The evolution of the three-dimensional (3D) morphology of a Si-based electrode upon cycling (1st discharge, 1st charge and 2nd discharge) is studied by in-situ synchrotron X-ray tomography. The Si-based electrode is constituted of silicon/carbon black/carboxymethylcellulose (Si/CB/CMC) embedded in a commercial carbon fiber paper, acting as a flexible 3D current collector. Its initial areal discharge capacity is 4.9 mAh cm−2. A reconstructed volume of 293 × 293 × 137 μm3 is analyzed with a resolution of ∼0.3 μm. Three phases are identified: (i) the solid phase (C fibers + Si + CB + CMC), (ii) the electrolyte phase (pores filled with electrolyte) and (iii) the gas phase (electrolyte-free pores). Their respective volume fraction, size distribution and connectivity, and also the dimensional changes of the electrode along the three axes are quantified during cycling. At the beginning of the 1st discharge (lithiation), the formation of gas channels attributed to the reductive electrolyte decomposition is observed. During the 1st charge, large cracks are formed through the electrode, which reclose during the subsequent discharge. The electrode expansion/contraction due to the Si volume change is partially irreversible, occurs mainly in the transverse direction and is much larger in the bottom part of the electrode.