The preparation of lithium-ion battery electrode slurries and the assembly of coin (or button) cells

1. Basic Introduction to Coin-Cell Batteries

Lithium-ion coin-cell batteries are mainly composed of the following parts: positive shell, negative shell, (positive/negative) electrode sheets, separator, gasket, spring, and electrolyte.

Commonly used coin-cell batteries include CR2032, CR2025, CR2016, etc. "C" represents a coin cell type, and "R" indicates the battery shape is round. The first two digits represent the diameter (in mm), and the last two digits represent the thickness (in 0.1 mm), with approximate numbers used for both. For example, the approximate dimensions of a CR2032 are 20 mm in diameter and 3.2 mm in thickness.

1.1 Battery Shell

The CR2032 coin-cell battery shell has a larger positive shell and a smaller negative shell with a mesh structure on the surface. Therefore, the assembly process generally starts with the negative shell.

CR2032 coin-cell battery positive shell (left), negative shell (right)

1.2. Electrode Sheet

The preparation process of the has a significant impact on whether the electrochemical performance can be fully realized.

Positive electrode sheet (left) and aluminum foil (right)

The preparation process for both positive and negative electrode sheets is the same, with the difference being that the positive electrode is coated onto aluminum foil, while the negative electrode is coated onto copper foil. Why is this the case?

Firstly, both materials have relatively good conductivity, are soft in texture, and are relatively inexpensive.

Secondly, aluminum is highly reactive. At low potentials, aluminum can undergo lithium intercalation, forming lithium-aluminum alloys, making it unsuitable as the current collector for the negative electrode. If aluminum foil is used as the current collector for the negative electrode, it will form an alloy with lithium, then powder, which severely impacts the battery’s lifespan and performance.

Finally, copper tends to oxidize at high potentials, making it unsuitable as the current collector for the positive electrode. The oxide layer on the copper surface is a semiconductor and allows electron conduction. When the oxide layer becomes too thick, the impedance increases. Additionally, lithium does not form lithium alloys with copper at the same location.

What makes a good electrode sheet ?

It should meet the following criteria: (1) The slurry coating is uniform, with no noticeable thickness variations, and especially in the thinnest areas, the aluminum foil is visible as a bright color; (2) The electrode sheet should maintain a complete round shape without damage, and there should be minimal burrs around the edges; (3) The coating area of the electrode sheet should be free of particles and there should be no significant powder shedding.

1.3 Separator

The separators used in the laboratory are generally Celgard 2400 or other products from the Celgard series, which are punched into small discs and used, with the diameter slightly larger than the positive and negative electrode sheets . The separator can be chosen based on the battery's performance requirements. For more details on the selection and parameters of separators, please refer to the later section on choosing lithium battery separators and understanding their parameters.

What is the function of the separator?

Simply put, without the separator, the positive and negative electrodes would directly contact each other, causing a short circuit. This is why some batteries need to suppress the formation of lithium dendrites, to prevent them from puncturing the separator and causing local short circuits, which could lead to safety incidents. Separators are typically made from polymer materials such as polyethylene, which are non-conductive. Their structure contains many micropores that allow lithium ions to pass through. Although they are insulators, the statement that they "do not allow electrons to pass through" is inaccurate.

1.4 Lithium Foil (i.e., Negative Electrode)

The diameter of the negative electrode sheet is slightly smaller than that of the negative shell. The diameter of the lithium foil for CR2032 is 15.8mm, and the corresponding positive electrode sheet is also 15.8mm. It is important to note that lithium foil is relatively soft and prone to deformation. Therefore, before assembling the battery, the deformed lithium foil can be flattened using the positive shell (since it is larger). Additionally, metallic lithium is highly prone to oxidation and deterioration in the air, and it can explode when exposed to water. Therefore, the metallic lithium foil purchased must be opened in a glove box, and care should be taken not to damage the gloves during the process.

Metallic lithium foil suitable for CR2032 batteries

1.5 Gasket

The gasket is a round aluminum piece with a diameter the same as that of the lithium foil. In experiments, different specifications and thicknesses can be purchased based on requirements.

Note: Components such as gaskets, positive and negative shells, etc., must be repeatedly cleaned with alcohol using ultrasonic cleaning before being dried in a blow-dryer or drying oven.

1.6 Spring Clip (Support Plate)

The spring clip primarily serves to support the battery. Without the spring clip, the battery would be compressed too much during the pressing process, potentially damaging the internal components. The spring clip is only added on the negative side. However, if spring clips are added to both the positive and negative sides, the battery cannot be sealed during the pressing process, which would expose the electrolyte to air and result in a failed experiment.

