I. Basic Introduction to Button Batteries
A lithium-ion button cell battery mainly consists of the following parts: positive electrode shell, negative electrode shell, (positive/negative) electrode sheets, separator, gasket, spring sheet, and electrolyte.
Commonly used coin cell battery casings include CR2032, CR2025, and CR2016. "C" represents the coin cell system, and "R" indicates the battery is circular. The first two digits represent the diameter (in mm), and the last two digits represent the thickness (in 0.1 mm), using the closest approximation. For example, the approximate dimensions of CR2032 are a diameter of 20 mm and a thickness of 3.2 mm.
1.1 Battery casing
The image below shows the CR2032 button cell battery casing. The positive electrode casing is larger, while the negative electrode casing has a mesh structure on its surface and is smaller. Therefore, the assembly process usually starts with the negative electrode casing.
1.2 electrode
The electrode fabrication process has a significant impact on the full realization of electrochemical performance, which we will discuss in detail in section 2.1. Here, we will only briefly introduce it. The figure below shows the electrode fabricated from the positive electrode material.
The preparation processes for the positive and negative electrodes are the same, the difference being that the positive electrode is coated on aluminum foil, while the negative electrode is coated on copper foil. Why is this?
First, both have relatively good conductivity, are relatively soft in texture, and are relatively inexpensive.
Secondly, aluminum is relatively reactive. At low potentials, aluminum will intercalate lithium, forming a lithium-aluminum alloy, which is unsuitable as a current collector for the negative electrode. If aluminum foil is used as the current collector for the negative electrode, the aluminum will form an alloy with lithium and then pulverize, severely affecting the battery's lifespan and performance.
Finally, copper is easily oxidized at high potentials, making it unsuitable as a current collector for the positive electrode. The oxide layer on the copper surface is a semiconductor, allowing electron conduction; however, if the oxide layer is too thick, the impedance will increase. Furthermore, lithium will not form a lithium-intercalation alloy with other materials at the same location.
What kind of high-quality film is considered a good high-quality film?
The following aspects should be met: (1) The slurry coating is uniform, and no obvious unevenness in thickness can be observed. In particularly thin areas, even bright aluminum foil can be observed; (2) The electrode remains intact and round without damage, and there are as few burrs as possible around it; (3) There are no particles in the electrode coating area and no obvious powder shedding.
1.3 Diaphragm
The separators used in the laboratory are generally Celgard 2400 or other products in the Celgard series, stamped into small discs with a diameter slightly larger than the positive and negative electrode plates. Separators can be selected according to different battery performance requirements. For information on separator selection and parameters, please see the later section on the selection of lithium-ion battery separators and the significance of their parameters.
What are the uses of a diaphragm?
Simply put, without a separator, the positive and negative electrodes would be in direct contact, leading to a short circuit. This is why some batteries suppress the formation of lithium dendrites—to prevent them from puncturing the separator, causing a localized short circuit, and resulting in a safety hazard. Separators are generally made of polymer materials such as polyethylene, which are non-conductive. Their structure contains many micropores that allow lithium ions to pass through. While it is an insulator, the statement that it does not allow electrons to pass through is inaccurate.
1.4 Lithium sheet (i.e., negative electrode)
The diameter of the negative electrode sheet is slightly smaller than that of the negative electrode shell. The diameter of the lithium sheet in CR2032 is 15.8mm, and the corresponding positive electrode sheet is also 15.8mm. It is worth noting that lithium sheets are relatively soft and easily deformed. Therefore, before installing the battery, you can use the positive electrode shell (because it is relatively large) to flatten any deformed lithium sheets. Furthermore, metallic lithium is extremely prone to oxidation and deterioration in air, and it can easily explode when exposed to water. Therefore, purchased metallic lithium sheets should be opened in a glove box, and care should be taken not to damage the gloves during opening.
Figure 3 shows lithium metal sheets suitable for use with CR2032 batteries.
1.5 gasket
The gasket is a circular aluminum sheet with the same diameter as the lithium sheet. In the experiment, different specifications and thicknesses can be purchased according to requirements.
Note: Gaskets, positive and negative electrode shells, and other components should be repeatedly ultrasonically cleaned with alcohol before use, and then dried in a forced-air drying oven.
