Chemical Syntheses And Electrochemical Performances Of Polyanion Compounds As Positive Electrodes For Lithium And Sodium Ion Batteries

As one of the wide energy form, it is important to realize a clean and efficient utilization of energy for the production and storage techniques of electricity. Lithium ion batteries （LIBs）, which dominated the portable electronic device market, is an irreplaceable energy carriers. At the same time, LIBs is also a prominent option for windy, optical and electrical energy storage. However, it is still necessary to improve the energy and power density of LIBs. However, the natural resource of lithium is not abundance in the Earth’s crust, the rapidly increasing demand on LIBs could continually increase the price of lithium, and then hinder the development of LIBs in large-scale energy storage systems such as electric vehicles and power grids. Sodium is an abundance natural resource,inexpensive and unlimited everywhere on earth. Therefore, Na-ion batteries （SIBs） are regarded as promising alternatives to the well-developed Li-ion batteries. The lack of longlife cycle performance of stable-cathode material is the main bottleneck to hinder the energy storage and conversion of SIBs. It is still a challenge in electrochemical performance and practical applications until develop a stable insertion/desertion material.As a member of cathode family, such as transition-metal oxides, spinel and organic compounds, polyanion materials possess higher thermal stability, operated potential and security. Polyanion compounds have attracted great interest since the first report on the electrochemical performance of LiFePO4. Among the types of polyanion compounds, lithium orthosilicate Li2MSiO4 has attracted considerable attention due to its high theoretical capacity,333 mAh g-1 if two lithium ions in per unit formula can be extracted reversibly. Unfortunately, Polyanion compounds suffer from their poor intrinsic electronic conductivity and slow lithium ion diffusion rate, resulting in large polarization and poor rate performance, and then limit the large scale application in LIBs and SIBs. The dissertation should focus on strategies of the controllable synthesis of mirco-nanostructures and surface structures of phosphates and silicates via simple solution and solid method. The effect of mirco-nanostructures and surface structures of these materials on the electrochemical performance have been also investigated in this dissertation, no matter applications on LIBs or SIBs. The main points are summarized as follows:（1） The hollow carambola and jujube-seed shaped Li2FeSiO4 assembled by nanoplates have been successfully synthesized by a simple hydrothermal method. The different morphologies are induced by the two different iron precursors, which determines the final morphologies. Uniform carbon coverage at the surface of Li2FeSiO4 using beta-cyclodextrin as a carbon source has been achieved via annealing. The intermediate particles were detected by XRD and TEM at different reaction times to confirm the possible formation mechanism. As lithium-ion battery cathodes, the hollow carambola. Li2FeSiO4/C composite exhibits better electrochemical performances than the jujube-seed one. This morphology with inner hollow and outside open structure provides accessible electronic and ionic channels, supplies a lager specific surface area enhancing the contact area between the electrolyte and active particles, indicating more active sites during the electrochemical reaction procedure.（2） The Li₂MnSiO₄ is successfully synthesized by hydrothermal reaction, and then annealed in the presence of citric acid to obtain its carbon-coated composite. Interestingly, the hydrothermal synthesized Li₂MnSiO₄ shows a shuttle-like shape, which is composed of primary particles with average size of 50-100 run. After carbon coating, these nanoparticles are separated from the secondary particles, and the thickness of carbon layer is about 2-4 nm. The optimized hydrothermal reaction is at 150℃ for 48 h by comparing the electrochemical performance of LIBs. The Li₂MnSiO₄/C nanocomposite delivers a high discharge capacity of 206 mAh g-1 at the first cycle, and slowly decreases tol33 mAh g-1 at the end of 50 cycles.（3） In order to improve the cycling stability of Li₂MnSiO₄ and the discharge capacity of Li2FeSiO4, partial substitution of Mn-site with Fe has been performed improve the electrochemical performance. Hierarchical mesoporous Li2Mn0.5Fe0.sSiO4 consisted of nanoparticles or nanoplates has been successfully synthesized for the first time by different reactive temperature and pressure. Their carbon-coated composites are obtained by a heat treatment in the presence of citric acid. The initial capacity of 330 mAh g-1 of should indicate the second lithium ion is basically extracted/inserted, approaching the theoretical capacity of 333 mAh g-1. The electrochemical impedance spectra, together with the calculation of the lithium diffusion coefficient, reveals that the charge-transfer resistance is the smallest and the Li-ion diffusion coefficient is the largest of hierarchical mesoporous Li2Mn0.5Fe0.5SiO4/C consisted of nanoparticles. This superior electrochemical property could be as cribbed to mesoporous structure, the shorten lithium diffusion length and the enhancement of the electronic conductivity as well as the diffusion coefficient of lithium ions.（4） Mn-doped LiTi₂（PO₄）₃ nanoparticles, LiTi2-xMnx（PO4）3, were prepared by a hydrothermal reaction and then a high-temperature annealing in air. If the final annealing was conducted in Ar/H2, disordered surface with Ti4+/3+ and Mn3+/2+, and oxygen vacancies would be formed in the nanoparticles. The following electrochemical measurements suggest that the disordered surface and the oxygen vacancies of these nanoparticles effectively promote the charge transfer not only across the interface between electrolyte and the active material, but also inside the active material, then greatly improving their cycling stability and rate capability. This strategy is promising to be developed as a general strategy for other polyanion compounds, such as Li3V2（PO4）3, LiVPO4F, Na3V2（PO4）3, Na3V2（PO4）2F3. This synthesis is convenient and cost-effective, avoiding surface coating with carbon, which is at the expense of the tapping density and volume energy density.（5） Tunnel-structured Na0.54Mn0.5OTi0.51O2 nanorods have been synthesized by a facile molten salt method. These nanorods are grown in the direction normal to the sodium-ion tunnels, greatly shortening the diffusion distance of sodium ions and benefiting the transfer kinetics. Thus, the nanorods show significant enhancements in terms of reversible capacity, cycling stability and rate capability. The electrochemical performance could be further promoted via carbon coating.