Highly Improved Rate Capability for a Lithium‐Ion Battery Nano‐Li4Ti5O12 Negative Electrode via Carbon‐Coated Mesoporous Uniform Pores with a Simple Self‐Assembly Method

Abstract

A mesostructured spinel Li 4 Ti 5 O 12 (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 sp 2‐hybridized carbon‐containing hydrophobic block is employed as a structure‐directing agent. Then the assembled composite material is crystallized at 700 °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.