Simultaneous Enhancement of Lithium Transfer Kinetics and Structural Stability in Dual-Phase TiO 2 Electrodes by Ruthenium Doping.

Dual-phase TiO 2 consisting of bronze and anatase phases is an attractive electrode material for fast-charging lithium-ion batteries due to the unique phase boundaries present. However, further enhancement of its lithium storage performance has been hindered by limited knowledge on the impact of cation doping as an efficient modification strategy. Here, the effects of Ru 4+ doping on the dual-phase structure and the related lithium storage performance are demonstrated for the first time. Structural analysis reveals that an optimized doping ratio of Ru:Ti = 0.01:0.99 (1-RTO) is vital to maintain the dual-phase configuration because the further increment of Ru 4+ fraction would compromise the crystallinity of the bronze phase. Various electrochemical tests and density functional theory calculations indicate that Ru 4+ doping in 1-RTO enables more favorable lithium diffusion in the bulk for the bronze phase as compared to the undoped TiO 2 (TO) counterpart, while lithium kinetics in the anatase phase are found to remain similar. Furthermore, Ru 4+ doping leads to a better cycling stability for 1-RTO-based electrodes with a capacity retention of 82.1% after 1200 cycles at 8 C as compared to only 56.1% for TO-based electrodes. In situ X-ray diffraction reveals a reduced phase separation in the lithiated anatase phase, which is thought to stabilize the dual-phase architecture during extended cycling. The simultaneous enhancement of rate ability and cycling stability of dual-phase TiO 2 enabled by Ru 4+ doping provides a new strategy toward fast-charging lithium-ion batteries.