Lithium-ion Battery Manufacturing Process
Lithium-ion battery manufacturing is a complex process. In this article, we will discuss each step in details of the production, meanwhile present two production cases with specific parameters for the better understanding:
The production of cylindrical wound 18650 battery (capacity 1400mA h) and winding type 383450 battery (capacity 750mA·h) .
Lithium-ion Battery Manufacturing Process Design
The production of lithium-ion battery cells includes four links:
Pole piece production, cell assembly, cell formation, and battery packaging.
The process is shown in Figure 1.
Every process in the cell production process is very important. Improper operation will directly affect the performance of the batteries and increase the rate of defective products.
1. Electrode production process
1.1 Batching process
Weigh according to the ingredient list of the positive and negative electrode materials, and bake at the corresponding temperature.
Qualified positive and negative electrode materials can be pulped according to the positive and negative electrode slurry mixing process flow chart (Different mixers have different mixing parameters).
And the temperature of the slurry should be kept under 30°C at any time. The required equipment is a double planetary mixer with dispersing function.
Notes on ingredients are as follows:
(1) Prevent other impurities from mixing.
(2) Prevent the slurry from splashing.
(3) The concentration (solid content) of the slurry should be adjusted gradually from high to low, so as not to avoid trouble.
(4) Scrape the sides and bottom between stirring to ensure an even distribution.
1-Lithium cobaltate 2- Conductive agent 3- Anode material mixer 4- Glue
5-Binder 6- Solvent 7- Glue tank 8- Slurry trolley 9- Graphite
10-Conductive agent 11- Adhesive SBR 12- Anode material mixer
13- Glue 14- Slurry trolley 15- CMC 16- Solvent 17- Glue tank
(5) The slurry should not be left for a long time to avoid precipitation or decrease in uniformity.
(6) The materials to be baked must be sealed and cooled before use to avoid changes in the properties of the component materials.
(7) The length of the stirring time is mainly based on the performance of the equipment and the amount of materials added.
The use of stirring paddle: replace it according to the difficulty of slurry dispersion. For those that cannot be replaced, the speed can be adjusted from slow to fast to avoid damage to the equipment.
(8) Screen the slurry before discharging, as shown in Figure 2.
The purpose of sieving the positive and negative electrode slurry is to remove metal impurities in the positive and negative electrode materials, mainly iron, and remove large particles to prevent belt breakage during coating.
1.2 Coating process
Coating is to coat the positive and negative electrode materials that have been prepared on the aluminum foil and copper foil according to their respective requirements.
Then it is dried in an oven to evaporate the solvent, and finally the positive electrode sheet and the negative electrode sheet are produced.
The principle of the coating machine is: The coating roller of the coating machine drives the slurry to rotate.
According to the surface density required by the process standard, adjust the size of the scraper gap to control the amount of slurry transfer, and then control the coating surface density. And the slurry is transferred to the surface of the foil by the simultaneous rotation of the rubber roller and the coating roller.
Finally, the pole piece is put into an oven for drying, and the solvent in the slurry is evaporated.
In this way, the materials of the positive and negative electrodes can be well adhered to the surface of the current collector of the positive and negative electrodes.
The coating principle and coating process of pole piece are shown in Figure 3.
1.3 Pole piece rolling process
Rolling is to use the automatic rolling machine to evenly compact the pole piece to a certain thickness under the appropriate pressure and roll gap. Then the active material is in close contact with the grid and current collector to reduce the moving distance of electrons.
In addition, the thickness of the pole piece is reduced, the loading capacity is increased, and the utilization rate of the battery volume is improved.
Before rolling, the pole pieces must be baked at 60°C for several hours. Otherwise, it is easy to lose powder or film layer after the pole piece is rolled. The pole piece rolling is shown in Figure 4.
The rolling process requires that：
- The surface and section of the foil are smooth, uniform in color, without obvious bright lines, obvious bumps, dark streaks, etc.
- No obvious warping and wrinkles on the edges.
- No powder falling.
- No rust on the core.
