Battery Technology | Button Cell Battery Manufacturing Process (Includes Practical Testing Tutorial)

Introduction:

Button cells are one of the commonly used battery types in laboratories, widely applied in material development, electrochemical performance testing, and other fields. This article provides a detailed introduction to the complete process of button cell electrode preparation and battery assembly, including required materials, equipment, steps, and precautions, offering comprehensive guidance for relevant researchers.

一. Overview of Button Cells

Button cells consist of a set of button cell casings and internal components. Stainless steel battery casings are widely used in laboratory testing due to their good electrochemical stability, excellent sealing, small size, simple assembly, and low cost. Common models include CR2032, CR2025, CR2016, etc., with CR2032 battery casings (20mm in diameter, 3.2mm in thickness) often used in laboratories. Additionally, there is a reusable Swagelok battery, with a stainless steel casing and PTFE liner, suitable for battery disassembly and analysis.

A set of CR2032 battery casings includes: positive and negative shells, snap springs, and washers. The basic steps for assembling a button cell include: slurry preparation, coating, drying, cutting, and assembly. The following text will detail the process using the CR2032 battery casing as an example.

二．Electrode Preparation:

Electrode preparation consists of two key stages: mixing and coating.

(1) Mixing Process Selection Criteria:

Use the mass threshold method to decide the process path:

Manual grinding method (0.1-5.0g range): Suitable for small amounts of active materials (such as preliminary screening of new electrode materials).

Mechanical mixing method (>5.0g): Achieved through laboratory mixers for slurry homogenization.

(2) Coating Technology Adaptation Principles:

Dynamically select based on slurry volume:

Manual coating: Standard approach (slurry volume <10mL), requires controlling coating speed at 0.5-1.0 mm/s.

Mechanical coating: Use a small coating machine (slurry volume ≥10mL), with blade gap precision of ±5μm.

1. Slurry Material Preparation:

(1) Active Material: Cathode and anode materials are generally in powder form, and particle size should not be too large (for laboratory research, the maximum particle diameter should not exceed 50μm; for industrial applications, it should not exceed 30μm). If there are larger agglomerated particles, grinding is necessary.

(2) Conductive Agent: Common carbon-based conductive agents such as acetylene black (AB), conductive carbon black, Super P, etc.

(3) Binder: Commonly used PVDF system or PTFE system, as well as SBR emulsion.

(Common mass ratio: active material: conductive agent: binder = 8:1:1 (or 8:1.5:0.5). This ratio can be adjusted according to the materials, but generally, the cathode material should not be less than 75%, and the conductive agent and binder should not be less than 5% each.)

2.Slurry Preparation Steps:

(1) Prepare the Solution: Add PVDF to NMP and stir below 50°C until fully dissolved, resulting in a clear, transparent solution.

(2) Add Conductive Agent: Weigh out the conductive agent and slowly add it into the weighing bottle, stirring for 20 minutes.

(3) Add Active Material: Weigh the active material and add it to the weighing bottle, stirring for 4-5 hours until the slurry reaches a thick consistency.

(Note: During the addition of the conductive agent and active material, try to avoid contact with the sides of the bottle.)

3.Electrode Coating:

(1) Current Collector Selection: For lithium-ion battery electrodes, aluminum foil is used for the cathode and copper foil for the anode. If the foil is smooth on one side, the coating should be applied to the rough side to enhance the adhesion between the current collector and the material.

Coating Method: Use a doctor blade or a casting machine for coating, ensuring the material is even and clean. It is recommended to thoroughly clean both the materials and equipment with alcohol and degreased cotton before coating.

4.Electrode Drying and Pressing:

● Electrode Drying:

(1) To remove the large amount of solvent NMP and any moisture in the slurry, a combination of blower drying and vacuum drying is necessary. Note that the baking temperature for NMP should exceed 100°C; however, it is advisable to lower the baking temperature and extend the baking time to ensure effective drying.

(2) When using blower drying, the highest temperature can be set at 100°C to eliminate moisture. Due to the low moisture content, the drying time can be shortened. Two temperature stages can be set during blower drying, with differing durations for each stage. 

   Note: The drying temperature for the anode should be lower than that for the cathode to prevent copper foil oxidation. For blower drying, the cathode should not exceed 120°C, and the anode should not exceed 90°C. The drying time should not be too long to avoid powder loss.

(3) After blower drying, vacuum drying is recommended, generally set at 120°C for about 10 hours.

Note: Do not skip blower drying and go directly to vacuum drying, as this will lead to NMP filling the vacuum drying chamber, affecting the drying efficacy. Alternatively, static drying can be used but it requires extended time.)

● Electrode Pressing:

Use a roller press or a flat press for pressing. The roller press can compress the cathode coating to a thickness of 15-60μm. The flat press can apply a pressure of approximately 80-120 kg/cm²

5. Electrode Cutting and Weighing:

● Cutting: Use a punch press to cut small electrodes, adjusting the diameter according to the battery case model.

● Weighing: Use a high-precision balance to measure the mass of the electrodes and record the data.

