Preparation And Electrochemical Performance Of Transition Metal Oxides/Carbon As Anode Materials For Lithium Ion Batteries

Lithium ion batteries (LIBs) are becoming more and more valued by researchers and enterprises with the development of portable electronic devices, electric vehicles and hybrid electric vehicles, due to the advantages of high working voltage, high energy density, long repeat service life, low self-discharge rate, non-memory effect and environmental benignity compared to other secondary batteries such as lead acid batteries, nickel cadmium batteries and nickel metal hydride batteries. Nowadays, most widely used commercial graphite anode materials have low capacity, which could not meet the demand of high energy density. Transition metal oxides have high theoretical capacity, which are one of the most promising anode materials for LIBs. However, transition metal oxides suffer from poor cycling performance and rate capability due to large volume shrinkage/expansion during the charge/discharge processes, the aggregation of metal particles and the poor conductivity. The strategies to deal with the problems mentioned above include constructing unique structures (nanostructure or porous structure), doping and compositing with carbon materials etc. Among them, compositing with carbon materials to construct hybrid nanocomposites can not only enhance the electrical conductivity and the diffusion of lithium ion, but also maintain the structural integrity of anode materials since carbon served as a buffer to cushion the stress from volume expansion and prevented the aggregation of nanoparticles.Therefore, in this work, MnO/N-doped carbon (MnO/N-C) nanocomposites have been successfully synthesized by a facile thermal-decomposing method. Porous Co3O4 nanoflakes/CNTs (Co3O4/CNTs) nanocomposites have been prepared by a hydrothermal method and post-processing in Ar atmosphere. The nanocomposites of CNTs with homogenously anchored MoO2 nanoparticles have been successfully synthesized by a hydrothermal method. The nanocomposites of Fe3O4 encapsulated into CNTs have been produced by immersing and thermal-decomposing method. And then detailed analysis about structure and composition has been made. The enhanced electrochemical performance of anode materials in this paper has been verified through contrast experiments. The main research contents are listed as following:(1) MnO/N-C nanocomposites have been synthesized by thermal-decomposing the mixture of manganese acetate and glycine, which was employed as carbon source to obtain N-doped carbon. MnO nanoparticles with sizes of 10-20 nm were dispersed in N-doped membranous carbon matrix to form loosely agglomerated small clumps of composites. Besides, agglomerated MnO nanoparticles with sizes of 20-60 nm were prepared without glycine as comparison sample. At the current density of 100 mA g-1, after 150 cycles, MnO/N-C and MnO deliver stable reversible capacities of 562 and 634 mAh g-1, respectively. The results suggest that the N-doped carbon in MnO/N-C reduced the specific capacity of MnO/N-C. However, the rate capability of MnO/N-C is better than that of MnO. The fitted results of EIS show that the N-doped carbon in MnO/N-C can improve the electronic conductivity and lithium ion transfer, indicating that the enhancing of electronic conductivity and lithium ion transfer has a far greater impact on the rate capability.(2) Porous Co3O4 nanoflakes with pore sizes of 20-80 nm intertwined with CNTs have been synthesized by a hydrothermal method and post-processing in Ar atmosphere. For comparison, the pure Co3O4 nanoflakes were synthesized following the same procedure without adding CNTs. The cycling stability and rate capability of Co3O4/CNTs nanocomposites are much better than that of Co3O4 and CNTs. The fitted results of EIS show that the electronic conductivity and lithium ion transfer of Co3O4/CNTs nanocomposites are enhanced compared with Co3O4 and CNTs. Furthermore, the intertwined CNTs with a certain amount of flexibility prevent the electrode from isolating or cracking during the lithiation/delithiation processes, resulting in a superior cycling stability and rate capability. Nyquist plots of the sample Co3O4/CNTs collected from varied cycles reveal that the impedance of the Co3O4/CNTs electrode tends to increase and then decrease with the cycles increased, corresponding to the specific capacity reducing in the initial several cycles and then increasing slowly within 40 cycles. The decrease in the impedance after the initial 5 cycles backs up the realization of the activation process. The structure evolution of C03O4/CNTs after cycling seen in SEM is responsible for the activation.(3) M0O2/CNTs nanocomposites have been synthesized by a facile hydrothermal approach with the assistance of glucose, which was employed as the reducing agent and the structure directing surfactant. The M0O2 nanoparticles anchored on CNTs had sizes of 20-50 nm. The preparation procedures of the composites of CNTs with M0O3 nanobelts (MoO3/CNTs) and MoO2 nanoparticles were the same as those of the MoO2/CNTs nanocomposites only without adding glucose or CNTs in the hydrothermal process, respectively. The cycling stability and rate capability of Co3O4/CNTs nanocomposites are much better than that of Co3O4 and CNTs. The CNTs network in the nanocomposites not only provides a continuous longdistance pathway for electron transfer, but also acts as a favorable buffer to the aggregation of the anchored MoO2 nanoparticles and the volume change, which maintains the integrity of electrode during the lithiation/delithiation processes, finally resulting in the enhanced electrochemical performances. The hybridization of MoO2 nanoparticles with CNTs is proved to be an efficient way to improve the electrochemical performances of LIBs anode.(4) Fe3O4/CNTs nanocomposites with Fe3O4 confined into CNTs have been synthesized by the combination of immersing approach and thermal decomposition. Fe3O4-11/CNTs and Fe3O4-32/CNTs with mass fraction of Fe3O4 is 11.4% and 32.1%, respectively, which were prepared by changing the solution concentration in immersing process. For Fe3O4-11/CNTs, most of the Fe3O4 nanoparticles are encapsulated into CNTs, while for Fe3O4-32/CNTs, a large amount of nanoparticles are anchored onto the outside walls of CNTs. In addition, Fe3o4 (1:3) nanocomposite was chosen as control sample to evaluate the advantages and/or disadvantages of Fe3O4 into or outside CNTs, in which all of the Fe3O4 nanoparticles were anchored onto the outside walls of CNTs. Electrochemical performance analysis indicates that the cycling stability and rate capability of Fe3O4-11/CNTs is the best. Therefore, Fe3O4 nanoparticles encapsulated into CNTs can efficiently enhance the electrochemical performance. In the structure of Fe3O4 nanoparticles encapsulated into CNTs, CNTs can not only efficiently restrain the expansion of nanoparticles during cycling, but also provide 3D network channels for electrons and lithium ions. In this way, the electrode can ensure its structure integrity and achieve better electrochemical kinetics.