Electrochemical Performance And Preparation Of Poly (Vinylidene Fluoride)-based Microporous Polymer Electrolyte

In recent years, there has been a growing demand for high-energy density rechargeable batteries for portable electronic products. The science of polymer electrolytes is a highly specialized interdisciplinary field which encompasses the disciplines of electrochemistry, polymer science, organic chemistry, and inorganic chemistry. The field has attracted ever-increasing interest both in academia and industry. Many microporous polymer electrolytes comprising polymer matrices, plasticizing organic solvents and alkali metal salts have been intensively studied for applications in rechargeable lithium batteries and other electrochemical devices. From the aspect of industrialization, microporous polymer electrolytes (MPEs) have significant advantages due to their high ionic conductivity and excellent mechanical properties. Some MPEs have high ionic conductivities of 10-3 S cm-1 at room temperature, but their thermal stability is relatively poor.Various approaches to increasing the mechanical strength have been proposed recently. Inorganic fillers, such as fume silica, zeolite, clay, Al₂O₃ or fiber have been added to strengthen the dimensional stability of gel polymer electrolytes. Missing, however, is a detailed analysis of the contribution to ionic conductivity of different fillers.Up to date, most microporous polymer electrolyte contains ethylencarbonate (EC), dimethylcarbonate (DMC) and diethylcarbonate (DEC) with lithium hexafluorophosphate as lithium salt. EC/DMC/DEC based electrolytes permit numerous charging and discharging cycles without significant loss in capacity but their thermal stability is a safety problem at high temperature (60 oC and more) owing to their volatility (DEC and DMC) and flammability. Based on the above mentioned shortage of polymer electrolyte, we have done these works as follows:1. Composite microporous polymer electrolyte membranes comprising poly (vinylidene fluoride), PVDF, poly (ethylene glycol), PEG, and 1-ethyl-3-methylimidazolium tetrafluoroborate functionalized montmorillonite (EMIm-MMT) were prepared by a phase inversion method. The EMIm-MMT was subjected to FTIR, WXRD, TGA and element analysis in order to better understand the intercalation of imidazolium cations in sodium montmorillonite (Na-MMT) through exchange with intermellar sodium ions and improved thermal stability compared to organic-montmorillonite (O-MMT). The studies of porosity, weight uptake, and ionic conductivity of composite microporous membrane were performed as a function of EMIm-MMT content. The addition of EMIm-MMT can greatly enhance the thermal stability of the polymer electrolyte membrane. The maximum conductivity at 25 oC was found to be 7.77 mS cm-1 for PVDF–PEG/6wt% EMIm-MMT.2. A novel composite microporous polymer electrolyte based on poly (vinylidene fluoride), PVDF, poly (ethylene glycol), PEG, and Li-exchanged vermiculite (Li-VMT) was prepared by a simple phase inversion technique. The prepared membrane was subjected to XRD, SEM, impedance spectroscopy. The incorporation of Li-exchanged vermiculite greatly enhanced the ionic conductivity and solvent uptake as compared to the membrane without Li-exchanged vermiculite. The Li-exchanged vermiculite plays a active role in ion transport since relatively large platelets serve as the anion and allowed for exceptionally large Li transference numbers.3. A mixture of flammable organic solvent, alkali metal salt and nonflammable room temperature ionic liquid (RTIL) has been used as a new concept electrolyte. A novel microporous polymer electrolyte based on poly (vinylidene fluoride), PVDF, poly (ethylene glycol), PEG, was prepared by a simple phase inversion technique. The mixed electrolyte was observed to be nonflammable at ionic liquid contents of 60 vol. % or more. The viscosity (range from 0.98 to 30.5 mPa·s) and conductivity (range from 9.9 to 22.25 mS cm-1) of the mixed electrolyte were discussed. The porosity, solution uptake and conductivity mechanism of polymer membranes were also discussed.