With the continuous development of downstream industries such as new energy vehicles, the production scale of lithium batteries is constantly expanding.
This series is divided into two parts. The first part focuses on the principles, formulations, and manufacturing processes of lithium batteries. The second part explains the production and performance of lithium batteries. This article is the first part of this series.
I. Working Principle
1. Positive electrode structure
LiCoO2 + conductive agent + binder (PVDF) + current collector (aluminum foil)
2. Negative electrode structure
Graphite + conductive agent + thickener (CMC) + binder (SBR) + current collector (copper foil)
3. Working principle
3.1 Charging process
A power source charges the battery. During this process, electrons (e) from the positive electrode travel through the external circuit to the negative electrode. Lithium ions (Li+) jump from the positive electrode into the electrolyte, crawl through the winding holes in the separator, and swim to the negative electrode, where they combine with the electrons that have already arrived. At this point:
The reaction that occurs at the positive electrode is:
The reaction that occurs at the negative electrode is:
3.2 Battery Discharge Process
There are two types of discharge: constant current discharge and constant resistance discharge. Constant current discharge involves adding a variable resistor to the external circuit that changes with voltage. Constant resistance discharge essentially involves adding a resistor to the positive and negative terminals of the battery to allow electrons to pass through. Therefore, the battery will not discharge as long as electrons at the negative terminal cannot move to the positive terminal. Electrons and Li+ ions move simultaneously, in the same direction but along different paths. During discharge, electrons travel from the negative terminal through the electron conductor to the positive terminal, while lithium ions (Li+) "jump" into the electrolyte from the negative terminal, "crawl" through the winding holes in the separator, and "swim" to the positive terminal, where they combine with the electrons that have already arrived.
3.3 Charge and discharge characteristics
The positive electrode of the battery cell uses LiCoO2, LiNiO2, and LiMn2O2. LiCoO2 is a crystal form with a very stable layer structure. However, when x Li ions are removed from LiCoO2, its structure may change. Whether it changes or not depends on the size of x.
Research has revealed that when x > 0.5, the structure of Li1-xCoO2 becomes extremely unstable, leading to crystal collapse, which manifests externally as the complete termination of the battery cell. Therefore, during battery cell use, the x value in Li1-xCoO2 should be controlled by limiting the charging voltage. Generally, if the charging voltage is no greater than 4.2V, then x is less than 0.5. In this case, the crystal structure of Li1-xCoO2 remains stable.
The negative electrode C6 has its own characteristics. After the first formation, Li in the positive electrode LiCoO2 is charged into the negative electrode C6. When discharging, Li returns to the positive electrode LiCoO2. However, after formation, some Li must remain in the center of the negative electrode C6 to ensure the normal insertion of Li in the next charge and discharge. Otherwise, the cell voltage will be very short. In order to ensure that some Li remains in the negative electrode C6, it is generally achieved by limiting the lower limit voltage of discharge: the upper limit voltage of safe charging ≤ 4.2V, and the lower limit voltage of discharge ≥ 2.5V.
The principle behind the memory effect is crystallization, a reaction that almost never occurs in lithium-ion batteries. However, the capacity of lithium-ion batteries still decreases after multiple charge-discharge cycles for a complex and varied reason. The most important factor is the change in the positive and negative electrode materials themselves. At the molecular level, the hole structures on the positive and negative electrodes that accommodate lithium ions gradually collapse and become blocked; from a chemical perspective, the active materials of the positive and negative electrodes become passivated, leading to side reactions that generate other stable compounds. Physically, there may also be instances of the positive electrode material gradually peeling off. In short, this ultimately reduces the number of lithium ions that can move freely within the battery during charge and discharge.
Overcharging and over-discharging will cause permanent damage to the positive and negative electrodes of lithium batteries. From a molecular level, it can be intuitively understood that over-discharging will cause the negative electrode carbon to release too many lithium ions, causing its layered structure to collapse. Overcharging will force too many lithium ions into the negative electrode carbon structure, making some of them unable to be released.
Inappropriate temperatures can trigger other chemical reactions inside lithium batteries, generating unwanted compounds. Therefore, many lithium batteries have protective temperature-controlled membranes or electrolyte additives between the positive and negative electrodes. When the battery temperature rises to a certain level, the composite membrane pores close or the electrolyte denatures, increasing the battery's internal resistance until the circuit is broken, preventing the battery from heating up further and ensuring the battery's charging temperature remains normal.
