How is a Lithium Ion Pouch Cell Manufactured in the Lab?

How is a Lithium Ion Pouch Cell Manufactured in the Lab?


As shown by this camera, lithium-ion batteries
are an indispensable part of any mobile device. They are lightweight and take up little space, while storing large amounts of energy. Therefore, they are ideal for use in portable electronics,
electric vehicles — but also for stationary use, such as off-grid storage of renewable energies. A lithium-ion battery is composed of one or several cells. The cells are stacked on top of each other in layers.
The most important layers are the electrodes, cathode and anode. To allow the transport of electricity, the cathode is placed on highly conductive aluminum foil, the anode on copper foil. The positively charged cathode consists of a lithium-metal oxide, the negatively charged anode is made of graphite. But what is happening inside the cell? A liquid electrolyte consisting of solvents
and of a conducting salt that contains lithium enables the lithium ions to travel between the electrodes. When the cell is charged via a battery charger, the plus and minus terminals are connected through metal contacts, and electrical voltage is applied. As a result, positively charged lithium ions are dissolved
from the lithium-metal oxide at the cathode and deposited at the anode. At the same time, negatively charged electrons are released from the metal oxide. Via the metal contacts, the electrons flow towards the anode,
in order to offset the positive charge of the embedded lithium ions. During discharge, electrons and lithium ions
flow in the reverse direction. The electric current generated in this way can be used to power an external device. To prevent contact between the electrodes, which could cause a short circuit, a plastic separator is placed between cathode and anode. The separator has a microporous structure,
and is therefore permeable to the tiny lithium ions. Performance and service life of the cell largely depend on material properties and processing techniques, which presents a challenge for the scientists working at the Institute for Applied Materials at the Karlsruhe Institute of Technology. They are producing laboratory samples of cells
rapped in plastic foil, so-called pouch cells. A scientist mixes together the ingredients
for the cathode to make a paste. The paste consists of the binding agent polyvinylidene fluoride and m-methyl pyrrolidone, which is a solvent. Also added are lithium metal oxide as the active cathode material, and auxiliary agents, such as carbon black or graphite,
which will increase the electrical conductivity of the electrode. The graphite anode is subsequently prepared
from a comparable mixture. The scientist first blends the ingredients roughly
and feeds them into a mixer. At high speed the heterogeneous mixture
is stirred to form a homogeneous paste. This process is performed under vacuum to prevent
air bubbles from getting whipped into the paste. The electrode paste is then taken to the coating machine,
where it is applied to a current collector. The paste is poured evenly onto a sheet of aluminum foil,
which is slowly moving through the machine. A sort of metal bar, the doctor blade, scrapes off excess paste. The wet cathode coating now passes through a number
of drying chambers, where the solvent is evaporated. A scientist now places the coated electrode between two rollers,
by which it is compressed at high pressure. This is called calendaring. The aim of this process is to achieve
the targeted porosity and layer thickness for the electrode. Next, the scientist cuts the calendared sheet into strips
which are suitable for a specific electrode size, and provides them with a small, uncoated strip, the conducting tab. The anode is cut from copper foil coated with graphite.
It is somewhat larger than the cathode. This is to prevent the formation of lithium deposits at the end of the anode during charging, which could result in short circuits. Some cell components are sensitive to moisture. Therefore, the researchers have to assemble the cells in a drying room, where an environment with extremely low humidity is created. Inside the drying room, a metal strip is welded by means of high-
frequency ultrasound to the uncoated conducting tab of the electrode. This strip is more stable than the thin current collector foil. At a later stage, the terminals for applying
electric voltage will be fixed to this strip. To package the cells, a moisture-impermeable
plastic casing is required, the pouch. A separator made of an electrically non-conductive
membrane is placed between the electrodes so as to prevent short circuits. Subsequently, the stacked cells are inserted in the pouch, and the sides of the pouch are joined together by means of heat sealing, leaving one side open. By using a pipette, the researcher fills the electrolyte,
which consists of solvents and the conductive salt, into the cell. This enables the lithium ions to move freely
from one electrode to the other. In an evacuable box, the cell is completely vacuum-sealed. Now that the cell has been completely assembled, it is put through various laboratory tests.
In a test box, the welded metal strips are fixed to a holder. The cell now undergoes several charge and discharge cycles. The voltage curve is recorded through a measuring tool
and graphically represented by means of characteristic curves. By experimenting with different materials and processes, the scientists are striving to increase the efficiency
and reliability of the cells, while reducing manufacturing costs. Since their work is oriented towards industrial workflows and materials,
the results of their research enable them to come up with production recommendations – for example on new storage technologies for renewable energies.

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