Synthesis And Electrochemical Performances Of ZnCo₂O₄-base Composite Materials As Anode For Lithium Ion Batteries

Recently, electrode materials with a high specific capacity, high rate performance, and long cycle life for Li-ion batteries (LIBs) are paid extensive attention due to ever-growing demand for portable electronic devices or electric vehicles. As a mainstay of anode materials for commercialized LIBs, Graphite could hardly meet the demand due to low theoretical capacity (374 mAhg-1). It is therefore necessary to find better alternative anode materials. Recently, there has emerged a great interest in Co-based spinels due to their high reversible capacities. Among Co-based spinels, ZnCo2O4 is quite attractive because of its various advantages such as low toxicity, lower cost, high thermal stability and high specific capacity. However, low conductivity and large volume change during the cycle process need to be further improved. In this presentation, ZnCo2O4 nanowires array, ZnCo2O4@CNT and ZnCo2O4@MnO2 composites were successfully synthesized. And the electrochemical performances of these ZnCo2O4-base composite materials as anode for lithium ion batteries were investigated. The main studies are as follows:(1) ZnCo2O4 nanowire synthesized by a simple hydrothermal method were characterized by SEM, XRD, TG and electrochemical method for its structure, morphology and electrochemical performance. It had been proved that the precursor ZnCo2(CO3)3 changed to ZnCo2O4 nanowire by the oxygen in air during the annealing process. The ZnCo2O4 nanowire, with a 100 nm width and 5 μm～8μm length, possessed the first discharge capacities as high as 1498 mAh/g. It still remained 612 mAh/g after 50 charge discharge cycles. Longer ZnCo2O4 nanowire can be obtained by extending the time of hydrothermal process, but its specific capacitance was lower.(2) To improve its conductivity and cycling performance of ZnCo2O4 nanowire, a layer of carbon nanotube was coated on the ZnCo2O4 nanowire, forming ZnCo2O4@CNT. The coating dynamics was proved by SEM, XRD and electrochemical measurement technology. The capacity of the composite structure was 1392 mAh/g at the first discharge, and remained at 735 mAh/g after 50 cycles. The capacitance was 570 mAh/g at a high current density of 0.5 A/g. The cycling performance and high rate capacitance of ZnCo2O4@CNT nanowire are much better than that of pure ZnCo2O4.(3) MnO2 nanoflate covered with ZnCo2O4 nanowire was synthesized by hydrothermal method by mixing KMnO4 and ZnCo2O4 together into autoclave. MnO2 nanoflate was confirmed by SEM, XRD, TEM, SAED method and so on. After electrochemical measurement, it was found that the cycling performance of this kind of composite material was dramatically enhanced, but the irreversible capacity loss also enlarge during the first cycle, and the cycling capacity was also low. The second cycling discharge capacity was only 314 mAh/g. the capacity remained 215 mAh/g after 50 cycling performance.(4) Toward to flexible substrate, we constructed flexible Li-ion battery with ZnCo2O4 nanowire composite growing on carbon fiber as anode and flexible LiCoO2 as anode. Further charge discharge performance was measured.