Do you know how does a button battery cell CR2032 produce?
Button batteries consist of a complete set of button batteries and internal components. Stainless steel batteries have good electrochemical stability, good sealing performance, small size, simple assembly, low price and suitable temperature of 40-80 C, which are suitable for a large number of tests.
Recently, domestic and foreign enterprises began to develop high-throughput button battery automatic assembly equipment for batch accelerated verification and research of key battery materials. CR2032, CR2025 and CR2016 are the common button battery cells. CR2032 battery cells are commonly used in laboratories (i.e. 20 mm in diameter and 3.2 mm in thickness). The button cell shell is scrapped, and the metal recovery link is needed to avoid wasting and polluting the environment.
A set of CR2032 battery shells includes negative shell, shrapnel and two gaskets. The basic steps of assembling a button cell include: pulping, coating, drying, cutting and assembling. The following are explained in detail.
1. Preparation of polar plates
The preparation process of polar plates in laboratory can be divided into two steps: mixing and coating. The mixing process mainly includes manual grinding and mechanical mixing, while the coating process includes manual and mechanical coating. When mixing in laboratory, manual grinding method or mechanical mixing method should be adopted according to the amount of material supplied. For example, manual grinding method is recommended when the mass of active material is 0.1-5.0 g. When the mass of active material exceeds 5.0 g, laboratory mixer is recommended for mixing. In the laboratory, the mixing quantity is limited, so manual coating is often used. When the slurry is enough, a small coating machine can be used. The whole process of making polar plates needs to be carried out in a dry environment, and the materials and equipment used need to be kept dry. Fig. 1 is the process of preparing electrodes by manual mixing and coating, including material preparation, weighing and grinding of active materials and conductive agents, adding binders, slurry grinding, manually coating the electrodes with slurry, and baking the electrodes.
(1) Active substances, conductive agents, binders, solvents, rotors and weighing bottles are needed in the pulping and pulping process.
Active substances: Positive and negative materials (active substances) for laboratory use can be purchased or prepared by themselves. Generally, they are powder materials. Particle size should not be too large to facilitate uniform coating. At the same time, it avoids the limitation of material dynamic properties and the non-uniformity of polar plates caused by larger particles. The problem. Laboratory studies generally show that the maximum particle diameter (Dmax) is not more than 50 microns, and industrial applications generally show that Dmax is not more than 30 microns. Larger particles, aggregates or nano scale need to be grind and sifted.
Conductive agent: Commonly used conductive agent is carbon-based conductive agent, including acetylene black (AB), conductive carbon black, Super P, 350G and other conductive materials.
Binder: commonly used binder systems include polyvinylidene fluoride oil system [poly (vinylidene fluoride), PVDF system] and polytetrafluoroethylene water system [poly (fluortetraethylene), commonly used as emulsion, referred to as PTFE system], SBR (styrene butadiene rubber) emulsion, etc.
Commonly used mass ratio is active substance: conductive agent: binder = 8:1:1 (or 8:1.5:0.5, can be adjusted according to the material, but generally speaking, cathode material is not less than 75, conductive agent and binder are not less than 5)
Solvent: NMP (N- methyl pyrrolidone) is often used.
The preparation of NMP and PVDF solution: The solution of NMP and PVDF can be prepared into three kinds: 0.02 g/mL, 0.025 g/mL and 0.03 g/mL. The suitable concentration of the material is chosen. The preparation method is very simple. It only needs to mix the two substances in a wide-mouth bottle. Through magnetic stirring, there is no white substance in the solution. It should be noted that after the preparation, the wide-mouth bottle should be sealed by sealing glue, because NMP is easy to absorb water or deteriorate. It should be noted that the binder (such as PVDF) should be added to the solvent NMP and stirred below 50 C until the PVDF is completely dissolved.
(2) sizing steps:
The first step is to take 0.025 g/mL NMP/PVDF solution of 2 mL with pipette gun and put it into D15 agitator for magnetic stirring.
The second step: weigh the 0.05 g conductive agent Super P slowly into the weighing bottle and stir 20 min. During the process of adding, we should try not to make the conductive agent touch the upper side of the bottle wall, nor let the conductive agent disperse out of the weighing bottle because of the too fast addition.
The third step: weigh 0.4 g active substance and add it into the weighing bottle. Ibid., mix for 4-5 hours after adding, the mixing time is not fixed, whichever slurry is sticky.
