1. Production process of battery cells
The production process of cylindrical batteries: front process, middle process and back process.
Pre-process : Process the positive and negative electrode materials and current collectors into a certain coating weight and size
The positive and negative electrodes
Middle process : Wind the pole piece and diaphragm made in the previous process into a winding core and then put it into the shell.
The electrolyte is injected and sealed to form a complete battery structure.
Post-process : An important battery process for the first charging, discharging and activation of cylindrical batteries
2. Stirring
1. Stirring: Prepare a uniformly dispersed and highly stable active material slurry to meet the processing requirements of the coating process
Positive and negative electrode slurry formula: active powder material, conductive agent, binder, additives and solvent
2. Factors affecting the mixing results:
Physical properties of solvents: density, viscosity and wettability of solvents with solid powders
Physical properties of solids: including density, particle size, geometric shape, etc.
Solid-liquid ratio: different fixed ratios and solid-liquid ratios
3. Geometric parameters of the equipment: groove diameter, geometric shape, shape and size of the agitator, number of agitator blades, size of the effective shear area
4. Process control points:
Mixer speed; Mixing time; Mixing temperature; Formula
5. Product control points:
Slurry viscosity: Rheometer, positive electrode: 3000-30000Pa.s negative electrode: 1000-15000Pa.s
Solid content: Take a small amount of slurry, mass W , apply it into a thin film in a container, dry the solvent at a certain temperature, and then weigh the mass w , then the solid content N=w/W , quickly measure the moisture meter, positive electrode: 50%-80% negative electrode: 40%-60%
Particle size: scraper fineness meter, positive electrode: 150-300 mesh , negative electrode: 100-200 mesh
Surface tension: The contact angle with the substrate directly affects the thickness of the coating edge; Spinning drop interfacial tension meter
3. Coating
Coating : A process of coating a slurry containing active materials onto a current collector.
Core control of coating: coating weight, coating length, coating width, coating appearance
Generally, there will be an online monitoring system, such as a β-ray online weighing instrument and a laser online thickness gauge.
Focus: Appearance inspection, no obvious pits, bubbles, powder loss, cracks and other abnormalities
3. Coating and drying: Evaporate the solvent to obtain a dry electrode sheet
Important process control parameters: baking temperature, baking time, wind speed
4. Roller Pressing
Rolling: The friction and positive pressure generated by the engagement of two rough rollers with the coated electrode sheet is used to make the electrode sheet move vertically and horizontally.
The role of roller pressing:
Low pole piece thickness: improve compaction density and energy density
Improve the electronic contact between active materials and conductive agents, between electrode powder and current collector, and effectively improve the conductivity
Improve the adhesion between electrode powder and current collector, improve electrode processing performance and battery life
3. Process control points: double roller gap, pressure, roller speed
4. Product control points: pole piece thickness, compaction density, appearance, general rolling equipment is equipped with online thickness measuring device
5. Striping
Slitting: According to the design requirements of the electrode sheets of the battery products, the positive and negative electrode sheets after rolling are cut into the designed width
Product control points: pole piece width; burr particle detection ( CCD equipment for microscopic image observation and detection)
Burr hazards:
When the film powder, particles and burrs appear in the electrode, the charging and discharging process is prone to form a micro short circuit, leading to self-discharge, which may seriously cause safety hazards.
Causes of burrs:
Cutter head wear, blade engagement accuracy, pole piece slitting tension, processing environment
6. Production
Production: In the cylindrical battery production process, the stripped pole pieces are cut and baked, and after the pole ears are welded on the pole piece collector, the positive and negative pole pieces that meet the design dimensions and the winding processing requirements are prepared.
Product control points: drying, removing moisture from the previous process, baking temperature and time
Tab welding: It is to weld a tab of a certain length to the uncoated empty foil area of the pole piece. The main function of the tab is to connect the positive and negative pole pieces with the positive and negative poles of the battery through the conductive tab, opening up the electronic channel between the positive and negative pole pieces and the external circuit.
7. Winding
Winding: The positive and negative electrodes and the separator are wound together in a spiral form to form a core structure in which the "negative electrode - separator - positive electrode" is periodically arranged alternately to obtain a pole piece structure that realizes the electrochemical function of charging and discharging of cylindrical batteries.
