Synthesis and electrochemical performance of graphene-Li₂MnSiO₄ composite

Graphene-Li₂MnSiO₄ composite cathodes for lithium ion batteries were successfully synthesized by hydrothermal assisted sol-gel method. XRD (X-ray diffraction), SEM (scanning electron microscope) and EIS (electrochemical impedance spectroscopy) were used to characterize the component, structure and morphology of the obtained composite materials. Electrochemical performance of the as-prepared materials was tested when composited with different amount of graphene oxide (2%, 4%, 6%, 8%, 10% and without graphene oxide). The experiment results indicated that composite materials belong to the orthorhombic Pmn21 space group and the addition of graphene oxide did not change the structure of the materials. Graphene and Li₂MnSiO₄ material were homogeneously composited with each other and it could be clearly observed the relatively transparent and thin layer graphene in the edges of the materials. The micro-scale particle of all the samples was agglomeration of nano-crystallites. When composited with appropriate amount of the graphene oxide, the particles became loose and some micropore structure was formed. The conductive network graphene could greatly promote the liquid electrolyte to pass through particles, facilitate the electron transport and restrain particles agglomeration. The sample with 6% graphene oxide yielded the best electrochemical performance in all the samples (the carbon content is 8.25%). Without calculating the carbon mass, this material delivered an initial discharging capacity of 166 mAh/g, and retained 101 mAh/g after 20 cycles at 1.5~4.8 V with a current density of 10 mA/g. Besides, compared with the pristine, the graphene-Li₂MnSiO₄ composite materials delivered an excellent rate performance. The improvement of specific capacity and rate performance was due to that the interconnected network structure of graphene oxide acted a key role in stabilizing the structure of composite materials and inhibiting structural damage in the charge-discharge process. Moreover, the surface charge transfer resistance of the Li₂MnSiO₄ was significantly decreased when composited with graphene oxide. It means that the enhancement of electronic conduction ability is one of the main reasons that contribute to improvement of the electrochemical performance of composite materials. Thus, the graphene is considered to be of significance in improving material electronic conductivity and enhancing the reversible lithium intercalation/deintercalation capacity of Li₂MnSiO₄ cathode materials.