Li4Ti5O12/graphene nanoribbons composite as anodes for lithium ion batteries

LIBs are also explored as promising for electric vehicles (EVs), hybrid electric vehicles (HEVs) and energy storage systems in terms of their high power density, long cycle life and high safety. Lithium titanate (Li4Ti5O12, LTO) is regarded as a favorable anode material, since the traditional carbon/graphite materials have shown some critical issues including poor cyclic life; and high reactivity with the electrolyte solution that easily contributes to the thermal runaway of batteries under certain reported conditions. LTO with a theoretical capacity of 175 mAh·g−1 has excellent Li+ insertion and extraction reversibility in the voltage range of 1.0–2.5 V. Additionally, LTO has a very flat voltage plateau close to 1.55 V (vs. Li/Li+), which sufficiently avoids the formation of metallic lithium, thus resulting in improvement of the safety of lithium-ion batteries. However, one practical problem associated with unmodified Li4Ti5O12 is its poor rate performance, resulting from its inherent low electronic conductivity and moderate Li+ diffusion coefficient. Graphene nanoribbons (GNRs) are strips of graphene with outstanding electronic properties. GNRs have been used in a wide range of device materials. Recent theoretical and experimental studies have shown that GNRs can enhance lithium storage capacity through edge effect. On the basis of the unzipping mechanism, a large number of edge sites are created during the formation of GNRs, which might produce more electrochemically active sites for charge transfer during charge/discharge when compared to graphene and carbon nanotubes. A modified Hummer’s method was used to synthesize graphene oxide nanoribbons (GONRs). The GONRs formed from the above process was then reduced thermally. Anatase TiO2 and Li2CO3 were used as raw materials. Li4Ti5O12 was prepared by a solid-state reaction method. The stoichiometric amounts of Li2CO3 and anatase TiO2 with molar ratio of Li:Ti = 0.82:1 were mixed with ethanol as dispersant by planetary ball-milling. The ball-milled mixture was heated in a furnace at 800 °C in air for 18 h, followed by mechanical crushing to obtain the final Li4Ti5O12. LTO/GNRs (5% wt GNRs) composites was formed by mechanical mixing method. The morphological and structural properties of the GNRs were determined using X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy coupled with energy dispersive spectroscopy (SEM/EDS). Battery performance was conducted on a Maccor testing system. Electrochemical performance was tested with a Biologic potentiostat. Coin cells assembly and electrochemical measurements were carried out between 1.0 and 2.5 V vs Li+/Li0 with CR2032 coin cells. The capacity of the cell was calculated on the basis of the total mass of the active material (LTO). The XRD micrographs of graphene oxide nanoribbons (GONRs), thermally-reduced graphene nanoribbons (GNRs), LTO, and LTO/GNRs materials were generated and are presented in Fig. 1. The GONRs exhibited a highly crystalline structure, with a pronounced peak at 2θ = 10.8° including a small peak at 2θ = 25.6°, indicating that traces of the starting material of carbon nanotubes were still present in the sample. Figure 1a also displays XRD of the GNRs materials reduced from GONRs without the peak at 2θ = 10.8°. The peak at 2θ = 25.2° suggests that GONRs were completely reduced to a graphite-like structure. Peaks for GNRs were absent in LTO/GNRs composites, and have shown the same typical peaks with spinel LTO as can be seen in Fig. 1b. The TEM and SEM structures of GONRs, LTO and LTO/GNRs are presented in Fig. 2. As observed from the TEM image in Fig. 2a, MWNTs were completely unzipped to form GNRs. The TEM image in Fig. 2b shows the morphology of LTO particles wrapped around GNRs. The SEM image in Fig. 2c shows the morphology of LTO/GNRs composites. The specific capacities determined of the obtained composite at rates of 0.2, 0.5, 1, 2, and 5 C are 206.5, 200.9, 188, 178.1 and 142.3 mAh·g−1, respectively. This is