The best way to cool the 4680 cylindrical battery

Since Tesla released the 4680 battery, various battery manufacturers have actively developed large cylindrical batteries such as 4680. Large cylindrical batteries have many advantages: (1) they can reduce the number of copper bars and control circuits in the battery pack, avoid energy losses caused by higher voltage drop and heat generation, and improve system reliability. (2) Due to the reduced proportion of inactive components such as shell materials and separators, the energy and power density at the battery pack level can be higher.

However, the main problem with large cylindrical batteries is heat dissipation, and without effective heat dissipation, larger batteries may produce higher internal temperatures and greater thermal gradients, which will limit battery performance and significantly accelerate aging. The heat dissipation rate of a battery is affected by the surface-to-volume ratio and thermal conductivity. For cylindrical batteries, due to the cylindrical geometry, the surface-to-volume ratio is inversely proportional to the radius and is inherently low. Moreover, the thermal conductivity has strong anisotropy, and the thermal conductivity in the radial direction is low due to the multi-layer structure such as positive electrode, negative electrode, and diaphragm. The 4680 battery diameter is increased to 46mm, which is challenging to dissipate compared to 18 mm (18650 battery) and 21 mm (21700 battery).? ? ? ? ?

Therefore, the 4680 battery electrode adopts a full-pole lug or no-pole tab design, leaving an uncoated current collector on the side of the electrode as the battery tab, which is directly welded to the current collecting plate on the battery end face. The continuous omnipolar lug design of the 4680 battery is able to reduce heat generation and increase heat dissipation to address thermal issues caused by large cylindrical cell size increases. The Department of Mechanical Engineering at Imperial College London created a model of a 4680 cylindrical battery and used it to study different ways of managing the heat. The results show that the thermal performance of the 4680 battery is no worse than that of the 2170 battery due to the omnipolar lug design, while the energy and power of the 4680 battery are 6.9 times that of the 2170 battery. Finally, the model proves that the optimal cooling method for the 4680 battery is both end face base cooling, while for the 2170 battery it is side cooling.

In a cylindrical battery, the internal structure of the cell with a single and omnipolar lug designs is shown in Figure 1. For Danji design batteries, the remaining space between the bottom of the jelly roll and the metal case is filled with an insulating sheet to prevent short circuits, except for the tabs. Since the thermal conductivity of plastic insulation sheets is orders of magnitude lower than that of metal casings and current collectors, tabs become the main heat dissipation path from the bottom of the jelly roll to the outer metal shell. For Tesla 4680 batteries, the negative current collector copper sheet is directly connected to the housing, and adding a large number of connection points is expected to greatly reduce the connection resistance and reduce the bottleneck of heat dissipation

Figure 1 (a) Cross-section of a cylindrical cell with a monopolar lug design. Local magnification of electrode cross-section, (b) monopolar lug design and (c) omnipolar lug design. (d) negative lugs of 2170 LG M50T batteries and (e) Tesla 4680 batteries

In the simulation calculation, the parameters of the battery are shown in Table 1, the 2170 battery model parameters refer to the 21700 LG M50T actual battery, the length of the electrodes in the model is different from the actual battery, mainly the length of the model is calculated by the shell size limit, which is different from the actual battery, and the difference of the 4680 battery is relatively large. The design of the tabs includes:

(1) Single pole ear or two pole ears;

(2) Full pole lug, continuous white space of current collector, welding one pole lug piece per turn, and finally connecting with the pole column of the external circuit of the battery;

(3) No tabs, continuous white space for current collectors, folded end faces, no welding of additional tabs, directly connected with the end cover, the same as Tesla 4680 battery design.?? ?

