Study On Novel Gel Polymer Electrolytes For Rechargeable Lithium Ion Batteries

Lithium ion battery based on gel-type polymer electrolyte (GPE) is the new generation of rechargeable system developed from liquid lithium ion battery. It possesses high energy density, long cycle life, negligible leakage, light weight and flexible shape. The polymer electrolyte lithium battery is attractive power source applied in electric vehicle, cellular phones and others. The main technical problems of polymer electrolytes include: first, although the ionic conductivity can reach the magnitude of 10-3S/cm, which can satisfy the practical request, it is still lower than that of liquid electrolyte (10-2S/cm). The low ionic conductivity will limit the discharge rate and low-temperature performance of the battery system. Second, swelling of the pure gel-type polymer membranes due to absorbing a lot of liquid electrolytes leads to the poor mechanical property of GPE. In general, the methods of enhancing the ionic conductivity are unfavorable for the mechanical strength. Third, porous GPE often faces potential safety issues arising from liquid leakage. The core problem of GPE is how to coordinate ionic conductivity and mechanical properties. The existing problems of different types of GPEs (non-porous or porous gels) are also different. In order to solve these problems, we have designed and prepared fluorine-containing polymer electrolytes and hybrid polymer electrolytes. In addition, A modification of the pore wall and pore center in multi-pore membranes have been also conducted. The more details about membrane structure, morphology and performance have been studied, and the ion transfer mechanism in GPE has also been investigated. They are summarized as follows:1. Fluorine containing gel polymer electrolytesNovel gel network polymer electrolytes containing fluorine and sulfonic acid lithium are prepared by the UV polymerization of poly(ethylene glycol) dimethylacrylate (PEGDMA)/ 2-acrylamido-2-methyl propanesulfonic acid (AMPS)/trifluoroethyl methacrylate (TFEM). They are activated by uptaking the solutions of 1.0 M LiPF6/EC/DMC to prepare gel network polymer electrolytes. The characteristics of the polymer networks including gel fractions, thermal stability, liquid electrolyte uptake and electrochemical properties have been studied. When the weight ratio of PEGDMA/AMPS/TFEM is 60/35/5, the network polymer electrolytes show the highest liquid electrolyte uptake (144%) and ionic conductivity (about the order of 10?3 S/cm) at room temperature (25℃). The network polymer electrolytes are electrochemically stable up to 5.0 V versus Li+/Li. The performances of the network polymer electrolytes suggest that they are suitable for application in high performance lithium ion batteries.2. Lanthanum oxide hybrid gel polymer electrolytesAs the atomic structure of rare earth elements with more of the 4s, 5d and 6s unoccupied orbit, and these can track the differential small, rare earth compounds (hard acid) in the lithium salt-and the anion (hard base) to form a stable complex through electrostatic attraction, to reduce mobile anion numbers in the electrolyte and suppress the polarization degree. To further enhance the acidity of rare earth compounds (lanthanum oxide), acid and lanthanum oxide root through the bond linking of the"super acid like".In order to prevent lanthanum oxide from aggregation in the polymer matrix, its bond"hanging"to the polymer network is set up. The measurements of ionic conductivity and Raman spectrum indicate that the content of anions decreases with the increase of lanthanum oxide. By examining the microstructure of the materials it is found that lanthanum oxide particles chemically bonded by"hanging"to the polymer network are uniformly dispersed in the polymer membrane.3. PMMA grafted PVdF non-woven fabric gel polymer electrolytesTo achieve high ionic conductivity in polymer electrolyte, dual phase poly(vinylidenefluoride) (PVdF) based fibrous polymer electrolytes membrane is prepared via pre-irradiation. The PVdF fiber serves as supporting phase providing dimensional stability. Poly(methyl methacrylate) (PMMA) as gel phase help for trapping liquid electrolyte and substitute non-conductive PVdF phase to contact with electrodes thus increasing conductive area. PVdF based fibrous membrane can be used as polymer electrolyte in lithium ion battery after it is activated by uptaking 1 M LiPF6/EC-DMC electrolyte solution. Its room-temperature ionic conductivity is enhanced greatly by introduction of PMMA phase, from 2.3×10-3 S/cm (pristine PVdF fibrous membrane) to 7.9×10-3 S/cm (DOG=111.8%). The coin cells containing the dual phase fibrous membrane demonstrate excellent rate performance. The combination PMMA phase to PVdF phase is a novel approach to achieve high ionic conductivity in polymer electrolytes. 4. Fluorosilicone rubber porous GPEIn the current knowledge of polymer materials, the elasticity of fluorosilicone rubber is the best. Thus, the electrolyte membrane based on fluorine silicone rubber is favorable for lighting the battery resistance fluctuation caused by volume expansion and contraction in the charge and discharge process. Moreover, the fluorosilicone rubber has excellent chemical stability and thermal stability. On the other hand, trifluoromethyl-side-groups on the fluorosilicone rubber molecular chains are favorable for lithium salt dissociation. Therefore, fluorosilicone rubber polymer is an ideal candidate for electrolyte membrane. But fluorosilicone rubber electrolyte and the weak capacity of affinity, not conducive to the absorption of electrolyte and maintain. In order to obtain the satisfaction of electrochemical properties of the base material of its prepared porous membrane, and use hole in the wall precipitated a loose pro-modified inorganic particles electrolyte layer. Study of the membrane morphology of the film, the type of lithium battery performance, and so on.5."Pore/bead"gel polymer electrolytesA special"pore/bead"composite membrane is prepared by facile approach. The combination of bead into pore is a novel approach to achieve polymer electrolyte with excellent comprehensive properties. The bead (MCM-41) in the composite membrane is considered as the key factor to improve puncture strength and suppress thermal shrinkage, which would greatly improve the safety of lithium ion battery. The composite membrane displays highly ionic conductivity which is attributed to the porous structure both in the binder and the bead. The battery based on the composite electrolyte show satisfactory C-rate performance. Primary results showed that it is promising as polymer electrolyte for scale-up lithium ion battery. In addition, this facile approach also provides a new avenue for further improve polymer electrolyte performances.