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What are the causes and consequences of explosions and fires in lithium iron phosphate batteries? In the past few years, electric vehicles using ternary lithium batteries have experienced multiple fires and explosions. As a result, lithium iron phosphate batteries, which had relatively less negative news, were labeled as "absolutely safe" and became the preferred choice for electric vehicle power batteries. But in recent years, there have been frequent rumors of explosions and fires in lithium iron phosphate batteries. Isn't lithium iron phosphate very safe?  

The causes and consequences of explosion and fire in lithium iron phosphate batteries

Lithium iron phosphate batteries generally do not experience explosions or fires. The safety of lithium iron phosphate batteries is relatively high during normal use, but there is no absolute guarantee, and danger can still occur in some extreme situations. This is closely related to the material selection, proportion, process, and later use of each company.

Although lithium iron phosphate material has the highest thermal stability and structural stability among all positive electrode materials in terms of thermodynamics, and has been verified in actual safety performance tests, it can be considered the least safe in terms of the possibility and probability of short circuits occurring within the material and battery.

Firstly, in terms of material preparation, the solid-state sintering reaction of lithium iron phosphate is a complex multiphase reaction, which includes solid-state phosphates, iron oxides, and lithium salts, as well as carbon precursors and reducing gas phases. In order to ensure that the iron element in lithium iron phosphate is positively divalent, the sintering reaction must be carried out in a reducing atmosphere. In the process of reducing trivalent iron ions to positively divalent iron ions in a strong reducing atmosphere, there is a possibility of further reducing the positively divalent iron ions to trace amounts of elemental iron.

Single element iron can cause micro short circuits in batteries, which is the most taboo substance in batteries. This is also one of the main reasons why Japan did not apply lithium iron phosphate to power lithium-ion batteries. In addition, a significant characteristic of solid-phase reactions is the slow and incomplete nature of the reaction, which increases the possibility of trace amounts of Fe2O3 in lithium iron phosphate. The Argonne Laboratory in the United States attributed the poor high-temperature cycling of lithium iron phosphate to the dissolution of Fe2O3 during charge discharge cycling and the precipitation of elemental iron on the negative electrode. In addition, in order to improve the performance of lithium iron phosphate, its particles must be nano sized. A significant characteristic of nanomaterials is their low structural and thermal stability, high chemical activity, which to some extent increases the probability of iron dissolution in lithium iron phosphate, especially under high temperature cycling and storage conditions. The experimental results also indicate that the presence of iron element can be detected through chemical analysis or energy spectrum analysis on the negative electrode.

From the perspective of preparing lithium iron phosphate batteries, due to the small size and high specific surface area of lithium iron phosphate nanoparticles, and the use of carbon coating technology, high specific surface area activated carbon has a strong adsorption effect on gases such as water in the air, resulting in poor electrode processing performance and poor adhesion of the binder to its nanoparticles. During the battery preparation process, as well as during the charging and discharging cycles and storage of the battery, nanoparticles are prone to detach from the electrodes, causing internal micro short circuits in the battery.

Of course, this is only a problem during the manufacturing process, as the process technology for lithium batteries is developing rapidly. Some craftsmanship techniques are already very excellent.

Working principle of lithium iron phosphate batteries

As a rechargeable battery, the requirements are: high capacity, high output voltage, good charge discharge cycle performance, stable output voltage, ability to charge and discharge large currents, electrochemical stability performance, safety during use (not causing combustion or explosion due to improper operation such as overcharging, over discharging, and short circuit), wide working temperature range, non-toxic or less toxic, and no pollution to the environment. The lithium iron phosphate battery using lithium iron phosphate as the positive electrode has good performance requirements, especially in high discharge rate discharge (5-10C discharge), stable discharge voltage, safety (non combustion, non explosion), lifespan (number of cycles), and no pollution to the environment. It is currently the best high current output power battery.

As the positive electrode of the battery, LiFePO4 is connected by aluminum foil to the positive electrode, with a polymer separator in the middle that separates the positive and negative electrodes. However, lithium ion Li+can pass through while electron e - cannot. On the right is the negative electrode of the battery composed of carbon (graphite), which is connected to the negative electrode of the battery by copper foil. The electrolyte of the battery is located between the upper and lower ends, and the battery is enclosed in a metal shell.

During charging of LiFePO4 batteries, the lithium ion Li+in the positive electrode migrates to the negative electrode through a polymer separator; During the discharge process, the lithium ion Li+in the negative electrode migrates to the positive electrode through the separator. Lithium ion batteries are named after the migration of lithium ions during charging and discharging.

The working temperature and charging environment temperature of lithium iron phosphate batteries: 10 ℃ to 55 ℃; Discharge environment temperature: 20 ℃~60 ℃.

Safety classification test for lithium iron phosphate batteries

1. Resistance to heavy object impact: Lithium iron phosphate battery packs shall be tested according to regulations and shall not ignite or explode.

2. Thermal shock resistance: The battery pack shall be tested according to regulations and shall not ignite or explode.

3. Anti overcharging: The battery module should be tested according to regulations and should not ignite or explode.

4. Short circuit resistance: The battery module should be tested according to regulations and should not ignite or explode.

5. High temperature storage: The battery cells should be tested according to regulations and should not leak, smoke, catch fire or explode.

6. Heat resistance: The battery module was tested according to regulations, and no part of the explosive battery penetrated the screen, and no part or all of the batteries protruded from the screen.

7. Anti puncture: Lithium iron phosphate battery packs shall be tested according to regulations and shall not ignite or explode.

Characteristics of lithium iron phosphate batteries

High energy density

Its theoretical specific capacity is 170mAh/g, and the actual specific capacity of the product can exceed 140mAh/g (0.2C, 25 ° C);

Security

It is currently the safest positive electrode material for lithium-ion batteries; Does not contain any harmful heavy metal elements to the human body;

Long lifespan

Under 100% DOD conditions, it can charge and discharge more than 2000 times

The service life of lithium iron phosphate batteries is closely related to their operating temperature. Using temperatures that are too low or too high can cause significant adverse hazards during their charging, discharging, and usage processes. Especially when used in electric vehicles in northern China, lithium iron phosphate batteries cannot provide normal power or the power supply is too low in autumn and winter. It is necessary to adjust the working environment temperature to maintain their performance. At present, space constraints need to be considered to solve the constant temperature working environment of lithium iron phosphate battery in China. The more common solution is to use aerogel felt as the insulation layer.

The above is the cause and consequences of the explosion and fire of lithium iron phosphate batteries. I hope it can help you!

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