?In the basic thermal model, the fixed discharge current of all batteries is 1.5C, the thermal boundary is convection cooling on all surfaces, the ambient temperature is 25°C, the heat transfer coefficient is 30 W/(m2 K), the irreversible and reversible (entropy) heat generated by the electrodes is taken into account, the current collector takes into account the irreversible heat generation, and the total heat generated by the battery is the sum of all elements of the battery. The simulation results of the 21700 battery are shown in Figure 2, compared with the case of single lugs and all-pole ears, the discharge voltage of the all-pole lug battery is higher and the battery polarization is smaller (Figure 2a); Moreover, the average temperature during discharge is also lower (Figure 2b), and the difference between the maximum and minimum temperatures is lower (Figure 2c); Compared with the heat generated by electrodes, copper foil and aluminum foil, the proportion of heat generated by the current collector of the monopole 21700 battery is high, while the heat generated by the current collector of the omnipolar lug battery is almost negligible (Figure 2d)

The thermal characteristics of the 4680 battery with different tab designs are shown in Figure 3. The discharge capacity of the unipolar ear 4680 battery under different magnifications C is shown in Figure 3b, the capacity under 0.1C discharge is close to the rated capacity (27.07Ah), and the discharge capacity decreases significantly as the C rate increases from 0.1C to 1.5C. The current path of the current collector of the pole piece is shown in Figure 3c, the electrical performance of the 4680 battery is extremely sensitive to the number of tabs, and the addition of tabs can greatly improve the performance by reducing the internal resistance. Unlike the 2170 battery, the 4680 battery must have a bipolar or omnipolar ear design. Compared to bipolar lug designs, omnipolar lugs can significantly reduce the heat generated by current collectors (Figure 3e)

he design of the tabs of the 4680 battery considers the two cases of all-pole lugs (continuous white space, one tabs welded per winding ring) and no-tabs (continuous white space, no welding tabs left white folded directly connected to the end cover pole).Side convection, side conduction, top/base convection and top/base conduction four cooling methods, the result is shown in Figure 4. Conduction cooling schemes significantly reduce the average cell temperature compared to convection schemes due to higher rates of heat transfer between the cell and the surrounding environment through metal conduction than metal-air convection boundary conditions (Figure 4a). Side conduction cooling causes the maximum temperature difference across the cell (Figure 4b), and although top/bottom convection cooling has the lowest temperature difference, this method has the highest average temperature. The temperature distribution of the 4680 battery at the end of discharge under the four cooling schemes is shown in Figure 4c, and top/bottom conduction cooling is the optimal cooling scheme for the 4680 plateless electrolyzer, enabling a small average temperature and thermal gradient

The design of all-pole lugs (continuous white space, each wound ring welded one tab) and non-polar lugs (continuous white space, no welding of the tabs left white folded directly connected with the end cover pole) design will also have an impact on the temperature rise of the battery, and an insulating sheet will generally be added between the core and the shell when the all-pole lug is designed, thus affecting the heat transfer; When the electrodeless lug is designed, the current collector white fold is directly welded to the current collector plate, which increases the heat dissipation rate. As shown in Figure 5, the average temperature is highest for the 4680 omniaural battery, reaching 48 °C at the end of discharge. In contrast, batteries with a tabless design have a significantly lower average temperature, about 35 °C at the end of discharge. The temperature difference ΔT (that is, the difference between the maximum temperature Tmax and the minimum temperature Tmin), the thermal gradient of the all-pole lug design is twice the design value of the no-pole lug. By cooling the top and bottom, the average battery temperature and temperature difference can be further reduced

In conclusion, the model simulation results show that for the monopole 2170 battery, the best cooling strategy is side cooling, while for the 4680 no-pole battery, the best cooling strategy is top/bottom cooling. But at present, Tesla's cooling method is mainly side cooling.

References:

Shen Li, Mohamed Waseem Marzook, Cheng Zhang, Gregory J. Offer, Monica Marinescu,How to enable large format 4680 cylindrical lithium-ion batteries,Applied Energy,Volume 349,2023,121548, https:// doi.org /10.1016/j.apenergy.2023.121548.