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 x 10(-15) to 1.3 x 10(-14) cm(2) 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.