II. Lithium-ion battery formulation and manufacturing process
1. Positive and negative electrode formulations
1.1 Positive electrode formulation: LiCoO2 + conductive agent + binder + current collector (aluminum foil)
LiCoO2 (10μm): 96.0%
Conductive agent (CarbonECP) 2.0%
Adhesive (PVDF761) 2.0%
NMP (Added Adhesive): The weight ratio of solids is approximately 810:1496.
a) Positive electrode viscosity controlled at 6000 cps (temperature 25°C, rotor 3°C);
b) The NMP weight must be adjusted appropriately to achieve the required viscosity;
c) Pay special attention to the effects of temperature and humidity on viscosity.
Positive electrode active material:
Lithium cobalt oxide: a positive electrode active material and lithium-ion source, providing lithium for batteries. It is a non-polar material with an irregular shape, a particle size (D50) typically of 6-8 μm, a water content ≤0.2%, and is usually alkaline with a pH value of around 10-11.
Lithium manganese oxide: a non-polar substance with an irregular shape and a particle size (D50) of 5-7 μm. It has a water content of ≤0.2% and is usually weakly alkaline with a pH of around 8.
Conductive agent: Chain-like material, water content <1%, particle size generally 1-5μm. Superconducting carbon black with excellent conductivity, such as Ketjen black CarbonECP and ECP600JD, is commonly used. Its applications include: improving the conductivity of the cathode material, compensating for the electronic conductivity of the cathode active material; increasing the electrolyte absorption of the cathode sheet, creating new reaction interfaces, and reducing polarization.
PVDF adhesive: a non-polar, chain-like substance with molecular weights ranging from 300,000 to 3,000,000; its molecular weight decreases and its adhesiveness weakens after absorbing water. It is used to bond lithium cobalt oxide, conductive agents, and aluminum foil or mesh together. A commonly used brand is Kynar761.
NMP: a weakly polar liquid used to dissolve/swell PVDF and dilute slurries.
Current collector (positive lead): made of aluminum foil or aluminum strip.
1.2 Negative Electrode Formulation: Graphite + Conductive Agent + Thickener (CMC) + Binder (SBR) + Current Collector (Copper Foil)
Negative electrode material (graphite): 94.5%
Conductive agent (CarbonECP): 1.0% (Ketjen superconducting carbon black)
Adhesive (SBR): 2.25% (SBR = Styrene-butadiene rubber latex)
Thickener (CMC): 2.25% (CMC = Sodium Carboxymethyl Cellulose)
The weight ratio of water to solids is 1600:1417.5.
a) Negative electrode viscosity control 5000-6000 cps (temperature 25°C, rotor 3)
b) The water weight should be adjusted appropriately to achieve the required viscosity;
c) Pay special attention to the effects of temperature and humidity on viscosity.
2. Positive and negative mixing
Graphite: A negative electrode active material, essential for the negative electrode reaction; it is mainly divided into two categories: natural graphite and artificial graphite. It is a non-polar material, easily contaminated by and dispersed in non-polar substances; it does not readily absorb water and is also difficult to disperse in water. Contaminated graphite, after being dispersed in water, easily re-aggregates. The typical particle size D50 is around 20 μm. Particle shapes are diverse and mostly irregular, including spherical, flake-like, and fibrous forms.
Conductive agent: Its uses are as follows:
a) Improve the conductivity of the negative electrode to compensate for the electronic conductivity of the negative electrode active material.
b) Improve reaction depth and utilization rate.
c) Prevent the formation of dendrites.
d) Utilize the liquid absorption capacity of conductive materials to improve the reaction interface and reduce polarization. (Addition or omission can be selected based on the graphite particle size distribution).
Additives: reduce irreversible reactions, improve adhesion, increase slurry viscosity, and prevent slurry sedimentation.
Thickener/anti-settling agent (CMC): a high molecular weight compound that is readily soluble in water and polar solvents.
Isopropanol: A weakly polar substance that, when added, can reduce the polarity of the adhesive solution and improve the compatibility between graphite and the adhesive solution; it has a strong defoaming effect; it easily catalyzes the cross-linking of the adhesive network, thereby improving the bonding strength.
