Fluorine-doped SnO2 nanoparticles anchored on reduced graphene oxide as a high-performance lithium ion battery anode
Graphical abstract
Introduction
Lithium ion batteries (LIBs) have got successful application in portable electronics owe to the rapid development of battery technique over the recent years [1], [2]. In addition, LIBs have been considered as one of the most promising power sources for large-scale applications, such as the booming electric vehicle and the industrial energy storage system [3], [4], [5]. The currently researches are mainly focus on the developing of new electrode materials with high energy density and long-term cycle stability to meet the increasing demand in large-scale energy storage applications [6], [7].
A class of transition metal oxides have been developed as potential alternatives to commercial used graphite anode for high-capacity LIBs due to their much higher theoretical capacities than graphite (372 mAh g−1) [8], [9], [10]. Among these metal oxides, tin dioxide (SnO2), which is a semiconductor and been widely used in optoelectronic devices and gas sensors, has attracted particular interest as anode material attributing to its high theoretical capacity (1494 mAh g−1), abundance, low cost, nontoxicity and environmental benignity [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]. However, SnO2 anode suffers from severe volume change (up to 300%) during the lithiation-delithiation process which is common to anode materials based on alloying/dealloying mechanism [11], [12]. This would lead to the stress-induced pulverization of the electrode and loss of electric contact between the active material and the current collector, resulting in rapid capacity fading upon cycling.
Tremendous efforts have been devoted to resolve this problem and the corresponding cycling stability of SnO2 has been markedly improved. The main modification approaches could be classified into two categories. One strategy is to synthesize nano-sized SnO2 with unique geometrical morphologies that could sustain the strain during the volume variation. Various nanostructured SnO2, including 1D nanorods/nanotube/nanowires/nanobelts, 2D nanosheets, 3D nanoparticles or hollow/porous structures, have been designed as anode material for LIBs with improved performances [13], [14], [15], [16], [17], [18]. Another effective way is to incorporate SnO2 with carbonaceous materials, such as graphene, carbon nanotubes and amorphous carbon [19], [20], [21], [22], [23], [24], [25], [26], [27]. It's suggested that the carbon matrix can not only restrict the volume strain (cushion effect) but also improve the electrical conductivity of the composite.
Beyond that, ion doping could be another effective mean to enhance the electrochemical performance of SnO2 anode. In this respect, it's worthwhile to note that the conductivity and electrochemical reversibility of SnO2 can be significantly improved by doping with Cl, Sb, Mo or F atoms [28], [29]. Due to the similar ionic size of F− and O2−, F has been regard as the most efficient doping agent for SnO2 among these dopants, which could achieve a resistivity as low as 5 × 10−4 Ω cm [30]. Hence, F doped SnO2 has gained increased interest and been extensively investigated for its diverse applications, such as gas sensors [31], transparent conductors [32] and photocatalyst [33]. However, F doping is seldom adopted for SnO2 modification and the performance of F doped SnO2 anode has not been well studied [34], [35]. Therefore, we expect to combine the advantages of F doping, nanostructure and carbonaceous material incorporation so that the composite material could exhibit outstanding battery performance.
Herein, we present a facile strategy to synthesize fluorine-doped SnO2 nanoparticles anchored on reduced graphene sheets by means of a hydrothermal method. The structure and morphology of the composite material were systematically studied and the electrochemical performance were evaluated using coin-type cells. The electron transportation and Li ion diffusion in the electrode material were greatly enhanced by the synergistic effect of F doping, nanostructure and reduced graphene oxide incorporation. As a result, the as-synthesized anode material exhibited excellent electrochemical performance, such as high reversible capacity, excellent cycling stability and superior rate capability.
Section snippets
Synthesis of F-SnO2/rGO composites
Graphene oxide (GO) aqueous suspension was prepared from natural graphite powders (Shanghai Yifan Graphite Co., Ltd) by means of a modified Hummers' method [36]. The homogeneous GO suspension (2.5 mg mL−1) was exfoliated through ultrasonication for 30 min before use. All other chemicals, purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), were of analytical grade and used without further purification. The composites of fluorine-doped SnO2 nanoparticles anchored on reduced
Results and discussion
F-SnO2/rGO nanoparticles were successfully synthesized by a simple hydrothermal method. SnCl2 and NH4F were used as the source of Sn and F, respectively. Sn2+ was first bonded with GO sheets through electrostatic force and then nucleated and grow into ∼8 nm F-SnO2 nanoparticles during the hydrothermal process. Finally, these nanoparticles were uniformly anchored on the surfaces of rGO sheets to form the F-SnO2/rGO composite. It should be noted that the GO could be partially reduced into rGO
Conclusions
In summary, F-SnO2/rGO composite was successfully synthesized through hydrothermal method with a subsequent annealing procedure. Fluorine-doped SnO2 nanoparticles were uniformly anchored on the surfaces of rGO sheets with a high mass loading. The electron transportation and Li-ion diffusion in the electrode material were greatly enhanced by the synergistic effect of F doping and rGO incorporation. As a result, the F-SnO2/rGO anode delivered a large reversible capacity of 1037 mAh g−1 after 150
Acknowledgements
This work was financially supported by National Natural Science Foundation of China (Grant No. 21471119).
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