Core-Shell-Type Carbon-Coated Lithium Titanium Oxide Composites Derived from Ti-Based Metal Organic Framework As Anode Materials for Lithium Secondary Batteries

Abstract

Lithium titanium oxide, Li4Ti5O12 (LTO), is one of the most promising anode materials for Li secondary batteries. Although it has a lower specific capacity of approximately 175 mAh/g than does graphite (372 mAh/g), this material shows the unique property of the negligible lattice change in the Li-ion insertion/desertion process, providing excellent high-rate cycling stability. The biggest obstacles of limiting the application of LTO material electrode are poor electrical conductivity and low lithium-ion diffusion coefficient. To overcome these problems, in this work carbon-coated LTO (LTO/C) composites with core-shell-type structure are synthesized and it is derived from Ti-based metal organic framework (Ti-MOF). For the synthesis of the LTO/C composites, Ti-MOF was chemically lithiated by n-BuLi and heated at different temperature (500-800 oC) in an inert atmosphere. According to powder X-ray diffraction analysis, the as-lithiated sample shows an amorphous structure, and the LTO structure is generated after heat-treatment of above 600 oC. Transmission electron microscopy clearly shows core-shell-type LTO particles surrounded by carbon layers with a thickness of 3-5 nm. The electrochemical measurements clearly demonstrate the promising functionality of the present LTO/C composite as anode for lithium secondary batteries. The LTO/C composite obtained by heating at 700 oC show the largest discharge capacity (cal. 152 mAh/g) with good cyclability. The present experimental findings underscore the validity of the use of MOF as a useful platform for new synthetic strategies of carbon coated LTO materials and also for the improvement of their functionality.