| With the fast development of technology and the world economy, people are willing to live a better life. The conflict between energy supply and energy demand is becoming more and more serious. In order to deal with the increasingly serious energy crisis and cater to the theme of environmental protection, developing sustainable and environmental friendly new energy system has become the urgent matter. Secondary battery or rechargeable battery draws a widely public attention due to its convenience for fast energy storage with the ability to transfer chemical energy to electrical energy without pollution. Among the batteries, lithium ion batteries (LIBs), with the advantages of high energy density, high output voltage, low self discharge, wide work temperature, no memory effect and long lifespan, have gained widely application as the energy storage of portable electronic devices. Furthermore, lithium ion batteries are also promising power sources for hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), pure electric vehicles (PEVs) and storage green sources representing with wind, solar, tide energy, geothermal energy in smart grid. However, the safety of lithium ion battery needs to be improved. Liquid electrolyte for lithium ion batteries present potential safety problems due to the existence of large amount of organic solvents which are easy to get on fire and cause explosion because of misusage, short circuits or local overheating. In order to greatly improve the security and reliability of lithium ion battery, it’s very urgent to develop new electrolyte systems. Using the polymer electrolyte can avoid the leakage problem that is caused by traditional liquid electrolyte used in lithium ion battery and improve the safety and energy density of the battery. Besides, it can make the battery become thinner, lighter and more ductile. Cellulose, as a natural polymer with rich resource, is environmental friendly and cheap which makes it a possibility that cellulose can be used as a material during the polymer electrolyte research.The dissertation includes three major parts. Firstly:the preparation of methyl cellulose polymer electrolyte. Secondly:the preparation of composite polymer electrolyte with high performance. Thirdly:the preparation of porous carboxyl methyl cellulose polymer electrolyte.1. The preparation of methyl cellulose polymer electrolyteCellulose is abundant, cheap and environmental friendly. Besides, it can be well dissolved and the solution has high viscosity and good film-forming performance. In this research, we used water-soluble cellulose, methyl cellulose (MC), as raw materials and water as solvent to make polymer membrane MC. The obtained MC membrane was characterize by infrared spectrum (FT IR), scanning electron microscope (SEM), thermogravimetric analysis (TG), Differential scanning calorimetry (DSC) and so on. After the MC membrane absorbing organic polymer membrane electrolyte LB315, the polymer electrolyte MC is prepared. The ionic conductivity of the electrolyte (0.2 mS cm-1) is close to the commercial membrane Celgard 2730 (0.21 mS cm-1) at room temperature. Moreover, the lithium ion transference number of the polymer electrolyte MC(0.29) is a little higher than the commercial one (0.27), which help the lithium ion mobile during the charge and discharge process in the battery and increase the electrochemical performance of the battery. This methyl cellulose polymer electrolyte used in lithium-ion battery with little damage to the environment, can greatly reduce battery costs. This makes the new polymer electrolyte MC into practical application.2. The preparation of porous carboxyl methyl cellulose polymer electrolytecarboxymethyl cellulose (CMC), as a kind of cellulose with better water solubility and film-forming properties, is closely related to our life. It is widely used in food, medicine, construction and other field. In this research, we use CMC as raw materials and water (H2O) as solvent,non solvent N, N-dimethyl formamide (DMF) as non-solvent to make a series of porous polymer membranes.The obtained polymer membranes were characterized by infrared spectroscopy (FT-IR), scanning electron microscope (SEM), thermogravimetric analysis (TG) and Differential scanning calorimetry (DSC). After absorbing organic electrolyte LB315, the polymer electrolyte CMC was prepared. Amonga the series of polymer membranes, CMC-2,which used 2.5mL non-solvent, has the highest ionic conductivity (0.48 mS cm-1) at room temperature. Besides, the the lithium ion transference number of the porous polymer electrolyte CMC-2 reaches 0.46,which is much higher than the commercial Celgard 2730 (0.27).3. The preparation of composite polymer electrolyte with high performancePolymer electrolyte membrane plays an important role on the performance of lithium ion battery. We can improve the comprehensive performance of polymer electrolyte membrane through modification of the polymer.In order to improve the safety, mechanical properties and electrochemical properties of the polymer electrolyte MC, we used the electrospun PVDF and achieved a new composite membrane PVDF/MC/PVDF with sandwich structure. The composite membrane showed high safety and good electrochemical performance. The cost of production is also very low. After absorbing the LiPF6 electrolyte, the PVDF/MC/PVDF electrolyte was obtained. The ionic conductivity of PVDF/MC/PVDF electrolyte reaches 1.5 mS cm’1, much higher than the commercial Celgard 2730(0.21 mS cm-1) at room temperature. Besides, the lithium ion transference number of composite polymer electrolyte PVDF/MC/PVDF(0.47) is much larger than the Celgard 2730 (0.27). The electrochemical performance of polymer electrolyte PVDF/MC/PVDF was evaluated by LiFePO4, the results showed the battery which used the PVDF/MC/PVDF as electrolyte delivered higher discharge capacity and better rate performance. This suggest that the composite polymer electrolyte PVDF/MC/PVDF shows great attraction to the large-capacity battery requiring high safety and low cost. |
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