Spring Clip (Support Plate)

2.Assembly of Coin-Cell Batteries

2.1 Preparation of Positive Electrode Sheet

The preparation of the electrode sheet is mainly divided into two steps: (1) preparation of the slurry; (2) coating, drying, and pressing steps.

2.1.1 Preparation of Slurry (using Lithium Iron Phosphate as an example)

The slurry is composed of solvent, cathode material, conductive agent, and binder. We will introduce the different materials included in each category in later issues.

In the laboratory, the typical mass ratio of cathode material: conductive agent: binder is 80:10:10. Of course, this ratio can be freely adjusted, but the adjustment process requires a lot of trial and error. Generally speaking, the cathode material should not be less than 75%, and the conductive agent and binder should not be less than 5%. Sometimes, in order to achieve high-rate performance, there have been reports where the ratio of conductive agent reaches 40%.

If the prepared cathode material is limited, the three substances can be mixed in proportion. Using a pipette, NMP is added drop by drop, and then ground in a small mortar. During this process, attention should be paid to the amount of solvent NMP. If too much NMP is added, it can be appropriately dried under infrared light.

When the active material is more, take 0.4g of active material, and the corresponding amounts of conductive agent and binder are 0.05g. Use a 2020 or 2040 weighing bottle and sequentially add NMP, active material, conductive agent, and binder. The order of adding materials affects the final quality of the electrode sheet . According to the "Preparation Process Study of Coin-Type Lithium-Ion Batteries" and the experimental experience of some researchers, the best results are achieved by adding the materials in the following order:

Preparation of NMP and PVDF Solution

First, prepare the NMP and PVDF solution to save time when preparing the slurry each time. Three concentrations can be prepared: 0.02g/ml, 0.025g/ml, and 0.03g/ml, and the appropriate concentration for the material can be selected. The preparation method is simple: mix the two substances in a wide-mouthed bottle and stir using a magnetic stirrer until there are no white particles in the solution. It should be noted that after preparation, the wide-mouthed bottle should be sealed with sealing glue, as NMP is prone to absorb water or deteriorate.

Steps for Preparing the Slurry:

Step 1: Use a pipette to measure 2ml of 0.025g/ml NMP/PVDF solution and place it in a D15 stirring bar for magnetic stirring.

Step 2: Weigh 0.05g of conductive agent Super P and slowly add it to the weighing bottle. Stir for 20 minutes. During the addition, avoid allowing the conductive agent to touch the top of the bottle and ensure that it doesn’t spill out due to being added too quickly.

Step 3: Weigh 0.4g of active material and add it to the weighing bottle. The same precautions as above should be followed, and stir for 4-5 hours after adding the material. The stirring time is not fixed and should depend on the slurry’s viscosity.

Additionally, reports suggest that after magnetic stirring, performing 15 minutes of ultrasonic stirring results in better performance.

What is the Ideal Slurry State?

Generally, when gently shaking the weighing bottle, the mixture should neither be too viscous to flow nor too fluid like water without sticking to the walls. If it is too thick, add a drop of NMP and continue stirring; usually, one drop is enough. If it is too thin, the weighing bottle can be placed in a drying oven for a while.

Another method used by Senior Wang Qi for mixing materials is available, but it takes more time.

Note: Try not to reverse the order of steps 2 and 3.

2.1.2 Coating of the Electrode Sheet

Typically, a scraper and a casting coater are used for coating, with the cathode material coated onto aluminum foil and the anode coated onto copper foil. If a coating machine is unavailable, a glass plate and scraper can be used for coating. The coating process is relatively simple, but the following points should be noted:

(1) The aluminum foil should be flat, with as few wrinkles as possible; (2) Before coating, carefully clean the aluminum foil and the coating platform with alcohol and degreased cotton; (3) After cleaning with degreased cotton, use tissue paper to carefully clean once more to remove any cotton fluff and avoid scratching the aluminum foil.

2.1.3 Drying and Pressing of the Electrode Sheet

This step is listed separately because it requires strict execution, though the drying temperature can vary. Additionally, the quality of the electrode sheet can be measured by the degree of powder shedding. If powder shedding occurs easily, during battery cycling, active material may fall off the aluminum foil and dissolve in the electrolyte, causing the separator to turn black.

