Lithium-ion batteries are a well-known form of battery with a wide range of consumer products. These batteries are known to be used in several fields providing benefits to a lot of industries and markets. 

One of the forms includes lithium-ion coin cell battery which has so many different functions and mechanisms with the involvement of different coin cell materials and cases. The entire preparation phase of coin cell batteries is documented in this article.The success that these cells have reached is due to the excellent features that the lithium-ion batteries are capable of possessing. The features have already been enhanced by the introduction of a coin cell battery in the said field.

Introduction

A broad number of consumer products utilized lithium-ion batteries in a huge amount. The major goal for the energy storage researchers is to increase the lithium-ion battery's energy density as the rise comes in the utilization of electric vehicles and grid storage applications. One major way is developing new positive electrode materials that are stable at higher voltages or/and have higher specific capacities. These materials need to be developed into lithium-ion batteries for prolonged testing for investigating the way of the performance of these new materials.

Extremely reproducible data is offered by the commercially produced, machine-made pouch cells, which makes them suitable for the development of comparative electrolyte and performance evaluations with fixed negative and positive electrode materials.

Lithium-Ion Coin Cell Battery Process

Assembling of pouch cells

It was seen that the discharge capacities of a batch of 26 replicate commercially formed pouch cells which were first filled, then made, then degassed, and then, at last, were cycled 12 times was in an extremely narrow range (r.s.d. = 0.5%) and their intrinsic metrics' precision like coloumbic efficiency was even more. Although, the production of machine-made cells demands major resources as it is made in large batches. Thus, ordering a whole run of commercial cells is impractical for testing every new possible electrode material.

However, coin cells are made with hand-made electrodes to usually evaluate the new materials. Lithium metal cells (as lithium half cells) are used by most research labs with a lithium foil piece as the negative electrode and the new material as the positive electrode.

Half coin cells

The making of the half coin cells is comparatively easy. They can give extremely repeatable data. They cant do an accurate prediction of the performance of a material in an actual lithium-ion cell (known as a 'fuel cell' too). Primarily this is since there is a lot of lithium in the half cells, which can mask the complications with side reactions using up the lithium that's available. Therefore, favorable results might be provided by half cells that are made with a particular positive electrode material whereas, in reality, the performance of the full cell might be poor if the same particular material makes up the full cells.

Interactions between negative and positive electrodes

Determining the impact of the interactions between the negative and positive electrodes that would turn in a full lithium-ion cell from the data on a half lithium-ion cell is not possible. A way of making repeatable and accurate full coin cells will be valuable for predicting the accurate performance of the new electrode materials in lithium-ion batteries. Although, there has yet not been any establishment of reliable methods for making precise cells and there is a sparse discussion on the usage of the coin cell full lithium-ion cells in the literature.

Methods are being developed in the present work for making full coin cells by utilizing graphite as the negative electrode material. Researchers are provided with detailed instructions so that they can prepare full coin cells of high quality with good reproducibility between the cells. There can be a combination of this present paper's methods with the electrode fabrication methods that Marks et al. presented for completely evaluating lithium-ion battery materials in full coin cells.

Coin cell formation

With the negative material being graphite, and LiNi0.6Mn0.2Co0.2O2 (NMC622) as the positive material, full coin cells were formed. Generally, there are two construction designs that are based on the coin cell designs that are established and provide good precision for the lithium metal half cells. Previously, there have been descriptions of the selection of the spacer, construction of the cell, etc.

The electrodes that were made in the full coin cells were the same electrode materials as the pouch cells that were made by machines for being utilized in this work. The machine-made pouch cells were attained without any electrolyte (dry) from LiFUN technologies, in Zuzhou city, China. There was vacuum sealing of the pouch cells in China in a dry room without electrolyte and then it was shipped for utilization to Canada.

Punching of electrode disks

There was punching of the electrode disks from the electrode's one-sided coated region, extracted from the pouch-type cells with the punch diameter staying the same for all of the electrodes being utilized in the full coin cells. The punch diameter was 1.27 cm. Al2O3 is usually utilized for enhancing the stability of the positive electrode and it was used to coat NMC622 material. The coating on the positive electrode was pressed to 3.3 g/cm2 of density and 21.3 mg/cm2 of mass loading was possessed by the positive electrodes. The formulation of the electrode was 2% Super S carbon black conducting diluent by weight, 2% PVDF binder, and 96% active material. Marks et al. promoted a similar formulation of the electrode as this. A 13.2 mg/cm2 of loading was possessed by the negative electrodes of graphite and the coating was pressed to 1.55 g/cm2 of density.

Formation of Electrode

The formulation of the electrode was as follows; 2% Super S carbon black conducting diluent by weight, 2% CMC/SBR binder, and 96% active material. Air was punched into the electrodes and then they were heated over midnight in a glove box antechamber under vacuum at 110 ◦C before their transfer into the glove box. There was the assembling of the coin cells in the glove box. As discussed in the article's main text, a vacuum pen (Virtual industries, V8901) was used to help to construct some of the cells. In this work, the same electrolyte volume was used for all of the coin cells for keeping the electrolyte solution's ratio to the surface area of the electrode constant. The electrolyte was added between coin cells in each layer, dropwise from a syringe.

