A bus in Nanjing caught fire. It was caused by a lithium battery that a passenger carried. Why do lithium batteries always catch fire?

Recently, a news report about a lithium battery explosion and fire became a hot topic. The general situation was that a bus caught fire because a lithium battery carried by a passenger caught fire, resulting in many deaths and injuries.

Previously, there was also surveillance footage of a lithium battery suddenly catching fire while an electric bicycle was driving in Hangzhou, and a father and daughter on the bicycle were seriously burned. The scene was heartbreaking.

News screenshots

In addition to these more serious incidents that have attracted widespread public attention, we also occasionally see sporadic reports of fires caused by lithium batteries.

This can't help but make people a little worried. After all, in our daily life, from mobile phones to laptops, from electric bicycles to new energy vehicles, the application of lithium batteries can be said to be pervasive. We can't live without lithium batteries, but using lithium batteries is also worried about safety hazards. How should we use lithium batteries correctly and ensure safety?

Let’s learn about it together today.

Why do we have to use lithium batteries?

Lithium batteries have become the preferred power source for electric vehicles and many other electrical appliances due to their high power density, light weight, and environmental friendliness.

Lithium battery (Photo provided by the author)

According to statistics from the China Electronics Standardization Association (CESA), lithium batteries accounted for 97.1% of China's new electrochemical energy storage installed capacity in 2020, giving it a certain advantage in the market.

Are lithium batteries really safe? Why do they explode and catch fire?

Why do lithium batteries explode?

Lithium batteries mainly rely on the movement of lithium ions between the positive and negative electrodes to achieve the charging and discharging process. In principle, the main factors causing lithium battery explosions are overcharging and short circuiting.

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Overcharging mainly occurs during the charging process of lithium batteries. Due to the resistance of the battery, a large amount of heat will be accumulated in the battery during the charging process . The protection device in the lithium battery can provide a certain degree of protection against overcharging by detecting the voltage.

However, when the overcharging time is too long and the voltage remains too high, dendrite short circuits are likely to occur inside the lithium-ion battery, causing the temperature and pressure of the lithium battery to continue to rise, creating the risk of explosion and fire .

Short circuit mainly occurs during the use of lithium batteries . When lithium batteries are in use, their own temperature will continue to rise, and the battery also maintains normal heat dissipation.

If the battery temperature is too high due to external factors, it will easily damage the battery diaphragm and cause a short circuit, which will cause excessive internal heat accumulation, triggering a chain chemical reaction and causing the battery to explode and burn .

Therefore, some friends have said that lithium batteries are unsafe and it is better to replace lead-acid batteries in time. Is this true or false? Read on...

Are lead-acid batteries safer?

Lead-acid batteries are used in electric vehicles, communication base stations and some portable devices.

Compared with lithium batteries, the management system of lead-acid batteries lacks overcharge protection during charging, and many chargers are unable to cut off the power after full charge. The probability of explosion when overcharged is higher than that of lithium batteries. In addition, the electrolyte of lead-acid batteries is highly corrosive sulfuric acid. When leakage or explosion occurs, the harm is greater than that of lithium batteries.

In general, lead-acid batteries are not as safe as lithium batteries .

In daily life, we need to avoid direct contact between lead-acid batteries and metal products to prevent corrosion. At the same time, we should avoid direct contact between batteries and heat sources to prevent excessive temperatures and fire.

How dangerous is it to replace the battery without permission?

The change in battery size can easily cause the internal battery to shake, affecting its service life. Chargers for lithium batteries and lead-acid batteries are not universal, and there will be safety hazards if they are mixed and charged.

It is difficult to ensure that the battery connection is fully in compliance with safety standards if it is modified privately, and there is a risk of short circuit or explosion during operation.

The general life of electric vehicle batteries is 1 to 2 years. If the battery heats up severely during charging, or is visibly damaged or deformed, such as bulging or damage to the battery shell, it should be replaced at a professional repair shop. In addition, "it is full when charged, but empty when running". This is the most typical feature of battery sulfation. At this time, the battery has failed and needs to be replaced in time .

Other battery usage guidelines

1. Mobile phone battery

When charging a lithium battery, you need to strictly follow the charging time provided by the manufacturer, stop charging in time after it is fully charged, and do not charge continuously for more than 8 hours . At the same time, do not charge in an environment with too high or too low temperature, stay away from the bedroom when charging, and eliminate flammable materials .

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Try to choose original chargers and avoid unqualified third-party chargers .

2. Alkaline batteries

Alkaline batteries are mainly used in household appliances such as clocks, remote controls, toys, etc., which are what we often call "AA batteries" and "AA batteries".

Alkaline batteries will leak when they are over-discharged. The alkaline solution is corrosive and will damage the equipment used.

Therefore, when using alkaline batteries, you need to pay attention to replacing low-power batteries in time. Do not store batteries in the device for a long time. Also, avoid placing batteries in open flames to avoid explosion .

How to avoid accidents?

1. Charge reasonably and avoid overcharging

When charging, plug in the battery first and then the power plug; when unplugging, unplug the power plug first and then the battery plug. Be careful not to leave it unplugged for too long.

2. Choose electric vehicles and battery products from regular manufacturers

Electric vehicles and battery products from regular manufacturers often undergo rigorous safety testing and are more secure.

Non-standard electric vehicles are often prone to certain problems with battery and circuit connections, and there are major safety hazards during charging and use.

3. Do not modify the electric vehicle without permission

It is difficult to guarantee the original quality of the assembly process and cell selection of batteries and storage batteries that are privately expanded, which makes the battery prone to short circuit problems during use.

4. Use the original charger that comes with the electric vehicle

The original charger is often compatible with the battery's charging protection function, but third-party chargers lack protection in this regard, which may cause battery damage and lead to safety accidents.

5. Try to choose charging and parking locations with unified configurations

When charging or parking electric vehicles, try to choose locations with uniform configuration and high security. Do not charge or park electric vehicles in civilian public areas such as corridors and public foyers.

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6. Avoid exposing the battery to direct sunlight

Excessively high external temperature can easily affect the heat dissipation of the lithium battery itself, causing the internal temperature of the battery to be too high, leading to diaphragm melting and short circuit.

Therefore, the battery should be kept away from direct sunlight and away from excessively high heat sources .

In summary, in daily life, we must pay attention to the correct use of batteries, do not replace the batteries of electric vehicles without permission, try to park them in uniformly configured parking locations, and remember not to park them in corridors and public foyers. When taking public transportation, we must also abide by regulations and not carry batteries that exceed the standard or are unqualified.

Author: Chu Yuhao, PhD from Beijing University of Chemical Technology, Engineer at Sinopec Beijing Research Institute of Chemical Industry

Review丨Li ​​Quan, PhD, Institute of Physics, Chinese Academy of Sciences