3) coating of polar plates
The positive and negative collectors of lithium ion batteries are aluminium foil and copper foil respectively. If the one-sided smooth foil is selected, it is suggested to coat on the rough side to increase the binding force between the collector and the material. There is no special requirement for the thickness of foil, but the uniformity of foil density is very high. If it is silicon-based anode material, carbon-coated copper foil can be used to improve adhesion, reduce contact resistance, increase the repeatability of test results and improve cycle performance.
Generally, scraper and tape coating machine are used for coating. The positive material is coated on aluminium foil and the negative material is coated on copper foil. Students without coating machines can use glass plates and scrapers for coating. The coating process is relatively simple, but we need to pay attention to the following points.
(1) aluminum foil needs to be leveled and wrinkles should be reduced as far as possible.
(2) carefully clean aluminum foil and coating machine platform with alcohol and absorbent cotton before coating.
(3) Degreased cotton should be carefully cleaned with toilet paper once after cleaning. First, remove the possible cotton floss to avoid scratching aluminum foil.
In addition, special attention should be paid to the fact that the area capacity of the general polar plate is set at 2-4 mA h/cm2, and the minimum is not recommended to be less than 1 mA h/cm2. This kind of active material load is closer to the industrial application, and it is convenient to accurately evaluate the material's multiplier and low temperature characteristics. In some cases, this load can be exceeded, for example, for thick electrodes. On the one hand, the weighing error of the electrodes fabricated under this capacity is large; on the other hand, because the electrodes are thin, the dynamic performance is good, the volume change is small, and the electrolytic liquid phase is far excessive, which is conducive to measuring the maximum capacity of the materials, but the rate and cyclicity measured by the half-cell may be significantly higher than that of the actual full-cell. The results of dynamic and cyclic data can not correspond well with the actual capacity of batteries under working conditions. Of course, even if there are differences with the actual system requirements, but if all materials are compared according to the same polar plate production conditions, it is also meaningful to compare the performance differences of materials. However, the reliability of dynamic and cyclic performance data under different conditions is often low, and the consistency of thin plates manually manufactured in laboratory is often difficult to guarantee.
2 polar piece drying conditions, rolling technology, cutting and weighing of the chip, vacuum baking.
A. Drying of polar plates
The drying of polar plates generally needs to consider three points: baking temperature, baking time and baking environment. For NMP, the baking temperature should be above 100 C. On the premise of being able to bake, the baking temperature should be lowered as far as possible and the baking time should be increased. For some materials which are easy to oxidize or unstable in high temperature air, they need to be baked in an inert atmosphere oven. The drying condition can also be determined directly by measuring the moisture content of the polar plates.
The purpose of plate drying is to remove a large amount of solvent NMP and moisture in the slurry, so it is necessary to go through two steps: blast drying and vacuum drying. The specific temperature and time of each step are reported differently in different jobs.
(1) The drying temperature of NMP does not need to be too high, but because of too many solvents and more heat, the drying time is longer.
(2) Because the boiling point of water is 100 C, the temperature of blast drying needs to be higher, but because the moisture content is less, the drying time can be shortened. When blast drying, two temperature zones can be set. Each temperature time is different, and the highest temperature can be set to 100 C. In addition, the drying temperature of the negative electrode should be lower than that of the positive electrode, and sometimes the phenomenon of copper foil oxidation occurs.
Note: When the drying temperature is too high and the drying time is too long, serious powder dropping will occur. As for the temperature of blast drying, the positive pole should not exceed 120 C, and the negative pole should not exceed 90 C.
(3) After blast drying, vacuum drying is required. The temperature is generally set at 120 C and the time is about 10 hours. However, vacuum drying can not be directly carried out without blast drying, which will result in NMP filling in the vacuum drying box, and the drying effect is not good. No vacuum drying is possible, but it is best not to omit this step.
B. tabletting and cutting
After coating, the dry composite coating is loose. If used directly, it is easy to peel off and damage after being infiltrated by electrolyte. Roller press or tablet press can be used for tablet processing. The positive plate coating can be generally pressed to 15-60 micron by roller. The tablet press can be pressed by about 80~120 kg/cm2 pressure. The stability, firmness and electrochemical performance of the electrodes after pressing were improved, and the test performance was better than that of the samples without pressing.
There are two main purposes of pressing: one is to eliminate burrs, make the surface smooth and smooth, and prevent short circuit caused by burrs piercing the diaphragm when charging batteries; the other is to enhance the strength of the plates and reduce ohmic impedance. Excessive pressure will cause the curl of polar plates, which is not conducive to battery assembly. Excessive pressure can not play the role of pressing.
