Preparation And Performance Of Graphene And SnO₂ Composite Anode Materials For Lithium-ion Batteries

Graphite has been considered as the main commercial anode material of lithium-ion batteries, however, it suffers from low capacity and poor high rate lithium storage performance. SnO2 is regarded as a potential new-generation anode material of lithium-ion batteries, due to its advantages of high capacity, low lithium inserted potential, good safety and environmental friendliness. However, its practical application has been limited due to the poor cycle life and high current charge-discharge performance, which rises from the large volume variation of SnO2 during the charge-discharge cycling. In this work, in order to obtain the new type of lithium-ion battery anode materials with excellent electrochemical lithium storage properties, graphene has been prepared by a microwave hydrothermal method using flaky graphite as raw material. Then SnO2/nano graphite sheets composite, SnO2/graphene composite and SnO2@C/graphene ternary composite have been produced.A simple approach was proposed to prepare graphene sheets by microwave hydrothermal reduced graphene oxide without any auxiliary reagents. This method was easily controlled and the graphene oxide as graphene precursor was not necessary to be neutral. The results showed that the microwave reaction pressure and reaction time were main influence factors on the degree of graphene oxide reduction. If the reaction pressure was big enough and the reaction time was long enough, the reaction temperature effect was relatively small. When the reaction pressure and time were greater than 1.5 MPa and 40 min, graphene oxide reduction degree was higher. The electrochemical performances of graphene anode for lithium-ion batteries were studied by cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge-discharge. The graphene as an anode material exhibited high capacities and excellent electrochemical performances in lithium-ion batteries. The first lithium insertion and lithium extraction capacities were 2114 and 1069 mAh·g-1 at current density of 100 mA·g-1, and the reversible specific capacity remained 733 mAh·g-1 after 100 cycles.The SnO2/nano graphite sheets composite and Sn O2/graphene composite have been prepared used nano graphite sheets and graphene sheets as buffered carrier by a one-step microwave hydrothermal method using a microwave reaction system, respectively. The lithium storage property of SnO2/graphene composite was better than that of SnO2/nano graphite sheets composite. Especially, when the content of SnO2 nanoparticles in SnO2/graphene composite was about 73.28%, its electrochemical properties are superior to the same kind of anode materials reported in literatures. The discharge specific capacity remained 1358 m Ah·g-1 after 100 cycles at current density of 100 mA·g-1. Even at a high current density of 1000 mA·g-1, the reversible specific capacity remained 677 mAh·g-1 after 1000 cycles. The size of SnO2 particles in composite was less than 3.3 nm, which can provide larger specific surface area and increase the lithium ion transport channel. In addition, the formation of the porous composite structure was advantageous to the penetration of the electrolyte, and favorable for charging and discharging at high current density.SnO2@C/graphene ternary composite material was prepared via a double layer modified strategy of carbon layer and graphene sheets. The size, dispersity and coating layer of SnO2@C were uniform. The SnO2@C/graphene had a typical porous structure. The electrochemical performances of SnO2@C/graphene were better than those of carbon spheres/graphene, SnO2@C and SnO2/graphene according to the results of comparative experiments. The first lithium insertion and lithium extraction capacities of SnO2@C/graphene were 2210 and 1285 mAh·g-1 at a current density of 1000 mA·g-1, respectively. The coulombic efficiency was 58.60%. The reversible specific capacity of SnO2@C/graphene anode was 955 mAh·g-1 after 300 cycles. SnO2@C/graphene anode had excellent rate performance. The reversible specific capacity still maintained an average of 394 mAh·g-1 even at the high current density of 5 A·g-1.The synergistic mechanism between Sn O2@C and graphene was investigated by cyclic voltammetry and electrochemical impedance spectroscopy.