Ru0.01Ti0.99Nb2O7 as an intercalation-type anode material with a large capacity and high rate performance for lithium-ion batteries

Abstract

Ru_xTi_1−xNb₂O₇ (x = 0 and 0.01) materials have been synthesized via a solid-state reaction method. X-ray diffraction combined with Rietveld refinements demonstrates that both samples have a Wadsley–Roth shear structure with a C2/m space group without any impurities, and that the unit cell volume increases after the trace Ru⁴⁺ doping. Scanning electron microscopy and specific surface area tests reveal that the Ru⁴⁺ doping decreases the average particle size. The Li⁺ ion diffusion coefficient and electronic conductivity of Ru_0.01Ti_0.99Nb₂O₇ are respectively 64% and at least two orders of magnitude larger than those of the pristine TiNb₂O₇. First-principles calculations show that the increased electronic conductivity can result from the formation of impurity bands after the Ru⁴⁺ doping. Ru_0.01Ti_0.99Nb₂O₇ exhibits a large initial discharge capacity of 351 mA h g⁻¹ at 0.1 C between 3.0 and 0.8 V vs. Li/Li⁺, approaching its theoretical capacity (388 mA h g⁻¹). At 5 C, unlike the pristine TiNb₂O₇ with a small charge capacity of 115 mA h g⁻¹, Ru_0.01Ti_0.99Nb₂O₇ delivers a large value of 181 mA h g⁻¹, even exceeding the theoretical capacity of the popular spinel Li₄Ti₅O₁₂ (175 mA h g⁻¹). After 100 cycles, Ru_0.01Ti_0.99Nb₂O₇ shows a large capacity retention of 90.1%. These outstanding electrochemical performances can be attributed to its improved Li⁺ ionic and electronic conductivity as well as smaller particle size.