Insight into effects of graphene in Li4Ti5O12/carbon composite with high rate capability as anode materials for lithium ion batteries

Li4Ti5O12/carbon composites have shown promising high rate capability as anode materials for lithium ion batteries. In this paper, unique effects of graphene in Li4Ti5O12/carbon composites on electrochemical performances are focused by means of comparing Li4Ti5O12/graphene with Li4Ti5O12/conductive carbon black (CCB) and Li4Ti5O12. The investigated anode materials are synthesized by a facile hydrothermal method. The amount of graphene or CCB in the Li4Ti5O12/carbon composites is about 3 wt% measured by thermogravimetric (TG) analysis. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) show that Li4Ti5O12/graphene consists of small sized Li4Ti5O12 nanocrystals supported on graphene nanosheets, while Li4Ti5O12/CCB comprises Li4Ti5O12 nanocrystal aggregates coated nearly by graphited carbon. The electrochemical performances of these samples as anode materials for lithium ion batteries are investigated by galvanostatic charge-discharge method. Li4Ti5O12/graphene provides a superior rate capability. At the high current density of 1600 mAg(-1), the reversible capacity after 200 cycles is still more than 120 mAh g(-1), which is about 40% higher than that of Li4Ti5O12/CCB. Cyclic voltammetry (CV) demonstrates that stronger pseudocapacitive effect occurs on Li4Ti5O12/graphene than on Li4Ti5O12/CCB. This derived from the structure features that graphene-supported small Li4Ti5O12 nanocrystals provide more surface active sites for the lithium ion insertion/extraction. The strong pseudocapacitive effect is responsible for the improvements of capacity and high-rate capability. Further, electrochemical impedance spectra (EIS) show that Li4Ti5O12/graphene electrode have lower charge transfer resistance and smaller diffusion impedance, indicating the obvious advantages in electrode kinetics over Li4Ti5O12 and Li4Ti5O12/CCB. The results clarify the positive effects of graphene in Li4Ti5O12/carbon composites as anode materials for lithium ion batteries. (c) 2013 Elsevier Ltd. All rights reserved.