Hierarchical hollow Li₄Ti₅O₁₂ urchin-like microspheres with ultra-high specific surface area for high rate lithium ion batteries

Large specific surface area is critical for Li₄Ti₅O₁₂ to achieve good rate capacity and cycling stability, since it can increase the contact area between electrolyte/electrode and shorten the transport paths for electrons and lithium ions. In this study, hierarchical hollow Li₄Ti₅O₁₂ urchin-like microspheres with ultra-high specific surface area of over 140 m²·g⁻¹ and diameter more than 500 nm have been successfully synthesized by combining the versatile sol–gel process and a hydrothermal reaction, and exhibit excellent electrochemical performance with a high specific capacity of 120 mA·h·g⁻¹ at 20 C and long cycling stability of < 2% decay after 100 cycles. Ex situ electron energy loss spectroscopy (EELS) analysis of Li₄Ti₅O₁₂ microspheres at different charge–discharge stages indicates that only a fraction of the Ti⁴⁺ ions are reduced to Ti³⁺ and a phase transformation occurs whereby the spinel phase Li₄Ti₅O₁₂ is converted into the rock-salt phase Li₇Ti₅O₁₂. Even after 100 cycles, the oxidation–reduction reaction between Ti³⁺ and Ti⁴⁺ can be carried out much more effectively on the surface of Li₄Ti₅O₁₂ nanosheets than on commercially available Li₄Ti₅O₁₂ particles. All the results suggest that these Li₄Ti₅O₁₂ microspheres may be attractive candidate anode materials for lithium ion batteries.