Facile Synthesis of Anatase TiO2 Quantum Dot/ Graphene Nanosheet Composites with Enhanced Electrochemical Performance for Lithium-Ion Batteries

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1. Introduction With rising interest in green electrode materials for lithium-ion batteries (LIBs), increasing attention has been paid to titanium dioxide (TiO2) anode material in recent years because of its long cycle life, low cost, and minimum environmental impact. Moreover, the relatively high lithium insertion /extraction voltage of a TiO2 anode (higher than 1.5 V vs Li+/Li) can efficiently avoid the formation of SEI layers and lithium plating on the anode, which improves the safety of the batteries as compared with its carbon-based counterparts. However, many potential electrode materials (e.g., TiO2) in Li-ion batteries are limited by poor electron transport, slow Li-ion diffusion in electrodes, and increased resistance at the interface of electrode/electrolyte at high chargedischarge rates. Graphene, which has exceptional electrical, mechanical, optical, and surface properties, is widely utilized to prepare various hybrid materials. The graphene substrate itself can be contributory to the improved electrochemical performance because it may enhance the electronic conductivity of the overall electrode[1]. It is noteworthy, on the other hand, that the size and dispersion of nanoparticles on graphene are crucial factors for improving cell performance because small particle size plus good dispersion (e.g., down to several nanometers) can endow the composite electrode a superior high surface area to buffer the volume change of the particles, but it could also bring the required conductivity to individual nanoparticles and shorten the diffusion length for Li ions, which are beneficial for high lithium storage and rate capability, respectively. However, most metal oxide/graphene composites prepared so far have the high level of metal oxides accompanied with the partial aggregation of particles may also result in the rapid capacity loss and poor cycle performance. Therefore, it remains a challenge to develop a facile and general approach for the synthesis of well-dispersed MOx (e.g., TiO2) Quantum Dots/Graphene composites with favored structures for high-performance lithium-ion batteries (LIBs). 2. Experimental Section 2.1 Synthesis of graphene oxide (GO) The graphite oxide was synthesized from natural graphite flake (Alfa Aesar, 325 mesh) by a modified Hummers method. As-prepared graphite oxide was dispersed in water by ultrasonication for 30 min, followed by a low-speed centrifugation to get rid of any aggregated GO.2.2 Preparation of TiO2 quantum dots /Graphene nanosheets (TiO2-QDs/GNs) Composites In a typical experimental procedure, 5.8 g of CTAB was dissolved in a mixture of 10 ml of n-pentanol and 60 ml of n-hexane; Then, the 10 ml GO aqueous dispersion (1 mg mL-1) was slowly poured and intensely stirred for 30 min at room temperature. Subsequently, with the formation of a golden water-in-oil emulsion. Then 0.8 ml of Titanium(III) chloride was added to golden water-in-oil emulsion while stirring. The achieved transparent microemulsions were poured into a Teflon-lined stainless steel autoclave (100 ml), and then placed in an oven maintaining 200°C for 6 h. The collected precipitates were treated under reduced pressure in a rotary evaporator to remove the volatile organic reagents and then repeatedly washed with water and ethanol to remove surfactants and other impurities. The final samples were dried at 80 °C for 2 h for further characterizations. 3. Results and Discussion In summary, we report a facile method to synthesize well-dispersed TiO2 quantum dots (6±2 nm) on graphene nanosheets (TiO2-QDs/GNs) in a water-in-oil (W/O) emulsion system (Figure 1). The prepared TiO2/graphene composites displayed high performance as an anode material for lithium-ion battery, such as high reversible lithium storage capacity (190 mA h g-1 after 100 cycles), high Coulombic efficiency (over 96%), excellent cycling stability and high rate capability (as high as 144 mA h g-1 at 10 C, 135 mA h g-1 at 20 C, 124 mA h g-1 at 30 C and 101 mA h g-1 at 50 C, respectively). Very significantly, the preparation method employed can be easily adapted and may offer an attractive alternative approach for preparation of the highly dispersed nanosized graphene-based nanostructured composites as promising applications in various energy-storage devices high performance electrodes for various energy-storage devices.Figure 1 Electrochemical measurements of (a) TiO2-QDs/GNs and (b) TiO2-QDs electrodes. Inset in Fiure 1 is the Schematic of synthesis steps for TiO2-QDs/GNs composite.