Template-free synthesis of hierarchical porous anatase TiO2 microspheres with carbon coating and their electrochemical properties

•A simple supercritical alcohol route to synthesis hierarchically mesoporous TiO2.•Primary TiO2 nanoparticles were coated with ultrathin carbon layer.•Mesoporous TiO2 exhibited excellent reversible capacity of 212 mA h g−1 at 0.1 C.

Hierarchically porous anatase titanium oxide (TiO2) microspheres were synthesized using a green supercritical methanol route over a very short reaction time of 15 min without using templates or surfactants. Primary nano-sized particles with diameters of 20–55 nm with organic coverage on the surface were loosely aggregated and formed secondary micron-sized particles 1.0–2.5 μm in diameter, creating a porous structure with average pores 9–15 nm in diameter. When the as-synthesized microspheres were calcined under a Ar/5% H2 condition, carbonization of the organic groups formed an ultrathin and uniform carbon layer on the nano-sized primary particles with a thickness of 0.5–1 nm and reduced some of the Ti4+ to Ti3+. Both the hierarchically porous structure and the conductive layer coating had positive effects by increasing Li ion storage capacity. The prepared TiO2 microspheres exhibited a high reversible discharge capacity of 212.3 mA h g−1 at 0.1 C, a high-rate performance of 77.9 mA h g−1 at 8 C, and an excellent capacity retention of >97% at the end of 100 cycles at 1.0 C, whereas TiO2 nanoparticles without porous structure and surface modification exhibited lower discharge capacities of 161.8 mA h g−1 at 0.1 C and 5.2 mA h g−1 at 8 C, and poorer capacity retention of 26%. The considerable improvement in the electrochemical performance was attributed to the nano-sized TiO2 primary particles, porous structure, and carbon coating and Ti3+ incorporation.