Nickel-Doped Titanium Oxide with the Rutile Structure for High-Performance Sodium Storage

We prepared rutile TiO2 particles doped with Ni2+, Al3+, Nb5+, and Ta5+ by hydrothermal synthesis as anode materials for Na-ion batteries and investigated the effect of doping cation valence on the anode performance and the Na+ diffusion behavior. In situ X-ray diffraction analyses confirmed the insertion and extraction of Na+ while maintaining the rutile structure. Among the various doped TiO2 electrodes, the Ni-doped TiO2 one exhibited the best anode performance with a high reversible capacity of 135 mA h g-1 even at 50C (16.75 A g-1). This electrode showed a very long cycle life: the capacity of 225 mA h g-1 could be attained even after 10,000 cycles. The first-principles calculation suggested the formation of impurity levels in the forbidden band of TiO2 by various cation dopings. Electrochemical impedance analyses revealed that the Ni-doped TiO2 electrode showed lower charge-transfer resistance (R ct) compared with other cation-doped TiO2 electrodes. Measurements using the galvanostatic intermittent titration technique found that the Na+ diffusion coefficient (D Na+) of Ni-doped TiO2 has a higher value of 1.2 × 10-13 cm2 s-1 compared with D Na+ of 4.8 × 10-14 cm2 s-1 in the case of undoped TiO2. The first-principle calculation supported this result: the Ni2+ doping could reduce the activation energy required for Na+ diffusion in rutile TiO2. Therefore, we suggest that an easier migration of Na+ was promoted in the Ni-doped TiO2, effectively enhancing the charge-discharge capacity and the cycle life. Although rutile TiO2 as an anode has had a difficult history, this study proved that impurity element doping such as Ni2+ can transform it into a very attractive anode material.