Effects of Tab Design in 21700 Li‐Ion Cells: Improvements of Cell Impedance, Rate Capability, and Cycling Aging

Abstract

Li‐ion cells of the industrially relevant 21700 format are investigated experimentally by systematic variation of their tab design. To observe the effects of the tab design only, the cells are built on pilot scale using the same electrodes, electrolyte, and separator. Tabs are varied regarding 1) conventionally welded tabs and 2) tabs made from the electrodes’ foils (“foil tabs”), as well as the tab number. Conclusions are drawn on cell impedances, rate capability (0.5C–3C), heating behavior, and cycling aging. Cells with a multitab design show a reduced impedance (1 kHz) by 62%, an improved charging rate capability by ≈17% for 3C, an improved cycle stability by 26.5%, and charge throughput by 22.8%. For the check‐ups at C/10, improvements in cycle life and charge throughput of 51.1% and 42.4% are observed, respectively. The improved cycle life is due to the reduction of inhomogeneities in the jellyrolls as observed by X‐ray computed tomography. The data are put into context with cells from other studies to conclude the order of magnitude of the effects of tab design variation. Based on experimental findings, estimations are made for 46800 format cells regarding energy content and impedance.

Multistep Improvement of Pilot-Scale 21700 Cells for Increased Fast-Charging Capability: Combining Optimized Electrolyte, Cell Design and Fast-Charging Protocol

“…The same electrodes were used in previous studies from our group and were manufactured at ZSW's research production line. 5,26,27 It must be noted that the electrodes had not been optimized for fast-charging.…”

“…The cell assembly process was already described in our previous papers. 26,27 The assembly of the cells was conducted in different batches. After filling the electrolyte, the cell impedance was measured using a Hioki BATTERY HiTESTER 3554 at a frequency of 1 kHz ± 30 Hz (Z at 1 kHz).…”

“…4,5 Even within one cell type, the cell design can highly influence the electrochemical performance. 5,26,27 In cylindrical cells, changes in the tab design can increase the cycle life of the cell, by for example, reducing the cell resistance. 27 Additionally, one layer of GF/A (260 μm) was used within the PAT-cells instead of Celgard (25 μm).…”

“…21 Changes in the tab design of larger format cells can counter such heating. [25][26][27] Frank et al 25 investigated the effect of foil tabs (tabs made from the electrode's metal foils, also called "tabless design") on the performance of different cylindrical cells (18650, 21700, 46800 design) in comparison to segmented tab design by simulations. They found both a decrease in anode polarization as well as a lower average cell temperature during charging.…”

“…25 Sturm et al 24 found in their simulations that cylindrical cells (18650, 21700, 26650 design) with a larger number of tabs have a higher fast-charging capability. Our group 26,27 demonstrated experimentally an increased cycle life in cylindrical 21700 cells, by changing the welded tab design with only a few single tabs to a foil tab design.…”