The purpose of formation is to convert the active substances in the battery into substances with normal electrochemical action with the help of the first charge, so that the electrodes (mainly negative electrodes) form an effective passivation film. This passivation film plays a very important role in the electrochemical reaction of lithium-ion batteries and for the stability of the battery.
Basic charge and discharge schematic diagram of lithium battery
There are generally two types of formation cabinets
One is the high-temperature pressure formation equipment of pouch cells, and the other is the negative pressure formation equipment of square cells, which has different working principles for different cells.
Hot pressing
The cathode and anode materials of lithium-ion batteries are hot-pressed with electrolytes at high temperatures to improve the energy density and cycle life of the battery
The main function of the hot pressing cabinet is to provide an environment with controlled temperature and pressure to ensure the safety and stability of the hot pressing process. Temperature and pressure changes during the process can be monitored and regulated in real time. Improve the convenience and accuracy of operation. Lithium battery hot pressing cabinet has been widely used in the field of lithium battery manufacturing and research and development, through hot pressing treatment, the charging and discharging performance and safety performance of lithium battery can be improved, providing a foundation for further improvement and innovation of battery technology.
Parametric design principle
For example, the following scenario is used:
1. Current and voltage design in the initial stage: SEI film formation is generally considered to be below 3.0V, which changes with the different types of additives at present, and the current in the initial stage of design is ≤ 0.1C, and the cut-off voltage is about 3.5-3.8V
2. Follow-up current design: Normalize the formation current, 0.2C-0.5C according to each process
3. Pressure design: < 10kg/cm2, too high pressure is easy to extrude the electrolyte between the separator and the plate, and at the same time, the conductive handle is easy to form a spatial barrier, resulting in local stress concentration and subsequent local lithium precipitation; The principle of tightness and looseness, the initial stage of SEI formation should not be too much pressure, the middle stage pressure needs to be large to avoid the disordered expansion of the negative electrode charging process caused by the deformation of the plate, the final stage due to the low rate of change of the negative electrode expansion and the middle stage has inhibited the disorderly expansion, the pressure can be moderately adjusted to give the plate normal re-expansion space, this practice is conducive to the shipping thickness is not too low to cause the battery to expand too much after the cycle
4. Temperature design: generally between 50-80 degrees, related to the selection of electrolyte additives, too high temperature leads to electrolyte decomposition, too low temperature is not conducive to the release of stress
Purpose and benefits of hot pressing:
1. Reduce the initial thickness, increase the adhesion between the plates, and drive away the gas generated during the formation process, so as to avoid the positive and negative electrodes from affecting the capacity due to the presence of bubbles
2. Eliminate the deformation caused by the uneven charging expansion of the anode material during the formation process, which is beneficial to the lithium-ion conduction path
3. The temperature and pressure effects brought by hot pressing contribute to the stability of the negative electrode SEI film and affect the stability of the subsequent life
4. It is conducive to reducing the overall thickness expansion caused by deformation after circulation
5. Release the stress concentration caused by tight tension in the automatic winding model (including plates and diaphragms)
6. Improvement of finishing capacity (improved fit) and energy density (reduction of thickness and overall volume)
7. Provide a pre-thickness design, which is helpful for the overall design
Problems and sequelae of hot pressing:
1. The decrease of liquid retention amount, the pressure effect leads to the reduction of the space in the prime, the decrease of the electrolyte storage space, and part of the electrolyte is squeezed out into the aluminum bag, resulting in an increase in the amount of degassing
2. The increase in the amount of degassing may lead to the electrolyte sticking to the aluminum bag, and the proportion of poor appearance will increase
3. The decrease in liquid retention amount, the rapid decline of cycle life in the later stage (maybe after 500, 600, 700 circles, it depends on the material selection and design, 4.4V has been more serious)
4. The increase of internal resistance and the temperature effect improve the stability and uniformity of SEI film-formation, but the relative thickness of SEI film leads to the increase of internal resistance
5. The expansion rate may not be much improved after life, because the initial thickness is lower than the general formation, such as the model does not have serious deformation and hot pressing but leads to an increase in the expansion rate
6. The generation of local lithium precipitation, the pressure effect leads to more concentrated stress at the guide handle, and the discharge of the extruded electrolyte leads to local lithium separation after the life cycle
7. Increase in overall process time
8. The cost of equipment investment is rising
9. The ascent of space
10. The cooling time of hot pressing is too long, resulting in incomplete stress release, size rebound, and cooling in a short time can help to release stress, and increase the purchase of fans or cold presses
11. Increase the proportion of poor pressure drop
Preparation before design verification
Data Collection & Validation Design:
The collection and analysis of data is the key to the success or failure of hot pressing, which can determine whether it is effective or ineffective and whether the model is suitable for hot pressing
1. The importance of the sample and the control group: The plate needs to be in the same batch with the same winding machine and the same storage environment
2. Have a clear understanding of what you want? reduce the initial thickness, improve the energy density, improve the capacitance, improve the deformation, improve the expansion after life, increase consistency, and then design the verification steps after understanding the requirements
3. The myth of the Taguchi method, the best parameter of the Taguchi method is not necessarily the best parameter, he is just a direction, omitting many experiments, unless there is a fine understanding of the experiment, otherwise the Taguchi method must not be as good as the full factor experiment, slow work out of fine work
4. Sufficient verification cycle and time, too short time to know the effect leads to the failure of verification
5. Professional analysis and discussion will help to locate and improve the direction
6. Things are always both positive and negative, and there will definitely be sacrifices and trade-offs behind improvements
What data needs to be collected for hot pressing?