Process control points:
The length and width of the diaphragm are the largest to avoid short circuit caused by contact between positive and negative electrodes
Winding tension
Pole piece alignment correction
Final pole piece cutting
3. Product control points
Appearance; X- ray projection imaging winding alignment inspection
8. Shelling
Shelling: Assemble the wound core into the steel shell. In order to prevent the bottom of the steel shell from contacting the positive electrode aluminum foil and causing a short circuit, upper and lower gaskets are usually placed between the steel shell and the core. The lower gasket is left empty to facilitate welding of the negative electrode to the bottom of the steel shell.
When the winding core is assembled into the steel shell, the negative electrode tab is at the bottom of the shell and cannot be welded to the bottom of the shell by ultrasonic welding equipment.
Resistance welding can be done by passing the welding needle through the middle of the coil core.
2. Key process control points:
Welding needle pressure, welding time, current
3. Product key control points:
Tensile test
9. Drying
Baking: The electrolyte is like the blood in a lithium-ion battery. It is an important material that connects the positive and negative electrodes and enables battery charging and discharging. The lithium salt in the electrolyte is very sensitive to moisture. A small amount of moisture will cause hydrolysis, which will affect the internal resistance and life of the battery.
10. Injection
Injection: inject a fixed mass of electrolyte into the steel shell
Liquid injection process: battery vacuum pumping - pressurized liquid
injection - normal pressure static
Process control parameters: vacuum degree; moisture control in the environment
Product control point: injection volume. When the electrolyte is insufficient, it is difficult to ensure sufficient and uniform distribution in the isolation membrane and the electrode, which in turn affects the distribution of current density during charging and discharging, resulting in increased internal resistance of the battery and low capacity. When the injection volume is too much, the period cap packaging is more difficult and prone to leakage.
11. Welding
Welding: Before sealing, the battery tabs must first be welded to the battery cap to connect the positive and negative poles. Laser welding is usually used.
Process control parameters: welding energy, welding time
Product control points: no false welding, welding coke, welding slag, no tab breakage and bending, tensile test
Sealing: The welded cap is driven by the booster cylinder to lower the mold to the forming end point, and the lower mold is closed to complete the first, second and third sealing processes of the battery.
Process control parameters: sealing size, end height, sealing pressure
Product control points: whether the battery shell is broken, scratched, and sealed (random inspection)
12. Formation
Formation: The process of charging the assembled battery for the first time with a small current rate from an external power source to activate the active materials in the positive and negative electrodes of the battery.
The role of chemical formation:
( 1 ) Lithium ions enter the electrolyte and negative electrode from the positive electrode for the first time, activating the active materials
( 2 ) Some lithium ions form SEI on the surface of the negative electrode , stabilizing the interface between the negative electrode and the electrolyte, which is of great significance for improving the cycle life.
( 3 ) Through the first charge, some batteries with abnormal assembly, such as excessive moisture content or short circuit, are excluded.
( 4 ) During the first charging process, gas is generated. After the gas is generated, it is extracted to avoid bulging.
3. Key process control points: current size, formation temperature
13. Let it stand
Standing: Before capacity separation, the formed cylindrical batteries are usually left standing at room temperature for 7-21 days. The OCV of the cylindrical batteries is recorded before and after standing , and the K value is calculated by the voltage drop before and after standing . The defective batteries with large self-discharge are removed based on the K value results.
Critical process control point: temperature
Professional term explanation
( 1 ) OCV Open current voltage, open circuit voltage, the static voltage of a lithium-ion battery when there is no current and no polarization state.
( 2 ) K value is the battery voltage drop per unit time, which is often used to evaluate the self-discharge rate of lithium-ion batteries.
14. Capacity Division
Capacity classification: The process of classifying capacity through testing under a certain discharge system is called capacity classification.
Key process control points: charge and discharge current, voltage, charging time
The capacity division process first charges the battery to full power, then discharges it at a certain rate ( 0.5C or 1C ) to the battery discharge lower limit voltage ( 2.5V or 2.8V ) and records the discharge capacity.
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