Synthesis of Li ₄Ti ₅O ₁₂ in supercritical water for Li-ion batteries: Reaction mechanism and high-rate performance

Nanosized and highly crystalline spinel lithium titanium oxide (Li ₄Ti ₅O ₁₂, LTO) particles are synthesized in supercritical water. The effects of various synthesis conditions - feed concentration, reaction time, and calcination - on the particle properties, including particle size, surface area, particle morphology, phase purity, and crystallinity, are carefully analyzed. Phase-pure LTO particles are obtained with a long reaction time of 6 h in supercritical water at 400 °C and 300 bar without subsequent calcination, while the anatase TiO ₂ impurity phase is detected at shorter reaction times of 5 min to 2 h. Particles synthesize in supercritical water with subsequent calcination at a relatively low temperature of 700 °C exhibit the highly crystalline LTO phase. Based on the analytical results using scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), and X-ray diffraction (XRD), an LTO formation mechanism in supercritical water is proposed. LTO particles prepare in supercritical water with subsequent calcination exhibit excellent long-term cyclability and high-rate performance. The discharge capacity after 400 cycles at 1C is 117.2 mAh g ^-1, which is approximately 80% of the initial discharge capacity (147.1 mAh g ^-1), and the discharge capacity at 10C is 100.5 mAh g ^-1. These electrochemical performances are significantly better than those of uncalcinated LTO synthesize in supercritical water and solid-state synthesize LTO.