When discussing lithium batteries, it is impossible to avoid focusing on their essential component, namely, the cells of which they are composed and which enable them to deliver power. The three most widely used lithium ion cell types, each employed for a distinct kind of application, are as follows:
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As can easily be inferred, cylindrical cells are cylinder-shaped, are the most commonly used and were among the first to be mass-produced. They can have different diameters, the most common being the 1865, where the number 18 indicates the diameter (18 mm) and the number 65 indicates the length (65 mm). There are, however, other formats, such as the 2170 or, again, the one most recently adopted by Tesla, the pioneer of lithium batteries for electric cars, with its 4680 used to power the Tesla Model Y. Apart from a few car manufacturers who have made this choice, cylindrical cells are routinely used in medium-small battery packs, e.g. in micro-mobility (bikes, scooters and motor scooters), portable tools, medical devices, and so on.
These types of lithium cell are so called because of their bag-like shape. They have a lightweight design and, as they have no inherent robustness, special protections, such as the addition of aluminium frames, must be inserted during production of the module to give them structural robustness. They come in various sizes which can be modified according to the manufacturer’s requirements. These cells are mainly used in smartphones, drones, laptops and the automobile industry.
Prismatic lithium cells have a solid rectangular casing made of aluminium or of a very strong plastic material. The internal components are layered. They come in different sizes, with a variety of formats depending on the field of application. Their individual components can reach a high capacity. Due to their structure, prismatic cells are best suited for the production of lithium batteries for the machinery and industrial vehicles industry, or the energy storage sector, all of which normally require medium-high capacities.
The benefits and drawbacks of the various cell types have already been discussed, but few people ever enquire about the lithium-ion battery cell production process and how it works.
Although the many cell types that make up a lithium battery appear very different from one another when viewed from the outside, it is astonishing to learn how similar their interiors actually are. The different types of battery cell production and assembly will now be explored in more detail.
In order to engineer a battery pack it is important to understand the fundamental building blocks, including the battery cell manufacturing process. This will allow you to understand some of the limitations of the cells and differences between batches of cells. Or at least understand where these may arise.
Lets Start with the First Three Parts: Electrode Manufacturing, Cell Assembly and Cell Finishing
Lets Take a look at steps in Electrode Manufacturing
The anode and cathode materials are mixed just prior to being delivered to the coating machine. This mixing process takes time to ensure the homogeneity of the slurry.
Cathode: active material (eg NMC622), polymer binder (e.g. PVdF), solvent (e.g. NMP) and conductive additives (e.g. carbon) are batch mixed.
Anode: active material (eg graphite or graphite + silicon), conductive material (eg carbon black), and polymer binder (eg carboxymethyl cellulose, CMC)
N-Methyl-2-pyrrolidone (NMP): this is a toxic substance, widely used in the plastics industry as it is nonvolatile and able to dissolve a wide range of materials. NMP residual will be a Quality Control test downstream (Gas Chromatography-Mass Spectrometry can be used to test sample) as that will affect cell performance reactively.
The anode and cathodes are coated separately in a continuous coating process. The cathode (metal oxide for a lithium ion cell) is coated onto an aluminium electrode. The polymer binder adheres anode and cathode coatings to the copper and aluminium electrodes respectively.
Immediately after coating the electrodes are dried. This is done with convective air dryers on a continuous process. The solvents are recovered from this process.
Infrared technology is used as a booster on Anode lines.
This is a rolling of the electrodes to a controlled thickness and porosity.
Lets Take a look at steps in Cell Assembly below
The electrodes up to this point will be in standard widths up to 1.5m. This stage runs along the length of the electrodes and cuts them down in width to match one of the final dimensions required for the cell.
It is really important that no burrs are created on the edges of the electrodes during this process as they can cause damage to the separator and a possible short-circuit at a later date.
The electrodes are dried again to remove all solvent content and to reduce free water ppm prior to the final processes before assembling the cell.
The final shape of the electrode including tabs for the electrodes are cut. At this point you will have electrodes that are exactly the correct shape for the final cell assembly.
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In a cylindrical cell the anode, cathode and separator are wound into a spiral. For pouch cells the electrodes stacked: anode, separator, cathode, separator, anode, separator etc.
Some prismatic cells have stacked electrodes and some have a flat wound jelly roll.
The anodes are connected to the negative terminal and the cathodes to the positive terminal. The process and robustness of this joint are important to understand as welding the cell to busbars can damage the internal welds.
The electrodes either as a roll or pack of stacked layers are loaded into the can or pouch. Depending on the cell format will change how this canning or enclosing process is completed.
Lets Take a look at steps in Cell Finishing below
The up until now dry cell is now filled with electrolyte. A partial vacuum is created in the cell and a pre-determined quantity of electrolyte is delivered to the cell. The partial vacuum helps the distribution and hence wetting of all layers within the cell.
The electrolyte is dispensed based on a defined volume of liquid. A second quality check is the weight of the cell before and immediately after filling.
The cell is charged and at this point gases form in the cell. The gases are released before the cell is finally sealed. The formation process along with the ageing process can take up to 3 weeks to complete.
During the formation process a solid-electrolyte interface (SEI) develops. The SEI can prevent the irreversible consumption of electrolyte and protect the anode from overpotential during fast charging.
Note: When the degas is applied can vary. Some apply it after Pre-formation charge others after Formation charge others after Ageing.
The cells are stored at a controlled temperature for a period of time. This allows the SEI to stabilize.
This step in the process ties up the cells for a length of time, this inventory of cells has a considerable value and hence ties up funds.
Alongside the charge/discharge cycle data a number of further finishing steps and checks will be done prior to shipment:
All data is recorded against the cells unique identification.
This is a first overview of the battery cell manufacturing process. Each step will be analysed in more detail as we build the depth of knowledge.
The cell manufacturing process requires 50 to 180kWh/kWh.
Note: this number does not include the energy required to mine, refine or process the raw materials before they go into the cell manufacturing plant.
What does 1 GWh of cells look like? How many cells? How much energy is that?
How much lithium metal is in 1 GWh of Lithium-ion cells?
First indicator in the breakdown of a total ~$36 million/GWh Capex cost. Of which 1/3 of that is for formation and aging.
A look at the Sodium Ion Cell Manufacturing Process, but perhaps more usefully a look at the differences compared to the lithium based cell manufacturing processes.
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