Figure 4 Gasket
1.6 spring clip (support piece)
The spring clip is primarily used to support the battery. Without it, the battery would be crushed during the pressing process, potentially damaging internal components. The spring clip is only added to the negative terminal. However, if spring clips are added to both the positive and negative terminals, the clasp cannot be properly sealed during the pressing process, causing the electrolyte to come into contact with air and resulting in experimental failure.
Figure 5. Spare slab (support piece)
1.7 Electrolyte
Different materials generally require different electrolytes. When conducting experiments, never compromise on quality or quantity. Small batches of electrolyte can be obtained from the company; they usually provide them. However, certain specialized electrolytes, such as low-temperature electrolytes, may need to be purchased separately, and these can be quite expensive.
II. Assembly of button batteries
2.1 Preparation of the positive electrode sheet
The preparation of electrode sheets mainly consists of two steps: (1) preparation of slurry; (2) coating, drying, pressing and other steps.
2.1.1 Preparation of slurry (taking lithium iron phosphate as an example)
solvent
cathode materials
conductive agent
adhesive
N-Methylpyrrolidone (NMP)
Lithium iron phosphate
SuperP
Polyvinylidene fluoride (PVDF)
The slurry consists of a solvent, a positive electrode material, a conductive agent, and a binder.
In laboratories, the mass ratio of positive electrode material: conductive agent: binder is generally 80:10:10. This ratio can be adjusted, but this requires extensive trial and error. Generally, the ratio of positive electrode material should be no less than 75%, and the ratio of conductive agent to binder no less than 5%. Sometimes, to achieve high rate performance, there are reports of conductive agent ratios reaching 40%.
If the amount of positive electrode material prepared is small, the three substances can be mixed in proportion, and NMP can be added drop by drop with a pipette. Then, it can be ground in a small mortar. During this process, attention should be paid to the amount of NMP solvent. If too much NMP is added, it can be dried appropriately under an infrared lamp.
When there is a large amount of active material, take 0.4g of active material, and the corresponding amounts of conductive agent and binder are 0.05g. Use a 20*20 or 20*40 weighing bottle, and add NMP, active material, conductive agent, and binder in that order. The order of addition affects the final quality of the prepared sheet. Based on the research on the preparation process of button lithium-ion batteries and the experimental experience of some fellow researchers, adding the substances in the following order yields the best results.
Preparation of NMP and PVDF solutions
First, prepare solutions of NMP and PVDF. This saves a lot of time each time you prepare slurry. You can prepare three concentrations: 0.02 g/ml, 0.025 g/ml, and 0.03 g/ml, choosing the concentration appropriate for your materials. The preparation method is very simple: just mix the two substances in a wide-mouth bottle and stir magnetically until there are no white substances in the solution. Note that after preparation, the wide-mouth bottle must be sealed with sealing tape, as NMP is prone to absorbing moisture or deteriorating.
Slurry preparation steps:
Step 1: Use a pipette to measure 2 ml of 0.025 g/ml NMP/PVDF solution, place it in a D15 stir bar, and stir magnetically;
Step 2: Weigh 0.05g of the conductive agent SuperP and slowly add it to the weighing bottle, stirring for 20 minutes. During the addition process, try to avoid letting the conductive agent touch the upper side of the bottle wall, and do not add it too quickly, causing the conductive agent to spill out of the weighing bottle.
Step 3: Weigh 0.4g of the active substance and add it to the weighing bottle. The precautions are the same as above. After adding, stir for 4-5 hours. The stirring time is not fixed; the stirring time should be adjusted according to the viscosity of the slurry.
Other reports indicate that performing ultrasonic stirring for 15 minutes after magnetic stirring results in even better performance.
What is the best state for the slurry?
Generally, the mixture should be neither too viscous to flow nor too fluid to adhere to the sides like water when gently shaken. If it's too thick, add a drop of NMP and stir for a while; usually, one drop is sufficient. If it's too thin, dry the weighing bottle in a forced-air drying oven for a short time.
You can also use Wang Qi's ingredient method, but this method takes longer.
Note: Try not to reverse the order of steps two and three.
