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

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