The easiest way to understand the overall process is to first divide the battery cell assembly into three large stages: First the electrode production, then the cell assembly and finally the cell conditioning. All three stages can be subdivided into smaller stages. It is important to keep in mind the basic structure of a lithium-ion battery cell.
At the very beginning... electrode assembly
From raw materials to the first cell: electrode assembly on the pilot line in Salzgitter is divided into four sections.
At the beginning there is the process step of mixing (powder processing/mixing) the anode and cathode formulations from powdery starting materials together with water or solvents. The production of the paste (slurry) is technically very demanding. Precise dosing and extreme cleanliness are essential. In Salzgitter at the CoE, carbon (graphite), binders and conductive additives are used as the most important ingredients for the anode and lithium metal oxide, binders and lead additives are used for the cathode. The individual raw materials are intensively mixed together to produce a homogeneous paste.
The second step is coating. The paste is transported in atmospherically sealed storage tanks, which ensure the homogeneous distribution of the paste by means of an agitator. During coating, the various slurries are applied to carrier foils, the anode to a copper foil and the cathode to an aluminum foil. The pilot line is equipped with coating technology that enables simultaneous double-sided coating. The drying of the paste on the metal strips takes place in a floating drying process – i.e. the coated strip has no contact with machine parts and “floats” through the line.
In step three, the coated film is compressed (roll pressing). This is done via a rolling mill – referred to as a calendar in technical terminology. In the pilot line, a calendar is used to process the coated strips with a maximum pressure of 200t. The resulting strips are compressed to their required thickness with maximum precision. The result is impressive: The thickness deviates by a maximum of 4µm from the target value – this corresponds to about one 25th of a human hair. The finished electrode strip (also called mother roll) is then removed from the calendar for further processing with special lifting devices and is ready for the subsequent processes.
For further processing of the electrode material, it may be necessary to divide the mother roll lengthwise. This process is called slitting. During the cutting process, the mother roll is divided into several narrower electrode strips. The knife arrangement and geometry are decisive for the quality of the resulting battery cell.
Cell assembly – how a cell is made from an electrode strip
Cell assembly divides itself into different processes. The type and sequence depend on the cell to be produced. The Center of Excellence for Battery Cells distinguishes between seven main steps.
In sheet cutting, anode, cathode and separator sheets are separated from the daughter rolls. These are also known as “sheets”. Highest technical availability and outstanding cutting-edge quality are essential for cell assembly. At the CoE in Salzgitter, a laser cutting system was therefore developed from scratch and set up in conjunction with partners.
After separating the rolls into sheets, they must be dried and made available for subsequent processes with minimal humidity in the ambient air. For this purpose, they are placed in vacuum drying ovens via airlock systems and further processed in so-called drying rooms following the drying process. The humidity in these rooms is 350 times lower than in the ambient air.
During stacking, the individual sheets of cathode, anode and separator are placed on top of each other with a precision of +/- 0.3mm. Several sheets are simultaneously removed from component magazines, aligned, placed on workpiece carriers and clamped – all in one second. The sheets are stacked in a repetitive cycle of anode, separator, cathode, separator, anode, etc. The result is a “cell stack”.
Stack drying then takes place at a constant temperature and with alternating cycles of vacuum and inert gas pressure. The change between vacuum and gassing with inert gases – such as nitrogen, helium, neon, argon, krypton and xenon – is necessary and sensible because it accelerates the drying process.
After drying, the plus and minus poles of the battery cell are welded. These metal sheets, known as “tabs”, are welded on the pilot line using a laser process.
During the assembly step, the stack of electrodes is inserted into the housing. In the pilot line, the basic shape of the so-called pouch cell or pouch bag is first created in a thermoforming line. In this cell design, the housing material consists of a vapor-tight film consisting of several layers. This type of cell can now be found in every jeans pocket – thanks to its flat and flexible design, it is an integral part of every smartphone. The cell stack is inserted into this basic shape and then sealed with a heat-sealing process.
During filling and sealing, the pouch cells are filled with electrolyte. A pouch cell is one of a total of three possible designs of a lithium-ion cell. “Pouch” refers to something similar to a bag. The housing of a pouch cell consists of a laminated aluminum foil, which is also used in a similar form for packaging coffee beans. Therefore, this design is also colloquially referred to as the coffee bag cell. In Salzgitter, the filling and sealing takes place in a process chamber that is manually loaded with a pouch cell. The electrolyte is then filled into the battery cell under vacuum with high precision by several filling nozzles. During subsequent wetting, all areas of the cell are brought into contact with the electrolyte. This process is regarded as one of the challenges in the development of high-performance lithium-ion batteries, because the electrolyte must penetrate through the edges of the stack and then spread inwards.
Step 3: Cell conditioning
Cell conditioning is divided into six steps:
The formation describes the first charging and discharging processes of the battery cell. The parameters (current and voltage curves) during formation influence the final cell performance. Here, too, the core know-how of a cell manufacturer is evident here.
Degassing and sealing are important because gas formation occurs in the cell during the electrochemically formation process.
The gas escapes into a dead space inside the pouch cell (also called gas pocket) and collects there. Next, the gas bag is separated, and the cell is brought into its final shape.
Finally, the pouch cell needs to be sealed – against water from the outside and electrolyte leakage from the inside. The quality of the seal determines the life span of the cell.
The cell is ready. Now it must be evaluated! The penultimate step of production is called aging. The aim is to identify cell-internal short circuits by regular measurements of the open circuit voltage of the cell. The process can take up to three weeks. No significant changes indicate a good cell condition in terms of quality.
The EOL Inspection (End of Line) includes the evaluation of the cell according to its most important electrochemical properties. The values measured allow conclusions to be drawn about the electrical storage capacity, the internal cell resistances and the losses.
Simply explained – Lithium-Ion battery
A lithium-ion battery cell consists of an anode (often also called “negative electrode”; graphite on copper foil), a separator (porous polyolefin foil, ceramic-coated), a cathode (often also called “positive electrode”; lithium metal oxide on aluminum foil) and an electrolyte (lithium salt and other additives dissolved in organic solvent). Basically, five main raw materials are required for the production of lithium-ion battery cells for electric cars. On the cathode side, a compound of the elements cobalt, nickel and manganese acts through its structure as a place for the storage of the charge carrier lithium, on the anode side this is graphite. Lithium is also contained in the electrolyte.