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How a lithium-ion battery works

To be able to assess the dangers posed by lithium energy storage devices, knowledge of how they work can be very helpful. Important to know: There is no “one” lithium battery. Instead, there is a large number of different energy storage systems in which lithium is used in pure or bound form. A fundamental distinction is made between primary (non-rechargeable) and secondary (rechargeable) lithium-ion cells. In common parlance, the latter is usually meant when we speak of lithium-ion batteries or, better, lithium-ion accumulators. In this article, you will learn more about the functionality and chemical properties of a lithium battery.

Functionality

A battery pack is composed of several cells depending on the power. Each lithium-ion cell consists of a positive and a negative electrode, the anode, and the cathode. Between them is an ion-conducting electrolyte. This guarantees the transport of lithium ions between the electrodes during the charging or discharging process. The best-known form of lithium energy storage device is lithium-ion batteries, in which a liquid electrolyte is used.

Another critical component is the separator. It prevents direct contact between the anode and cathode and thus contains a short circuit. When discharging, lithium ions and electrons are released on the anode side. The electrons flow through the external circuit and do the electrical work. At the same time, the lithium ions migrate through the electrolyte fluid and the separator to the cathode.

When charging, this process is reversed. Depending on the system, the structure and materials used may vary depending on the lithium-ion battery. In the lithium-polymer accumulator, the electrolyte is incorporated into the molecular framework of a polymer film. This makes it possible to dispense with a separate separator. Lithium-polymer energy storage can deliver only low discharge currents.

However, the polymer film allows a flat design, so much energy storage is found primarily in mobile phones and laptops. The thin-film lithium cell is an energy storage in which an ion-conductive gas replaces the electrolyte. This allows the use of lithium metal and, thus, an extremely high energy density. This technique is currently an essential part of lithium energy storage research.

Chemical properties

While the German Federal Institute for Occupational Safety and Health (BAuA) regards lithium-ion batteries as products under the REACH regulation, the American Occupational Safety and Health Administration (OSHA) classifies batteries as mixtures. In practice, many companies prepare and make available safety data sheets for lithium batteries even without a legal obligation. These usually provide valuable information on battery storage and handling. However, details of chemical composition can often also be found, which provide information on the hazard. A lithium battery can be divided into an anode, electrolyte fluid, and cathode.

As a rule, graphite (C) is used as the anode material, which must not be labeled under the CLP Regulation.

Many different materials are used for the cathode. The exact composition of the cathode material significantly determines properties such as lifetime, charging times, and performance. Iron, manganese, cobalt, and nickel are often used in the cathode.

The electrolyte fluid consists of an organic solvent and a conducting salt. While there is a large variety of possible solvents, lithium hexafluorophosphate (LiPF6) is almost exclusively used as the conducting salt.

Electrolyte liquid = organic solvent + conductive salt (LiPF6)

The exact chemical composition of the respective solvent mixture is usually a manufacturer's secret. By viewing various data sheets, however, you can get an overview of the components used. The flash points of the solvent components range from + 160 ° C to sometimes below 0 ° C. This explains the thermal instability of a lithium battery.

The conductive salt contains fluorine (F), among other things. The released hydrofluoric acid (HF) in non-concentrated form can lead to various hazardous situations in a damaged lithium battery.

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