Ethanol: A weakly polar substance that, when added, can reduce the polarity of the adhesive solution and improve the compatibility between graphite and the adhesive solution; it has a strong defoaming effect; it easily catalyzes adhesive chains and improves bond strength (isopropanol and ethanol have essentially the same uses; for mass production, cost factors can be considered when choosing which to add).
Waterborne binders (SBR): Bond graphite, conductive agents, additives, and copper foil or copper mesh together. Small molecule linear chain emulsions, highly soluble in water and polar solvents.
Deionized water (or distilled water): a diluent, added in appropriate amounts to change the fluidity of the slurry.
Negative lead: made of copper foil or nickel strip.
2.1 Positive electrode mixing:
2.1.1 Pretreatment of raw materials
1) Lithium cobalt oxide: Dehydrate. Generally, bake at 120°C under normal pressure for about 2 hours.
2) Conductive agent: Dehydrate. Generally, bake at 200°C under normal pressure for about 2 hours.
3) Adhesive: Dehydration. Generally, it is baked at 120-140°C under normal pressure for about 2 hours. The baking temperature depends on the molecular weight.
4) NMP: Dehydration. Dehydration is performed using a dry molecular sieve or by employing special material handling equipment, and the material is used directly.
2.1.2 Material ball milling:
1) After 4 hours, the balls are separated by sieving;
2) Pour LiCoO2 and CarbonECP into the hopper, and add grinding balls (dry material: grinding balls = 1:1). Perform ball milling on a roller mill, controlling the rotation speed to above 60 rpm.
2.1.3 Blending of raw materials:
1) Dissolution of the adhesive (at standard concentration) and heat treatment.
2) Ball milling of lithium cobalt oxide and conductive agent: This process initially mixes the powders, binds the lithium cobalt oxide and conductive agent together, and improves the agglomeration properties and conductivity. After being formulated into a slurry, it will not be isolated from the binder. The ball milling time is generally around 2 hours. To prevent impurities from being introduced, agate balls are typically used as the milling media.
2.1.4 Dispersion and wetting of dry powder:
Principle: When a solid powder is placed in the air, it will adsorb some air onto its surface over time. When a liquid binder is added, the liquid and gas begin to compete for the solid surface. If the adsorption force between the solid and the gas is stronger than that between the solid and the liquid, the liquid cannot wet the solid. If the adsorption force between the solid and the liquid is stronger than that between the solid and the gas, the liquid can wet the solid and squeeze out the gas.
When the wetting angle is ≤90°, the solid is wetted. When the wetting angle is >90°, the solid is not wetted.
All components of the cathode material can be wetted by the binder solution, so the cathode powder is relatively easy to disperse.
The impact of dispersion methods on dispersion:
1) Static setting method (long time, poor results, but does not damage the original structure of the material);
2) Stirring method: rotation or rotation plus revolution (short time, good effect, but may damage the structure of individual materials).
The effect of agitator impellers on dispersion speed: Agitator impellers generally include serpentine, butterfly, spherical, paddle, and gear shapes. Generally, serpentine, butterfly, and paddle-shaped agitators are used to deal with materials that are difficult to disperse or in the initial stage of batching; spherical and gear-shaped impellers are used for situations where dispersion is less difficult and are more effective.
The effect of stirring speed on dispersion speed. Generally speaking, the higher the stirring speed, the faster the dispersion speed, but the greater the damage to the material's structure and the equipment.
The effect of concentration on dispersion rate. Generally, the lower the slurry concentration, the faster the dispersion rate, but too low a concentration will lead to material waste and increased slurry sedimentation.
The effect of concentration on bond strength. The higher the concentration, the greater the flexural strength and the greater the bond strength; the lower the concentration, the smaller the bond strength.
The effect of vacuum degree on dispersion rate. High vacuum degree is conducive to the expulsion of gas from material gaps and surfaces, reducing the difficulty of liquid adsorption; the difficulty of uniformly dispersing materials under conditions of complete weightlessness or reduced gravity will be greatly reduced.
The effect of temperature on dispersion rate. At suitable temperatures, the slurry has good fluidity and is easy to disperse. If the slurry is too hot, it is prone to forming a skin, while if it is too cold, the fluidity of the slurry will be greatly reduced.
Dilution: Adjust the slurry to a suitable concentration to facilitate coating.
