway occurs, it will propagate downward. For example, after the first section of thermal runaway, there will be heat transfer, start to spread, and then the whole group will follow one by one like setting off firecrackers. For this kind of propagation, a model can be established, including the intermediate temperature rise rate, heat production of chemical energy and electric energy, heat transfer and convection, etc. The entire thermoelectric coupling model can be used for a related quantitative analysis with a calorimeter. With the propagation model, the R&D personnel of lithium-ion battery customization can design how to block and suppress, which requires adding a heat insulation layer; Layers are contradictory to cooling. These are all problems to be solved. In short, in terms of thermal runaway expansion and suppression, developers should start from two aspects: safety protection design and battery management.
Preventive measures for lithium battery thermal runaway
1. Set up a safety valve, but the pressure range of the safety valve needs to be strictly controlled.
2. Install a thermistor to prevent the battery from overcharging or short circuiting.
3. Precise thermal management of BMS, using water cooling, air cooling, etc. to cool down the battery during battery use.
4. The use of additives in the electrolyte reduces the flammability of the electrolyte.
5. Improve the quality of SEI film formation, such as: adding LiCF3SO3 to the electrolyte, so that there are more inorganic components in SEI.
6. Prevent the reaction between the positive electrode material and the electrolyte, such as: the use of additives in the electrolyte or the coating of the positive electrode material.
7. Increase the melting point of the diaphragm, such as coating ceramic layers on both sides of the diaphragm.
8. Standardize the use of lithium batteries to reduce or eliminate human factors such as overcharge and overdischarge.