| Polymer lithium ion battery has the higher specific energy than liquid lithium ion battery, moreover, it can resolve the problems of leakage and unsafety in liquid electrolyte lithium ion battery, and the shape of polymer lithium ion battery can be designed flexibly as well. But the high-rate performance decreases because the ionic conductivity of such electrolytes is low. Therefore, it has practical meaning to improve the performance for the polymer lithium ion battery. In order to provide the performance for the polymer lithium ion battery, new types of gel polymer electrolytes (GPEs) based on polymer are under developing; moreover, LiMn2O4 rods as cathode materials were prepared by using Poly(vinylidene fluoride) (PVDF) fibers membranes as template, and then their electrochemical properties were investigated. Some significant results of the research work were obtained.1. A novel gel polymer electrolyte based on star polymerα-CD-PMMA18 was prepared due to star polymer electrolytes have higher ionic conductivity than the linear polymer, and then its electrochemical properties were investigated. First, the functionalized macroinitiator based on theα-Cyclodextrins (α-CD) bearing functional bromide groups was synthesized. Then, well-defined star shaped polymers withα-Cyclodextrin (α-CD) core linking PMMA-block arms were synthesized by atom transfer radical polymerization (ATRP). The star polymerα-CD-PMMA18 was characterized with 1H NMR and FTIR analysis. Finally, gel polymer electrolytes (GPEs) were prepared by encapsulating electrolyte solution of 1 mol/L of LiClO4 /EC-PC (volume 1:1) into the obtained star shaped polymers host. The ionic conductivity of the GPEs and the cycling characteristics of LiCoO2/GPEs/Graphite cell were studied by electrochemical impedance spectroscopy and charge-discharge test, respectively. The results indicate that the GPEs has a high ionic conductivity up to 1.63×10-3 S cm-1 at room temperature and exhibits a high electrochemical stability potential of 4.8 V (vs. Li/Li+). The discharge capacity was 138.2 mAh/g for first cycle and reduced to 135.5 mAh/g after the 20th cycle, the reversible capacity maintained about 98% of its initial discharge capacity. Discharge capacity of the model cell with GPEs is stable with charge-discharge cycling. It is of great potential application in polymer lithium-ion batteries.2. In order to improve the porous structure of the PVDF macroporous membrane, thus facilitating electrolyte uptake and ionic migration, the PVDF/PEO-b-PMMA gel polymer electrolytes were prepared, and then its electrochemical properties were investigated. A block copolymer of poly(ethylene oxide)-block-poly (methyl methacrylate) was successfully synthesized via atom transfer radical polymerization (ATRP). First, polyethylene glycol monomethyl ether (PEO) terminated with a halogen atom (PEO-Br) was prepared by esterification of PEO with 2-bromoisobutyryl bromide. Next, bromo-terminated poly(ethylene oxide)-block-poly(methyl methacrylate) (PE0-b-PMMA) diblock copolymers were obtained by the polymerization of methyl methacrylate using PEO-Br as the macroinitiator and CuBr/bpy complex as the catalyst. The PE0-b-PMMA block copolymer were characterized with 1H NMR,gel permeation chromatography (GPC) and FTIR analysis. Subsequently, a novel macroporous polymer electrolyte based on PVDF/PEO-b-PMMA blends was prepared by a phase inversion technique, in which PE0-b-PMMA block copolymer obviously improved the porous structure of polymer membrane, thus facilitating electrolyte uptake and ionic migration. The highest ionic conductivity of macroporous PVDF/PEO-b-PMMA membrane at room temperature reached 2.79×10-3 S cm-1 at 30% of the blending mass fraction of PEO-b-PMMA in polymer matrix. At this account, the polymer electrolyte possessed the lowest activation energy for ions transportation. Results from electrochemical evaluation show that the electrochemical window of the GPE is above 5.0 V (vs. Li/Li+) , which can meet the requirements of polymer lithium ion batteries. All this shows that this kind of polymer electrolyte is of great potential application in polymer lithium-ion batteries.3. The PVDF/PMMA/PVDF gel polymer electrolytes were prepared due to the poor mechanical stability of PMMA gel polymer electrolytes and the leakage of electrolyte solution of the PVDF fibrous film, and then its electrochemical properties were investigated. The PVDF fibrous film was prepared by electrospinning method, then a novel kind of sandwiched polymer membrane was prepared, which consists of two outer layers of electrospun poly(vinyl difluoride) (PVDF) fibrous films and one inner layer of poly(methyl methacrylate) (PMMA) film. Its characteristics were investigated by scanning electron microscopy and X-ray diffraction. In this novel sandwiched structure, the solubility of PMMA into liquid electrolyte is greatly alleviated, and the mechanical stability is improved and the ionic conductivity is increased. The outer PVDF layers present porous structure, which provides pathways for ions into and from the GPEs, and the middle PMMA layer is solid and has good capacity to absorb liquid electrolytes. The membrane can easily absorb non-aqueous electrolyte to form gel polymer electrolytes (GPEs). The resulting gel polymer electrolytes had a high ionic conductivity up to 1.93×10-3 S/cm at room temperature and improve the leakage of electrolyte solution of the PVDF fibrous film. Results from electrochemical evaluation show that the electrochemical window of the GPE is above 4.5 V, which can meet the requirements of polymer lithium ion batteries. All this shows it is of great potential application in polymer lithium-ion batteries.4. In order or search for safe polymer electrolytes for lithium ion battery, the PVDF/PE/PVDF gel polymer electrolytes were prepared in this chapter, and then its electrochemical properties were investigated. The PVDF fibrous film was prepared by electrospinning method, a macroporous PE membrane was prepared by a phase inversion technique. Then a novel kind of sandwiched polymer membrane, which consists of two outer layers of electrospun poly(vinyl difluoride) (PVDF) fibrous films and one inner layer of poly ethylene (PE) film, was prepared. Its structure was investigated by scanning electron microscopy. The membrane can easily absorb non-aqueous electrolyte to form gel polymer electrolytes (GPEs). The resulting gel polymer electrolytes had a ionic conductivity up to 0.63×10-3 S/cm at room temperature, and exhibited a high electrochemical stability potential of 4.5 V (vs. Li/Li+). It is of great potential application in polymer lithium-ion batteries.5. The LiMn2O4 rods as cathode materials were prepared by using Poly(vinylidene fluoride) (PVDF) macrofibers membranes as template because nanomaterials have good electrochemical performance, and then its electrochemical properties were investigated. The PVDF fibrous film was by electrospinning method, then LiMn2O4 rods as cathode materials for lithium-ion batteries were prepared by using Poly(vinylidene fluoride) (PVDF) macrofibers membranes as template. The morphology, structure of LiMn2O4 rods were characterized by Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD). Subsequently, its electrochemical properties were studied by charge-discharge test. These results showed that LiMn2O4 rods possess typical spinel structure and exhibit an excellent cycling stability and rate capability as a cathode material for rechargeable lithium batteries. This method presents low cost and is easily for industrialization, which will produce great influence on the production of lithium ion batteries.
|
PDF Full Text Request