Preparation Of Mesoporous Manganese Oxides And Performance Study As Lithium Ion Anode

Lithium ion batteries(LIBs)have the features of high energy density, long cycling life, low self-discharge and excellent portability, serving as an ideal medium for highly effective storage and transformation of energy. However, along with the application of high energy-consuming facilities like electric vehicles, the specific n Graphite, as the anode material of commercialized LIBs, merely holds a theoretical capacity of 300-350 mAh g-1, which inevitably requires developing anode materials with higher specific capacity for the future LIBs. In this regard, ternary transition metal oxides have attracted lots of attentions as potential anode materials for LIBs, because of their excellent theoretical capacities, low cost and abundant reserves. Whereas, there still exists some problems of low electronic conductivity and poor cycling stability due to the large volume changes in charge/discharge process that need to be solved before the future application as anodes of LIBs. The nanostructured porous materials that own unique architectures can optimize charge-discharge characteristics of transition metal oxides and improve the lithium storage performance, not only able to sufficiently exploiting the theoretical capacity, but also showing a feasible path to figure out the above problems. In this thesis, via simple and controllable methods, we successfully prepared nanostructured mesoporous ternary transition metal oxides with tunable structures and morphologies, then systematically studied the influence of pore structure, grain size, specific surface and components on the specific capacity and cycle performance, and discussed the relevant reaction mechanisms. The main research results are summarized as follows:1. A relatively facile hydrothermal strategy has been successfully developed for the large-scale preparation of MOO3, MnMoO4·0.9H2O and MnMoO4 nanorods and nanocubes. It indicates that reaction temperature and additives greatly influence the dimension, size and growing mechanism of the products. The as-synthesized uniform rod- and cube-like MnMoO4·0.9H2O(JCPDS No.50-1286) with smooth surface before annealing transformed into mesoporous MnMoO4(JCPDS No.50-1287) with rough surface after annealing, but the profiles was well maintained. When evaluated as anode materials for LIBs, ternary MnMoO4 electrode shows higher specific capacity and more excellent rate capability than the pure MoO3 electrode. Especially, MnMoo4·0.9H2O nanocubes display high specific capacity of 497 mAh g-1 with a Coulombic efficiency of 99.4% after 60 cycles at a current density of 100 mA g-1, and when back to 50 mA g-1 after 60 cycles at different current densities, the reversible capacity could be stable at 770 mAh g-1, about 3 times of that of pure MoO3 electrode. The improved electrochemical performance of prepared ternary MnMoO4 electrodes is mainly attributed to good electrical conductivity of the electrode, which increased the transfer rate of ions and charges; high specific surface area and pore volume could provide more active sites for Li+ accommodation, and buffer the volume change, thereby improving the reversible capacity and cycling performance.2. Spinel structure ZnMn2O4 has been prepared via the facile coprecipitation and host-annealing process that could control the morphology to form nanoflowers, nanospheres, nanoplates by a simple variation of the solvent ratios of ethylene glycol to H2O in the hydrothermal process. The as-prepared ZnCO3-MnCO3 transformed to tetragonal ZnMn2O4 spinel(I4I/amd, JCPDS No.24-1133) through a host-annealing process. BET results showed that ZnMn2O4 possessed mesoporous structure and large specific surface area, which was beneficial to increase the contact area of the electrode/electrolyte. When evaluated as anode materials for LIBs, ZnMn2O4 electrode exhibits high specific capacity and excellent rate capability. ZnMn2O4 nanoflowers keep a reversible capacity of 1724 mAh g-1 at a current density of 100 mA g-1 after 50 cycles, and the capacity retention rate was about 81.6% of the first discharge capacity. When back to 200 mA g-1 after 80 cycles, the maintained reversible capacity of the nanoflowers still could be up to 1007 mAh g"1. Such excellent lithium-storage performance can be ascribed to excellent characteristics of spinel structure, the unique properties of nanoscale size, and the hierarchical structure, not only improving lithium storage capacity and shortening the diffusion path, but also maintaining the integrity of electrode during the lithium insertion/extraction processes.3. Nanocrystal mesoporous ZnMn2O4 has been synthesized by two kinds of cost-effective hard template methods, i.e., two-step hydrothermal method as ZMO-2s, and one-step water bath process as ZMO-ls, respectively. Both obtained ZnMn2O4 species exhibit tetragonal spinel structure with the average crystallite sizes of 9.8 and 18.4 nm, respectively. The mesopores come from the stacking and interconnection of tiny crystallite particles, are measured to be 7.46 and 8.96 nm, respectively. The electrochemical properties of ZnMn2O4 nanocrystals show that ZnMn2O4 by two-step exhibits higher specific capacity and better cycling performance. It could deliver a reversible discharge and charge capacity as high as 552 and 549 mAh g-1, respectively, even at the high current density of 200 mA g-1 after 100 cycles. Such good cycling performance of ZnMn2O4 electrode results from the abundant mesoporous channels between nanocrystals, which provided enough spaces for volume changes, and. avoided the smash of electrode.