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      FAQ
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      How to improve the capacity density of cells?
      493 2022-03-07
      The first task of designing a battery: improving the energy density of the battery (limited to capacity batteries) is the first task of designing a battery.
      Even if the capacity is not enough, the unit price is low, the cycle is good, and the safety is high, no one cares about the battery produced. So how can we improve the energy density of the battery?
      1. Increase the performance of materials that contribute to battery capacity;
      2. Reduce the volume of materials that do not contribute to battery capacity;
      3. Use more advanced production equipment;
      4. Improve the manufacturing process conditions and carry out stricter production monitoring;
      5. Optimize the miscellaneous items that affect the capacity;
      Let‘s start to discuss the categories separately:
      1. Increase the performance of materials that contribute to battery capacity: This is mainly for positive and negative active materials, which is the most direct method to increase capacity density. The main directions include:

      ① Use materials with greater gram performance: such as lithium-rich materials, high-voltage ternary materials, high-voltage lithium cobalt oxide materials, binary materials, etc. for the positive electrode; soft carbon, hard carbon, and silicene-based compounds for the negative electrode.
      ② Use positive and negative materials with higher compaction density.
      ③ Use active substances with better adhesiveness and conductivity: This can reduce the content of adhesives and conductive agents in the dressing, thereby increasing the capacity of the dressing per unit mass; in addition, the amount of adhesives and conductive agents is reduced. Processing properties such as compaction of the active material of the material can also be improved.
      ④ Use materials with smaller thickness rebound: After the lithium-ion battery is cycled, the thickness will rebound to a certain extent; the design needs to reserve the rebound thickness after the cycle; and when using materials with a smaller thickness rebound (according to what has been seen so far) See, these materials are also materials with good cycle performance), you can transfer the saved space for thickness rebound to the design thickness of the cell, thereby increasing the design capacity of the cell.
      ⑤Choose a material system with better matching performance: a single "good positive electrode", "good negative electrode" and "good electrolyte" together do not guarantee a "good battery". The combination of materials with poor matching will not only reduce the cycle performance of the battery, but may also affect the rate performance and even the performance of the positive and negative electrodes. Similarly, when the material matching is better, the performance, cycle, and expansion rate of the battery will be affected. The performance may be improved.
      With the increasing maturity of lithium-ion battery material technology, the potential of common lithium cobalt oxide and graphite has almost reached its limit. In the future, if other mature systems can be produced, it will have a revolutionary impact on the energy density improvement of lithium-ion batteries!
      2. Reduce the volume of materials that do not contribute to battery capacity: This item is more complicated, mainly including: use thinner aluminum-plastic film, use thinner battery separator, use thinner tabs, and use thinner copper foil and aluminum foil, use thinner transparent tape, etc.
      ① It can be seen that the face-to-face improvement is basically based on "thin" as the attribute. Under the limited volume, the reduction of the volume of the aluminum-plastic film, diaphragm, etc., which does not directly contribute to the capacity, means the increase of chemical substances that can provide capacity, thereby increasing the capacity of the cell.
      However, when these materials are thinner, their mechanical strength and safety performance will be affected: on the one hand, the auxiliary material manufacturer needs to reduce the volume without reducing the material performance, on the other hand, the battery cell manufacturer needs to change the process, Manufacturing parameters and even equipment (for example, thinner aluminum foil means higher roll elongation coefficient and increased probability of fragmentation during coating and rolling; thinner diaphragm has higher risk of short circuit rate; thinner aluminum plastic Membranes are more prone to pin breakage; thinner tabs reduce battery rate capability, etc.).
      Replacing these thinner materials requires a lot of certification from the battery factory. If the number of trial production is too small, it is impossible to accurately obtain statistical data. If the problem is too large, it will cause waste. Compared with improving the performance of active substances, reducing the content of inactive substances seems to have more ideas, and the aspects involved are equally broad.
      3. Use more advanced production equipment: advanced equipment is mainly to improve the consistency of production, reduce the fluctuation of production, and then increase the minimum value without changing the average value, and determine the quality of a batch of cells, precisely is the minimum value raised here. Take the following two examples:
      ①When the coating tolerance of the coating machine is smaller, the minimum capacity/design capacity can be increased. If the customer has a requirement for the minimum capacity, it is equivalent to increasing the minimum capacity when the design capacity is unchanged, that is to say The capacity of the battery cell evaluated to the customer is improved; at the same time, the coating deviation of the positive and negative electrodes is reduced. After considering the factors such as the first efficiency and the capacity decay after the cycle, the excess design of the negative electrode can be moderately reduced, so that the battery has more positive electrodes material, which is equivalent to having more capacity.
      ② When the accuracy of the rolling machine is improved, due to the reduction of the actual rolling error, the design compaction can be closer to the maximum compaction of the material, thereby increasing the design capacity and the capacity of the fabricated cell; rolling out the pole piece The thickness of the battery is more consistent, which is equivalent to increasing the consistency of the thickness of the cell, and then a smaller maxTHK-designTHK can be selected in the design, thereby improving the design capacity (width of slitting knife, width of tape measure, accuracy of sub-capacity cabinet) etc. are similar, and will not be repeated).
      4. Improve production conditions and carry out stricter production monitoring. There is a lot of knowledge in production control, to list one or two:
      ①Reduce the sequence time after rolling to before winding: This can reduce the thickness rebound of the pole piece, so as to better control the thickness of the cell. If the thickness of the cell is significantly lower than the maximum thickness, the design thickness can be appropriately increased, thereby increasing the design capacity of the cell.
      ② Stabilize the temperature of the separation room: The temperature has a great influence on the capacity of the cells. The low temperature may cause the cells with the qualified capacity to be transferred to the secondary capacity as low capacity, thereby wasting resources. Poor cells are shipped as qualified products.
      As with the short board principle, the capacity of a batch of cells is calculated by the lowest value of the cells shipped, and the thickness of a batch of cells is calculated by the maximum thickness of the cells shipped. In the labor-intensive industry of our country, human operation has a particularly important impact on the quality of products. In production, if the operator and quality personnel can minimize the error of human operation and reduce the fluctuation during production, it is equivalent to increasing the length of the shortest board in the barrel, thereby increasing the capacity of the battery and reducing the number of batteries. thickness of.
       5. Optimize the miscellaneous items that affect the cell capacity
      When the fully charged cell is disassembled, the main body of the negative electrode should be uniform and perfect golden yellow. However, when there are problems with human operation/materials and equipment and even design, various minor faults such as: lithium deposition in the first circle of the negative electrode of the winding core, lithium deposition in the wrinkled part of the diaphragm, uneven density of the tail film of the positive and negative electrodes during coating, lithium deposition Wait for the problem to appear. From the perspective of capacity, lithium precipitation is likely to cause a low capacity; if similar situations can be avoided, it is equivalent to increasing the capacity of the cells; if such situations cannot be avoided, it is necessary to predict its occurrence more accurately by design The probability and the size of the possible impact, so as to avoid some cells being scrapped due to problems that could have been predicted.