2.1.2 Electrode Coating
Generally, a doctor blade and a casting coating machine are used for coating. The positive electrode material is coated onto aluminum foil, and the negative electrode material is coated onto copper foil. Those without a coating machine can use a glass plate and a doctor blade for coating. The coating process is relatively simple, but the following points should be noted.
(1) The aluminum foil should be flat and wrinkles should be minimized. (2) Before coating, the aluminum foil and coating machine platform should be carefully cleaned with alcohol and degreased cotton. (3) After cleaning with degreased cotton, it should be carefully cleaned with toilet paper to remove any lint and avoid scratching the aluminum foil.
2.1.3 Drying and tableting of electrode sheets
The reason for listing this in a separate section is that it is a process that must be strictly followed, although the drying temperature can vary. In addition, the quality of the electrode sheets can be measured by the degree of powder shedding. If powder shedding is easy, the active material will fall off the aluminum foil during battery cycling and dissolve in the electrolyte, and the separator will be stained black.
The purpose of drying is to remove the large amount of solvent NMP and water from the slurry, so it involves two steps: forced-air drying and vacuum drying. The specific temperature and time for each step vary in different studies, but it is important to note that:
(1) The drying temperature of NMP should not be too high, but because there is too much solvent, a lot of heat is required, so the drying time is relatively long;
(2) Since the boiling point of water is 100℃, the temperature for forced-air drying needs to be relatively high. However, due to the low moisture content, the drying time can be shortened. Two temperature ranges can be set during forced-air drying, each with a different time. The highest temperature can be set to 100℃. Additionally, the drying temperature of the negative electrode should be lower than that of the positive electrode, as copper foil oxidation sometimes occurs.
Note: Excessive drying temperature and time will result in severe powder shedding. Regarding the temperature of the blower drying, the positive electrode should not exceed 120℃ and the negative electrode should not exceed 90℃.
(3) After air drying, vacuum drying is required, with the temperature generally set at 120℃ for about 10 hours. However, vacuum drying cannot be performed directly without air drying, as this will cause NMP to fill the vacuum drying chamber, resulting in poor drying effect. Vacuum drying is optional, but it is best not to omit this step if possible.
2.1.4 Tableting
After coating, the dried composite material coating is relatively loose. If used directly, it is prone to peeling and damage after being soaked in electrolyte. It can be pressed into tablets using a roller mill or tablet press. A roller mill can generally press the positive electrode coating to 15-60μm. A tablet press can use a pressure of approximately 80-120 kg/cm². The stability, firmness, and electrochemical performance of the pressed electrode are improved, and its test performance is better than that of unpressed samples. The important purposes of tablet pressing are twofold: first, to eliminate burrs, making the surface smooth and flat, preventing burrs from puncturing the separator and causing short circuits during battery assembly; second, to enhance the strength of the electrode and reduce ohmic impedance. Excessive pressure will cause the electrode to curl, which is detrimental to battery assembly; insufficient pressure will not achieve the purpose of tablet pressing.
The steps for cutting the diaphragm, electrode sheets, and calculating the active material content are omitted here as they are relatively simple. If you have any questions, please leave a message on the Materials People WeChat account or the Materials Bull website.
2.1 Assembly of button cells
2.1.1 Essential Items:
Inside the glove box: tablet press (preferably with digital display), 2 pairs of tweezers (at least one of which should be plastic), 1 spatula, electrolyte, lithium tablets, ground glass bottle (with dropper), syringe, dry paper towels, and other cleaning supplies;
Outside the glove box: button cell casing, current collector, spring sheet (or nickel foam), positive electrode plate, separator;
Note: Battery assembly components should undergo vacuum drying for approximately 4 hours before being placed in the glove box. The temperature should not be too high, ideally set between 60-80℃. Larger capacity glove boxes can store battery components beforehand for better hygiene. After materials enter the glove box, strictly follow the operating procedures for venting and energizing at least three times. It is recommended to place a small workbench inside the glove box to prevent reagents from corroding it. Electrolyte is highly corrosive to gloves and the inner walls of the glove box; therefore, care should be taken to prevent operational errors.