1.4 Pole piece cutting
Since the diameter of the 18650 battery is 18mm, the diameter of the battery core should be controlled to 17.5~18mm.
Therefore, the pole piece cannot be too long, otherwise the rolled battery core cannot be put into the cylinder.
But if the pole pieces are too short, the battery cells will not be able to fill the battery cylinder, causing potential safety hazards.
The cutting of the pole piece is shown in Figure 5.
2. Process design of assembly workshop
The battery assembly process is:
Winding → shelling → roll groove → electrode group baking → liquid injection → laser welding → sealing
The winding process design is as follows:
Winding is to use a separator to separate the positive and negative pole pieces, wrap them together, and prepare for entering the shell.
– Before winding, the pole pieces should be cut for positive and negative electrode burr detection. Requirement：
- The burr should not be greater than 12um.
- The positive and negative pole lug welding tension should not be less than 15N.
- The separator is smooth, free of folds and damage, attached to the core hole wall, and free of rebound plugging.
– When winding, pay attention to the alignment.
Requirement：
- From the edge of the core, the negative electrode completely wraps the positive electrode, and the size requirement should be (1±0.7)mm.
- The separator in the width direction completely wraps the negative electrode, and the size requirement should be (1±0.5)mm .
The fully automatic winding machine for lithium-ion batteries is shown in Figure 6.
The rolled battery cell is shown in Figure 7, and there should be no defects such as damage and burrs.
Subsequently, the electrolyte is injected into the assembled cell, and then sealed.
The humidity of the injection environment is 20%~40%RH.
The fully automatic liquid injection machine is shown in Figure 8.
Other parts of the process are shown in Figure 9~Figure 12.
3. Formation process
Lithium-ion batteries must be formed, tested, and sorted before leaving the factory.
The formation of lithium-ion batteries has two main functions:
(1) The active material in the battery is converted into a material with normal electrochemical action by means of the first charge.
(2) Make an effective solid electrolyte interface film on the surface of the electrode (mainly the negative electrode) to prevent the negative electrode from reacting spontaneously with the electrolyte. At the same time, there is a good contact between the active material and the electrolyte.
During the formation of the battery, the initial charge and discharge will reduce the discharge capacity of the battery due to the irreversible reaction of the battery. After the electrochemical state is stable, the battery capacity will tend to be stable.
Therefore, some batteries need to go through multiple charge and discharge cycles to achieve stability.
As with all Li-ion batteries, controlling the charging process is very important. Usually, it is carried out with a constant current first. And when the charging voltage of the battery reaches the set value, it is charged with a constant voltage.
If the lithium-ion battery is charged at a constant voltage with an improper termination voltage, it will easily affect the cycle life, and even cause the electrolyte to decompose and cause danger.
Lithium-ion battery charging usually uses constant current and constant voltage for charge and discharge tests, so the termination voltage of charging must be accurately controlled.
4. Capacity division process
During the manufacturing process of the battery, the actual capacity of the battery cannot be completely consistent due to technological reasons.
The process of classifying the battery by capacity is called capacity division after passing a certain charge and discharge system test.
5. Other processes
Other processes of lithium-ion battery preparation are shown in Figures 13 to 17.
[Case 1] Manufacturing process of cylindrical wound 18650 battery (capacity 1400mA h)
The height of the 18650 battery case is 65mm, the height of the effective inner cavity is 60mm, the outer diameter is 18mm, and the thickness of the steel case is about 0.25mm.
The negative electrode of the battery cell in the steel case is spot-welded on the bottom of the case through tabs. The positive electrode is spot-welded on the cap by laser spot welding to form a battery with the cap as the positive electrode and the shell as the negative electrode.
(1) Batching process
An oily formula system is generally adopted in the positive electrode, and the viscosity is generally controlled at 6000 ~ 10000mPa·s.
The negative electrode generally adopts a water-based formulation system.
The slurry ratio and ingredients are shown in Table 1~Table 4.
