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Lithium-ion batteries: why and when do they pose a risk of chemical exposure for people?

In recent years, there has been an increase in the use of motorised personal mobility devices (MPMDs) equipped with a non-combustion engine such as scooters, bicycles, unicycles, hoverboards and similar devices. Like more and more electric vehicles, most MPMDs contain lithium-ion batteries, which are appreciated for their excellent longevity and energy storage efficiency.

How do lithium-ion batteries work?

Marketed since the 1990s by Sony, lithium-ion batteries allow chemical energy to be converted into electrical energy. They store electrical energy so that it can be charged or discharged. Lithium-ion batteries consist of one or more cells (energy compartments). These cells are protected by packaging that can vary in size and shape. Each cell consists mainly of a positive electrode (cathode), a negative electrode (anode), an electrolyte (a liquid ion conductor containing a solvent and a salt conductor) and a separator (a physical barrier between the anode and cathode).

The main chemical compound of the cathode is lithium metal, such as lithium cobalt dioxide (LiCoO2). The anode is mainly composed of lithium carbonate, such as graphite. The oxidation reaction in the anode creates electrons and the reduction reaction in the cathode absorbs them.

When the battery is charged, the lithium ions (Li+) stored in the positive electrode (cathode) are transported by the electrolyte to the negative electrode (anode) while electrons move from the anode to the cathode. When the battery discharges and produces an electric current (energy in the form of electricity is removed from the battery cell), the lithium ions (Li+) move in the opposite direction. This electric current is then converted in order to operate a motor or an electronic device.

Does the use of lithium-ion batteries present a risk?

Thermal management of lithium-ion batteries plays a key role in their lifespan, performance and safety risk.

The optimal operating temperature for a lithium-ion battery is between 20 and 40°C. A battery management system (BMS) is included in most lithium-ion batteries to stop the battery from operating above a certain temperature threshold (usually 60°C).

If the temperature of the battery rises above this threshold, the anode coating will start to decompose. Above 70°C, the electrolyte will start to evaporate and increase the pressure in the cell, which can cause mechanical failure inside the battery. Heat increases the rate of reaction, which further increases its temperature. As a result of a succession of exothermic chemical reactions within each cell, a thermal runaway can be triggered, leading to a domino effect until every cell in the battery is degraded. Overcharging, short-circuiting or the presence of external heat can cause this thermal runaway. The uncontrolled increase in temperature and pressure within the battery results in degradation of the electrolyte which can leak (liquid or gas), ignite and explode.

The electrolyte is composed of a solvent of mainly organic carbonates and a conductor salt such as lithium hexafluorophosphate (LiPF6). Organic carbonates in liquid or gaseous form have a high flammability risk, while lithium hexafluorophosphate is harmful if ingested and causes severe skin and eye damage in case of contact. It can also cause organ damage in case of prolonged or repeated contact. If the electrolyte leaks and reacts with moisture or water, or if it ignites, hydrofluoric acid (HF) in liquid or gaseous form may be created. Its concentration will depend on the temperature of the combustion and the amount of electrolyte ignited.

Hydrofluoric acid (HF) represents a double threat to the human body. It is a corrosive product due to the hydrogen ions (H+) of the acid, which destroy the surface layers of the human body, and very toxic due to the fluoride ions (F), which penetrate deep into the body and cause cell necrosis. The chelation of fluoride ions on calcium and magnesium causes hypocalcaemia and hypomagnesaemia as well as the destruction of underlying tissues. The depletion of these elements leads to an excess of potassium and causes a biological imbalance which can present as heart arrhythmias.

The higher the concentration of HF, the sooner the victim feels the consequences of exposure. For example, when the concentration of hydrofluoric acid is less than 20%, the pain appears only 24 hours after contact with the tissue. The severity and consequences of exposure will depend on the amount and concentration of HF and the surface area affected. Contact with HF can cause severe systemic effects (including cardiac arrest), severe burns to the skin, eyes or digestive tract, as well as irritation of the respiratory tract in the event of inhalation, including pulmonary oedema.

In addition to hydrofluoric acid, other toxic gases (carbon oxides) are also created and released during combustion of the electrolyte.

What to do and how to protect yourself when a lithium-ion battery fails

Although lithium-ion batteries are reliable and the risk of combustion is relatively rare, it is important to be aware of safety advice when handling, storing or disposing of them (refer to the instruction manual that came with your electrical device). In the event of a lithium-ion battery failure where outgassing is observed, it is very important to move away from the battery to avoid harmful gases, flames and a possible explosion.

In the event of eye or skin contact with hydrofluoric acid (gas or liquid), it is essential to begin decontamination as soon as possible, either with water (ideally within 10 seconds of exposure) followed by the application of calcium gluconate, or with HEXAFLUORINE® solution. Water is a passive, hypotonic solution that removes the chemical from the surface of the biological tissue. After washing with water, the application of calcium gluconate limits the action of fluoride ions.

HEXAFLUORINE® solution is an active and hypertonic solution. Like water, HEXAFLUORINE® solution enables HF to be removed from the surface of the tissue but, unlike water, it also facilitates the extraction of HF from the tissue. The active agent in HEXAFLUORINE® solution helps to stop the chemical action of hydrogen and fluorine ions. HEXAFLUORINE® solution allows the user to immediately begin the process of extracting the penetrated hydrofluoric acid and to employ a longer time for intervention when dealing with a product which can be lethal. For a decontamination protocol to be effective, clothing and accessories must be removed. In all cases, medical advice is imperative.

Warning: even if the lithium-ion battery fire is extinguished, it is common for the fire to start again a few hours or days after the initial incident due to thermal runaway, which may occur again.