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. <i>In situ</i> 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 50<i>C</i> (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 (<i>R</i> _ct) compared with other cation-doped TiO₂ electrodes. Measurements using the galvanostatic intermittent titration technique found that the Na⁺ diffusion coefficient (<i>D</i> _Na+) of Ni-doped TiO₂ has a higher value of 1.2 × 10^-13 cm² s^-1 compared with <i>D</i> _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.