Table 1 Proportion of cathode slurry
Positive active material | Adhesive | Conductive agent | Solvent |
Solid content |
LiFePO4 | PVDF | S-P | NMP | |
92% | 4% | 4% | 55% | 40% -50% |
Table 2 Cathode ingredients
Slurry mixing | Rotation / Hz | Revolution / Hz | Time / h | Temperature / ℃ | Vacuum / MPa |
PVDF + NMP (solid content 7%) | 15 | 15 | 1 | 20 -55 |
|
1 | -0.09 – 0.1 | ||||
S-P | 15 | 15 | 0.5 | 20 -55 |
|
30 | 30 | 1.5 | 20 -55 | -0.09 – 0.1 | |
LFP+NMP | 15 | 15 | 0.5 | 20 -55 |
|
30 | 30 | 1.5 | 20 -50 | -0.09 – 0.1 | |
Solid content | 45% | ||||
Table 3 Proportion of negative electrode slurry
Negative active material | Adhesive | Conductive agent | Solvent |
Solid content | |
Graphite (BRT-H1) | SBR | CMC | S-P | Deionized water | |
94.5% | 2% | 1.5% | 2% | 55% | 45% – 50% |
Table 4 Negative electrode ingredients
Slurry mixing | Rotation / Hz | Revolution / Hz | Time / h | Temperature / ℃ | Vacuum / MPa |
CMC + H₂O | 15 | 15 | 0.5 | 20 – 55 |
|
28 | 40 | 1 |
| ||
S – P | 15 | 15 | 0.5 | 20 – 55 |
|
28 | 40 | 1.5 |
| ||
C | 15 | 15 | 0.5 | 20 – 55 |
|
28 | 40 | 1.5 |
| ||
SBR + H₂O | 28 | 40 | 1 | 20 – 55 | -0.09 – 0.1 |
Solid content | 46% | ||||
(2) Coating process
The positive and negative electrode slurries were passed through a 150-mesh sieve.
Then they are uniformly coated on the aluminum foil (positive electrode current collector) and copper foil (negative electrode current collector) respectively, and the uncoated area is left as the tab area.
The coated pole piece enters the coating machine oven for baking.
See Table 5 and Table 6 for the baking temperature, which is subject to complete drying.
Table 5 Positive electrode coating oven setting unit: ℃
1st stage temperature | 2nd stage temperature | 3rd stage temperature |
80 – 100 | 90 – 110 | 80 -110 |
Table 6 Negative electrode coating oven setting unit: ℃
1st stage temperature | 2nd stage temperature | 3rd stage temperature |
80 – 100 | 90 – 110 | 80 -100 |
The positive and negative coating parameters are shown in Figures 18 and 19
(3) Rolling
Pass the pole piece through the roller press, adjust the gap between the rolls and the hydraulic pressure, and press it to the specified compaction density and thickness.
The production specifications are shown in Table 7.
Table 7 Production specifications
Polarity | Pieces/ mm | Compaction density/ (g·cm-3) | Roll thickness/ μm | Tab specification |
Positive electrode | W56*1.850 | 2.20 | 133 | 80*3 |
Negative electrode | W57.5*1.917 | 1.53 | 81 | 70*3 |
(4) Liquid injection process
There are two calculation methods for liquid injection volume design: space coefficient method and volume method.
Generally speaking, it is more reasonable to calculate the injection volume according to the volume.
The different electrolyte manufacturers have different injection volume, but this factor may not be considered in the design.
Generally, 1g of electrolyte is equivalent to a capacity of 300~330mA·h, which can be slightly adjusted according to the cycle performance and process capability of the battery.
(5) Formation and volume separation
① Formation
Battery formation serves two purposes:
- One is to activate the active material inside the battery.
- The second is to form a stable and dense solid electrolyte interface film on the surface of the negative electrode.
The formation process includes multiple charging processes. Generally, low current charging is carried out first, and then the high current charging.
After the formation, the battery has no serious swelling, leakage, low pressure and zero pressure. Then, it is a qualified battery.
