Mesoporous Spherical Aggregates Consisted of Nb-Doped Anatase TiO₂Nanoparticles for Li and Na Storage Materials

Mesoporous spherical aggregates of Nb-doped anatase TiO2 nanoparticles were synthesized by a simple one-pot solvothermal method in supercritical methanol and were evaluated as anode materials for Li-ion and Na-ion batteries. A Li-ion battery electrode of mesoporous undoped TiO2 showed a significantly improved performance compared to that of a commercial TiO2 without mesoporous structure. An electrode of the 7 at. % Nb-doped TiO2 exhibited a further improved performance with the discharge (Li-extraction) capacity of 147 mA h g–1 even at the 1000th cycle and with that of 110 mA h g–1 at a high current rate of 10 C, which is comparable to the performances of Li4Ti5O12 in practical use for Li-ion battery anode. On the other hand, it achieved an outstanding cycle life with the discharge (Na-extraction) capacity of 128 mA h g–1 up to almost 1000 cycles. These excellent performances are attributed to Nb-doping in addition to the large reaction area of the mesoporous structure. The Nb-doping presented two important effects: (i) the improvement in the electronic conductivity and (ii) the broadening of ion-diffusion path in TiO2. Consequently, we believe that the mesoporous spherical TiO2 is very promising as Li and Na storage materials for rechargeable batteries.