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

摘要

Lithium titanium oxide, Li_4Ti_5O_(12) (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°C) 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°C. 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°C 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.