2.1.2 Determining the water oxygen content
Most glove boxes have digital displays to monitor water and oxygen levels. Brian's glove boxes can control water and oxygen levels below 0.05 PPM, but standards vary between different glove boxes. The battery assembly process takes place in a glove box that has undergone strict venting and intake procedures, rigorously isolating it from any potential oxidation, moisture, or other interference. If the water and oxygen levels in the glove box remain high, the gloves should be checked for damage, or the glove box should be regenerated.
2.1.3 Assembly process
There are two important order for assembling button cells. In our lab, we usually start with the negative electrode casing, but you can also start with the positive electrode casing. There is no right or wrong answer; it all depends on personal preference.
| Negative electrode shell | Spring | Gasket | Lithium sheet | Electrolyte | Separator | Electrolyte | Positive electrode sheet | Gasket | Positive electrode shell |
Step description
Step-by-step instructions
1
The positive electrode shell opening faces upwards, flat.
Place on a glass plate
No explanation needed
2
Use tweezers to place the pad and positive electrode plate into the positive electrode shell in sequence, with the positive electrode plate in the center.
Using tweezers, place the pad into the positive electrode shell with the burr side facing down. Then, carefully pick up the positive electrode sheet, placing it with the coating layer facing up in the center of the positive electrode shell. This step should be practiced repeatedly to ensure that the tweezers apply the appropriate force to avoid damaging the positive electrode sheet. Take care not to bend or twist the positive electrode sheet, and keep it flat in the positive electrode shell.
3
Use a dropper or syringe to draw up the electrolyte and wet the surface of the positive electrode.
Use an extremely fine glass dropper to carefully draw up a small amount of electrolyte, aiming to completely and evenly wet the electrode surface. Note that during the wetting process, the dropper/needle and the electrode must not touch.
4
Clamp the separator and cover the positive electrode plate.
Use tweezers to pick up the separator. Since the cut separator has the same diameter as the inside of the battery casing, it can be inserted perfectly into the positive electrode casing. This step requires extra care; do not allow the separator to come into contact with the electrolyte prematurely. Align the separator with the edge of the battery casing first, then slowly withdraw the tweezers, evenly covering the electrolyte.
Down
5
Use a dropper/syringe to draw up electrolyte again and wet the diaphragm surface.
Since the separator is an inert and clean material, you can gently touch it with the tip of the dropper to make it smoother, more even, and ensure a tighter seal between the edges and the battery casing. Try to prevent wrinkles in the separator.
6
Lithium sheet is clamped and placed on the separator.
middle
The lithium sheet, with a radius of 15.8mm, must be placed precisely in the center of the battery casing. This is the most difficult step and must be completed on the first attempt. Because the lithium sheet adheres to the electrolyte and separator, misplacement makes adjustment extremely difficult, essentially meaning the failure of the simulated battery assembly (although this isn't always the case; adding a little more electrolyte can allow for fine-tuning).
7
Place the clamping pad onto the lithium sheet, ensuring precise alignment.
If the shim is slightly misaligned at this step, it can be carefully adjusted.
8
Pick up the clip and place it on the pad, aligning it precisely.
Use tweezers for all steps whenever possible. If the component is accidentally misplaced, this step allows for minor adjustments. If the component's position is incorrect in previous steps, use tweezers in both hands to make slight adjustments. If tweezers are inconvenient to use, a spatula can be used.
9
Tweezers to pick up the negative electrode shell cover
No explanation needed
III. Reasons for Some Problems
After battery assembly, some issues may arise during testing. Below are some potential problems and explanations.
3.1 Reasons for low open-circuit voltage
(1) Burrs on the electrode sheet puncture the separator, causing a short circuit in the battery;
(2) During battery assembly, the positive and negative terminals are misaligned, causing a short circuit;
(3) Errors in the battery pressing process resulted in loose battery assembly, and the positive and negative battery casings were not properly connected to the positive and negative electrode plates, causing a short circuit.
3.2 High electrochemical impedance
(1) Insufficient amount of conductive agent added;
(2) The membrane has low porosity, which prevents lithium ions in the electrolyte from passing through smoothly;
(3) The electrolyte decomposes, and the lithium salt in it decreases.
