The main disadvantage of hard carbons when they are used for the negative electrode of Lithium-ion batteries is a large irreversible capacity generally attributed to their important specific surface area. Our goals were to estimate the amount of residual lithium, i.e. not involved in the solid electrolyte interface (SEI) after a full deinsertion, and to reduce the irreversible capacity due to the SEI formation. Galvanostatic charge/discharge of carbon lithium cells was performed with carbon materials to which an additional pyrolysis has been applied in various conditions. Electrode materials made from as-received carbon have been analysed ex situ by electron paramagnetic resonance (EPR) at different steps of the insertion/deinsertion process. We found that 20% of the irreversible capacity is due to residual lithium trapped in the carbon matrix. Post-treatments up to 900 degreesC resulted in decreasing the irreversible capacity from 260 to 65 mAh/g thanks to modifications of the surface functionality and possible improvement of the carbon network structure.