Due to their extreme volume expansion, Si/C-composites suffer from fracture or delamination and consequent capacity fading during the Li-ion cell operation. One approach to reduce the electrical contact loss and improve the performance is the application of mechanical pressure on the cell. Therefore, a comprehensive aging study of Si/C vertical bar NMC811 pouch cells is conducted with cells in different compression configurations as uncompressed and under flexible and fixed compression at pressure levels in the range of 0.08 MPa, 0.42 MPa, and 0.84 MPa. In-situ swelling measurements by dilation as well as in-operando mapping of the pressure distribution on the cell surfaces reveal the positive influence of the low pressure fixed and the middle pressure flexible compression on the cycle life. For the heavily fixed compressed cells, an inhomogeneous pressure distribution and occurring pressure hot-spots close to the cell current collectors of up to 5.2 MPa are found. An extensive post-mortem analysis including SEM cross-sectioning and EIS measurements of the aged anodes and the separator confirms cell failure by different aging mechanisms depending on the type of compression. Aging experiments of Si/C vertical bar NMC811 cylindrical 18650-cells show local differences along the jelly roll which are explained by the help of the pouch cell results. (C) The Author(s) 2019. Published by ECS.