2.1.5 Operating Procedures
a) Pour NMP into a 100L power mixer to 80°C, weigh out PVDF and add it to the mixer, then turn it on; parameter settings: speed 25±2 rpm, stir for 115-125 minutes;
b) Turn on the cooling system and add the pre-ground positive electrode dry material in four equal portions, with an interval of 28-32 minutes between each addition. Add NMP as needed for the third addition, and add NMP after the fourth addition. Power mixer parameter settings: speed 20±2 rpm.
c) After the fourth addition, perform high-speed mixing 30±2 minutes later, for a total of 480±10 minutes; power mixer parameter settings: revolution speed 30±2 rpm, rotation speed 25±2 rpm;
d) Vacuum mixing: Connect the power mixer to a vacuum system and maintain a vacuum level of -0.09 MPa. Stir for 30 ± 2 minutes. Power mixer parameter settings: revolution time 10 ± 2 minutes, rotation time 8 ± 2 rpm.
e) Take 250-300 ml of slurry and measure its viscosity using a viscometer; Test conditions: rotor number 5, rotation speed 12 or 30 rpm, temperature range 25°C;
f) Remove the positive electrode material from the power mixer for colloid milling and sieving. At the same time, affix a label to the stainless steel basin and hand it over to the slurry pulling equipment operator before it flows into the slurry pulling process.
2.1.6 Precautions
a) Complete and clean the machinery, equipment, and work environment;
b) When operating the machine, pay attention to safety to prevent head injuries.
2.2 Negative electrode mixing
2.2.1 Pretreatment of raw materials:
1) Graphite:
A. Mixing makes the raw materials homogenized and improves consistency.
B. Baking at 300~400°C under normal pressure removes surface oily substances, improves compatibility with water-based adhesives, and rounds the edges of the graphite surface (some materials cannot be baked to maintain their surface properties, otherwise their performance will be reduced).
2) Water-based binders: Dilute appropriately to improve dispersion ability.
2.2.2 Blending, wetting, and dispersing:
1) Graphite and binder solutions have different polarities and are not easily dispersed.
2) The graphite can be initially moistened with an alcohol-water solution before being mixed with the binder solution.
3) The stirring concentration should be appropriately reduced to improve dispersibility.
4) The dispersion process reduces the distance between polar and nonpolar materials, increasing potential energy or surface energy, thus it is an endothermic reaction. The overall temperature decreases during stirring. If conditions permit, the stirring temperature should be appropriately increased to facilitate heat absorption, while also improving fluidity and reducing the difficulty of dispersion.
5) Adding a vacuum degassing process during the stirring process can remove gas and promote solid-liquid adsorption, resulting in better performance.
6) The dispersion principle and method are the same as those in the positive electrode formulation.
2.2.3 Dilution:
Adjust the slurry to a suitable concentration to facilitate coating.
2.2.4 Material ball milling
1) Pour the negative electrode and KetjenblackECP into the material bucket and add a ball mill (dry material: grinding balls = 1:1.2). Perform ball milling on a roller, controlling the speed to above 60 rpm;
2) After 4 hours, the balls are sieved and separated.
2.2.5 Operating Procedures
1) Heat purified water to 80°C and pour it into a power mixer (2L).
2) Add CMC and stir for 60±2 minutes; power mixer parameter settings: revolution time 25±2 minutes, rotation time 15±2 rpm;
3) Add SBR and deionized water, and stir for 60±2 minutes;
Power mixer parameter settings: revolution time 30±2 minutes, rotation time 20±2 revolutions/minute;
4) Add the negative electrode dry material in four equal and sequential batches, adding purified water at the same time, with an interval of 28-32 minutes between each addition; power mixer parameter settings: revolution speed 20±2 rpm, rotation speed 15±2 rpm;
5) After the fourth addition of materials, stir at high speed for 30±2 minutes, for a total of 480±10 minutes;
Power mixer parameter settings: revolution speed 30±2 rpm, rotation speed 25±2 rpm;
6) Vacuum mixing: Connect the power mixer to the vacuum system and maintain a vacuum level of -0.09 to 0.10 MPa, then stir for 30 ± 2 minutes;
Power mixer parameter settings: revolution time 10±2 minutes, rotation time 8±2 revolutions/minute
7) Take 500 ml of slurry and measure its viscosity using a viscometer;
Test conditions: Rotor number 5, rotation speed 30 rpm, temperature range 25°C;
8) Remove the negative electrode material from the power mixer for grinding and sieving. At the same time, affix a label to the stainless steel basin and hand it over to the slurry pulling equipment operator before it flows into the slurry pulling process.
