Prismatic, Cylindrical and Pouch lithium battery cells

What are the manufacturing process behind a lithium battery cell?

The aim of this article is to have a quick overview of cell manufacturing process. This short description of lithium cell manufacturing will allow you to understand the basic process flow.

The lithium ion battery cells are in different shape:

• Prismatic Lithium Battery cell
• Cylindric Lithium Battery cell
• Pouch Lithium Battery cell
• Coin Lithium Battery cell

The main difference between them is the shape, max dimension avaialable in the market, the case type and theperformance which affect the final application. All of them except for pouch have hard metal case. The first three can have application in automotive and electronic devices as well. We can find coin cells in small electronic devices like headphone, smartwatch etc.

The overall lithium ion battery cell manufacturing process foreseen three different macro area:

• Powder Treatment and Electrode manufacturing
• Cell assembly
• Formation and Aging

We don’t consider the module and pack assembly process, since we are talking about cells.

Powder Treatment and Electrode Manufacturing: from powder to electrode reel. Mixing, coating, drying, calendering and slitting in lithium battery cell manufacturing.

The electrode manufacturing usually take place in normal environment condition, stable temperature but not dry conditions. This still valid for NMC and anode, but is not valid for LiFePO4 cathode.

Since LiFePO4 is hygroscopic, a mild dry environment is required for the manufacturing activities.

Here the raw material in powder are mixed together with conductive material (usually carbon) and polymeric binder in order to get the “Slurry”. The first process forseen mixing binder with solvent (or water). In parallel the mixing of active material and conductive agent take place.

Once the active material is completely covered by conductive agent, final mixing with binder solution take place in order to prepare the slurry. The slurry for electrode manufacturing should have specific characteristics:

• Viscosity
• Homogeneity
• Particle size distribution of active material inside the slurry.

As slurry of active material we have:

• LFP, NMC, NCMA for cathode;
• SiC , Graphite etc for anode;

Coating:

A die casting apply the slurry on both side of a thin web of metal sheet (copper for anode and aluminum for cathode). Most critical topic of coating process is keep a constant load of slurry on metal web foil.

The continuous web of metal foil covered by slurry pass through a long oven where the solvent/water can everapote and creare micro porosity in the slurry structure. Calendering process occour in order to get a very precise thickness of slurry. The calendering rollers work at medium high temperature (over 60°C) in order to active the binder. Binder activation is necessary to create adhesion between coating layer and web metal foil.

Now the electrodes in form of jumbo reel, pass through a slitting process in order to obtained multiple reel with the right dimension (related to cell dimension).

Cell assembly: from electrode and separator reel to prismatic and cylindrical lithium battery cell. Lamination and stacking, Z-Folding and Winding process

Cell assembly is the most costumize group of process for lithium ion battery manufacturing. Cell assembly area strictly depends on the type of lithium battery cells we want to produce and vary with manufacturer. All the listed below process occur in dry room which garantee low level of humidity. Indeed water can effect negatively the battery cell performance. In general we can say there are some macro general common area:

Lithium ion Cell manufacturing:

Winding: Electrode and separators reels which are unwinded separately and then rewinded together in order to produce wound element. The wound element is the Jelly Roll which represent the mostly cylindrical cell. The common lithium battery cylindrical cell format are 18650, 21700 and now the new 4680 and similar (4695-46120 – 32105). Coin Cells foreseen same process but with complete different dimensions of final cell.

Lamination and stacking: Electrode reels are notched on bare foil area (metal area uncoated with slurry) and then laminated in single sheet to the separator in order to create monocells. Monocells are then stacked together to create the final prismatic cell. Usually the prismatic cell have between 20 and 60 monocells.

Z folding: a pick and place, place electrode sheet on continuous separator web which separate all anode and cathode layer in order to create a prismatic cell

Prismatic winding: is the same process of winding for Cylindrical cell, the only difference is the winding mandrel shape: for prismatic winding cell, the mandrel is not round but flat.

After the above process, the welding process bound together the current collectors to the cells tab (metal barefoil). Then the cell can be placed inside the case which is pouch for prismatic and metal can for cylindrical, prismatic and coin cells. The case foreseen mechanical closing process (crimping process for cylindrical can), thermical closing process (using hot conductive bars for pouch) or laser welding technology (which weld the metal case creating a bound between the metal case and its cover). At the end the internal volume of the case, still avaialable for the electrolyte filling process.

After the electrolyte filling, laser welding technology or mechanical forming are the most commong technique to seal the case before the battery cell leave the dry room

Formation and Aging: all the steps from inactive to active lithium ion battery cells

The formation and aging process occured in a separate area of production floor. The scope of formation and aging is activate the battery cell after electrolyte filling process. After the electrolyte filling, in some cases, there a soaking area. The electrolyte should be penetrate in the whole phorosity of battery cells material before go forward with the following steps.

Before the formation there is a pre-charging process. During this process, usually the battery generate gas: for this reason vacuum chambers are used to remove it from the cell. The formation process then occured in several week (days in Tesla manufacturing process). The elecolyte generate a thick interphase layer on anode surface. This layer (SEI) prevent the irreversible electrolyte and lithium ion consumption.

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