Mixed Solvothermal Synthesis Of Cobalt Oxalates Anode Materials For Lithium Ion Batteries

Along with the extensive application of lithium ion batteries, anode materials forlithium ion batteries have also received more and more attention. Recentlytransition-metal oxalates are found to be potential anode materials for their goodperformance of high reversible capacity and rate capacity.In this paper，cobaltoxalates were prepared by mixed solvothermal method in the mixed solution ofethanol and water.The influences of reaction temperature, reaction time, proportion ofethanol and water and the doped Ni on structure and morphology of products wereinvestigated by means of X-ray diffraction (XRD)， scanning eletronmicroscopy(SEM) and transmission electromicroscopy (TEM). The electrochemical test wasstudied on LAND battery tester and CHI electrochemical workstationby assemblinghalf cells. The main contents are as follows:It was found that precursors were in two crystal structures. When the reactiontime was5hours, the precursors prepared at90℃,120℃,150℃and180℃were orthorhombic β-CoC2O4. While the temperature was raised to210℃, theprecursors turned into monoclinic α-CoC2O4. When the reaction temperature wasfixed at180℃, the crystal structures of precursors changed with the reaction time.The precursors prepared in2h and5h were orthorhombic β-CoC2O4. The precursorswere monoclinic α-CoC2O4when the reaction times were10h,15h and20h. Theresults indicated that temperature and time had great influences on the structure andmorphology of precursors. Monoclinic α-CoC2O4could be obtained by raisingtemperature or extending reaction time. But it was very easy to get larger thanorthorhombic β-CoC2O4.The influence of proportion of ethanol and water on the structure andmorphology of products was also studied. The result indicated that ethanol had noinfluence on the crystal structure of precursors. But the grain size decreased with theincrease of the proportion of ethanol. When the proportion of alcohol/water was morethan1:2, the grain size of the product was almost constant. The results indicate thatthe sample prepared at the condition of V(C2H5OH):V(H2O)=1:2had the bestelectrochemical performance.The results of electrochemical tests indicated that the sample prepared at180℃for5h in the mixed solvent of V(C2H5OH):V(H2O)=1:2had the best electrochemicalperformance. Its first discharge capacity was1585.1mAh g-1at0.5C and it could stillretain700mAh g-1after70cycles.Co1-xNixC2O4was successfully synthesized by mixed solvothermal method.Theinfluences of Ni on the crystal structure and electrochemical performance had beenstudied. When x=0.1and0.3, the precursors were monoclinic. When x=0.5, theprecursors were orthorhombic. That indicated that a little amount of Ni couldpromotethe formation of monoclinic precursors. Electrochemical test results revealedthat Co1-xNixC2O4had better electrochemical performance than CoC2O4. When x=0.1,the product had the best electrochemical performance. Its first discharge capacity was1442.4mAh g-1at0.5C and it could still retain851.1mAh g-1after70cycles. Theresults indicated that a little amount of Ni could promote the decomposition of SEI incharge-discharge process. So the Ni-doped Co1-xNixC2O4had good electrochemicalperformance.