Battery Assembly

1. Material Preparation

(1) Assembly Components: Anode case, lithium metal sheet, separator, gasket, spring sheet (foam nickel), cathode case, electrolyte.

(2) Tools: Compression mold, pipette, insulated tweezers.

2. Assembly Steps (Anode case | Spring sheet | Gasket | Lithium sheet | Electrolyte | Separator | Electrolyte | Cathode sheet | Gasket | Cathode case)

(1) Place the Cathode Case and Gasket: Position the cathode case with the opening facing upwards, and place the gasket with its rough side facing down.

(2) Insert the Cathode and Wet: Place the cathode with its coated side facing up in the center of the cathode case and wet it with electrolyte.

(3) Cover with Separator and Wet: Use tweezers to place the separator over the cathode and wet it again with electrolyte.

(4) Place the Lithium Sheet and Gasket: Position the lithium sheet with the smooth side facing down, aligning the gasket.

(5) Place the Spring Sheet and Anode Case: Align the spring sheet and cover with the anode case.

(6) Seal the Battery: Use a sealing machine to compress and seal the battery, ensuring the appearance is intact.

四．Coin Cell Testing

1. Active Material Mass Calculation Method:

   Active material mass = (electrode mass - foil area density * electrode area) * loading.

2. Resting: After the coin cell assembly is complete, it needs to rest for 4 hours before testing (for some membranes and electrolytes with poor wettability, additional resting time may be required).

3. Test Procedures:

   The Neware multi-channel battery testing system integrates various operating modes: 

   (1) Charging modes: constant current charging, constant voltage charging, constant current-constant voltage charging, constant power charging; 

   (2) Discharge modes: constant current discharge, constant voltage discharge, constant current-constant voltage discharge, constant power discharge, constant resistance discharge; 

   (3) Direct current internal resistance (DCIR) testing; 

   (4) Cycle testing; 

   (5) Cycle nesting: features nested cycling; 

   (6) CT-8002S-5V100mA-124 supports cyclic voltammetry testing, supporting up to 3 levels of nesting.

4. Testing Parameter Setup

Example 1: Lithium-ion Coin Cell Cycling Performance Step Setup

(1) Select the constant current cycling mode in the Neware BTS software.

(2) Test the assembled battery within the appropriate voltage range at a current density of 0.1 A/g. (Assuming the active material mass is 0.01g, the test current is set to 1mA)

(3) The setting of the voltage range for the cathode depends on the electrode material (for example, the test range for lithium iron phosphate//Li batteries can be set to 2~4.2V). This voltage window can be determined by reviewing literature or testing polarization curves.

(4) When using anode materials, the battery usually undergoes a discharge step first, following the sequence "resting - constant current discharge - constant current charging steps." Input a current of 1mA, with a discharge cut-off voltage of 0.01V, a charging cut-off voltage of 2.0V, and a cycle count set to 2000.

The test voltage range in the screenshot below is for reference only; specific voltage ranges should be determined based on the specific materials used.

Example 2: Lithium-ion Coin Cell Rate Performance Step Setup

(1) Allow the battery to stabilize internally by resting it for 1 hour, then select the constant current rate mode in the Neware BTS software.

(2) Input the theoretical specific capacity and active material mass to automatically calculate the current value for constant current charging and discharging. (For example, the theoretical specific capacity for lithium iron phosphate is 170mAh/g, and set the active material mass to 2mg).

(3) Set rate gradients as 0.1C, 0.2C, 0.5C, 1C, 2C, 5C, 10C, 20C, and 1C.

(4) Refer to literature to determine the voltage window, which is 2.0~4.2V. Set the charge cut-off voltage to 4.2V and discharge cut-off voltage to 2.0V, with 10 cycles at each rate. Click start after completing the step parameter settings.

五．.Precautions

1. Material Selection and Handling

   (1) Lithium Metal Sheet: Purity not less than 99.9%, diameter and thickness chosen as needed.

   (2) Separator: Select an insulating membrane with nanopores, larger than the lithium sheet and electrode.

   (3) Electrolyte: Choose according to experimental needs, generally using an excess amount.

2. Electrode and Battery Selection

   (1) Electrode Selection: Surface must be smooth, with no obvious material loss, mass, and thickness uniform.

   (2) Battery Selection: Case must be smooth and undamaged, open-circuit voltage normal.

3. Operational Precautions

   (1) Prevent Short Circuits: Use insulated tweezers to avoid contact between positive and negative electrodes.

   (2) Cleaning and Drying: Components must be cleaned and dried to avoid contamination.

   (3) Assembly Quantity: The number of cells assembled with the same material should be at least 5, considering error and operational mistakes.

六. Common Issues and Causes

1. Low Open-Circuit Voltage

   Causes: Electrode burrs piercing the separator, misalignment of electrodes, battery assembly not tight.

2. High Electrochemical Impedance

   Causes: Insufficient conductive agent addition, low separator porosity, electrolyte decomposition.

七．Conclusion

Summary: Coin cell electrode preparation and battery assembly is a complex and delicate process requiring strict control of conditions and operations at each stage. This guide aims to provide practical guidance and assistance to researchers, improving the efficiency of coin cell preparation and testing.

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