High precision measurement of reversible swelling and electrochemical performance of flexibly compressed 5 Ah NMC622/graphite lithium-ion pouch cells

A novel mechanical compression test bench was developed and validated, to measure the reversible swelling and the electrochemical performance at pressures between 0.075 MPa to 1.75 MPa, applied on 5 Ah NMC622/graphite lithium-ion pouch cells. Different cell setups were utilized in this study, including full-cells, symmetrical cells, and dummy cells, which consisted exclusively of anode, cathode, or separator layers. Symmetrical Swagelok T-Cells were measured for comparison. The reversible swelling was independent of the compression, whereas the porosity in the anode, the cathode, and the separator was reduced by the compression. Consequently, the ionic pore resistance in the anode increased by 19 %, if the compression was increased from 0.075 MPa to 1.75 MPa, whereas no significant impact was found on the cathode and the separator. The hindered ionic transport in the anode increased the full-cell direct current internal resistance (DCIR) by up to 14 %, while the capacity retention at elevated currents was lowered by 9 % for the highest compression. The results of this study suggest that the cells should be operated at 0.2 MPa maximum, since the electrochemical performance deteriorates for higher compression. To detect the influence of the compression with reduced time effort, DCIR pulse tests ≥ 60 s at elevated currents and low SoCs are recommended.