| The intercalation/deintercalation of Li-ion in transition metal oxide Co3O4 is based on conversion reaction, so it has a higher theoretical capacity(890 m Ah/g). And thus Co3O4 is considered as one promising potential anode material in future mass energy storage systems. But compared to the graphene which is mostly used as anode material in commercialized lithium ion battery, Co3O4 has some problems such as the large volume changes during the charge/discharge process and its semiconductor property, leading to the poor cycle stability and low electronic conductivity.To solve these two main problems, in this paper, hydrothermal/solvothermal reaction has been used to prepare different morphology of Co3O4, which may limit the effects of volume changes during the conversion process. And then Co3O4/graphene composite material was prepared by high energy ball-milling method, to increase the conductivity of the original material. The results of phase analysis and morphology characterization were mainly achieved by XRD, SEM and TEM. The electrochemical performances of Co3O4 and Co3O4/graphene composite were tested by constant current charge-discharge experiment and cyclic voltammetry testing. So we can find out the relationship between morphology and electrochemical properties, and also the modification of Co3O4. The main research methods and results of the paper are as follows:(1)Solvothermal reaction at 150 ℃, 170 ℃ and 190 ℃ was used to prepare Co3O4. SEM and TEM results indicate that morphologies of Co3O4 from low temperature to high temperature followed by porous floc, transition from porous floc to sheet-like and non-porous sheet. CV results show that all three kinds of material have four current peaks in the first week of discharge, which are related to the Pre-embedding of Li+, the transition from Co3O4 to Co O, the transition from Co2+ to Co, the formation of SEI film and interfacial lithium storage, respectively. The results of charge-discharge experiment show that all the anodes have the high discharge capacity in the first week, which can respectively reach 1634.91 m Ah/g, 1521.67 m Ah/g and 1795.77 m Ah/g. But only Co3O4 prepared by solvothermal reaction at 150 ℃ has a good cyclical stability which can keep 1249.64 m Ah/g reversible capacity, and also has a good rate capability. Both of 170 ℃ and 190 ℃ has a severe attenuation of capacity.(2)Three kinds of Co3O4 were prepared by hydrothermal reaction at 950 ℃, 110 ℃ and 120 ℃. The phase characterization show that as the temperature increases, the morphologies are dandelion, transition from dandelion to lamellar-like and lamellar. And all of them have a lot of porous on the micro-surface morphology. CV results are similar to solvothermal reaction, the electrochemical reaction at each peak are identical to the former. The charge-discharge experiments indicate that these Co3O4 have a quite good performance. Discharge capacity of first week can up to 1957.82 m Ah/g, 1860.85 m Ah/g and 2005.50 m Ah/g, respectively. And after cycle 30 weeks, the capacity shows an ascending trend, almost with no attenuation. But the coulombic efficiency of these Co3O4 in the recycling process remains low, only 92.50 %, 95.65 % and 94.10 % at average.(3)Using high energy ball milling compounded graphene and Co3O4 with the morphologies of porous floc, non-porous sheet, dandelion and lamellar. By the results of SEM and TEM, the original morphologies of the four Co3O4 have different degrees of damage, but the combinations with graphene are fine. The results of electrochemical performance test show that the exist of graphene improves the performance of Co3O4 greatly. Graphene makes the cyclical stability of Co3O4 in the process of charge and discharge cycles better, and the rate performance has also improved a lot. In addition, graphene improves the conductivity of Co3O4, and the coulombic efficiency of composite materials in the recycling process has also been improved. |
