A Facile Supercritical Alcohol Route for Synthesizing Carbon Coated Hierarchically Mesoporous Li4Ti5O12 Microspheres

Abstract

Hierarchically mesoporous Li4Ti5O12 (LTO) microspheres with a conductive layer coating are considered one of most promising structures to enhance high-rate performance as well as to retain high volumetric energy density. Herein, hierarchically mesoporous LTO microspheres with carbon coating are synthesized through a simple, supercritical alcohol route. The influence of varying synthesis conditions including concentration, solvent, reaction time, and calcination on the physicochemical and electrochemical properties of the LTO microspheres is carefully examined. Mesoporous LTO are synthesized at a short reaction time of 15 min in supercritical alcohols without using any structure-directing chemicals or templates. The use of supercritical methanol (scMeOH) results in a higher degree of surface modification, which retards the crystal growth more effectively when compared to supercritical ethanol (scEtOH) and supercritical isopropanol (scIPA). During heat treatment under a 5% H-2/Ar condition, carbonization of the organic groups attached to the surface of LTO effectively restricts particle growth and reduces the surface Ti4+ to Ti3+. At rapid charge-discharge rates of >8 C, or at long cycles of >50, the discharge capacities of the carbon-coated LTO are ordered scMeOH > scEtOH > scIPA. The higher degree of surface modification from scMeOH results in LTO with higher carbon content, higher Ti3+ content, larger BET surface area, smaller average pore size, and larger porosity when compared to scEtOH and scIPA, which resulted in better electrochemical performance. The formation mechanism of the unique, hierarchically mesoporous structure in the supercritical alcohols is also discussed.