Carbon-coated, hierarchically mesoporous TiO2 microparticles as an anode material for lithium and sodium ion batteries

Highlights: • Hierarchically porous anatase TiO₂ microparticles synthesized in sc-methanol. • Nanosized TiO₂ particles were carbon-coated using organic surface modifiers. • The fast nucleation and carbon coating reduced the crystal growth. • Small crystallite size and electronic network enhanced electrochemical performance. • As an anode in NIBs, 275 mAh g⁻¹ at 0.1 C and 40 mAh g⁻¹ at 10 C were delivered. -- Abstract: Hierarchically porous anatase TiO₂ microparticles are synthesized in supercritical methanol (scMeOH) in the presence of organic surface modifiers such as oleylamine, oleic acid, and poly(ethylene glycol)methyl ether/citric acid (PEGME/CA) mixture. Primary TiO₂ nanoparticles (5–9 nm) that loosely aggregate to form secondary micron-sized particles (0.2–1.5 μm) are obtained in the presence of PEGME/CA. The surface modifier aids the effective suppression of undesirable crystal growth because their molecules cap the surfaces of growing particles in scMeOH. An ultrathin, conformal and uniform carbon layer with 1–2 nm thickness is then formed on the surface of the TiO₂ particles by heat treatment. The carbon-coated TiO₂ particles delivers 231 mAh g⁻¹ at 0.1 C after 50 cycles and 85 mAh g⁻¹ at 10 C in a lithium-ion battery cell, 275 mAh g⁻¹ at 0.1 C after 50 cycles, 40 mAh g⁻¹ at 10 C, and high capacity retention of 94% after 450 cycles in a sodium-ion battery cell. The excellent electrochemical performance of the TiO₂ particles is attributed to the small crystallite size, continuous electronic network formed by the close contact of individual carbon-coated primary TiO₂ particles, and the effective penetration of the mesopores by the electrolytes.