2.2.6 Precautions
1) Complete and clean the machinery, equipment, and working environment;
2) When operating the machine, pay attention to safety to prevent head injuries.
Ingredient Notes:
To prevent the introduction of other impurities;
Prevent slurry splashing;
The concentration (solid content) of the slurry should be adjusted gradually from high to low to avoid creating new problems;
During the intermittent stirring, pay attention to scraping the sides and bottom to ensure even dispersion;
The slurry should not be left to stand for a long time to avoid sedimentation or reduced homogeneity;
Materials requiring baking must be sealed and cooled before being added to prevent changes in the properties of the components;
The mixing time depends mainly on the equipment performance and the amount of materials added;
The mixing paddle should be replaced according to the difficulty of dispersing the slurry. If it cannot be replaced, the rotation speed can be adjusted from slow to fast to avoid damaging the equipment.
The slurry is sieved before discharge to remove large particles and prevent tape breakage during coating.
Personnel handling ingredients should receive enhanced training to ensure they possess the necessary professional knowledge, in order to prevent serious accidents.
The key to proper ingredient mixing is even distribution; once this is achieved, other methods can be adjusted as needed.
I. Parameters required for battery manufacturing
1. Electrode size
2. Slurry pulling process
a) Current collector size
Positive electrode (aluminum foil), intermittent coating
Negative electrode (copper foil), intermittent coating
b) Requirements for grouting weight
3. After positive electrode pulping, the following processes are performed:
After cutting large sheets into small sheets, weighing the sheets (matching the sheets), baking the rolled sheets, welding the tabs to the negative electrode, and drawing the slurry, the following processes are performed:
Cutting large pieces into small pieces, weighing pieces (matching pieces), baking, rolling, electrode tab welding.
4. Rolling requirements
5. Film preparation method
6. Electrode baking
Note: The vacuum level of the vacuum system is -0.095 to 0.10 MPa; the protective gas is high-purity nitrogen with a pressure greater than 0.5 MPa.
7. Electrode manufacturing
a) Positive electrode:
The positive electrode tab is ultrasonically welded at the positive electrode plate. The end of the aluminum strip is flush with the edge of the electrode plate.
b) Negative electrode:
Nickel bar dimensions: 0.10×3.0×48mm. The nickel bar is directly spot welded using a spot welding machine, requiring 8 spot welds. The right side of the nickel bar and the right side of the negative electrode are aligned, and the end of the nickel bar is flush with the edge of the electrode.
8. Diaphragm dimensions: 0.025×44.0×790±5mm
9. Needle width: 22.65±0.05mm
10. Cell pressing: After the battery is wound, first attach a 24mm wide transparent tape to the bottom of the cell, and then use a flattening machine to cold press it twice.
11 Requirements before battery cells are installed
Adhesive tape 1: 10.0×38.0±1.0mm, the adhesive tape is evenly distributed on both sides of the battery cell;
Adhesive tape 2: 10.0 × 38.0 ± 1.0 mm, with the nickel strip in the center;
Adhesive tape 3: 24.0×30.0±2.0mm, the adhesive tape is evenly distributed on both sides of the battery cell;
The distance between the right side of the nickel bar and the right side of the battery cell is 7.0±1.0mm.
12-pack
When installing the battery, use both hands to apply force simultaneously and slowly insert the battery cell into the battery casing. Do not scratch the battery cell.
13 Negative electrode tab welding
The negative electrode nickel bar and the steel shell are welded using a spot welding machine. Welding strength must be ensured and incomplete welding is prohibited.
14 Laser Welding
When performing laser welding, the fixture should be carefully installed, and welding can only be carried out after the battery case and the top cover are properly fitted. Care should be taken to prevent welding deviation.
15-cell vacuum baking
Remark:
(1) The vacuum level of the vacuum system is -0.095 to -0.10 MPa;
(2) The protective gas is high-purity nitrogen, with a gas pressure > 0.5 MPa;
(3) Vacuum pump and nitrogen injection once per hour.
16. Injection volume: 2.9 ± 0.1 g
Relative humidity of the injection room: ≤30%, temperature: 20±5℃. Sealing tape: 6mm wide red tape. When applying the tape, be careful to wipe the electrolyte at the injection port clean. Use two rubber bands to fix the cotton at the injection port.