What necessary data must be collected:
1. Comparison of the thickness of the plate before and after the experiment and the control group (generally refers to the negative plate, the volume change rate of the positive electrode material before and after charging is low, and it is helpful to compare it together if it is fine, it should be remembered that the complete life is not only the negative electrode charge and discharge, but also the positive electrode charge and discharge will increase the uniformity due to hot pressing), and the dismantling of the battery cell must be carried out after the test
2. Comparison of internal resistance, internal resistance is an important part of the verification of differences, hot pressing involves the stability of SEI film formation, and the improvement of internal resistance under the temperature effect helps to verify the success or failure of hot pressing to provide a reference (the purpose is to minimize the impact and improve the most)
3. The performance of capacity, in terms of ME verification, can only be compared with the overall capacity performance, and the actual performance and N/P ratio will be compared for the weight of the plate in more detail, but it is too energy-consuming
4. The difference and slope of cycle life are not only the life cycle at room temperature, but also the life cycle at 45 degrees (the difference between 45 degrees and room temperature is to accelerate the experiment, the room temperature may run well, and the high temperature may decline early)
5. The expansion rate of the thickness after the life cycle
6. X-ray observation of the change of the shape of the element before and after (deformation)
7.6090 test, accelerate the experiment, and verify whether there is leakage due to the local pressure thinning of the aluminum bag due to hot pressing, and whether the liquid retention volume of the electrolyte is due to the reduction of hot pressing and resulting in a large difference in capacity before and after
8. Degassing & liquid retention capacity: The key is related to life
9. Pressure drop: 28 days, if it is too long, you can observe the high temperature pressure drop for 7-14 days, and the hot pressing may accelerate the internal short circuit, resulting in a poor increase in pressure drop
10.85oC 4hrs test, observe the rebound in a short period of time at high temperature, if the stress is not released, the rebound will be very violent
11. Crush, the increase in energy density may lead to the failure of the security test for items that are prone to failure
Reasons why hot pressing doesn't work:
1. Wrong experimental design: Usually the experimental parameters need to have a large difference interval to show the difference, if the results confirm that the difference is not large, it may be that the parameter difference is too small for the system to discern the effect
2. Differences in the selection of material systems, the models of different material designs may not be suitable for the same set of hot-pressing parameters, such as: voltage system, anode material selection (aqueous oil system capacity 330or350, artificial graphite or MCMB), electrolyte additives, copper foil substrate thickness, and materials under the current sustainable development of lithium battery are very important
3. Process differences: semi-automatic winding (it is not recommended to use him for verification), automatic winding, whether the space in the design of the aluminum bag is sufficient or the deformation is fundamentally due to the automatic bag small cell (this hot pressing cannot save the deformation)?the negative plate is rolled once or twice (the proportion of secondary rolling and rolling is 40%-100% or 75%-100%)
4. Source of deformation (process, material design?)
5. What is to be improved?Post-life expansion?Volume energy density increase?Initial thickness reduction?Life or capacity to play consistency?
Meaningful experiments promote improvement, not every experiment can be improved, so the design and requirements of the experiment need to be clarified, not a single department can do it, and it requires teamwork to achieve the goal together!