The production process of a Gigafactory is characterized by its complexity and high technical elements. In addition to this, high production volumes generally associated with economies of scale, and the large dimensions that these factories have also present a challenge. Therefore, we are talking about highly digitalized and automated procedures that seek to combine, in a unique process, different routes and complex sub-processes. The manufacture of batteries depends, precisely, on the development of these.

On this basis, and in order to simplify, we could generally divide the battery manufacturing process generally into three major phases or "blocks" of activities, which result in the production of the required storage devices.

THE THREE MAIN PHASES OF THE BATTERY PRODUCTION PROCESS

As detailed below, the 3 main phases are (i) electrode manufacturing, (ii) cell assembly and (iii) training, aging and test that validates the right performance of the assembled battery cells.

1. ELECTRODE MANUFACTURING

Whatever the format (pouch, cylindrical or prismatic), the first step when manufacturing a battery is the production of the two covered layers known as electrodes. At this stage, it is vital to avoid contamination between materials, which is why Gigafactories have two identical and separated production lines: one for the anode and the other for the cathode.

Generally, the anode is made of a copper foil coated with graphite, while the cathode is composed of an aluminum foil coated with the chosen chemistry (NMC, NCA, etc.), as we have already analyzed in a previous article.

In total, within this block, we find 4 large activities that determine the production of the electrodes:

1.1. Mixing

In the electrode production process, the first step is to produce a mix known as "slurry", which has a significant impact on the battery´s final performance. This procedure is key for the subsequent bonding of the active material to the current collector, that will then transfer the electrochemical energy through the cell tabs.

The slurry is a mixture of powders (mainly active material) combined with a solvent (liquid) and a binder.

There are two types of equipment to produce the slurry: batch production equipment, usually planetary mixers, or continuous production equipment, which combine the basic dosing operations along the mixing chamber by means of automated gravimetric feeding systems.

1.2. Coating & Drying

Once the slurry is produced, it is pumped through a piping system to the coating area, where the mix is printed on a metal foil that is unrolled to the coating head. There, the slurry is deposited, and the coated foil continues its process through a drying oven where the solvent evaporates leaving the active material attached to the foil and evenly distributed. Gradual drying is key to obtain a good quality electrode, which requires ovens that can be as long as 80m.

The coating, which is applied on both sides of the foil, can be intermittent or continuous depending on the cell size and format to be produced. In general, the width of the printed strips on the roll limits the dimensions of the cell and therefore, it directly affects the production capacity of the line.

1.3. Calendering

The next step in the battery manufacturing process is calendering, which acts as the finishing process for the coated rolls. Like the previous step, it is a roll-to-roll process, where the coated rolls travel through two heated rollers to compress the material and thus, ensure constant thickness, density and better adherence.

1.4. Slitting

Slitting is the first cutting process used to limit the foil to the size of the individual electrodes that will be required for the final assembly. This is, the rolls coming from the calendering process go through a bank of blades, which cuts them into multiple smaller rolls to fit with the final design (daughter rolls).

2. CELL ASSEMBLY (DRY ROOM)

Once the electrode manufacturing phase has been completed, the process moves on to a second phase where the cells are assembled.

One of the most relevant aspects of this phase is that it must be carried out in a dry environment to avoid any humidity remaining in the electrode, which can lead to increased degradation and capacity loss. Therefore, electrodes go through a drying process that reduces the remaining moisture and are transported to a climatically controlled environment that ensures the quality of the cells.

This environment is called a dry room. Such rooms are generally maintained at a dew point of -40°C, although lower temperatures are required for more moisture-sensitive chemistries such as, NMC811 or lithium metal.

At this stage electrodes are cut and assembled into their cases. Although this process varies according to the cell format (pouch, prismatic, cylindrical), there are three main activities in this block:

2.1. Notching

For pouch cells, the next step corresponds to a cutting process that converts the coated rolls into individual electrode sheets.

The cutting machine (which is still different for anode or cathode production) unrolls the foil and produces rectangular electrodes with an uncoated area left, this area will be the tabs required later in the assembly.

The cutting process can be performed with two types of technologies: mechanical cutting (formed by a die with blades) and laser cutting. Although the mechanical system usually reduces the cost, it requires regular sharpening and replacement of the blades. On the other hand, the laser avoids direct contact with the electrodes and offers more flexibility.

2.2. Stacking

Once the individual sheets are produced, they go through a stacking process, which is usually the trickiest and often a bottleneck in cell assembly. This is the first stage in which the cathode and anode lines are combined. The goal is to alternately stack anode layers, the separator and cathode layers, while leaving the uncoated tabs exposed.

The most common methodology for this is Z-stacking, where the separator is folded over each electrode layer in a zigzag movement. The alignment is key in this process, as misalignment can cause the electrodes to extend beyond the separator, creating short-circuit once the cell is completed.

Another alternative is stacking through lamination. This method, similar to single sheet stacking, joins two of the components together (separator/anode/separator) and these are later stacked in between cathode layers in an alternating manner.  

2.3. Pouch Assembly

Once stacking is complete, the exposed electrode tabs must be attached to the main terminals through a welding process.

The cell is then placed into a preformed packing material and sealed, leaving an open edge for the electrolyte filling.  Once vacuum sealing the remaining edge, the product is left to soak for hours prior to forming, aging and test phase.

3. FORMING, AGING AND VALIDATION

Once assembled, the cell undergoes a conditioning phase. The formation, aging and test phase, is the critical phase in which the cell is initially charged, and undergoes several tests to evaluate its characteristics and performance.

The final sequence, this is, pre-charging, degassing, forming, high temperature aging etc., may differ in time, order and repetitions depending on the manufacturer. Depending on the validation protocol the cells could spend weeks in this last phase.

The equipment consists of a fully automated system filled with towers full of channels, which resemble large automated warehouses. The equipment required has a large impact on the final dimensions for the production plant, due to the high volumes of cells being processed simultaneously.  

Once this last phase is completed, the resulting device is ready to be used in different applications that require batteries for their functioning.

A SET OF HIGHLY DIGITALIZED AND AUTOMATED PROCESSES TOWARDS SUSTAINABILITY

The battery manufacturing process is made up of diverse and complex processes that have a high technical and precision element attached to it. As mentioned at the beginning, the battery production industry is also characterised by its high degree of digitalisation and automation, which are key for process optimisation and productivity.

Thus, solutions based on machine learning and artificial intelligence will be very common in the future gigafactory generations. Based on the data richness that a battery manufacturing plant has, gathering and processing this information will be key to enable the improvement and optimisation of the processes.

But beyond the benefits that these technologies bring in terms of efficiency and productivity, it is also necessary to have in mind the possibilities that this solutions offer to the battery industry in terms of sustainability and environmental impact. For instance, through waste analysis, further studies can be carried out on how to reduce waste generation with the aim of minimising the environmental impact and maximising the "circularity" of the sector.

To sum up, battery production is characterised by its complexity and diversity of activities. However, a large part of the future energy transition depends on the development and optimisation of these processes, as well as their proliferation through the famous gigafactories. That is why from CIC energiGUNE we support the industry in the research and development of this type of activities and their challenges, to boost one of the industries that will be key for the future.