Study On Solid Polymer Electrolytes Modified By Hybridization And Comb-grafting And Their Applicability

All-solid-state lithium ion battery is one of the hotspots in the development of next generation battery. The key technology is to develop an eligible solid electrolyte. Currently, varieties of electrolytes are under research, which can be categorized into inorganic material and polymer. Due to flexibility, solid polymer electrolyte (SPE) can be adapted to the widely used coating and roll-to-roll technologies in present lithium ion battery industry. This is the major reason for this thesis focusing on the SPE. Specifically, the thesis prepared two promising electrolytes:one is based on poly(ethylene oxide)(PEO) hybridized with metal-organic framework Zn4O(1,4-benzenedicarboxylate)3(MOF-5), the other is mainly composed of a novel comb polysiloxane grafted with two side chains. The compatibility of electrolyte/electrode interface was investigated to reveal the working mechanism of solid polymer battery. Finnally, solid batteries with good cycling performance were prepared, which laid a good foundation for future practical application. The major results are summarized as follows:(1) MOF-5samples with different size-levels were synthesized through solvothermal method. The effects of deprotonation agent, temperature, solvent quantity and the mixing method of reactants on its properties were investigated, indicating that the deprotonation of organic ligand is the major limiting factor of MOF-5formation. When H2O is used as deprotonation agent, MOF-5is formed only at temperature higher than80℃and the crystal size is increased with increasing temperature, reaching around20μm at100℃. When employing TEA as the deprotonation agent, MOF-5with20nm can be obtained at25℃. The average size of the sample prepared by dripping method is about4μm. The Langmuir specific surface areas are1152m2g-1,920m2g-1and784m2g-1for size-levels of20μm,4um and20nm, respectively, demonstrating the sample with bigger crystal size tends to have larger specific surface area.(2) The PEO-based electrolyte was first modified through hybridization with nano-sized MOF-5. The ionic conductivity, lithium ion transference number, compatibility with electrodes and battery performance are significantly improved. A highest ionic conductivity of3.16×10-5S cm-1at25℃for the composition of PEO-LiN(SO2CF3)2(EO:Li=10:1, denotes molar raio of ethylene oxide unit in PEO to lithium salt)/10wt.%MOF-5is obtained, which is four times higher than the electrolyte without MOF-5. Meanwhile, the lithium ion transference number is increased from0.17to0.29at60℃. The anodic steady window of the composite electrolyte is4.57V at60℃, which is decided by the limit of MOF-5. The interfacial stability of electrolyte/lithium interface becomes more stable after incorporation of MOF-5. The reversible capacities and Coulombic efficiency of LiFePO4/Li solid cells are also increased at both60℃and80℃. At80℃, the cycling performance is significantly improved, retaining45%capacity after100cycles at1C rate, while there is a sharp decay after30cycles in the cells without MOF-5filling.(3) A kind of novel comb polysiloxane (PCSMS) grafted with two side chains with different polarity was designed and synthesized. Based on PCSMS, a kind of solid electrolyte membrane, which can be easily cut and rolled, was prepared and assembled into solid cells with good cycling performance at room-temperature (25℃). As for PCSMS, one side chain has a cyclic ethylene carbonate group which helps solvate lithium ion, and the other side chain is tripodal siloxane with six oxygen atom which benefits lithium ion transportation due to its flexibility and coordinating ability. The ionic conductivity of the best modified composition reaches7.43×10-5S cm-1at25℃with lithium ion transference number of0.204. The electrochemical window is4.95V. Moreover, the electrolyte does not decompose apparently until temperature is as high as259℃. The assembled LiFePO4/Li solid cell can operate at25℃very well, delivering specific capacity of124mAh g-1at a0.3C rate and retaining78%capacity after150cycles.(4) The fading mechanism of PCSMS based LiFePO4/Li solid cell at80℃was revealed based on micro-analysis of electrolyte/electrode interface and a new method for AC impedence spetra resolution of solid battery. After failure of the solid cell, the ionic conductivity of the electrolyte and the charge transfer resistance at the anode do not experience notable change, whereas the charge transfer resistance at the cathode increases sharply and the related lithium ion diffusion coefficient decreases more than3orders of magnitude, which is proved to be related to the formation of passivated film at cathode interface. The passivated film is a result of decreased electrochemical stability at higher temperature. By sandwiching between PCSMS-based electrolyte and LiFePO4cathode a layer of PEO-LiTFSI/MOF-5composite electrolyte, the cycling performance is significantly improved. Additionally, the Li4Ti5O12/Li cell using PCSMS-based electrolyte can retain92%capacity after500cycles.