There are three stages to the production of lithium batteries in the UK, including electrode manufacturing, cell assembly, and cell finishing. Within these stages, there are sub-processes, such as coating the anode and cathode, assembling various components, and packing and testing the cells.
The most common cells include pouch, cylindrical, and prismatic, and while the cell design can vary, the manufacturing processes aren’t too dissimilar. What’s more, manufacturing accounts for around a quarter of the cost of lithium-ion batteries.
Four main elements make up lithium-ion cells, including an electrolyte medium, a separator, an anode, and a cathode. The electrolyte medium facilitates the movement of lithium ions between the electrodes, the separator prevents contact and shorting, the anode holds charged lithium ions, and the cathode holds discharged lithium ions.
Usually, the anode is crafted from graphite, and the cathode is typically a metal alloy containing lithium, cobalt, nickel, and more. Each of the mentioned components then gets packed in a casing with exposed tabs, creating positive and negative terminals. Following this, cells are organised and linked to form a battery pack.
First and foremost, electrode materials need to be mixed with a conductive binder, resulting in the formation of a uniform slurry with the solvent. Since the anode is formed from carbon and the cathode is a lithium metal oxide, it’s important to ensure contamination is avoided between these active materials. This can be achieved by the processing of each taking place in separate rooms.
Following the combining of electrode materials and a conductive binder, the formed slurry is coated either sporadically or continuously on each side of the current collector. This is applied via a specialist tool, with the coating’s thickness being controlled by the machine.
Next, the solvent needs to be evaporated, which takes place in a long drying oven. The solvent in the cathode coating is extremely flammable, and this gets used or recovered for thermal recycling. In contrast, the anode is water-based, meaning only vapour is produced.
After this, coated foils undergo compression from a pair of rotating rollers, which is otherwise known as calendaring. This is done to alter the electrode’s physical properties, including permeability, concentration, conductivity, bonding, and more.
The next step involves the electrode being cleaned, cut into thin strips, and recoiled via a slitting machine. All residual solvent and moisture are then removed from the coils by way of a vacuum oven.
Following the preparation of electrodes, the sub-assembly process can take place in the dry room. This involves the cell’s internal structure being formed, with the separator being placed between the anode and cathode. For cylindrical and prismatic cells, the process is called winding. For pouch cells, the chosen method is stacking. Regardless of the required process, highly automated equipment tends to be a necessity.
Following this, a laser or ultrasonic welding method is employed to connect the assembled cell structure to the cell tabs/terminals and safety devices. Next, the sub-assembly is placed in the cell housing before a heating or laser welding method is employed to seal the casing while leaving a gap for the electrolyte injection.
Lastly, the housed cell is filled with the electrolyte and sealed. This occurs in dry environments only, as moisture can result in toxic gases being emitted, as well as the decomposition of the electrolyte. The process is carried out via a high-precision dosing needle; the capillary effect in the cell is activated due to the application of pressure.
Once lithium polymer batteries have been labelled accordingly, the time comes to begin the formation process. This is a description of the initial charging and discharging processes after the electrolyte has been injected into the battery. This leads to the cells being placed in information racks and contacted by spring-loaded contact pins.
From the defined voltage and current curves, the cells can then be charged and discharged appropriately. The process sees the Solid Electrolyte Interface (SEI) being formed between the electrolyte and the electrode. This is achieved by the lithium ions being embedded in the crystal structure of the graphite on the anode side, creating this protective layer. The layer is responsible for Li-ion batteries’ low self-discharge rate, impacting the life and performance of the battery.
The initial charging results in the strong evolution of gas for larger pouch cells. The gas is then pushed out of the cell into dead space (the gas bag). This process is called degassing, in which the gas bag is pierced into a vacuum chamber, resulting in escaping gases being sucked away. The gas bag is then removed and discarded safely while the cell gets sealed under a vacuum.
Following this, the ageing process takes place in order to guarantee quality. During this time, consistent tracking of cell performance and characteristics takes place, which involves the measurement of the open-circuit voltage (OCV) of the cell over three weeks. Similarly, normal temperature (NT) and high temperature (HT) ageing are differentiated between, with HT ageing usually commencing first. After the cells have been stored in ageing cabinets and no significant changes occur, they are deemed fully functional.
The end of the ageing process sees the end-of-line (EOL) test rig take place. This involves the cells being moved to a testing station, in which discharging to the shipping state of charge occurs. Additional leakage tests, OCV tests, optical inspections, internal resistance measurements, and further pulse tests may also be conducted.
After the successful completion of testing, the battery packs are good to go.
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