Mass of electrode drops

2.0 ± 0.1 mg of mass is possessed by one electrolyte drip from the syringes that were utilized. As compared to DMC- containing solvent blends, the DEC-containing solvent blends evaporate at a slower rate and that is why the chosen one as the solvent was EC/DEC. DMC-containing solvent blends possess good rheological characteristics but comparatively, they are volatile.

Using two Celgard separators for Configuration

Once the first Celgard layer was added to the cell, 6 drops were evenly placed on the top of the first Celgard layer and the positive electrode's top for the configuration with two Celgard separators. 4 drops are placed on top after the addition of the second Celgard layer. 3 drops are evenly placed across the negative electrode before the placement of the negative electrode on the separator’s top. Then, the negative electrode is added to the full coin cell in such a way that the positive electrode is in alignment with it. Thus, 19 drops × 2.0 mg/drop = 38 mg, could be the total electrolyte mass. 6 drops are evenly placed on the positive electrode’s top for configuration with one BMF (Polypropylene Blown Micro Fiber available from 3M Company) separator

BMF separator

Another 12 electrolyte drops were added to the separator's top once the BMF separator is added. 3 drops are evenly placed across the negative electrode before the placement of the negative electrode on the separator's top and then they add the negative electrode to the full coin cell in such a way that it's in alignment with the positive electrode. Here, 42 mg was the total electrolyte mass. The BMF separators are extremely compliant and soft. They are around 90% porous and have a thickness of 0.25 mm. Maccor 4000 Series automated test system (Maccor Inc., USA) was used to cycle all cells between 3.0-4.3 V in the temperature-controlled boxes (30.0 ± 0.1◦C). Cells were either cycled with a C/10 formation cycle followed by C/5 cycling or a C/5 formation cycle followed by C/3 cycling.

Disassembling of the cells

Some of the cells were disassembled after cycling for checking for the issues of electrode alignment for helping explanation of the observed testing behavior. 1% LiPO2F2 in a 1 mol L−1 LiPF6 solution in a 1:1 EC/DEC solvent blend by weight, and 1.00 ± 0.02 g of the electrolyte solution was used to fill the above-described pouch cells. A compact vacuum sealer (MSK-115A, MTI Corp.) was used to immediately seal the cells at -90 kPa gauge pressure and held suddenly at room temperature of 21-25 C at 1.5 V for preventing copper current collector’s corrosion during the wetting period of 24 hours that followed.

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Loading of the cells

Cells were loaded into the temperature-controlled boxes then. The cells were connected to a Maccor 4000 Series automated test system in the end. Soft rubber was utilized to clamp the pouch cells at 25 kPa pressure as the formation of gas was supposed to occur without production, which has been observed previously to considerably enhance the precision of the experiments. Cells were charged to 3.8 V and stopped following the first full cycle between ∼C/10 and between 2.8 to 4.3 V. Cells were open cut in an argon atmosphere glove box and they are then vacuum sealed for removing the gas that’s made during production. Cells were finally taken for cycling testing.

First step

This project's first step was building full coin cells following the conventional Li metal half-cell design. A Li-metal foil piece is utilized as the negative electrode when the conventional half cells are being made. Usually, the size of the Li foil disk is more than the working positive electrode or the negative electrode, allowing for a way larger error margin in the alignment of the electrode disks. Although, using the negative and positive electrode disks of the same size is optimal when fuel cells are being made. Thus, negative and positive electrode disks with the same diameter were utilized in this work. Additionally, the assembly method and the initial design were similar to that for the half coin cells.

Repeat full coin cells

Many repeat full coin cells were formed and used for prolonged cycling tests for determining the full coin cells’ reproducibility. Considerable variation was seen between the cells. These cells were then moved into an argon atmosphere glove box for considering the variance’s cause, then they are taken apart and photographed.

Alignment of the negative and positive electrodes

According to observations, negative and positive electrodes weren’t aligned exactly in cells III and II. The misalignment can be easily seen. According to more observations, there was an occurrence of the lithium plating in the areas where the negative electrode didn't cover the positive electrode. Plating lessens the available lithium inventory and thus the cell's available capacity is reduced.

Practical results

Out of 3 identical cells, after 100 cycles, cell III retained 80%, 90% was retained by cell II whereas 95% of its initial capacity was retained by cell I. Here we can see a direct relationship between the capacity loss with the extent of the misalignment of the electrode for these 3 cells. This method of assembling is not suitable for comparative electrode material testing. The first full coin cells’ low precision is because of the misalignment of the electrode disk. Thus it was found what was causing it and then it was prevented.

Cell’s Construction

Considerable enhancements were shown in repeatability and performance by the cells that were made with the help of a vacuum pen, although they still were not within the required limits of precision for the high-quality electrode materials comparisons. Then there were considerations of the role that the usage of two Celgard separators possessed. A cell goes a little curved when it is crimped. The pliable lithium foil bends with the cell when it's happening in the half cell. The bending maintains pressure and contact throughout the cell.

