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 TiO 2 with doping amounts of 1–2 at % were synthesized by a hydrothermal method. The effect of divalent cation (Ni 2+, Cu 2+, and Zn 2+ ) 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 TiO 2 particles due to their larger doping amounts. The Ni-doped TiO 2 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 cm 2 s –1 . In contrast, the charge transfer resistance of the Ni-doped TiO 2 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 TiO 2 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 TiO 2 to enhance charge–discharge capacities. These results demonstrate the outstanding effect of Ni doping on high-rate and long-term performances.