Preparation And Electrochemicai Performance Of Transition Metal Oxides As Anode Materials For Lithium Ion Batteries

Transition metal oxides (MO, M=Fe, Mn, Co, Ni, Cu, etc.) have been paid more and more attention due to its abundance, safety,reliability and high theoretical specific capacity. They have been widely researched as anodes for lithium-ion batteries (LIBs). But they also have some disadvantages, such as poor electrical conductivity, large volume changes due to the lithiation and delithiation reactions, which result in the structural collapse and influence the electrochemical performances of anode materials. Some measures have been investigated to solve these problems. In this paper, we improve the electrical conductivity and electrochemical performance of iron oxides by zinc doping. The electrochemical performances of carbon coated manganese oxides and lithium titanate anode materials were investigated by carbonizing different carbon sources. Mesoporous ZnO was fabricated by solvothermal method. The formation of ZnO mesoporous architecture is systematically studied. And then it was coated with carbon using glucose as carbon sources. The structure of ZnO and carbon coating are researched for the influence of the electrochemical performance. The electrochemical performance can be greatly enhanced by doping and coating carbon. The main contents are summaried as following:(1) Zn-doped Fe3O4 with carbon coating was synthesized by the simple co-precipitation method and subsequent carbonization process. First, Zn-doped Fe2O3 was fabricated by co-precipitation method. The Zn/Fe molar ratio can be easily controlled by adjusting the addition amount of Zn and Fe precursors. The Zn-doped Fe3O4 particles with carbon coating were obtained after a carbonization process using pyrrole as carbon source, during which Fe2O3 was reduced to Fe3O4. XRD, XPS and HRTEM were used to investigate the structural information of the Zn-doped Fe3O4 with carbon coating. The electrochemical performances were studied using the cyclic voltammogram, electrochemical impedance spectra and galvanostatic charge-discharge test. The results show that the Zn-doped FesO4 particles with carbon coating exhibited superior cycling and rate performance. The carbon coated sample with a Zn/Fe molar ratio of 0.05:1.95 achieves the best cycling performance and rate capability. It delivers a reversible capacity of 713.6 mAh g-1 after 60 cycles at a current density of 100 mA g-1, showing an enhancement of 2.7 times than the sample without carbon coating, indicating that the carbon coating is favorable to the enhancement of cycling performance. It achieves the capacities of 670.9,625.0,599.1, 566.2,454.2, and 223.3 mAh g-1 at current densities of 100,200,400,800,1600 and 3200 mA g-1, respectively, which is 1.6 to 3.4 times compared to the undoped samples with carbon coating at the corresponding current densities. This result indicates that the doping process can improve the rate capability. Thus, the enhanced electrochemical performance of the Zn-doped Fe3O4 with carbon coating is attributed to the Zn-doping in Fe3O4 and the nitrogen-doped carbon coating for improving the electronic conductivity and Li-ion diffusion kinetics, and the nitrogen-doped carbon coating for buffering the volume change.(2) Manganese (II) oxide (MnO) nanoparticles with nitrogen-doped (N-doped) carbon coating were fabricated via an efficient carbonization process using pyrrole, acrylonitrile and pyridine as different carbon sources, respectively. The galvanostatic charge-discharge test, cyclic voltammogram and electrochemical impedance spectra were used to investigate the electrochemical performance, combing with the measurements of XRD, XPS, TG and Raman tests, which can explain the relationship between the electrochemical performance and the different carbon from various carbon sources. HRTEM images reveal the uniform carbon coating on the MnO nanoparticles. XPS spectra demonstrate the presence of pyridinic N and pyrrolic N in the carbon coating. The nanocomposite using pyridine as carbon source exhibited superior cycling and rate performance, which could deliver a reversible capacity of 634 mAh g-1 after 100 cycles at the current density of 100 mA g-1 and the capacities of 59、510、440�'�310 mAh g-1 at current densities of 100,200,400 and 800 mAg-1, respectively. From the comparison of the electrochemical performances of the MnO and N-doped carbon nanocoposites derived from different carbon sources combing with XPS results, pyridinic N contributes more to the improvement of cycling performance and rate capability than pyrrolic N.(3)The nanocomposites of LTO with nitrogen-doped carbon coating were prepared at different temperatures using two carbon sources of pyrrole and pyridine. XRD, XPS, Raman and TG testing were used to analyze the structural characterization. The galvanostatic charge-discharge test, cyclic voltammogram and electrochemical impedance spectra can investigate the electrochemical performance, on which the influence of different carbon sources and carbonization temperature can be illustrated combing with the structural information. The nanocomposite derived from carbonizing pyridine at 600℃ reveals superior cycling and rate performance.It could deliver a reversible capacity of 322.1 mAh g-1 after 100 cycles at the current density of 100 mA g-1 and the capacities of 336.2,294.2,271.3,246.6 and 208.5 at 100,200, 400,800 and 1600 mA g-1, respectively. Compared with the electrochemical performance of pure LTO and the N-doped nanocomposites derived from carbon sources of pyrrole and pyridine under different temperatures, the carbon content and carbonization temperature play a key role in cycling performance and rate capability.(4) Strawberry-like ZnO mesoporous architectures with size of about 1 μm have been synthesized by a one-pot solvothermal approach in the presence of methanol, zinc nitrate hexahydrate (Zn(NO3)2·6H2O), and polyvinyl pyrrolidone (PVP), which was utilized to control the size, morphology, and agglomeration of ZnO particles. This approach is based on the hydrolysis and condensation of zinc nitrate hexahydrate in methanol under a solvothermal condition. The morphology, dispersion and porosity can be obtained by the measurement of SEM, HRTEM and BET. The as-obtained strawberry-like ZnO mesoporous particles are monodispersed and the size distribution is narrow. The strawberry-like ZnO with carbon coating was obtained by the pyrolysis of glucose. SEM, HRTEM and XRD were used to analyze the influence on the morphology and components by carbon coating. The electrochemical performance of strawberry-like ZnO with carbon coating was measured by the Land battery test system and electrochemistry workstation for the galvanostatic charge-discharge test, cyclic voltammogram. After carbon coating, the strawberry-like ZnO could deliver a reversible capacity of 463.5 mAh g-1 after 200 cycles at the current density of 100 mA g-1 and the capacities of 416.3,253.8,157.1 and 101.5 mAh g-1 at current densities of 100,200,400 and 800, respectively. When the current density increased to 500 mAh g-1, the capacity still maintain at ca.224.5 mAh g-1 after 800 cycles. Comparing to the electrochemical performance of strawberry-like ZnO without carbon coating, the electrochemical performance can be increased by carbon coating and the special structure of ZnO is beneficial to the enhancement of electrochemical performance.