In the process of roll compaction, it is necessary to compact the plates, and the compaction density should be as close as possible to the compaction density of the plates in industry. In order to measure the dynamic limit of materials, compaction density can be controlled according to the purpose of research.
The prepared pole sheet is clamped up and down with weighing paper and punched out on the punch. The diameter of the pole sheet can be adjusted according to the punch die size of the punch. The punch die with diameter of 14 mm (corresponding to CR2032 button battery) is often used in the laboratory. Choose the good or bad of the punched microplate, and try to select the microplate with regular shape, smooth surface and edge. If there are burrs or raw materials on the edge of the microplate, the microbrush can be used for slight treatment. The number of small plates prepared by stamping is adjusted according to test requirements and smear area. Generally, the number of plates used for charge-discharge test is not less than 5 (it is recommended to select more than 8 complete test plates).
The qualified microplates were moved to a balance with high accuracy (accuracy not less than 0.01 mg) and weighed. The microplates were placed in the bags of the batteries to be loaded, and the corresponding data were recorded. In addition to the weight of the polar plate, when the thickness of the polar plate is measured by a thickness gauge, the uniformity of the thickness of the polar plate is considered to be good and the average thickness of the polar plate is recorded when the measurement error of the thickness of the polar plate is less than 3%.
The weighed plates are put into the vacuum drying box, vacuum to 0.1 MPa, set the drying temperature and time, and can be baked at 120 C for 6 hours. The purpose of this step is to further remove the moisture in the plates. After warming up, it is suggested to mark experimental information (Fig. 5) to prevent others from mishandling.
C. double sided pole piece processing method
In the laboratory test and analysis, the electrochemical performance of the electrodes prepared on some industrial production lines and dismantled from the cores are also analyzed and evaluated. Most of the electrodes mentioned above are coated on both sides, so it is necessary to treat the two-sided electrodes into one-sided electrodes before assembling button batteries for testing. The usual processing methods include scraper method, wiping method and back glue sticking method.
Scraper method mainly uses scalpel to scratch the side of the target pole piece, which can be directly operated in glove box. However, this method is easy to damage the fluid collection and takes a long time, so it is not recommended to use.
In the wiping method, water is used as solvent to wipe the side of the negative plate, while NMP (N-methyl pyrrolidone) is used as solvent to wipe the positive plate. After wiping to the back without obvious active material (visual observation can be done), the stamping machine is used to stamp the sheet, and the standard size of the single-sided polar sheet is prepared. This method is simple to operate, but it is easy to penetrate solvent or atmosphere, which affects the other side of the plate. In addition, the method is difficult to prepare samples at the edges of the polar plates, and is used for preparing the samples in the center area of the polar plates.
Back gluing method is a new method developed recently by the failure analysis team of Institute of Physics, Chinese Academy of Sciences. It uses edge folding and conductive glue on the back to wrap the back bread of the target plate in the collector and conductive glue to form a single-sided plate. This method is easy to operate and can be easily sampled and fabricated on any part of the double-sided polar plate. The whole process can be completed in the glove box.
The target plate sample after unilateral treatment needs to be cleaned in order to remove lithium salts and residual electrolytes on the surface of the plate. The commonly used method is to immerse the single-sided plate in DMC and other solvents for 6-8 hours, or to pick up the sample of the target plate with blunt-head tweezers, and to absorb DMC with a pipette or dropper, to wash the side of the target plate containing active substances several times, or to combine the two methods. After cleaning, the pole piece is placed in the vacuum chamber, and the vacuum drying is used to remove the solvent. Cleaning and vacuum drying are carried out in glove box. The dried pole sheet can be placed in a flat die to keep the pole sheet flat and facilitate the assembly of button batteries in the later stage.
3 button battery assembly method
Transfer the prepared electrodes to the glove box in inert atmosphere. Prepare button battery assembly components: negative shell, lithium metal sheet, diaphragm, gasket, spring sheet (nickel foam), positive shell, electrolyte, in addition to pressing die, pipette and insulating tweezers.
There are two main assembling orders of button batteries. Our laboratory usually starts from the negative shell, but also from the positive shell. There is no right or wrong, it depends on personal habits.
CR2032 norminal capacity 210mAh, more and more applications needs high power, now we can make max 260mAh, voltage 3.0v non-rechargeable.