Safety of Lithium ion batteries

In recent years, lithium ion batteries have received lots of publicity and been discussed in media – in both positive and negative context. They have replaced other batteries in portable consumer devices and moreover, they have been adopted for propulsion system of electric and hybrid vehicles. Lithium ion batteries have been selected for these technologies because large amount of energy can be stored in them in relation to the size. Therefore, mobile phones can be equipped with a large color displays and good internet connections, and electric vehicles reach reasonable driving ranges. On the other hand, several accidents involving lithium ion batteries have been reported including battery fires in Boeing 787 Dreamliner planes in 2013 and self-ignition of Samsung Galaxy Note 7 mobile phones. This kind of issues can always arise when packing high amount of energy in small space, for example chemical energy in oil or electrochemical energy in a battery. Hence, we investigate safe use and lifetime effects of lithium ion batteries in the CloseLoop project.

Thermal runaway is a process often causing lithium ion battery fires or explosions. It is induced by factory defects or changes in the battery structure generated by aging or misuse of the battery. The process is often triggered by normally higher environmental temperature, which accelerates chemical reactions releasing thermal energy. This additionally released thermal energy accelerates chemical reactions even further and eventually energy is released in an uncontrolled manner resulting in, for example, melting of battery components or even explosion.  

The main components of a lithium ion battery are a positive electrode, a negative electrode, an organic electrolyte and a separator between the electrodes as sketched in Figure 1. The terminal voltage of a single lithium ion battery vary from 2.4 to 3.7 V, depending on the chosen electrode materials. Typically, batteries used in various applications consists of several lithium ion battery cells. Mobile phone batteries comprise of a few single cells whereas laptop computers have some ten units and hybrid and electric vehicles require even thousands single cells. The needed operating voltage is obtained by connecting the cells in series, and the charge capacity by connecting the cells in parallel.

Figure 1. The structure of a lithium ion battery.

 

Both relatively harmless and very nasty substances are used for constructing lithium ion batteries. In general, all the electrode materials are quite safe and can be handled in air. Naturally, exposure to these should be avoided. The electrolyte, however, is considerably more hazardous. It typically consists of lithium containing salt, such as LiPF6, and a mixture of organic solvents. If the electrolyte gets into contact with the airborne water, harmful compounds, such as very corrosive hydrogen fluoride (HF), is formed. Because of this, the lithium ion batteries are always waterproof, and must not be opened. In case of a battery fire, both water and fire extinguishers should be avoided and rather smother the fire with sand, for example. Water and the commonly used extinguisher chemical can even sustain the fire if, for example, metallic lithium has been formed inside the battery. However, water does cool down the battery and the environment efficiently, and works better than nothing.

Causes for battery safety hazards:

  • Formation of lithium dendrites at the negative electrode during over-charging or charging a cold (about 0 °C or cooler) battery. In these conditions, lithium might be reduced to the negative electrode and this leads to formation of dendrites. If the dendrites grow large enough, they can pierce the separator and short-circuit the cell. This quickly leads to thermal runaway.

 

Comment: The manufacturers install a battery management system to their devices for prevent the over-charging and the charging in too cold conditions. Therefore, you should always use the original battery in the appliance. If you suspect that the battery has cooled down e.g. during transportation, do not charge the battery before it has warmed through to the normal room temperature.

 

  • The electrode materials decomposes during over-charging. This problem concerns mainly lithium cobalt oxide (LiCoO2) that is used at the positive electrodes. If too much lithium is extracted from this material, its structure collapses. These kind of situations are prevented with the battery management systems or using alternative materials such as lithium iron phosphate (LiFePO4).

 

  • The electrodes get into contact with each other, and this leads to short circuit and thermal runaway. This is usually caused by a factory defect. It is also a possibility if a battery is damaged e.g. falling on a ground or in a traffic accident.

 

Comment: Factory defects are very rare. However, this did occur for a small part of the batteries in the case of Samsung Galaxy Note 7. If you notice your battery heating unusually much (during charging) or swlling, detach the battery immediately from the device and switch the device off. Transfer the battery to a fire safe, well-ventilated space. Never use a damaged battery.

Figure 2. Battery testing in the laboratory of Aalto University. Sand is kept at hand in case of a fire.

 

Despite all of these warnings, the lithium ion battery related safety hazards are very rare. Most of us have several batteries in use daily, and most have not witnessed dangerous situations. Batteries are used more and more in new applications every day, and therefore the amount of stored energy is increasing. The manufacturers are doing their best to make the battery technology even more safer to use. However, if an unexpected situation occurs, the instructions above will help you through.

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