17. Transformation System
(1) Opening formation process
a) Constant current charging: 40mA×4h; 80mA×6h
Voltage limit: 4.00V
b) Perform a full voltage test. Batteries with a voltage ≥ 3.90V are sealed. Batteries with a voltage < 3.90V are subjected to a constant current of 60mA to 3.90~4.00V before sealing, and then subjected to steel ball injection.
c) Battery cleaning, using acetic acid and alcohol as the cleaning agent.
(2) Continued transformation into a system
Continue the conversion process as follows:
a) Constant current charging (400mA, 4.20V, 10min)
b) Dormancy (2 min)
c) Constant current charging (400mA, 4.20V, 100min)
d) Constant voltage charging (4.20V, 20mA, 150min)
e) Hibernation (30 min)
f) Constant current discharge (750mA, 2.75V, 80min)
g) Dormant (30 min)
h) Constant current charging (750mA, 3.80V, 90min)
j) Constant voltage charging (3.80V, 20mA, 150min)
(3) Detection of volume
Batteries should be classified according to the following levels of capacity:
After the batteries are removed from the cabinet, their voltage is checked. Batteries with a voltage <3.77V are recharged using a programmed charging method.
(1) Constant current charging (750mA, 3.80V, 10min)
(2) Sleep (2 min)
(3) Constant current charging (750mA, 3.80V, 30min)
(4) Constant voltage charging (3.80V, 20mA, 60min)
18 Battery Re-inspection
After the batteries are removed from the cabinet and subjected to capacity testing, they are placed at room temperature for 20 days before re-inspection. The steps are as follows:
a) Reshape the battery using a shaping machine;
b) Inspect all battery thickness, voltage, and internal resistance. The classification method is as follows:
II. Battery Manufacturing Process
1. (Positive and negative electrodes) Dry mixing → Wet mixing → Roller coating of paste onto conductive substrate → 3-step drying → Winding → Trimming (cut to a certain width) → Roll pressing → Winding (for later use). Dry mixing is performed using a ball mill, with glass balls or zirconia ceramic balls as the grinding balls.
Wet mixing is employed. A planetary mixer with blades mounted on 2-3 shafts provides better mixing. The amount of solvent in the wet mix must be appropriate to create a suitable rheological state for a smooth coating. The electrode paste applied by roller coating must maintain a certain viscosity, applied to both sides of the aluminum or copper foil. The coating thickness depends on the battery model. The coating is then dried sequentially through three heating zones. NMP (or water) evaporates from the coating with the flow of hot air or dry nitrogen, allowing the solvent to be recycled. Rolling is used to increase the coating density and ensure the electrode thickness conforms to the battery assembly dimensions. The pressure during the rolling stage must be moderate to prevent powder from scattering during winding.
2. Battery assembly
Assembly process of cylindrical battery: Insulating bottom ring into cylinder → winding cell into cylinder → inserting mandrel → welding negative current collector to steel cylinder → inserting insulating ring → rolling steel cylinder → vacuum drying → liquid injection → welding assembly cap (PTC element, etc.) to positive lead → sealing → X-ray inspection → numbering → formation → cycling → aging.
Square battery assembly process: Insulating bottom is placed into steel box → sheet-shaped combined cells are placed into cylinder → negative current collector is welded to steel box → sealing gasket is installed → positive current collector is welded to rod lead → combined cover (PTC element, etc.) is welded to swivel lead → combined cover positioning → laser welding → vacuum drying → liquid injection → sealing → X-ray inspection → numbering → formation → cycling → aging.
Assembly process description: Taking a cylindrical battery as an example (the basic process is the same for square batteries). Before winding the core into the cylinder, aluminum strips (0.08-0.15 mm thick, 3 mm wide) and nickel strips (0.04-0.10 mm thick, 3 mm wide) are ultrasonically welded to designated locations on the positive and negative conductive substrates as current collectors.
Battery separators are generally composed of PE/PP2 layers or PP/PE/PP3 layers. The separators are heat-treated at 120℃ to increase their barrier properties and improve their safety.
The positive electrode, separator, and negative electrode are stacked together and wound into a cylinder. Since a paste electrode is used, the paste material and the substrate must be well bonded to form a high-density electrode. In particular, it is necessary to prevent powder from falling off, so as to avoid it penetrating the separator and causing a short circuit inside the battery.
