Evaluation of undoped and M-doped TiO2, where M = Sn, Fe, Ni/Nb, Zr, V, and Mn, for lithium-ion battery applications prepared by the molten-salt method

Abstract

The molten-salt method was used to synthesize a series of transition-metal containing titanium dioxides. Some of the transition metals were found to substitute into the TiO₂ lattice, such as (Ti_0.9Fe_0.1)O₂, (Ti_0.9Zr_0.1)O₂, (Ti_0.9V_0.1)O₂, and (Ti_0.9Mn_0.1)O₂, while others were formed as composite electrodes (in addition to relatively minor substitutions), namely 0.1SnO₂–0.9TiO₂ and 0.05NiO–0.1Nb₂O₅–0.9TiO₂. Although identical synthesis-conditions were used the different transition metals yielded different phases. A comparative study of the electrodes relating surface area and composition (via X-ray photoelectron spectroscopy, XPS), and electrochemical behaviour is presented in this work. Among the substituted single phase electrodes, (Ti_0.9Zr_0.1)O₂ exhibited the best reversible capacity of ∼160 mA h g⁻¹, at the end of the 60^th cycle in the voltage range 1.0–2.6 V, with a capacity fade of 24% from the 2^nd to the 60^th cycle. Among the composite electrodes, 0.05NiO–0.1Nb₂O₅–0.9TiO₂ shows the best performance which is comparable to pure TiO₂ but with a slower capacity-fade on extended cycling. The worst performing electrode is (Ti_0.9V_0.1)O₂ with a reversible capacity of only ∼70 mA h g⁻¹ at the end of 70 cycles with a current density of 130 mA g⁻¹ in the voltage range 1.0–2.6 V and a capacity drop of 52% from the 2^nd to the 70^th cycle. The composite 0.1SnO₂–0.9TiO₂ features the highest irreversible capacity-loss. Zr-substitution into TiO₂ gives the best electrochemical performance.