Why do lithium batteries spontaneously ignite?

Principle of self-ignition

Lithium is the most active metal in the world. Lithium-ion batteries are small in size, high in density, and high in energy density, making them the first choice for electric vehicles. In the work of lithium-ion batteries, the use of lithium ions to gain and lose electrons and migration and accumulation to achieve the storage of electrical energy.

When the battery is charged, the lithium atoms in the positive electrode lose electrons and become lithium ions, causing a potential difference. The lithium ions in the electrolytic medium migrate to the negative electrode under the action of the potential difference. When discharging, the entire program is reversed. The whole working process is completed by the loss of lithium metal 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 the air and produces a strong oxidation reaction with oxygen, which causes combustion and explosion.

Therefore, in the practical application of lithium batteries, scientists have tried every means to prevent the conversion of lithium ions in the electrolyte to lithium metal, and to lock metal lithium in graphite or lithium compounds. It is often heard that lithium iron phosphate and cobalt acid Lithium is a material that stores lithium atoms.

Power battery safety issues: Electric vehicle lithium batteries will not easily self-ignite

At the same time, in order to prevent air from entering the battery inside, a series of protective measures have also been taken. This causes the lithium metal to not explode in contact with oxygen.

In use, the reason why the lithium battery will spontaneously ignite is because the protective measures are not in place or a serious external force is destroyed, resulting in protection failure, which causes the metal lithium to contact with the air.

Common protective measures

outer casing is protected. In order to prevent air from entering, the lithium battery is packaged in a closed container and is usually equipped with a stainless steel casing and an aluminum alloy casing to prevent external force damage. For example, Tesla's electric vehicles even use titanium alloy shields to prevent damage to the battery container during use, especially in traffic accidents.

The diaphragm block protection protects against damage from the inside of the battery while preventing external force damage.

Usually, in order to prevent the positive and negative poles of the battery from being directly touched and short-circuited, there is a diaphragm in the battery, which separates the positive and negative electrodes on one hand and allows charged ions to pass on the other hand.

However, in lithium batteries, the diaphragm also carries another protective function. When the battery temperature is too high, the diaphragm gap will automatically close, so that lithium ions can not pass through, thus terminating the reaction of the entire battery. Therefore, the battery is prevented from being subjected to high pressure due to excessive temperature, thereby causing a problem that the battery sealing structure is broken.

Overcharge protection, not only to block the air, but also to prevent metal lithium from leaking out of the electrode.

Scientists use the nanovoid and material lattice mechanisms of electrode materials to store and lock the metal lithium formed during charge and discharge.

In this way, even if the battery casing is broken and oxygen enters, the oxygen molecules are too large to enter these fine cells, and spontaneous combustion is avoided.

However, charging with excessive voltage or charging for a long time may cause very dangerous damage to the lithium battery.

After the charging voltage of the lithium battery is higher than the rated voltage (usually 4.2V), if the charging continues, the lithium ion will accumulate on the surface of the negative electrode material because the negative electrode has been filled with lithium atoms. Due to the polarization, these lithium ions form electron transfer, form metallic lithium, and grow dendrites from the surface of the negative electrode toward the lithium ions.

These metal lithium without electrode protection are extremely active on the one hand, and are prone to oxidation reaction and explosion. On the other hand, the formed metal lithium crystal breaks through the separator, short-circuiting the positive and negative electrodes, thereby causing a short circuit and generating a high temperature. At high temperatures, materials such as electrolytes are cracked to produce gas, causing the battery casing or pressure valve to bulge and rupture, allowing oxygen to enter and react with lithium atoms deposited on the surface of the negative electrode, causing an explosion.

When charging the lithium battery, be sure to set the upper voltage limit and overcharge protection. Such a protection circuit is installed in a lithium battery produced by a regular battery manufacturer. Automatically power off when the voltage exceeds the standard or the battery is full.

In fact, spontaneous combustion is not easy.

Under the protection of qualified shells, diaphragms and circuits, it is not easy to spontaneously burn lithium batteries.

Especially in an inorganic material lithium battery, for example, a lithium iron phosphate battery , since the battery does not use an organic material like a ternary lithium battery , even if material destruction and short circuit occur inside the battery, the high temperature generated does not cause the inorganic material to decompose. Produce high-pressure gas, which in turn explodes spontaneously. In the experiment, even if the qualified lithium iron phosphate battery is put into firewood (the firewood temperature is lower than 600 °C), there will be no explosion and natural conditions.

At the same time, compare with the situation that the pre-development of new energy vehicles has not recognized the protective measures in actual use of vehicles, at present, after decades of exploration and practical use of millions of new energy vehicles worldwide, from battery manufacturers to automobile manufacturers. Have accumulated enough experience.

The protective strength of the outer casing of the lithium battery is greatly enhanced, and the protection of the battery can be effective even in the case of a serious traffic accident such as disintegration of the entire vehicle.

In the selection of battery separators, manufacturers are also fully aware of the importance of using a diaphragm with self-protection. At present, this technology has been used in general high-quality lithium battery products. For the application of anti-overcharge protection devices, it has already become a national mandatory standard. Qualified electronic protection devices and mature battery management systems have greatly reduced the chances of spontaneous combustion of new energy vehicle lithium batteries.

In the case of the current spontaneous combustion of new energy vehicles, none of them were caused by spontaneous combustion explosion of the battery pack under normal driving conditions. The explosion of self-ignition of more new energy vehicle lithium batteries comes from extremely serious traffic accidents and improper modification of electrical equipment in the car. After nearly a thousand degrees of high temperature roasting, the lithium battery was finally ignited.

For consumers, the lithium battery of new energy vehicles is generally very safe. However, it is necessary to guard against the fact that in the case of a major industrial outbreak, the indiscriminate use of individual manufacturers and the price war of products have led to the neglect of battery protection and the hidden dangers caused by the illegal modification of electric vehicles.

[Electric Vehicle Times Network Short Comment] Battery Natural and Explosion For new energy vehicles, people always think that driving a new energy vehicle is like a time bomb of a carrier, affecting the promotion and development of new energy vehicles. In fact, lithium battery is not so terrible.