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

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High Resolution Image Download MS PowerPoint Slide We prepared rutile TiO 2 particles doped with Ni 2+, Al 3+, Nb 5+, and Ta 5+ 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 TiO 2 electrodes, the Ni-doped TiO 2 one exhibited the best anode performance with a high reversible capacity of 135 mA h g –1 even at 50 C (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 TiO 2 by various cation dopings. Electrochemical impedance analyses revealed that the Ni-doped TiO 2 electrode showed lower charge-transfer resistance ( R ct ) compared with other cation-doped TiO 2 electrodes. Measurements using the galvanostatic intermittent titration technique found that the Na + diffusion coefficient ( D Na+ ) of Ni-doped TiO 2 has a higher value of 1.2 × 10 –13 cm 2 s –1 compared with D Na+ of 4.8 × 10 –14 cm 2 s –1 in the case of undoped TiO 2 . The first-principle calculation supported this result: the Ni 2+ doping could reduce the activation energy required for Na + diffusion in rutile TiO 2 . Therefore, we suggest that an easier migration of Na + was promoted in the Ni-doped TiO 2, effectively enhancing the charge–discharge capacity and the cycle life. Although rutile TiO 2 as an anode has had a difficult history, this study proved that impurity element doping such as Ni 2+ can transform it into a very attractive anode material.