Highly improved rate capability for a lithium-ion battery nano-Li 4Ti 5O 12 negative electrode via carbon-coated mesoporous uniform pores with a simple self-assembly method

Abstract

A mesostructured spinel Li4Ti5O12 (LTO)-carbon nanocomposite (denoted as Meso-LTO-C) with large (>15 nm) and uniform pores is simply synthesized via block copolymer self-assembly. Exceptionally high rate capability is then demonstrated for Li-ion battery (LIB) negative electrodes. Polyisoprene-block-poly(ethylene oxide) (PI-b-PEO) with a sp2-hybridized carbon-containing hydrophobic block is employed as a structure-directing agent. Then the assembled composite material is crystallized at 700 degrees C enabling conversion to the spinel LTO structure without loss of structural integrity. Part of the PI is converted to a conductive carbon that coats the pores of the Meso-LTO-C. The in situ pyrolyzed carbon not only maintains the porous mesostructure as the LTO is crystallized, but also improves the electronic conductivity. A Meso-LTO-C/Li cell then cycles stably at 10 C-rate, corresponding to only 6 min for complete charge and discharge, with a reversible capacity of 115 mA h g-1 with 90% capacity retention after 500 cycles. In sharp contrast, a Bulk-LTO/Li cell exhibits only 69 mA h g-1 at 10 C-rate. Electrochemical impedance spectroscopy (EIS) with symmetric LTO/LTO cells prepared from Bulk-LTO and Meso-LTO-C cycled in different potential ranges reveals the factors contributing to the vast difference between the rate-capabilities. The carbon-coated mesoporous structure enables highly improved electronic conductivity and significantly reduced charge transfer resistance, and a much smaller overall resistance is observed compared to Bulk-LTO. Also, the solid electrolyte interphase (SEI)-free surface due to the limited voltage window (>1 V versus Li/Li+) contributes to dramatically reduced resistance.