The purpose of drying is to remove the large amount of solvent NMP and moisture from the slurry, so two steps are required: blow-drying and vacuum drying. The specific temperature and time for each step vary in different studies, but it is important to note:

(1) The temperature for drying NMP does not need to be very high, but because there is too much solvent, more heat is needed, so the drying time is longer.

(2) Since the boiling point of water is 100°C, the temperature for blow-drying needs to be higher, but because the water content is relatively low, the drying time can be shortened. During blow-drying, two temperature stages can be set, with each stage having a different duration, and the highest temperature can be set to 100°C. Additionally, the drying temperature for the negative electrode should be lower than the positive electrode, as sometimes copper foil can oxidize.

Note: If the drying temperature is too high or the drying time is too long, severe powder shedding may occur. For blow-drying, the temperature for the positive electrode should not exceed 120°C, and the negative electrode should not exceed 90°C.

(3) After blow-drying, vacuum drying should be carried out. The temperature is generally set to 120°C, with a drying time of about 10 hours. However, vacuum drying should not be performed directly without blow-drying, as this would cause NMP to fill the vacuum drying box, resulting in poor drying. Although it is possible to skip vacuum drying, it is better not to omit this step if conditions allow.

2.1.4 Pressing

After coating, the composite material coating that has been dried is relatively loose. If used directly, it is prone to peeling off and being damaged after being soaked in electrolyte. Pressing treatment can be performed using a roller press or a tablet press, etc. A roller press can generally press the positive electrode coating to 15-60μm. The tablet press can use a pressure of about 80-120kg/cm². After pressing, the electrode’s stability, firmness, and electrochemical performance are improved, and its test results are better than those of unpressed samples. The main purposes of pressing are: (1) to eliminate burrs, smooth the surface, and prevent burrs from puncturing the separator during battery assembly, causing short circuits; (2) to enhance the strength of the electrode sheet and reduce the ohmic resistance. If the pressure is too high, it will cause the electrode sheet to curl, which is not conducive to battery assembly. If the pressure is too low, it will not achieve the desired effect.

The steps for cutting separators, electrode sheets , and calculating the active material content are omitted here, as they are relatively simple. If you have any questions, feel free to leave a message.

2.1 Assembly of Coin-Cell Batteries

2.1.1 Essential Items:

Inside the glove box: tablet press (preferably with a digital display), two tweezers (at least one should be plastic), a medicine spoon, electrolyte, lithium foil, ground glass bottle (with rubber-tipped dropper), syringe, dry tissues, and other cleaning supplies.

Outside the glove box: coin-cell battery shell, current collectors, spring clip (or foam nickel), positive electrode sheet , separator sheet.

Note: The battery assembly components must undergo a vacuum drying treatment for about 4 hours before being placed in the glove box. The temperature should not be too high, ideally set between 60-80°C. For larger glove boxes, these battery components can be pre-stored for cleanliness. After the components are placed into the glove box, strict exhaust-inlet operations must be followed, at least three times. It is recommended to place a small workbench inside the glove box to prevent reagents from corroding the glove box. Electrolyte can severely corrode both gloves and the inner walls of the glove box, so care should be taken to avoid operational mistakes.

2.1.2 Determining Water and Oxygen Content

Most glove boxes have a digital display for monitoring water and oxygen content. For example, the water and oxygen content in a Brian glove box can be controlled to below 0.05 PPM, but the standards may vary for different glove boxes. The battery assembly process should be conducted inside a glove box that has undergone strict exhaust-inlet operations, ensuring strict isolation from potential oxidation, humidity, and other interferences. If the water and oxygen content in the glove box remains high, check for glove damage or perform a regeneration of the glove box.

2.1.3 Assembly Process

There are two main sequences for assembling coin-cell batteries. In our laboratory, we generally start with the negative shell, but it can also be started with the positive shell. There is no right or wrong in this regard; it depends on personal preference.

3.Reasons for Some Issues

3.1 Causes of Low Open Circuit Voltage

(1) Burrs on the electrode sheet puncture the separator, causing a short circuit in the battery;

(2) During the battery assembly process, misalignment of the positive and negative electrodes leads to a short circuit;

(3) An error during the pressing step causes the battery assembly to be loose, resulting in poor contact between the positive and negative electrode shells and electrode sheets , leading to a short circuit.

3.2 Causes of High Electrochemical Impedance

(1) Insufficient amount of conductive agent;

(2) Small porosity of the separator, preventing lithium ions in the electrolyte from passing through smoothly;

(3) Decomposition of the electrolyte, reducing the lithium salt content.

4.Canrd Brief Introduce

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