Table 8 Formation process
Procedure | Current / mA | Time / min | Limit voltage / mV | Termination current / mA |
1 | 28 |
| 3450 |
|
2 | 140 | 600 | 3550 | 14 |
After formation, they were left at room temperature for 2 days, and 5 batteries were randomly selected to test the open circuit voltage, see Table 9.
Table 9 Open circuit voltage data
No. | 1 | 2 | 3 | 4 | 5 |
Open circuit voltage | 3.312 | 3.326 | 3.311 | 3.315 | 3.322 |
If the voltage range is within a reasonable range, then the next step of capacity division process can be carried out.
② Capacity division
The capacity division of the battery refers to:
Due to slight differences in the production process of the battery cells, the actual capacity of each battery is not exactly the same, so the capacity level of the battery need to be detected by charging and discharging the battery.
After the formation of battery, it should be stored at room temperature for 2 days, or aged at 50°C for one day. Until then, the capacity can be divided.
See Table 10 for the capacity-dividing process.
Table 10 Capacity division process
Procedure | Operating mode | Current / mA | Time / min | Limit Voltage / mV | Termination Current / mA |
1 | Constant current discharge | 140 | 200 | 2500 |
|
2 | Constant current and constant voltage charging | 700 | 420 | 3850 | 20 |
3 | On hold |
| 3 |
|
|
4 | Constant current discharge | 700 | 150 | 2500 |
|
5 | Constant current and constant voltage charging | 700 | 200 | 3850 | 14 |
After the capacity separation, the batteries are sorted.
Sorting out qualified products in terms of capacity, voltage and appearance.
[Case 2] Manufacture process of winding type 383450 battery (capacity 750mA·h)
Raw materials: ternary cathode material, graphite anode material.
(1) Batching process
The positive electrode generally adopts an oily formula system, with the viscosity generally controlled at 5000~7000mPa·s.
The negative electrode generally adopts a water-based formula system, with the viscosity of the negative electrode controlled at 2000~3000mPa·s.
The mixer is shown in Figure 22.
The slurry ratio of positive and negative electrodes is shown in Table 11.
Table 11 Slurry ratio of positive and negative electrodes
Cathode material | Adhesive | Conductive agent | Solvent | Negative material | Dispersant | Conductive agent | Adhesive | Solvent |
LNMCO | PVDF | Super-P | NMP | Graphite | CMC | Super-P | SBR | Deionized water |
95% | 2% | 3% | 33% – 38% | 95% | 1.5% | 1% | 2.5% | 50% – 55% |
(2) Coating process
The positive and negative electrode slurries were passed through a 150-mesh sieve, and then uniformly coated on aluminum foil (positive electrode current collector) and copper foil (negative electrode current collector), respectively. The uncoated area was left as the tab area.
The coated pole piece enters the coating machine oven for baking.
The coating process parameters of the positive and negative electrodes are shown in Figure 22, and the main parameter values of the positive and negative electrode coatings are shown in Table 12.
See Table 13 and Table 14 for the baking temperature, subject to complete drying.
Table 12 Main parameter values of positive and negative electrode coating (unit: g/m2)
Positive double-sided density | Aluminum foil areal density | Negative double-sided density | Copper foil areal density |
422.6 | 40.5 | 218.6 | 88.0 |
Table 13 Positive electrode coating oven setting (unit: ℃)
1st stage temperature | 2nd stage temperature | 3rd stage temperature |
80 -100 | 90 – 110 | 80 – 110 |
Table 14 Negative electrode coating oven setting (unit: ℃)
1st stage temperature | 2nd stage temperature | 3rd stage temperature |
80 -100 | 90 – 110 | 80 – 100 |
(3) Rolling and slitting process
See Table 6.15 and Table 6.16 for the parameters of positive and negative roll cutting,
and the cutting of positive and negative pole pieces is shown in Figure 6.23 and Figure 6.24.
