Continuous synthesis of Li4Ti5O12 nanoparticles in supercritical fluids and their electrochemical performance for anode in Li-ion batteries

• Continuous synthesis of Li4Ti5O12 (LTO) using supercritical fluids.
• Addition of Li source prior to calcination is required to obtain phase-pure LTO.
• Supercritical water/air-calcined LTO exhibits excellent capacity at below 4 C.
• Supercritical methanol/H2-calcined LTO exhibits better rate performance at above 8 C.

A continuous supercritical fluid process is adopted for the synthesis of lithium titanium oxide (Li4Ti5O12, LTO) nanoparticles for applications in lithium ion batteries. The effect of various synthetic conditions, including concentration, residence time, precursor ratios, and supercritical fluids on the phase purity and particle properties are examined. The as-synthesized samples in supercritical water (scH2O) or in supercritical methanol (scMeOH) exhibit nanoparticles with sizes of 4–10 nm, but retain an amount of Li of 6.8–8.6 mol.% less than that of the stoichiometric Li content in LTO. The deficient amount of Li is added and calcined in an air or H2/Ar flow at 600 °C. The calcined LTO exhibits phase-pure LTO with high crystallinity. The air-calcined LTO synthesized in scH2O exhibits an initial discharge capacity of 174.2 mAh g−1 at 0.1 C, good rate performance of up to 4 C (133.4 mAh g−1), and excellent long-term cyclability for up to 200 cycles. The H2/Ar-calcined LTO synthesized in scMeOH exhibits an ultrathin and uniform carbon layer on the nanosized LTO with a thickness of 0.5–1 nm. It thus shows much better high-rate performance for charge–discharge rates of above 8 C compared to the air-calcined LTO synthesized in scH2O.