Synthesis Of ZnFe₂O₄（Zn₂SnO₄）/Graphene Composite And The Doping Modification Of LiFePO₄at Low Temperature

In order to meet the increasing requirements for EV and HEV applications, great attention has been paid to design new anode and cathode materials. Compared with conventional carbon materials, transition metal oxides (TMO) can yield a rather high specific capacity through conversion reactions, such as tin, zinc oxides and Iron oxides. Besides simple oxides, some mixed oxides with the AB2O4(or A2BO4) structure also show promising applications as anodes for Li-ion batteries. However they show poor cycling stability due to large volume changes upon Li uptake/release reaction. Graphene can not only buffer the volume changes during reaction but also prevent the aggregation of the nanoparticles upon long-term cycling due to its large specific surface area and high mechanical strength.As a promising cathode material, LiFePO4has attracted the eyes because of its long cyclic life, high safety and low cost. However, the problems such as bad low temperature performance, poor conductivity, low energy density and processing performance have largely limited its Factory practical applications.Two kinds of Zn2SnO4nanocrystals/graphene (Zn2SnO4/G) nanohybrid have been prepared by a facile one-pot hydrothermal route. As the first one, Microsized Zn2SnO4crystals with an octahedral shape are firmly confined by the graphene sheets. Another Zn2SnO4nanocrystals are uniformly dispersed and immobilized by the graphene nanosheets reduced from GO. The direct restacking of the hydrophobic graphene sheets is inhibited by loading Zn2SnO4nanocrystals as the spacers. Zn2SnO4/G shows an improved electrochemical performance than bare Zn2SnO4due to the conducting, dispersing and immobilizing effects of graphene.A nanohybrid based on ZnFe2O4nanospheres and graphene nanosheets (ZnFe2O4/G) has been synthesized by a facile one-pot in situ solvothermal route. Spheric ZnFe2O4nanoparticles with a size of100nm (10nm) are confined in between the graphene sheets, forming a unique hybrid nanostructure. ZnFe2O4/G shows excellent rate capability and high-rate cycling stability for lithium storage. It also shows a high capacity when used as an anode for a ZnFe2O4/G-LiFePO4C full cell.LiFe0.98M0.02PO4doped materials were synthesized by two step solid state reation using the acetate of (Co, Mn, Ni and Zn) as doping soureces to improve its performance of LiFePO4at low temperature. The electrochemical measurements show that the2%Co-doped sample show the best performance at-20℃. This is attributed the fact that Co occupies the corresponding Fe site and weakenes the olivine structure, and that makes lithium ion esay access.