Table 15 Positive electrode roll cutting parameters
Positive electrode | Slitting / mm | Compacted density / (g cm-3) | Rolling / mm | Number of slices |
Value | 42 | 3.40 | 0.139 | 5 |
Tolerance | ±1 |
| ±0.003 |
Table 16 Negative electrode rolling and cutting parameters
Negative electrode | Slitting / mm | Compacted density / (g cm-3) | Rolling / mm | Number of slices |
Value | 43 | 1.50 | 0.156 | 5 |
Tolerance | ±1 |
| ±0.003 |
(4) Tab welding and gluing process
① Tab welding
After the positive and negative pole pieces are respectively rolled and slitted, the next process is the tabs welding.
At present, aluminum strips are generally used for positive pole tabs, while nickel strips are used for negative pole tabs.
The specifications of the aluminum and nickel strips used in this design are 0.1 x 3 mm, and the tabs are equipped with high-temperature glue.
The welding of the positive pole lug adopts an ultrasonic welding machine, and the welding of the negative pole lug adopts an electric welding machine. The reason is that aluminum is relatively soft, and the use of spot welding can easily lead to weak welding or weld penetration.
The welding parameters are shown in Figure 25 and Figure 26.
The requirements for tab welding are:
- During the welding process of the pole piece, the other areas of the pole piece must not be scratched.
- There are no serious burrs, warped edges, and protrusions after the tabs are welded, and the welding is firm.
- For the positive electrode, after the tab is welded, the welding strength is greater than 15N. Besides,the aluminum foil can be seen to be torn by hand tearing. There should be no cracks and no wrinkles on the aluminum foil welded to the lug position.
- For the negative electrode, after the tab is welded, the welding strength is greater than 4N. Besides,the copper foil can be seen to be torn by hand tearing, and the copper foil at the tab must not have cracks or ruptures.
② Gluing.
The purpose of pasting the glue is to further prevent the short circuit after the positive and negative electrodes are wound.
Paste high-temperature insulating tapes on the specific positions of the positive and negative pole pieces, using two specifications of tapes of 8mm and 15mm respectively.
The location of the tape is shown in Figure 27 and Figure 28.
(5) Winding and packaging process
① Winding.
The width of the separator is 45mm, and it is wound manually.
The schematic diagram of the parameters of the cell after winding is shown in Figure 29.
② Package.
At present, the commonly used flexible packaging material is aluminum-plastic film. Its structure is mainly divided into three parts: nylon layer, Al layer and PP layer,
and its schematic diagram is shown in Figure 30.
The aluminum-plastic film die process is shown in Figure 31.
Figure 32 shows the schematic diagram of the cell after top-side sealing.
The control points of the packaging process are shown in Table 17.
Table 17 Control points of packaging process
Item | Requirement | Influence |
Unsealed top area width | （1±0.5）mm | If it is too small, it will cause PP glue to overflow, pollute the head or battery, and there may be a short circuit between the tab and the aluminum-plastic film. If it is too large, it will be sealed to the separator and affect the packaging area. |
Distance from side seal to top edge | （1.0±0.5）mm | If it is too small, the PP glue will overflow and pollute the head or battery. If it is too large, the top and side seals cannot be overlapped. |
Distance from side seal to cell main body | （1.5±0.5）mm | Too small may cause poor insulation resistance. If excessive, the effective seal will be not enough after slitting. |
Exposed CPP size | （1±0.5）mm | If it is too small, it may cause a short circuit between the tab and the aluminum-plastic film. |
Package Parallelism | Tear off the battery seal to reveal a complete strip of milky white, without the original color of the aluminum-plastic film. | Affect the encapsulation effect. |
Top seal thickness | 180 – 205μm | If it is too small, the PP layer will melt too much. And if it is too large, the aluminum-plastic film can not seal the whole part. |
Side seal thickness | （196±15）μm |
(6) Liquid injection process
① Liquid injection and vacuum pre-sealing
Cells must be vacuum baked at 85°C for 36~48 hours before liquid injection.
When baking, the mouth of the air bag must face up. Besides, the mouth of the air bag must be opened to a certain size to ensure that the moisture inside the battery core can escape more easily. Otherwise, the battery will be seriously inflated after liquid injection.
