Lithium‑ion battery production involves multiple highly‑controlled, automated procedures to guarantee safety and performance. Key stages include electrode preparation, drying and calendaring, electrode slitting, cell assembly, electrolyte filling, formation & aging, as well as testing & grading.
Electrode Manufacturing
① Slurry Mixing
Active materials and additives are fed into a mixing vessel for dry mixing. Solvent is then added to homogenize the mixture. Optionally, pre‑dispersed binder solution can be incorporated for further blending. Vacuum is applied to remove entrapped air from the blend. The final output is known as electrode slurry.
② Coating
Slurry is evenly applied onto metal foil via coating tools such as slot‑dies or doctor blades. Continuous or intermittent coating modes are available; both sides of the foil may be coated simultaneously or sequentially. Tandem coating processes are commonly adopted for sequential double‑side coating. After coating, the foil web travels through a drying tunnel supported by rollers or suspension systems. Heat is supplied to evaporate solvent from the coated layer. The tunnel is divided into independent temperature zones to implement customized temperature profiles. Upon drying, electrodes are cooled down to ambient temperature and wound into rolls.
③ Calendaring & Slitting
During calendaring, double‑sided coated copper or aluminum foil is compacted by a pair of rotating rollers. Prior to calendaring, electrodes go through electrostatic discharge and cleaning via brushes or airflow. The rotating rollers exert precisely‑defined pressure to compress the coating. After calendaring, electrodes are cleaned and rewound in a roll‑to‑roll workflow.
Slitting cuts wide master electrode rolls into narrower daughter rolls with target widths, mostly using rotary slitters. After slitting, individual daughter rolls are cleaned and rewound in roll‑to‑roll operation.
④ Vacuum Drying
Daughter electrode rolls are placed on dedicated trays and transferred into vacuum dryers for 8‑48 hours of treatment. Residual moisture and solvent inside electrodes are removed through low‑temperature evaporation under reduced pressure. This step also mitigates residual stress and residual moisture. Upon vacuum drying completion, electrodes are directly transferred into a dry‑room environment or hermetically sealed under vacuum.
Cell Manufacturing
⑤ Die‑cutting
Continuous electrode rolls are cut into discrete electrode sheets. A notching process may be performed prior to full separation to form tab features. During notching, dried electrode webs are unwound, and tabs are created by laser cutting. After notching, electrodes can be rewound or fed directly to the cutting station. Both notching and die‑cutting can be implemented by laser processing or mechanical dies.
⑥ Stacking / Winding
For stacking: alternating layers of negative electrode, separator, positive electrode and separator form a cell stack. A single stack may contain up to 120 individual electrode sheets. Vacuum suction tools pick, transport and position electrode pieces. Z‑stacking is widely adopted in industry: positive and negative sheets are fed from opposite sides while the separator folds in a Z‑shaped pattern. Once stacking finishes, the stack bundle is wrapped with separator film, which is then cut and secured with adhesive tape.
Winding is used for cylindrical cells; flat‑winding applies to prismatic cells. For cylindrical jelly‑roll production, positive rolls, negative rolls and two separator webs are concurrently fed into winding equipment. Winding proceeds around a central mandrel for cylindrical cells or a winding core for prismatic cells. Upon completion, the jelly‑roll is fastened with adhesive tape.
⑦ Welding
Pouch cell: Uncoated tab regions of the wound core are trimmed to proper length and ultrasonically welded to positive and negative current collector tabs respectively. Aluminum‑plastic laminate film is unfolded and deep‑drawn to form pocket cavities. The wound core is inserted into the formed pocket with tabs protruding outward. Three sides are heat‑sealed, leaving one opening for electrolyte injection. The tab‑exit edge is sealed afterwards.
Cylindrical cell: An insulating ring is fitted onto the negative end of the jelly‑roll, which is then inserted into the metal can with the insulating ring sitting against the can bottom. The negative tab is resistance‑welded to the can base. Another insulating ring is placed at the positive end, and a groove is formed by rolling. The positive cap assembly is laser‑welded to the positive tab. After electrolyte injection, the cap is pressed into the grooved can for final hermetic sealing.
Prismatic cell: Uncoated tab sections of the wound core are trimmed and laser‑welded to the positive and negative terminal assembly of the cell cover. The wound core is usually wrapped with insulating foil to prevent internal short‑circuit against the metal housing. The core is inserted into the casing, and housing‑to‑cover joints are completed via laser welding. The cover assembly reserves a filling port as the sole access after cover welding.
⑧ Electrolyte Injection
A precision metering nozzle is positioned at the filling opening; accurate positioning prevents housing contamination by electrolyte. Precisely‑dosed electrolyte is dispensed into the cell housing. Negative pressure is applied during and after injection to trigger capillary action, enabling full electrolyte penetration into voids within electrode stacks. The injection opening is sealed after filling.
⑨ Wetting & Soaking
Post‑filling soaking improves electrode wet‑out and avoids dry zones during formation. Ambient temperature is raised to 30 °C‑50 °C. Higher temperature lowers electrolyte viscosity and enhances contact between liquid electrolyte and porous structures of electrodes and separators, accelerating liquid infiltration.
⑩ Formation
Formation refers to the initial charge‑discharge cycle of fresh cells. Cells are loaded onto formation racks and electrically contacted by probe pins. Controlled charge and discharge are executed according to predefined current‑voltage profiles. At early‑stage formation, lithium‑ions react with electrolyte and deposit on graphite anode surfaces to form the Solid Electrolyte Interphase (SEI), a protective interfacial layer between electrolyte and electrodes. Pouch cells are normally held under mechanical clamping pressure throughout formation.
⑪ Degassing (Pouch & Prismatic Cells)
Large‑format cells generate significant gas during the first charging cycle. For pouch cells, evolved gas accumulates in pre‑attached gas bags. During degassing, gas bags are punctured under vacuum to extract by‑product gas. The junction between gas bag and cell body is vacuum‑sealed; gas bags are cut off and disposed as hazardous waste. For prismatic cells, temporary sealing plugs are removed and degassing is performed under vacuum. A secondary electrolyte refill may take place before final sealing.
⑫ Aging
Aging serves as one of the quality‑judgement and final‑inspection procedures. Cells are stored on aging racks or tower cabinets under high‑temperature and room‑temperature conditions respectively. Open‑circuit voltage is measured periodically for up to three weeks to monitor voltage drift. Stable open‑circuit voltage indicates sound cell status. Aging duration varies by cell manufacturer and battery chemistry system.
⑬ Testing and Grading
Before factory shipment, cells undergo comprehensive final inspections, including AC/DC internal resistance measurement, visual inspection, open‑circuit voltage test, capacity verification and short‑circuit test. Cells are discharged down to shipping‑state SOC. Manufacturers sort and grade cells based on measured performance parameters. Cells passing all checks proceed to packaging and delivery.