Before the wound battery cell is inserted into the steel cylinder, an insulating base is placed at the bottom of the steel cylinder to prevent short circuits inside the battery. This is the same for most batteries.
The electrolyte is generally LiPF6 and a non-aqueous organic solvent. Before vacuum liquid injection, the battery must be vacuum dried for 24 hours to remove moisture and humidity from the battery components, so as to prevent LiPF6 from reacting with water to form HF and shorten its life.
The battery sealing process involves applying sealant, inserting gaskets, and then rolling the edges and shrinking the cross-section—the basic principle is the same as for alkaline rechargeable batteries. After sealing, the battery is treated with a mixture of isopropanol and water to remove oil and spilled electrolyte, and then dried. An odor sensor or "sniffer" element is used to check for battery leakage.
After the entire battery assembly is completed, the battery's internal structure is inspected using X-rays to check for abnormalities such as misaligned cells, cracks in the steel casing, solder joints, and short circuits. Batteries with these defects are eliminated to ensure battery quality.
The final process is formation. During the first charge, a protective film called the solid electrolyte interphase (SEI) forms on the anode. This film prevents reactions between the anode and the electrolyte and is a key factor in the battery's safe operation, high capacity, and long lifespan. After several charge-discharge cycles, the batteries are aged for 2-3 weeks. Micro-short-circuited batteries are discarded, and the batteries are then sorted by capacity and packaged to become commercial products.
III. Battery Performance
1. Electrical properties:
(1) Rated capacity: 0.5C discharge, the discharge time of a single cell is not less than 2 hours, and the discharge time of the battery pack is not less than 1 hour and 54 minutes (95%);
(2) 1C discharge capacity: 1C discharge, the discharge time of a single cell is not less than 57 min (95%), and the discharge time of the battery pack is not less than 54 min (90%);
(3) Low-temperature discharge capacity: at -20°C, the discharge time of a single cell or battery pack at 0.5C is not less than 1 hour and 12 minutes (60%).
(4) High-temperature discharge capacity: At 55°C and 0.5C discharge, the discharge time of a single cell is not less than 1 hour and 54 minutes (95%), and the discharge time of the battery pack is not less than 1 hour and 48 minutes (90%);
(5) Charge retention and recovery capability: After being fully charged and left at room temperature for 28 days, the charge retention discharge time is not less than 1 hour and 36 minutes (80%), and the charge recovery discharge time is not less than 1 hour and 48 minutes (90%).
(6) Storage performance: The individual cells or battery packs used for storage tests should be selected from those manufactured less than 3 months ago. Before storage, they should be charged to 50%–60% capacity and stored for 90 days in an environment with an ambient temperature of 40℃±5C and a relative humidity of 45%–75%. After the storage period, the battery packs should be removed, fully charged at 0.2C, left to stand for 1 hour, and then discharged at a constant current of 0.5C to the termination voltage. The above test can be repeated 3 times, with a discharge time of not less than 1 hour and 12 minutes (60%).
(7) Cycle life: The battery or battery pack is charged at 0.2C and discharged at 0.5C for two consecutive cycles. The test is stopped when the discharge capacity is lower than 72 minutes (60%) for two consecutive cycles. The cycle life of a single cell is not less than 600 cycles and the cycle life of the battery pack is not less than 500 cycles.
(8) High-Temperature Shelf Life: The high-temperature shelf life test should be conducted on individual batteries manufactured within the last three months. Before shelf life, the batteries should be charged to 50% ± 5% capacity and then left at an ambient temperature of 55℃ ± 2C for 7 days. After 7 days, the batteries should be removed and left at an ambient temperature of 20℃ ± 5C for 2–5 hours. First, discharge the batteries at 0.5C to the termination voltage, then charge them at 0.2C after 0.5 hours. After resting for 0.5 hours, discharge them again at a constant current of 0.5C to the termination voltage. This capacity is used as the recovery capacity. This process is repeated for one week until the discharge time in a certain week is less than 72 minutes (60%), at which point the test ends. The shelf life should be no less than 56 days (8-week cycle).
2. Safety performance
(1) Continuous charging: Charge the individual cells with a constant current of 0.2 ItA. When the voltage of the individual cells reaches the charging limit voltage, switch to constant voltage charging and maintain it for 28 days. After the test, there should be no leakage, gas leakage, cracking, fire or explosion (equivalent to full charge float charging).
