How to enable large format 4680 cylindrical lithium-ion batteries

Reference to：Shen Li a c 1, Mohamed Waseem Marzook a, Cheng Zhang b, Gregory J. Offer a c, Monica Marinescu

Original article link on sciencedirect.com

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Abstract

The growing demand for large-format lithium-ion batteries presents challenges related to heat management. While 1865 and 2170 cylindrical cells can be efficiently cooled, the newer 4680 cells, popularized by Tesla, require effective thermal management. This study creates a model for 4680 cylindrical cells to explore various thermal management strategies. It reveals how the "tabless design" improves 4680 cell performance and enables larger formats. The tabless 4680 cell matches or surpasses the thermal performance of the 2170 cell while offering significantly higher energy and power. The model also proposes a procedure for selecting the optimal thermal management approach. For example, base cooling is recommended for 4680 tabless cells, while side cooling is ideal for 2170 LG M50T cells. The study concludes that any viable large-format cylindrical cell should incorporate a continuous tab design and base cooling. These findings are relevant to cell manufacturers and battery pack designers, with broader applications for energy storage system design.

1. Introduction and motivation

Cylindrical battery cells struggle with heat dissipation because they have a poor surface-to-volume ratio (it gets worse as the cell gets larger) and low thermal conductivity sideways. Trying to make them bigger while keeping good radial heat dissipation is challenging.

However, cooling from the ends of these cells, although it has less surface area, is better because the jellyroll inside conducts heat well lengthwise. This end cooling method also keeps a consistent surface-to-volume ratio when making the cells wider while keeping the same height.

Improving the design of battery tabs can help manage heat in cylindrical cells. Traditional designs can lead to hotspots, but more tabs can reduce this issue. Tesla's recent patent introduced a "tabless" design for large 4680 cells, expected to reduce heat and improve performance. While the benefits of this design have been demonstrated in some aspects, other factors like current paths and cooling methods have not been fully assessed for the 4680 cell.

2. Model description

The Department of Mechanical Engineering at Imperial College London created a 4680 cylindrical battery model and used it to study different ways of managing heat. The results show that due to the tabless design, the thermal performance of the 4680 battery is no worse than that of the 2170 battery, while the energy of 4680 cell is 6.9 times that of 21700 cell. Finally, the model proves that the best cooling method for the 4680 battery is two-sided base cooling, while for the 2170 battery it is shell cooling.

A cylindrical cell is composed of a jellyroll (current collector, electrodes, separator soaked), internal tabs and the outside metal can filled with electrolyte. Fig. 1 shows the schematic description of cylindrical cells with single and continuous tab (or tabless). The cross-section schematic of a single-tab cylindrical cell is shown in Fig. 1(a). For the single-tab design, the negative side of the jellyroll is electrically and thermally connected to the metal can by the single tab, as illustrated in Fig. 1(b). Fig. 1(d) shows the negative side for the disassembled LG M50T cell. As it can be seen, other than this single tab, the remaining space between the jellyroll base and metal can is filled with separator to prevent short-circuit. Since the thermal conductivity of the separator is orders of magnitude lower than that of the metal can and the current collector, this single tab forms the main heat rejection path from the jellyroll base to the outside metal can.

The study uses a 3D electro-thermal modeling framework to simulate these cells, with dimensions specified in Table 1. The 2170 cell's dimensions match those of the LG M50T cell, and slight discrepancies are due to integer layer constraints. For the 4680 cell, required dimensions are lacking, so a virtual tabless 4680 cell model is created, assuming the same electrode materials and thickness as the LG M50T cell. The resulting 4680 cell has a capacity 5.4 times greater than the 2170 cell.

3.1. Cell voltage performance

The electrical performance of the all-tab and the single-tab designs is studied for both the 2170 and 4680 cells, for a fixed discharge current of 1.5C. This value was chosen as extreme enough to show the effect of tab design, while still within the limits of the LG M50T datasheet. The thermal boundary is in all cases convective cooling on all surfaces with a heat transfer coefficient of 30Wm?2K?1 and ambient temperature of 25?°C.

In the thermal model, both irreversible and reversible (entropic) heat generation by the electrodes is considered. Irreversible heat generation is considered for current collectors. The total heat generation from a cell is calculated as the sum over all the sub-elements of a cell. The detailed equations and assumptions are listed in the previous modelling work by Li et al. The total heat generation from current collectors and the electrode/separator/electrode unit is illustrated in Fig. 2(d) during the 1.5C discharge process. The heat generation from the current collectors for the all-tab design is negligible compared with their contribution to heat generation in the single-tab design. For all the usual criteria, such as larger discharge energy, lower average temperature and lower temperature gradient, the all-tab design performs better than the single-tab design.

The thermal gradient for the all-tab design is twice the value of tabless design, indicating that the all-tab cell does not benefit from base cooling as much as the tabless cell. The temperature distribution at the end of discharge is shown in Fig. 7(c). The temperature difference between the jellyroll and the metal can at the negative side is significant for the all-tab cell, indicating a thermal bottleneck created by the poor thermal connection pathway, unlike in the tabless cell.

Fig. 8. Model predictions of the thermal performance of a 4680 tabless cell during a 1.5C discharge under four thermal management conditions: side convection, side conduction, top/base convection and top/base conduction. (a) Volume-averaged temperature and (b) the maximum temperature difference within the cell. (c) Internal temperature distribution at the end of discharge.

In summary, the model simulation results show that for the single-lug 2170 battery, the optimal cooling strategy is side cooling, while for the 4680 non-lug battery, the optimal cooling strategy is top/bottom cooling. But currently, the cooling method used by Tesla is mainly side cooling.

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