Controllable Synthesis And Lithium Storage Performance Of Porous Structured Transitional Metal Oxide

With the current world non-renewable energy gets increasly depleted, electric /hybrid vehicles and portable equipments are growing popularity. Lithium ion battery has been the major choice for the energy storage system because of its many advantages, such as high energy-to-weight, power-to-weight ratios and low self-discharge rate. However, the lithium ion technology of conventional graphite based anode can not meet the requirement for high specific energy, long cycling life and fast charge/discharge of the lithium ion batteries. It is necessary to develop novel lithium ion anode materials.Herein, through wet chemical method and subsequent heat treatment, the OMC/TiO2, metal doped NiO and NiCo2O4, C@ZnMn2O4/C were designed and synthesized by tuning the structure and composition. In addition, we systematically studied lithium storage performance and discuss the charge/discharge mechanism. The main content is summaried as follows:(1) Ordered mesoporous carbons (OMCs) supported well-dispersed TiO2 nanocrystallines, denoted as OMCT5 and OMCT15 hybrids, were prepared via solvothermal synthesis route. The electrochemical performance and charge/discharge mechanism are investigated for the lithium ion batteries based on the OMCT anodes. Under the condition of current density of 100 mA g-1, the hybrid OMCT15 electrode can retain a specific capacity of 541 mA h g-1 over 60 cycles. Even when cycled at 1600 mA g-1, it could still achieve a capacity of 260 mAh g-1 and a coulombic efficiency of 98%, only a capacity of 49 mA h g-1 for OMC and 12 mA h g-1 for nanocrystalline TiO2 sample. During charge/discharge process of OMCT15 electrode, the three-dimensional ordered mesoporosity connected net facilitates the liquid electrolyte diffusion into the bulk of the electrode material, hence provides short transport length for lithium ions and electrons and fast conductive ion transport channels. The OMC in the obtained composite not only buffers the volume expantion/contraction under intercalation/deintercalation of lithium ions for nanocrystalline, but also efficiently prevents the aggregation of TiO2 nanocrystalline and the cracking or crumbling of the electrode material upon continuous cycling. Moreover, the synergetic effect between the conducting OMC matrix and TiO2 nanocrystallines is responsible for the excellent electrochemical performance of the hybrid electrode.(2) Well-dispersed Cu ions doped hierarchical hollow NiO spheres were prepared by solvothermal and susequent annealing process, using auxiliary glycine as surface active agent. This technology is simple, effective and easy to realize metal ion doping. We realized fine tuning on the hierarchically mesoporous Cu3%NiO structure via controlling the calcination temperature, also homogeneously adjustable doping of Cu component into lattice of NiO. Furthermore, we studied the interface contact impedance of fresh CuxNiO electrode and the charge/discharge cyclic stability. It is shown that an optimal chemical doping component of 3%Cu and calcination temperature of 350℃ are determined for the 3D mesoporous hierarchical CuxNiO nanostructres as LIBs anode materials with excellent Li ion storage performance. The Cu3%/NiO sample calcinated at 350℃ delivers the highest specific capacity of 550 mA h g-1 after 100 cycles with a coulombic efficiency of 97.1%, which is 2.98 and 2.0 times higher than the reversible capacity of the pure NiO and Cu5%NiO electrodes. The great improvement of the electrochemical performance can be attributed to the synergetic effects of Cu doping into the NiO lattice with an optimal level, facilitate the charge transition kinetics of the interface between the electrode and liquid electrolyte, increased the specific surface area and electrochemical active sites, accommodate the stress come from the volume change during charge/discharge. However, in the process of redox reaction, the 2D mesoporous nanosheet assembly 3D hollow CuxNiO hierarchical architectures provide the space to buffer the volume expand and short the lithium ions diffusion path length. The above aspects contribute to excellent electrochemical performance of CuxNiO electrode.(3) Mn doped hexagonal porous NiCo2O4 nanosheets were synthesized by co-precipitation method and post heat treatment prcocess. An optimal level metal doping can enhance the electron conductivity and structural stability of electrode materials. When tested as anode materials for lithium ion batteries,5%Mn-NiCo2O4 sample manifested high reversible capacity of 1425 mA h g-1 under 0.5 A g-1 current density after 80 cycles. Even cycled at 2 Ag-1 and 5 A g-1, a stable specific capacity of 968 mA h g-1 can be retained, which is approximately about 2.8 times than that of pure NiCo2O4 (358 mA h g-1) samples. Besides, ex-situ XRD and TEM measurements of 5% Mn-NiCo2O4 sample on the crystalline phase, morphological and structural changes upon lithiation/delithiuation can useful to reveal the electrochemical mechanism. The results reveal that in situ generated CoO and metallic Co, Ni nanoparticles at the discharge process, are conducive to form the interconnected porous electronic conductive network, and facilitate reversible reaction of Li2O. In addition, the newly 2D porous surface-to-surface packed microstructure of NiCo2O4 nanosheets with metal Mn doping, efficiently accommodate the volume change and prevent metal and metal oxide particles aggregation generated from the redox process of electrode.(4) The porous C@ZnMn2O4/C hybrid nanospheres were fabricated by combining solvothermal, solution dipping and post heat processing techniques. In order to allevaite the volume change and improve rate capability of the metal oxide electrode materials during charge/discharge, the carbon modification on internal and surface of porous ZnMn2O4 has been obtained by adsorption and impregnation route. The internal carbon and surface carbon layer connect ZnMn2O4 particle to form a hybrid nanospheres, enhances the structural stability of ZnMn2O4 electrode. Then, the electron conductivity and ion diffusion rate of ZnMn2O4 electrode are also enhanced. The internal carbon layer in the framework of the C@ZnMn2O4/C porous hybrid electrode, can greatly reduce mechanical stress under electrochemical process, prevent the agglomeration of ZnMn2O4 particles and the cracking or crumbling of the electrode. Furthermore, in situ nitrogen atom doping on the carbon layer generate defects, leading to enhancemewnt of the charge transition conductivity and rate capability of the electrode materials. The uniquely porous nanostructure C@ZnMn2O4/C hybrid electrode shows outstanding cycle capacity and rate ability when tested as anode electrode for lithium ion battery. Under the current density of 0.5 A g-1, the porous C@ZnMn2O4/C nanospheres electrode can retain a specific capacity of 1225 mA h g"1 over 150 cycles. Even cycled at 5 A g-1, a capacity of 621 mA h g-1 could still be achieved.