(2) Overcharging: Charge the individual cells with a constant current power supply at 3C. After the voltage reaches 10V, switch to constant voltage charging. Stop charging when the battery explodes or catches fire, or when the charging time is 90 minutes or the battery surface temperature stabilizes (temperature difference ≤ 2°C within 45 minutes). The battery should not catch fire or explode (3C 10V). Charge the battery pack with a constant current power supply at 0.5ItA. After the voltage reaches n×5V (n is the number of individual cells in series), switch to constant voltage charging. Stop charging when the battery pack explodes or catches fire, or when the charging time is 90 minutes or the battery pack surface temperature stabilizes (temperature difference ≤ 2°C within 45 minutes). The battery should not catch fire or explode.
(3) Forced discharge (reverse charging): First, discharge the single cell at a constant current of 0.2 ItA to the termination voltage, and then reverse charge the battery at a current of 1 ItA. The charging time should be no less than 90 minutes. The battery should not catch fire or explode. Discharge one single cell in the battery pack to the termination voltage, while the rest are fully charged. Then discharge the battery at a constant current of 1 ItA until the voltage of the battery pack is 0V. The battery should not catch fire or explode.
(4) Short-circuit test: Short-circuit a single cell externally for 90 minutes, or stop the short circuit when the cell surface temperature stabilizes (temperature difference ≤ 2°C within 45 minutes). The external circuit resistance should be less than 50mΩ, and the cell should not catch fire or explode. Connect the positive and negative terminals of the battery pack with copper wires with a resistance less than 0.1Ω until the battery pack voltage is less than 0.2V or the battery pack surface temperature stabilizes (temperature difference ≤ 2°C within 45 minutes). The cell should not catch fire or explode.
3. Mechanical properties
(1) Compression: Place a single cell between two compression planes and gradually increase the pressure to 13kN. For cylindrical cells, the compression direction is perpendicular to the longitudinal axis of the cylinder. For prismatic cells, compress the wide and narrow sides of the cell. Each cell can only be subjected to compression once. The test results should comply with the provisions of 4.1.2.1. Place a steel rod with a diameter of 15cm on the battery pack and compress the wide and narrow sides of the battery pack until it reaches 85% of the original size of the battery pack. Hold for 5 minutes. Each battery pack can only be subjected to compression once.
(2) Needle penetration: Place the single cell in a steel fixture and penetrate it with a steel nail of φ3mm～φ8mm from the direction perpendicular to the battery plate (the steel nail stays in the battery) for 90 minutes, or stop the test when the battery surface temperature is stable (temperature difference ≤2℃ within 45 minutes).
(3) Heavy object impact: Place the single cell on a rigid plane, and place a steel rod with a diameter of 15.8 mm flat in the center of the cell. The longitudinal axis of the steel rod is parallel to the plane. Let a weight of 9.1 kg fall freely from a height of 610 mm onto the steel rod in the center of the cell. When the single cell is cylindrical, the impact direction is perpendicular to the longitudinal axis of the cylindrical surface. When the single cell is square, the wide and narrow sides of the cell must be impacted. Each cell can only be impacted once.
(4) Mechanical impact; The battery or battery pack is rigidly fixed to the test equipment using a method that supports all fixed surfaces of the battery or battery pack. It is subjected to one impact of equal magnitude in three mutually perpendicular directions. At least one direction must be perpendicular to the wide face of the battery or battery pack. Each impact is performed as follows: within the first 3 ms, the minimum average acceleration is 735 m/s². The peak acceleration should be between 1225 m/s² and 1715 m/s².
(5) Vibration: The battery or battery pack is directly installed or mounted on the vibration table using a clamp for vibration testing. The test conditions are: frequency 10Hz～55Hz, acceleration 29.4m/s2, XYZ sweep frequency cycle number 10 times in each direction, and sweep frequency rate 1oct/min.
(6) Free fall: Drop a single cell or battery pack from a height (lowest point height) of 600mm onto a 20mm thick hardwood board on a concrete surface, once each in the XYZ directions. After the free fall is completed.
4. Environmental adaptability
(1) High temperature baking: Place the individual battery in a high temperature explosion-proof box and heat it to 130°C at a heating rate of (5±2C)/min, and keep it at this temperature for 10min.
(2) High temperature storage: Place the single cell or battery pack in an oven at 75±2C for 48 hours. The battery should not leak, leak gas, crack, catch fire or explode.
(3) Low pressure: (UL standard).
