The electrochemical study of the empty tunnel structure Ti2Nb2O9 shows that it can intercalate lithium reversibly, through a solid-solution process, leading to the phase Li4Ti2Nb2O9. The high-resolution electron microscopy study of this phase shows that Ti2Nb2O9 exhibits an exceptional nanoporosity, involving the formation of aligned holes throughout the whole microcrystal, which may favor, together with the presence of tunnels, the diffusion of lithium during the insertion. The reversible capacity of this phase, of 125 mAh/g at 1.57 V vs Li/Li+, after 30 cycles, shows its great reversibility.