Can lithium batteries in electric cars spontaneously ignite?

New energy vehicles with lithium batteries as energy storage devices have now become the mainstream of the market. However, with the promotion and application of new energy vehicles, the problem of self-combustion of lithium batteries and the safety of car use have become concerns.

Why does lithium battery spontaneously ignite? Can electric cars be used safely and safely? The small editor combed for everyone.

The principle of spontaneous combustion

Lithium is the most active metal in the world. Lithium-ion batteries have a high density of small capacity and high energy density, making them the first choice for electric vehicles. Lithium-ion batteries use lithium-ion gain and loss electrons and migration aggregation to store electrical energy in their work.

When the battery is charged, the lithium atom in the positive pole loses electrons and becomes a lithium ion, resulting in a potential difference. Lithium-ions in electrolytic media accumulate to negative poles under the action of potential difference. When discharge, the whole program is reversed. The whole work process is accomplished by the loss and loss of lithium metals in the electrode and the migration of electrons and lithium ions in the electrolyte.

However, the chemical properties of lithium are too active. Lithium metal is exposed to air and produces a fierce oxidation reaction with oxygen, resulting in combustion and explosion.

Therefore, in the practical application of lithium batteries, scientists have tried their best to prevent the conversion of lithium ions into lithium metals in electrolytes and lock lithium metals in graphite or lithium compounds. lithium iron phosphate and lithium cobalt acid are materials that store lithium atoms.

At the same time, a series of protective measures have been taken to prevent air from entering the interior of the battery. This causes lithium metal to explode without contact with oxygen.

In use, the reason why lithium batteries spontaneously ignite is due to the fact that the protective measures are not in place or serious external forces are destroyed, resulting in the failure of the protection and causing the metal lithium to contact the air.

Common protective measures

Shell protection. In order to prevent air from entering, lithium batteries are enclosed in closed containers and are usually equipped with stainless steel shells and aluminum alloy shells to prevent external force damage. Tesla's electric vehicles, for example, even use titanium protective panels to prevent damage to battery containers in car use, especially in traffic accidents.

The diaphragm block protection, while preventing external forces from destroying, but also to prevent damage from the inside of the battery.

Usually, in order to prevent the battery's positive and negative poles from directly touching and short-circuiting, there will be a layer of the diaphragm in the battery, on the one hand separating the positive and negative poles, and on the other hand allowing charged ions to pass through.

However, in lithium batteries, the diaphragm also performs another protective function. When the battery temperature is too high, the diaphragm gap will automatically close, so that lithium ions can not cross, thus terminating the entire battery reaction. This prevents the battery from vaporizing the electrolyte due to high temperature, causing high pressure and destroying the battery seal structure.

Overcharge voltage protection not only prevents the air from being cut off but also prevents the leakage of metal lithium from the electrode.

Scientists use the nanoscale and lattice mechanisms of electrode materials to store and lock the metallic lithium formed in charge and discharge.

In this way, even if the battery shell is broken and oxygen enters, the oxygen molecules will not enter these small cells because they are too large to avoid spontaneous combustion.

However, using too high voltage or charging for too long after filling can cause very dangerous damage to lithium batteries.

After the lithium battery charging voltage is higher than the rated voltage(usually 4.2 V), if the charge continues, since the negative cell is already filled with lithium atoms, the subsequent lithium ions will accumulate on the surface of the negative electrode material. Due to polarization, these lithium ions form electron transfer, form metallic lithium, and dendritic crystals grow from the negative surface to lithium ions.

This metallic lithium without electrode protection is extremely active on the one hand and are prone to oxidation reactions and explosions. On the other hand, the formed metallic lithium crystal penetrates the diaphragm, short-circuits the positive and negative poles, causing short circuits and high temperatures. At high temperatures, materials such as electrolytes crack to produce gas, causing the battery shell or pressure valve to swell and burst, allowing oxygen to enter, and react with lithium atoms that accumulate on the negative surface, and then explode.

When charging lithium batteries, be sure to set the voltage cap and overcharge protection. Lithium batteries produced by regular battery manufacturers are equipped with such protective circuits. Power off automatically when the voltage is excessive or full.

In fact, spontaneous combustion is not easy.

Lithium battery spontaneous combustion is not an easy task under the protective conditions of qualified housing, diaphragm, and circuit.

Especially in inorganic materials lithium batteries, for example, lithium iron phosphate batteries, because the battery does not use organic materials in the lithium ternary battery, even if the material is damaged and short-circuited inside the battery, the resulting high temperature will not cause the inorganic material to decompose and produce high-pressure gas., The explosion then spontaneously ignited. In experiments, qualified lithium-iron phosphate batteries would not explode and be natural even if they were put into firewood(which is below 600 °C).

At the same time, compared with the situation in which manufacturers of new energy vehicles did not understand the protection measures in the actual use of vehicles in the early stages of the development of new energy vehicles, at present, after decades of exploration and the actual use of millions of new energy vehicles in the world, from battery manufacturers to auto manufacturers have accumulated enough. Experience.

The protective strength of the shell of the lithium battery has been greatly strengthened. Even in the case of serious traffic accidents such as the disintegration of the vehicle, battery protection can still be effective.

In the selection of battery diaphragm, manufacturers are also fully aware of the importance of the use of the self-protected functional diaphragm. At present, high-quality lithium battery products have already used this technology. For the application of anti-charging electronic protection devices, it has long become a national mandatory standard. Qualified electronic protection devices and mature battery management systems have greatly reduced the chance of spontaneous combustion of lithium batteries in new energy vehicles.

In the current case that caused the spontaneous combustion of new energy vehicles, none of them is due to the spontaneous explosion of the battery pack under normal driving conditions. The explosion of more lithium batteries in new energy vehicles is due to extremely serious traffic accidents and improper modifications of in-vehicle electrical equipment. In the heat of nearly a thousand degrees, the lithium battery was eventually ignited.

For consumers, lithium-ion batteries in new-energy cars are generally safe. However, it is necessary to guard against the fact that in the case of an industrial explosion, individual manufacturers 'crude manufacturing and product price wars have brought about the neglect of battery protection and the hidden dangers brought about by the illegal modification of electric vehicles.

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