After baking, put the cell into the glove box for manual liquid injection.
After the liquid injection is completed, vacuumize the cell (-65kPa). Its purpose is to make the electrolyte immediately enter the main body of the cell to prevent the electrolyte from leaking out during pre-sealing.
Finally, pre-seal the cells (vacuum degree -80kPa, temperature 170°C).
The injection process parameters are shown in Table 18.
Table 18 Liquid injection process parameters
Baking time / h | Baking temperature / ℃ | Vacuum / Mpa | Electrolyte model | Injection volume / g |
36 – 48 | 85 | -0.08 – 0.09 | TC-261R | 1.95±0.1 |
② Hot and cold pressing
After the battery is injected, it needs to be aged at room temperature for 12~20h.
When standing, the mouth of the air bag faces up, making it easier for the electrolyte to fully infiltrate the pole piece.
After the aging, the hot and cold pressing process is carried out. The purpose of this process is to make the appearance of the battery cell more flat and compact.
After hot and cold pressing, the contact distance between the positive and negative electrodes is reduced, which can reduce the internal resistance of the battery cell and shorten the conduction path of Li-ion.
The hot and cold pressing process parameters are shown in Table 19.
Table 19 Process parameters of hot and cold pressing
Hot pressing | Cold pressing | ||||
Pressure / Mpa | Temperature/ ℃ | Time / s | Pressure / Mpa | Temperature/ ℃ | Time / s |
0.14 ± 0.02 | 80 ± 5 | 40 | 0.14 ± 0.02 | 25 ± 5 | 40 |
Precautions:
Excessive hot and cold pressing will easily cause the pole pieces at the corners of the battery to wrinkle and increase the internal resistance of the battery. Serious ones are more likely to cause the positive and negative pole pieces to be broken and form an open circuit.
If the pressure is too small, the battery cannot be effectively reshaped.
(7) Formation and capacity division
① Formation
The designed formation process parameters are shown in Table 20.
Procedure | Item | Time / min | Current / mA | Termination Voltage / mV | Terminatio current / mA |
1 | Constant current charging | 10 | 15 | 3000 |
|
2 | Constant current and constant voltage charging | 1300 | 37.5 | 3950 | 8 |
② Sealing and cutting and folding
After the formation of battery, it must be subjected to secondary aging (ie hot and cold pressing). Its purpose is to reshape the battery again so that it is denser and not soft. More importantly, it is to make the solid electrolyte interfacial film formed during the formation process more stable.
The second sealing is to extract the gas generated after the battery is formed, then package it on the right side of the battery body, and finally perform the cutting and folding process.
If it is placed for too long after the secondary aging without performing the second-sealing pumping, the capacity of the battery may be reduced, resulting in liquid corrosion and battery softening.
Its process parameters are shown in Table 21.
Table 21 Process parameters for second sealing and cutting and flolding
Second sealing | Cutting and folding | |||||
Head temperature / ℃ | Vacuum / kPa | Package thickness / μm | Unsealed area / mm | Residual liquid volume / g | Trimming width / mm | Cutting edge melt glue width / mm |
170 | -85 | 196±15 | 1 | 1.31 | 3.5-32 | ≥1.5 |
③ Capacity division
The functions of the capacity division process are:
- Classify and match the battery according to the required voltage and capacity.
- The electrodes are fully soaked in the electrolyte through the charge-discharge cycle.
- The battery is fully activated through charge and discharge cycles, making the performance of the battery more stable.
The capacity-dividing process parameters used in this design are shown in Table 22.
Table 22 Capacity division process parameters
Procedure | Item | Current / mA | Time / min | Limit voltage / mV | Termination current / mA |
1 | Constant current discharge | 375 | 90 | 3000 |
|
2 | Constant current and constant voltage discharge | 150 | 420 | 4200 | 8 |
3 | On hold |
| 10 |
|
|
4 | Constant current discharge | 375 | 150 | 3000 | 4 |
5 | Constant current and constant voltage charging | 375 | 200 | 3950 |