The whole production of LiFePO4 cells is extremely complicated. It can be separated into over 50 small processes. To make it easier, we will explain it in 3 major stages.
Due to the high safety demands of LiFePO4 batteries, there are highly high-level standards for the precision, stability, and automation of lithium equipment in battery manufacturing.
Cell Stage I – Electrode Sheet Manufacturing
The primary purpose of Cell Stage I is to produce a completed usable electrode sheet, including both positive and negative electrode sheets.
The stage mainly includes slurry mixing, coating, roller pressing, slitting, and making.
Slurry Mixing (Equipment: Vacuum Mixer)
With a vacuum mixer, the powder positive and negative materials are mixed together with special solvents. After high-speed stirring, the slurries of positive and negative materials are produced. The slurry requires to be completely evenly and bubble-free.
Electrode Sheet Coating (Equipment: Coating Machine)
Coating is to evenly coat the mixed slurry on the metal sheet and dry it to make positive and negative electrode sheets.
As the core process of Cell Stage I, the quality of the coating process has a deep influence on the consistency, safety, and life cycle of the final battery. So the coating machine is the highest value in the equipment in stage 1.
The coating process requires no particles, debris, or dust to be mixed into the electrode sheet, otherwise, the quality of the electrode will be greatly reduced. After generations of technology development, the best coating machine is now the slit extrusion type, which can control the precision of coating thickness of slurry to under 2 µm. In the meantime, the coating technology, tension control, deflection technology, and drying technology demand higher standards.
Calendering – Roller Pressing (Equipment: Roller Presser)
Roller Pressing is to compact the coated electrode sheet again in order to increase the energy density of the battery.
Appropriate compacted density can increase battery capacity, reduce internal resistance, reduce loss of polarization, and extend battery cycle life.
The flatness of the electrode sheet after calendering will directly affect the processing effect of the slitting process. And the evenness of the active material on the electrode sheet will also affect the performance of the battery cell.
Slitting (Equipment: Slitter)
Slitting is the continuous slitting of a wide roll of electrode sheets into a number of narrow sheets of the desired width.
The flatness of the edge after slitting (no burr, no flex) is the key to examining the performance of the slitting machine.
Electrode Sheet Making (Equipment: Forming Machine)
This last step is to make the finished electrode sheet for the next stage.
Electrode sheet making includes welding lugs to the slit electrode sheet, attaching protective tape, lug wrapping, or laser cutting to make the lugs for the subsequent winding process.
Cell Stage II – Cell Integration
The goal of Cell Stage II is to complete the manufacturing of battery cells. The technical route and production equipment of this stage vary for different types of lithium batteries.
The essence of Cell Stage II is the assembly process. Specifically, it is the orderly assembly of the (positive and negative) electrode sheets made in the previous stage, with the separator and electrolyte.
Due to the different energy storage structures of square (rolled), cylindrical (rolled), and soft pack (stacking) batteries, there are obvious differences in the technical routes and production line equipment for different types of LiFePO4 batteries in Cell Stage II.
Winding (Equipment: Winding Machine)
Winding is to wind the electrode sheets into LiFePO4 battery cells. This process is for the production of Square (Prismatic) and Cylindrical LiFePO4 batteries.
There are 2 types of winding machines, square winding machines, and cylindrical winding machines for the two types of LiFePO4 batteries.
Tension control is the core technology.
Compared with cylindrical winding, square winding process demands much higher tension control. So it is more difficult for square battery in this process.
Stacking (Equipment: Stacking Machine)
Stacking is to stack the electrode sheets into battery cells. Stacking machines are mainly used in the production of soft pack batteries.
Compared with square and cylindrical cells, soft pack cells have obvious advantages in terms of energy density, safety, and discharge performance.
However, when the stacking machine is doing the single stacking task, multiple sub-processes and complex mechanism synergy are involved. To enhance the stacking efficiency will need to deal with complex dynamics control problems. Meanwhile, the winding machine speed is directly related to the winding efficiency, it is relatively simple to increase the efficiency.
At present, the production efficiency and quality rate of stacking cells are lower than winding cells.
Liquid Injection (Equipment: Liquid Injection Machine)
This process is to inject the electrolyte into the cell in a certain quantity.
The cell sealing (equipment: shell-in machine, slot-rolling machine, sealing machine, welding machine) is to place the rolled cells into the shell.
Cell Stage III – Formation
The goal of Cell Stage III is to complete the formation and packaging.
The LiFePO4 Battery Cell is almost finished in stage II. Stage III is to activate it by first charge & discharge.
After testing, sorting, and assembling, the LiFePO4 cells will be safe and stable.
The Formation (Equipment: Charge/Discharge Machine)
Formation is the activation of the battery cell by the first charge. During the first charge, an effective passivation film (SEI film) is generated on the negative electrode surface to “initialize” the LiFePO4 battery cell.
Capacity Division (Equipment: Charge/Discharge Machine)
i.e. “capacity analysis”, is to charge and discharge the battery cells according to the designed standards, in order to measure the capacity of the cells.
The charging and discharging of the cells runs through the process of formation and Capacity Division, so the Charge & Discharge Machine is the most used key equipment in Stage III.
The smallest working unit of a charge & discharge machine is a “channel”. A “unit” (BOX) is made up of several “channels”. Several “units” are combined together to form a charge & discharge machine.
Testing (Equipment: Testing Equipment)
Before and after charging, discharging, and resting.
Sorting is classifying and picking the cells according to certain standards after formation and capacity division according to the test results.
The purpose of the testing and sorting process is not only to remove unqualified battery cells but also to sort out the battery cells with similar performance. It will help optimize the overall performance of the battery pack since the battery cells are always connected in parallel and series in the actual applications.
The LiFePO4 Battery production cannot be done without the equipment and facilities. In addition to the raw materials, the manufacturing process and production equipment are also important factors in the performance of the battery.
After years of rapid development, China’s lithium equipment enterprises have gradually overtaken the Japanese and Korean equipment company in technology, efficiency, stability, and many other aspects. And, they have the advantages of cost performance, after-sales maintenance, etc.
Battery Assembly I – cell assembly
According to the designed scheme, assemble the cells and holders together and fix them.
Synchronizing all cells to a full state, which is generally 3.65V.
According to the designed connection scheme, weld the terminals of the battery cells. Connect the balance wires without connecting the BMS.
At present, our factory is using laser welding machines. Welding quality directly affects the long-term use of LiFePO4 batteries. Especially for the high current/power applications, unqualified welding will cause the terminal connection to fall off after a time of use, while laser welding prevents the possibility of false welding.
Battery Assembly II – BMS
According to the designed scheme, fix the BMS. Apply silicon gel, which can help to dissipate heat effectively.
Connect the balance wires, and connect the temperature sensors.
If there are other optional parts, such as Bluetooth, such as heating film, connect, install, and fix.
Battery Assembly III – Packaging
Place the module into the shell.
Make it fixed and secured.
Connect to the external terminals.
Tests, such as capacity tests, high current tests, vibration tests, etc.