Positive Effects of Tab Design in Cylindrical Li-ion Cells – A Systematic Study on Pilot-Scale

Cylindrical cells (18650, 21700) are produced in the order of billions per year worldwide. In these cells, the electrodes are connected to the outer positive and negative connectors via tabs. However, not much is known about the effect of the tabs on cell performance and aging from experiments. The difficulty in such investigations is that they cannot be performed with different commercial types of cells, since the tab design and chemistry are different.

To best of our knowledge, we show the first systematic experimental investigation of tab design in 21700 cylindrical cells. The direct comparison of the tab designs becomes possible by using the same electrodes, separator, and electrolyte. The tabs are varied regarding their number on anode and cathode, as well as by their type (conventionally welded vs. tabs made from the electrode’s foils). All cells are tested regarding impedance (@1kHz), heating behavior, fast-charging capability, long-term cycling stability, and X-ray computed tomography (CT) after cycling. All experiments were reproduced with additional cells. The results from pilot-line built cells are set into context with commercial cells.

We find the trend of decreased impedance with increasing tab number. From a design change from one welded tab on each electrode (typical in commercial high-energy cells) to electrodes completely covered with foil tabs (similar to Tesla’s tabless design), a decrease of 62% in impedance was observed. Furthermore, the tab number reduces the effect of the C-rate on the maximum temperature for a complete discharge, however, the temperature differences within the cells remain similar.

Commercial cells with welded tabs typically show mechanical deformation of the jellyroll after cycling [1]. These deformations start usually at the tabs, since they introduce inhomogeneities to the jellyroll since they are thicker than the electrodes. Long-term cycling experiments with pilot-line built 21700 cells with the same tab number show that foil tabs lead to increased cycle life by a factor of 1.26 for 1C cycling at 25°C [2]. For check-ups at C/10, the cycling stability is even increased by a factor of 1.51%. The increases in total charge throughput are in similar orders of magnitude. X-ray CT measurements of the cells after cycling show that the cells with welded tabs show deformations of the jellyroll, while the cells with foil tabs are not deformed. It is likely that the increased homogeneity of the jellyroll by the foil tabs (instead of welded tabs) leads to the absence of the roll-over of the capacity fade and therefore to the significantly increased cycle-life.

References

[1] T. Waldmann, V. Knoblauch, M. Wohlfahrt-Mehrens et al., J. Electrochem. Soc. 161 (2014) A1742. https://dx.doi.org/10.1149/2.1001410jes

[2] T. Waldmann, R.-S. Scurtu, M. Wohlfahrt-Mehrens et al., Energy Technol. (2022) 2200583. https://dx.doi.org/10.1002/ente.202200583

Acknowledgment

Funding of the project RollBatt (03XP0245A) by the German Federal Ministry of Education and Research (BMBF) is gratefully acknowledged.