Preparation Of Ternary Metal Oxides/Carbon Nanocomposite As Anode Materials For Lithium Ion Batteries

In the past decades, lithium ion batteries (LIBs) have been widely used in our daily life due to the high power density, long cycle life, no memory effect and environmental friendliness. Anode material is an important component in LIBs. considerable efforts have been made recently in finding ideal anode materials. Although graphite has served as a reliable anode material for commercial LIBs with high reversibility, its low capacity (372 mAh g-1) can hardly meet the increasing demand for high energy/power density of next generation LIBs. Ternary metal oxides have attracted the interest of the researchers due to high theoretical capacity. However, ternary metal oxides have poor electrical conductivity and suffer from large volume change during lithiation/delithiation process, which leads to rapid capacity degradation.In this thesis, we report three kinds of ternary metal oxides/carbon composites to enhance the performance of batteries. The main details and results are as following:(1) MnFe2O4 microspheres were obtained using Mn(CH3COO)2·4H2O and FeCl3·6H2O as the raw materials under alkaline environment. MnFe2O4@C composites were successfully synthesized using glucose as carbon source with subsequent heat treatment. The as-prepared products were measured using SEM and TEM. The MnFe2O4 microspheres range from 300 nn to 400 nm in diameter and the thin carbon shells are 3-5 nm in thickness. Electrochemical properties were tested by coin cells. The MnFe2O4@C composite retains a capacity of 646 mAh g-1 after 50 cycles. However, the pristine MnFe2O4 microspheres only maintain 300 mAh g-1. The MnFe2O4@C composite deliveres 252 mAh g-1 at a high current density of 1000 mA g-1. However, the pristine MnFe2O4 microspheres deliver only～40 mAh g-1 at the current density of 1000 mA g-1. The poor rate performance of the pristine MnFe2O4 microspheres is mainly caused by their low electronic conductivity and large volume variations during the charge/discharge process. The carbon shells in MnFe2O4@C composite can not only enhance the conductivity of the electrode but also serve as protective layers to prevent the inner active materials from pulverizing, and thus maintain their high capacities(2) Nitrogen-doped three-dimensional graphene was prepared with ethylenediamine as the nitrogen source. ZnCo2O4/N-doped three-dimensional graphene composite was successfully prepared by a facile two-step method involving a solvothermal process and subsequent annealing treatment. The TEM images of ZnCo2O4/N-3DG composite further confirm that high density of ZnCo2O4 nanoparticles of 20-40 nm are uniformly dispersed on graphene nanosheets. Electrochemical properties were tested by coin cells. The initial discharge and charge capacity of ZnCo2O4/N-3DG composites is 1654 and 931 mAh g-1. The composite can maintain 1102 mAh g-1 after 150 cycles. The specific capacity of ZnCo204/N-3DG composite is 727,544,436,300, and 272 mAh g-1 after each ten cycles for the current density of 80,160,400,800, and 1600 mA g-1, respectively.(3) CUCo2O4/G composites were successfully synthesized using CoCl2·6H2O and CuCl2·2H2O as the raw materials under a solvotbermal process. The TEM images of CuCO2O4/G composites observed that high density of CUCo2O4 nanoparticles of 10 nm are uniformly dispersed on graphene nanosheets. Electrochemical properties were tested by coin cells. The initial discharge and charge capacity of CuCo2O4/G composites is 1500 and 844 mAh g-1. The composite can maintain 1040 mAh g-1 after 80 cycles. The CUCo2O4/G composite delivers a reversible capacity of 976,1010,714,419 mAh g-1 for the current of 0.1C,0.2C, 0.5C and 1C after each ten cycles, respectively. Even at the current density as high as 2C, the reversible capacity still retains approximate 211 mAh g-1.