(593c) Electrochemical Performance of Three Dimensionally Structured Sn/SnO2/Graphene Nanocomposites for Lithium Ion Battery Anode for Enhanced Reversible Capacity
A three-step method was developed to fabricate nanocomposites of Sn/SnO2/graphene for a negative electrode of lithium-ion battery. Theoretically tin oxides and graphene as a negative electrode can store over twice as much lithium as graphite. In situ of the negative electrode a nanometric matrix of Li2O is generated by the electrochemical reduction of SnO2 which can provide a facile environment for the reversible alloying of lithium with tin to a maximum stoichiometry of Li4.4Sn. But the generation of the matrix leads to a high first-cycle irreversible capacity. With the aim of reducing this irreversible capacity and improving the cyclic performances of the battery, Sn/SnO2 nanocomposites were investigated. Graphene sheets were included to this nanocomposites to reduce the volume expansion of Sn nanoparticles during lithium insertion. Lithium can be stored on both sides of graphene sheets (LiC3), inducing in a theoretical capacity of 744 mAh/g. Sn/SnO2 nanoparticles can be homogeneously distributed in the graphene nanosheets matrix. In the three dimensional structure of Sn/SnO2/graphene nanocomposites , Sn/SnO2 nanoparticles can act as a spacer to effectively separate graphene nanosheets and also Sn/SnO2 nanoparticles can be dimensionally confined by the surrounding graphene nanosheets which can limit the volume expansion of Sn nanoparticles upon lithium insertion. In this research, at first graphite oxides were prepared from graphite powders with the presence of strong oxidizing agents of anhydrous mixture of sulfuric acid, sodium nitrate and potassium permanganate. Graphite oxides were then reduced by SnCl2 to graphene sheets in the presence of HCl and urea. The reducing process was accompanied by the generation of SnO2 nanoparticles. To produce Sn/SnO2/graphene nanocomposite powders, as-synthesized SnO2/graphene powders were heat-treated under 5% H2 and 95% Ar mixed gas atmosphere at a temperature of 600°C for 1-4 h. The structure and composition of Sn/SnO2/graphene nanocomposites were characterized by means of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and electrochemical measurements by cyclic voltammetry, charge/discharge profile and cyclic performances.