The production process of square lithium - ion batteries is a complex and multi - step procedure that requires precision and strict quality control.

The first step is electrode preparation. For the anode, graphite powder, along with binders and conductive additives, is mixed in a suitable solvent to form a homogeneous slurry. This slurry is then coated onto a copper foil substrate using techniques such as slot - die coating or doctor - blade coating. After coating, the electrode is dried in an oven to remove the solvent, and then calendared to achieve the desired thickness and density. The cathode preparation follows a similar process. Cathode materials like lithium - nickel - manganese - cobalt - oxide (NCM) or lithium - iron - phosphate (LFP) are mixed with binders, conductive agents, and a solvent to form a slurry, which is coated onto an aluminum foil. The coated cathode is also dried and calendared.

Next is the cell assembly. A separator, typically made of polypropylene or polyethylene, is placed between the anode and the cathode. The electrode - separator - electrode stack is carefully rolled or folded into a rectangular shape to fit the square battery casing. In some cases, multiple electrode - separator layers may be stacked to increase the battery's capacity. Once the stack is formed, it is inserted into a pre - fabricated square aluminum or steel casing.

The electrolyte filling process comes after cell assembly. Liquid electrolyte, composed of a lithium - salt (such as lithium hexafluorophosphate) dissolved in an organic solvent mixture, is injected into the battery casing. The electrolyte wets the electrodes and separator, enabling the lithium - ion transport during charge and discharge. After filling, the battery is sealed tightly to prevent electrolyte leakage and the ingress of moisture.

The final step is the formation and activation process. The newly assembled battery is subjected to a series of controlled charge - discharge cycles. During the initial charge, a solid - electrolyte - interphase (SEI) layer forms on the anode surface. This layer is crucial for the battery's performance and stability. The formation process helps to activate the battery, optimize its capacity, and improve its overall performance. After formation, the battery may go through additional aging and conditioning steps to further stabilize its performance before it is ready for use.