Nickel-Doped Titanium Oxide with Layered Rock-Salt Structure for Advanced Li-Storage Materials

As advanced anode materials for Li-ion batteries, single-crystalline particles of Ni-, Cu-, and Zn-doped rutile TiO₂ with doping amounts of 1–2 at % were synthesized by a hydrothermal method. The effect of divalent cation (Ni²⁺, Cu²⁺, and Zn²⁺) doping on the Li⁺ diffusion behavior was clarified after the phase change from a rutile structure to a monoclinic layered rock-salt structure. The larger oxygen vacancy amounts were detected for Ni- and Zn-doped TiO₂ particles due to their larger doping amounts. The Ni-doped TiO₂ electrode exhibited the best high-rate performance with a high reversible capacity of 115 mA h g^–1 even at a very high current rate of 100C (33.5 A g^–1). This electrode showed an excellent long-term cycling performance with 170 mA h g^–1 even after 24,000 cycles. No significant difference was observed depending on the type of doping element: the Li⁺ diffusion coefficient ranged from 8.8 × 10^–15 to 1.3 × 10^–14 cm² s^–1. In contrast, the charge transfer resistance of the Ni-doped TiO₂ electrode was lower than those of the other electrodes. The first-principles calculation confirmed that the oxygen vacancy donor levels were formed in the forbidden band of the cation-doped layered rock-salt TiO₂ to improve its electronic conductivity and that the activation energy required for Li⁺ diffusion could be reduced by Ni doping. Therefore, we considered that Li⁺ transfer was promoted in Ni-doped TiO₂ to enhance charge–discharge capacities. These results demonstrate the outstanding effect of Ni doping on high-rate and long-term performances.