Battery assembly is a process that requires practice to master. It's perfectly normal to break a few batteries the first couple of times, so don't be discouraged.
Second, pouch batteries
Soft-pack cells are essentially battery cells that use aluminum-plastic packaging film as the packaging material. Relatively speaking, lithium-ion battery packaging falls into two main categories: soft-pack cells and metal-cased cells. Metal-cased cells include steel-cased and aluminum-cased cells, etc. In recent years, some cells with plastic casings due to special requirements can also be classified into this category.
The difference between the two, besides the different outer casing materials, also determines their different packaging methods. Pouch cells use heat sealing, while metal-cased cells generally use welding (laser welding). The reason pouch cells can use heat sealing is because they use an aluminum-plastic packaging film.
Aluminum-plastic packaging film
The composition of aluminum-plastic packaging film (hereinafter referred to as aluminum-plastic film) is shown in the figure. Its cross-section consists of three layers: nylon layer, Al layer and PP layer.
Each of the three layers has its own purpose. First, the nylon layer ensures the shape of the aluminum-plastic film, preventing it from deforming before it is manufactured into a lithium-ion battery.
The Al layer is composed of a single layer of metallic Al, and its purpose is to prevent water penetration. Lithium-ion batteries are very sensitive to water, and the water content of the electrodes is generally required to be in the PPM range, so the packaging film must be able to block moisture penetration. Nylon is not waterproof and cannot provide protection. Metallic Al reacts with oxygen in the air at room temperature to form a dense oxide film, preventing moisture penetration and protecting the inside of the battery cell. The Al layer also provides plasticity for perforation during the aluminum-plastic film molding process, as detailed in point 3.
PP is an abbreviation for polypropylene. This material is characterized by melting at temperatures above 100 degrees Celsius and has a sticky consistency. Therefore, the heat sealing of batteries relies heavily on the PP layers melting and bonding together under the heating of the end cap. After the end cap is removed, the material solidifies and bonds upon cooling.
Aluminum-plastic film looks simple, but in practice, bonding the three layers of material evenly and firmly together is not an easy task. Unfortunately, most high-quality aluminum-plastic film is currently imported from Japan. While domestically produced versions exist, their quality needs improvement.
Aluminum-plastic film forming process
Soft-pack battery cells can be designed in different sizes according to customer needs. Once the external dimensions are designed, a corresponding mold needs to be made to form the aluminum-plastic film. The forming process is also called punching (I personally think it should be punching, but since everyone writes it that way, let's go with the common usage). As the name suggests, it involves using a forming mold to punch a hole in the aluminum-plastic film under heating to accommodate the core. See the picture below for details.
After the aluminum-plastic film is punched and cut into shape, it is generally called a Pocket bag, as shown in the picture below. Generally, when the battery cell is thin, a single dent is punched (left in the picture below), and when the battery cell is thick, a double dent is punched (right in the picture below), because if the deformation on one side is too large, it will exceed the deformation limit of the aluminum-plastic film and cause it to break.
Sometimes, depending on the design requirements, a small indentation will be punched at the location of the air bag to increase its volume.
Top and side sealing process
Finally, we've gotten to the main topic (you're really good at going off-topic!). The top-side sealing process is the first packaging process for soft-pack lithium-ion cells. Top-side sealing actually includes two processes: top sealing and side sealing. First, the wound core is placed into the punched indentation, and then the packaging film is folded in half along the dotted line, as shown in the picture below.
The diagram below shows the sealing areas after the aluminum-plastic film is loaded into the roll core, including the top sealing area, side sealing area, first sealing area, and second sealing area. These will be described in detail below.
After placing the roll core into the pit, the entire aluminum-plastic film can be placed into the fixture for top and side sealing in the top and side sealing machine. The top and side sealing machine works as follows:
1506415991871021742.png
The top and side sealing machine shown in the picture has four clamps. The station on the left is for top sealing, and the station on the right is for side sealing. The two yellow metal pieces are the upper sealing heads, and there is a lower sealing head below them. During the sealing process, the two sealing heads are heated to a certain temperature (usually around 180℃). When they are closed, they are pressed onto the aluminum-plastic film, and the PP layer of the aluminum-plastic film melts and then adheres together, thus completing the sealing.
