| This thesis is composed of two parts. The first part is study on electrochemical properties, capacity fading mechanism and improving cycle life method of aqueous lithium ion batteries. The second part is study on preparation, characterization and electrochemical properties of ammonium polyphosphate based composites for intermediate temperature fuel cells.Aqueous lithium ion batteries have the advantages of non-aqueous lithium ion battery and traditional secondary batteries with aqueous electrolyte, which will eliminate burning, explosive crisis resulted from organic electrolyte. It has competitive potential compared with lead-acid, nickel-cadmium and some other low voltage batteries. However, the cycle life is short and the reasons on capacity fading are still not very clear at present. Moreover, the candidate with flat charge/discharge curve for the anode is very limited.Intermediate temperature fuel cells are very attractive because they combine the advantages of high- and low-temperature fuel cells. This kind of fuel cells avoids the catalyst poisoning at low temperature and high requirement on assembling materials at high temperature. Ammonium polyphosphate based composites are promising electrolyte for this kind of fuel cells. However, the studies on conductive stability and structure transformation during measuring process are very little. Research and development on ammonium polyphosphate based composites with high conductivity and high stability will be significant to the application of intermediate temperature fuel cells.In this paper, the following contents are included:First, selection and investigation on current collector, active materials, and the pH value of the electrolyte were conducted. Reasons of capacity fading of LiNi1/3Mn1/3Co1/3O2/5 M LiNO3 (pH 11) / LixV2O5 lithium ion cell based on structure and dissolution of transition metals were investigated preliminarily. XRD and ICP results showed that the properties of the anode have more impact on the cycle life of the cell. In order to stabilize the cycling capacity of the anode, thus improve the cycle life of the cell, coating with an ionic conductive polyaniline (PAn) on the surface of the anode was proposed. Cycle tests revealed that the stability of the lithium ion cell with surface coated anode has greatly improved. The improving mechanism of surface coating by PAn was also investigated.More details on capacity fading during cycling process of LiMn2O4/ LixV2O5 lithium ion cell with 5 M LiNO3 aqueous solution as electrolyte was investigated. In order to improve the cycle performance of the as-assembled cell, coating with an ionic conductive polypyrrole (PPy) on the surface of the anode was proposed via in-situ polymerization method. Cycling tests revealed that the stability of the lithium ion cell with surface coated anode was greatly improved. Moreover, the effect of different coating amount of polypyrrole on the cyclability of the lithium ion cell was investigated. The force between electrode and current collector affected by PPy coating was also investigated.Some polyanionic compounds, e.g. TiP2O7 and LiTi2(PO4)3 with 3D framework structure were proposed to be used as anodes of lithium ion battery with aqueous electrolyte. The TiP2O7 and LiTi2(PO4)3 give capacities of about 80 mAh g-1 between potentials of -0.50 V and 0 V (vs. SHE) and 90 mAh g-1 between -0.65 V and -0.10 V (vs. SHE) , respectively. A test cell consisting of TiP2O7/5M LiNO3/ LiMn2O4 delivers approximately 42 mAh g-1 (weight of cathode and anode) at average voltage of 1.40 V, and LiTi2(PO4)3/ 5M LiNO3/ LiMn2O4 delivers approximately 45 mAh g-1 at average voltage of 1.50 V. Lithium ion diffusion coefficient in LiTi2(PO4)3 was investigated by Cyclic Voltammogram (CV) method, and the results are 6.08×10-5,2.24×10-5cm2 s-1 respectively during charge and discharge process. The lattice parameter change with x of Li1+xTi2(PO4)3 was investigated. The capacity fading maybe related to deterioration of anode material.A proton-conductive composite of (NH4)2SnP4O13 was synthesized by solid-state method. X-ray diffraction (XRD), scanning electronic microscopy (SEM) and thermal gravimetric analysis (TGA) investigation were performed on this composite. The conductivity of (NH4)2SnP4O13 measured by impendence spectroscopy with the temperature range of 125 - 275℃under different atmosphere. Maximum conductivities of 13.8 mS cm-1 at 275℃under dry H2 and 35.5 mS cm-1 at 225℃under wet H2 respectively were obtained.We prepared (NH4)2Si1-xTixP4O13 composites, which have potential application in intermediate temperature fuel cells as electrolyte. The effect of Ti content on the conductivity of the composite was investigated systematically. Maximum conductivities of 0.68 mS cm-1 under dry atmosphere and 30.8 mS cm-1 under wet atmosphere respectively were obtained. Among them, (NH4)2Si50Ti50P4O13 gave good conductivity both in dry and humid atmosphere. The (NH4)2Si50Ti50P4O13 composite also showed good conductivity stability when changing the atmosphere between dry and humid gas. Moreover, the XRD results showed that the structure of (NH4)2Si50Ti50P4O13 changed after measuring in dry gas and humid gas. |
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