Negative electrode disk

Although, when fuel cells were being made, the negative electrode disk wasn’t that pliable which made people wonder whether the pressure was not even throughout the cell. As compared to the Celgard separators, BMF separators are more compressible and thicker. They were used for helping more evenly maintain the pressure within the full coin cells. According to the results, the reproducibility of the triplicate full coin cells was considerably increased by the usage of a single BMF separator. Glass fiber separators have been utilized by various researchers.

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Cycling behavior of the full coin cells

At last, there was a comparison of the performance of the machine-made pouch cells with the full coin cells' cycling behavior that were made with a single BMF separator and electrodes of equal diameter, and a vacuum pen was used to assemble them. Then under identical conditions, the same electrolyte was used to prepare cells.

There are two datasets for both types of cells:

i) In winter, the electrochemical tests are performed that are stopped after 100 cycles (A, C)

ii) in summer, electrochemical tests are performed for 200 cycles and then stopped (B, D).

This second testing was done in summer when the temperatures increased so that there is a slight rise in the temperature-controlled environments to above the 30◦C set point. Those variations in temperature further display the significance of controlling these experiments' every aspect. Also, the result shows that as compared to the first batch, dramatically superior precision is possessed by the full coin cells. These cells' reproducibility is enough for the testing of the comparative materials, with considerable benefits over the conventional half coin cell designs.

1. Working electrode’s Preparation

Make a ~6 wt. % polyvinylidene fluoride (PVDF) binder’s mixture in N-methyl-2-pyrrolidone (NMP). Weight 80 wt. % active material which is LiCoO2 in this case and 10 wt. % C black which is acetylene. 99.9+ %), and then for 1 minute, mix them in a vortex. Add NMP-binder mixture like that the binder constitutes 10 wt. % of the mixture’s total weight. Then transfer the mixture into a small glass vial and mix it for 30 minutes at maximum rpm in the vortex mirror. For better mixing, two 5mm diameter zirconia balls can be utilized as the media. Add more NMP if needed for achieving slurry of the desired consistency.

Spread the current collector’s metal foil on a glass plate. Typically, the current collector is copper for the anode and aluminum for the cathode. Acetone should be used and it should be made sure that no air bubbles are present between the glass plate and the foil. Masking tape’s two layers should be used for forming a track and defining the region that needs to be coated. A stainless steel spatula should be used to apply the slurry to the metal foil and then a razor blade is used to uniformly spread it on the track.

The coating should then be dried at 90-120 C in vacuum or air for around 2-8 hours. The time is adjusted based on the binder and material that’s utilized. The coated metal foil is then placed between the two steel plates (two weighing papers too for protecting the coating) and then a hydraulic press is used to press it under ~3000 lb of weight.

The dried coated metal foil should then be punched into 8mm of discs, inside a glovebox for instance. Then the cathodes should be weighed and wrapped before they are transferred into the glove box. Then the same material’s uncoated metal foil should be punched into 8 mm of discs and then those discs should be weighed.

2. Electrolyte’s Preparation

Store the electrolyte in a Nalgene bottle that's wrapped using an aluminum foil as the electrolyte is photosensitive.

3. Counter electrode’s preparation (in this case, Lithium foil)

Use a stainless steel scalpel/nylon brush to clean the lithium foil's surface and keep cleaning until there is the appearance of a shiny silvery surface (inside an argon glovebox). Lithium foil should then be punched into discs that are half-inch in diameter (inside an argon glovebox too)

4. Coin Cell Assembly

The Celgard C480 membranes are punched into 19mm of discs and they are then utilized as separators. Then the working electrodes, separators, spacers, springs (from MTI Corp.), and coin cell cases are transferred into the glove box (after using the argon five times to flush the exchanger).

5. Assembling the coin cells in the glove box

Place the working electrode on the cell cup after adding two drops of electrolyte. Then place two separators with 2 electrolyte drops between them. Now before you place the lithium counter electrode, add two more drops of electrolyte. Use the cell cap to close the cell and use the compact crimping machine (from MTI Corp.) to crimp 3-4 times. Use plastic tweezers to handle the finished cells after the cells are assembled. Now the cell is ready to be tested.

6. Representative Results

A coin cell was made as an example by utilizing LiCoO2 for the working electrode as the active material. The cell was tested at a C/5 rate after construction. For this coin cell, the voltage window was between 3-and 4.3 V.

The most important step in the working electrode’s preparation in our experience is forming good slurries with consistency.

Conclusion

As a result, the introduction of the coin cell batteries following the lithium-ion batteries has proved to be a successful step in the industrial field occupying lithium-ion batteries and related products. A lot of research has been conducted in the same regard and almost all have had a positive remark about the same. 

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References:

https://iopscience.iop.org/article/10.1149/2.1171902jes/pdf

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3476756/

https://www.targray.com/li-ion-battery/equipment/coin-cell-manufacturing

https://www.landtinst.com/how-to-make-coin-cells/

https://www.osti.gov/servlets/purl/1373751