There's not much to say about the side seal (we'll skip the side voltage stuff, it's a long story), but let's focus on the top seal. A schematic diagram of the top seal area is shown below. The top seal is responsible for sealing the tabs, which are metal (aluminum for the positive electrode, nickel for the negative electrode). How are they integrated with the PP package?
This is accomplished using a small component on the tab: tab adhesive. I'm not entirely clear on the specific structure of the tab adhesive, and I hope someone knowledgeable can provide further information. I only understand that it also contains PP, meaning it melts and bonds when heated. The encapsulation at the tab position is shown in the circled area in the image below. During encapsulation, the PP in the tab adhesive melts and bonds with the PP layer of the aluminum-plastic film, forming an effective encapsulation structure.
Liquid injection and pre-sealing processes
After the top side of the pouch cell is sealed, an X-ray inspection is performed to check the parallelism of the core. Then, it goes into a drying room to remove moisture. After standing in the drying room for a certain period of time, it enters the liquid injection and pre-sealing process.
As we learned from the above introduction, after the top-side sealing of the battery cell is completed, only one opening remains on the air bag side. This opening is used for liquid injection. After liquid injection, the air bag side must be pre-sealed immediately, also known as one-sealing. After one-sealing, the battery cell is theoretically completely isolated from the external environment. The one-sealing principle is the same as that of top-side sealing, so it will not be repeated here.
Settling, chemical formation, and fixture shaping processes
After electrolyte injection and sealing are completed, the battery cell must first be left to stand. Depending on the process, this can be divided into high-temperature standing or room-temperature standing. The purpose of standing is to allow the injected electrolyte to fully wet the electrode sheets. Then the battery cell can be used for formation.
The image above shows the formation cabinet for pouch cells, which is essentially a charging and discharging device. I searched for a long time but couldn't find a picture with the cells attached, so just imagine the cells clipped to it. Formation is the initial charging of the cells, but it doesn't charge them to the maximum usable voltage, and the charging current is very small.
The purpose of formation is to create a stable SEI film on the electrode surface, which is essentially an activation process for the battery cell. During this process, a certain amount of gas is generated, which is why an air bag is reserved in the aluminum-plastic film. Some manufacturers use a fixture-based formation process, where the battery cell is clamped in a fixture (sometimes a glass plate with steel clamps is used for simplicity) before being placed in a cabinet for formation. This way, the generated gas is fully squeezed into the adjacent air bag, resulting in a better electrode interface after formation.
After formation, some battery cells, especially thicker ones, may deform due to higher internal stress. Therefore, some manufacturers will set up a shaping process after formation, also known as fixture baking.
Second sealing process
As mentioned earlier, gases are produced during the formation process, so we need to extract the gases before performing a second sealing. Some companies call this two processes: Degassing and secondary sealing, plus a later process of cutting the gas bag. I'll just refer to them all collectively as secondary sealing.
During the second sealing process, the gas bag is first punctured with a guillotine while a vacuum is drawn, extracting the gas and a small portion of the electrolyte from the bag. Then, the second sealing head is immediately applied to the second sealing area to ensure the cell's airtightness. Finally, the sealed cell is cut away from the gas bag, and a basic soft-pack cell is formed. The second sealing is the final packaging process for lithium-ion batteries, and its principle is the same as the previous heat sealing, so it will not be described again.
Subsequent processes
Since the questioner asked about packaging, and the subsequent steps are not closely related to packaging, I will discuss the processes after the second packaging together.
After the air bag is cut at the second seal, the edges need to be trimmed and folded. This involves trimming the first and second seal edges to the appropriate width and then folding them to ensure the width of the battery cell does not exceed the standard. After folding, the battery cell can be sent to the capacity testing cabinet for capacity testing, which is essentially a capacity test to see if the battery cell's capacity has reached the specified minimum value.
In principle, all battery cells must undergo capacity testing before leaving the factory to ensure that cells that fail to meet capacity standards are not delivered to customers. However, when battery cell production is high, some companies may conduct partial capacity testing to determine the pass rate of the batch of cells based on statistical probability.
