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Lithium-ion batteries are one of the most powerful energy storage devices on the market. Their high energy density makes them durable, and they require little maintenance, with a capacity that remains constant even during long periods of operation. The structure and materials used in the batteries make intermediate charging possible at any time. Lithium-ion batteries work according to one principle, with electrical energy stored through a chemical process, making them a suitable option for powering devices such as electric stacker trucks.
Compact power pack – the structure of lithium-ion batteries
The construction of a lithium-ion battery consists of numerous individual cells, each with the same structure. It contains the following components:
- Positive electrode: The cathode consists of lithium metal oxide, which may contain variable amounts of nickel, manganese and cobalt. These metal oxides are also called transition metals.
- Negative electrode: The anode is usually made of graphite.
- Electrolyte: In order for the lithium ions to move as charge carriers in the cell, anhydrous electrolytes are also included. These contain salts such as lithium hexafluorophosphate dissolved in an aprotic solvent such as diethyl carbonate. In lithium polymer batteries, a polymer of polyvinylidene fluoride or polyvinylidene fluoride-hexafluoropropylene is also used at this point.
- Separator: To prevent short circuits, a separator made of nonwovens or polymer-films is installed between the electrodes. The separator is permeable to lithium ions and can absorb large quantities.
The design allows lithium to move back and forth between the electrodes in ionized form. Depending on the electrode materials used, lithium-ion batteries are divided into different groups. Operation remains the same in each, but the energy density, cell voltage, temperature sensitivity, capacity, and charge capacity and discharge current can vary with different transition metal ions.
The structure of a lithium-ion battery can be manufactured as:
- Lithium-polymer batteries: The electrolyte used is a polymer-based film with a gel-like consistency. This structure makes it possible to manufacture particularly small batteries (less than 0.1 mm thick) and in various designs. With an energy density of up to 180 Wh/kg, they are very powerful, but mechanically, electrically and thermally sensitive.
- Lithium cobalt dioxide batteries: The positive electrode of this type of battery is made of lithium cobalt dioxide. The anode is made of graphite. These types of batteries are prone to thermal runaway when overloaded.
- Lithium titanate batteries: Negative electrodes are not made of graphite, but of sintered lithium titan spinel. These enable a superfast-charging capacity as well as operation at temperatures as low as -40°C. The positive electrodes are again made of lithium titanium oxide.
- Lithium iron phosphate batteries: Cells each have a cathode made of lithium iron phosphate. The electrolyte is present in solid form. These batteries have a lower energy density of up to 110 Wh/kg, but are not prone to thermal runaway if mechanically damaged. The discharge voltage curve indicates a memory effect, but this is very low compared to Ni-Cd alternatives.
Charging and discharging – how lithium-ion batteries function
The operation of lithium-ion batteries is based largely on the constant movement of ionized lithium between the electrodes. The lithium-ion flow balances the external current flow during charging and discharging so that the electrodes themselves remain electrically neutral.
If the battery is discharged, for example when energy is used by a device, the lithium atoms at the negative electrode each emit an electron. This electron is returned to the positive electrode via the external circuit. In the same process, the same number of lithium ions move from the negative electrode through the electrolyte and separator to the positive electrode. The electrons are picked up by the positive electrode through strongly ionized transition metal ions. These can be different depending on the battery type. Unlike lithium ions, they are not mobile.
When charging the accumulator cells, the non-ionized lithium atoms move from the positive electrode through the separator back to the negative electrode. Here they are inserted between graphite molecules. The process is also known as intercalation and is triggered by charging with a constant current until the rated current is reached. When the end-of-charge voltage is reached, it is maintained while the charging current decreases. To prevent damage to the cells or overheating (thermal runaway), most lithium-ion batteries are fitted with charging or protection electronics. This is adapted to the design of the cells and ensures that neither overcharging nor deep discharging can occur.
The separator – essential for the safe function of lithium-ion batteries
The in-built separator controls and safeguards the electrochemical reactions inside lithium-ion batteries. It works by isolating the two electrodes from each other so that internal short circuits can’t occur. At the same time, the special permeable design ensures that only lithium ions can pass through, moving between the negative and positive electrodes. In addition, the separator ensures gas exchange in the closed lithium battery cells.
As a result, separator components must be made of microporous membranes, which can vary depending on the battery power and size. Either polymeric films (as in lithium polymer batteries) or heat-resistant ceramic separators are used for this purpose. By combining nonwovens with a ceramic coating, separators are particularly flexible and yet temperature-resistant up to 700° C.
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