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Lithium iron phosphate batteries have advantages such as high energy density, high safety, high temperature resistance, low cost, many cycles, and long life, and are widely used in the fields of electric vehicles and energy storage. In order to meet the actual application needs, a large number of single cells are usually connected in series or in parallel to form a battery pack. However, this high-energy system is likely to cause more serious consequences when thermal runaway occurs.
Overcharge and overheating are considered to be one of the main factors leading to battery thermal runaway, but usually only a single factor test is performed, and whether it catches fire or explodes is used as the criterion for passing or failing. In the actual battery thermal runaway scenario, it may be driven by a single factor in the early stage, but as the thermal runaway process develops, it will gradually evolve into a situation where multiple factors are coupled and driven, making the consequences of thermal runaway more serious. It is of practical significance to study the coupling effect of lithium iron phosphate lithium-ion batteries under various abuse conditions. Overcharging is considered to be an important cause of battery fires. The early overcharging standard for consumer batteries was 3C/4.8V. Since consumer batteries are usually used alone and with the advancement of battery technology, thermal runaway caused by overcharging has been significantly reduced, and the current standards have significantly relaxed the parameter requirements for overcharging. Overheating tests usually simulate the melting of the diaphragm or the decomposition of the solid electrolyte interface (SEI) film at a temperature of 130°C or 85°C.
The safety problem of batteries is essentially the thermal characteristics of batteries. D.P. Kong et al. studied the thermal characteristics of lithium iron phosphate lithium-ion batteries after local heating at different positions and found that heating the bottom of the battery is more likely to cause thermal runaway than heating other positions. P.J. Liu et al. studied the effects of two abuse methods, overheating and overcharging, on the thermal runaway of lithium iron phosphate lithium-ion batteries. The results showed that compared with overheating, the risk of fire caused by overcharging is higher, and the battery burns more violently during the test. Thermal runaway is also significantly related to the state of charge (SOC) of the battery. For example, P.J. Liu et al. used a heating plate to simulate the thermal runaway process triggered by the abuse of adjacent batteries in the module and found that the trigger temperature of 50% SOC is higher than that of 100% SOC batteries. K. Wang et al. conducted an overcharge test on lithium iron phosphate lithium-ion batteries at 0.5C, and captured that the gases emitted by the battery thermal runaway were mainly hydrogen (H2), carbon monoxide (CO) and carbon dioxide (CO2), as well as various alkanes.
The large-scale series and parallel connection of batteries greatly increases the probability of overcharging and overdischarging risks, because if a battery has a problem, the heat spread may cause the entire battery pack to catch fire and explode. Not only that, the generated flammable gas mixed with air will also cause a more serious explosion.
In order to improve the safety of batteries in actual application scenarios, it is necessary to strengthen the battery's tolerance to multiple abuses, which is becoming a focus of academic and industrial attention.
In addition, when using the battery, it should also be avoided to charge it after it is completely out of power, which is easy to cause irreversible damage to the battery.
When the lithium iron phosphate battery is fully charged, the charging should be stopped immediately, otherwise the battery will be overcharged, resulting in a shortened battery life or even dangerous situations such as short circuits.
Using the battery in a high temperature environment will also affect its performance.
The constant current discharge method refers to controlling the size of the discharge current so that the battery releases energy at a constant rate. It is suitable for the rapid discharge of lithium iron phosphate batteries.
