Modification Of PVdF Fibrous Gelled Polymer Electrolyte And Its Electrochemical Study

Up to now, high-quality polymer membranes are difficult to satisfy the demand of low cost at the same time they improve the electrochemical properties of lithium-ion batteries(LIBs). Nanofibrous polymer membranes prepared by high-voltage electrospinning method have same advantages such as high porosity, good ionic conduction and low cost, which are regarded as the ideal separator for LIBs. Poly(vinglidene flonride)(PVd F) is usually used to prepare fibrous gelled polymer electrolyte(FGPE) because of its good chemical stability, high dielectric constant, and strong hydrophobicity. The resultant FGPE possesses relatively high ionic conductivity at room temperature and good mechanical strength. In fact, electrospun PVd F fibrous membrane acts as the supporting backbone and adsorption of the electrolyte in the FGPE, which has been caused much attention for its application in polymeric LIBs.It is due to possessing a large number of micrometer-sized with inter-connectivity that the absorbed electrolyte suffers from the potential issue of leakage from polymer matrix during battery operation. As a result, the electrochemical performances of the assembled polymer LIBs go to bad. Therefore, how to retain the electrolyte in the FGPE is a problem. Many attempts have been taken to resolve this problem. For example, the PVdF fibrous membrane was blended with other polymers compatible with the electrolyte based on organic carbonates. However, the blended fibrous polymer membrane would be fracture due to the dissolution of the blended polymer matrix, resulting in a potential safety accident from the short circuiting inner the battery. Accordingly, based on the mentioned problem, some modifications of the PGPE have been carried out in this article and the electrochemical properties of the modified FGPE were studied.1. Multilayer fibrous polymer membranes based on PVdF/PMMA were prepared by electrospinning technology. They were characterized by scanning electronic microscopy(SEM), thermal gravimetric(TG), linear sweep voltammetry(LSV), electrochemical impedance spectroscopy(EIS) and galvanostatic charge-discharge. The results revealed that the trilayers PVdF/PMMA/PVdF fibrous membrane had 197% of electrolyte absorption ratio and 72% of electrolyte retention ratio, respectively. The assembled Li/LiFePO4 cells with this PGPE could deliver an initial capacity of 147 mAh g-1 at the 0.1C rate, and maintained 95% of the initial capacity after 50 cycles. The results suggest the sandwich PVdF/PMMA/PVdF fibrous membrane has a good cycling performance.2. MMA monomers were polymerized in situ within the pores of the PVdF/PMMA/PVdF fibrous membrane, forming(PVdF/PMMA/PVdF)-b-PMMA polymer membrane. The morphology of the as-prepared membrane was observed by SEM and the crystallinity degree was determined from DSC analysis. The mechanical properties of the obtained membrane were also measured. The electrochemical performances were evaluated by LSV, EIS and galvanostatic charge-discharge measurement. The results suggest that the PVdF/PMMA/PVdF trilayers integrate into a whole due to crosslinked polymerization of MMA monomers in the pores of fibrous membrane, in which the middle PMMA fibrous membrane acts as the “rivet” to enforce the FGPE. The obtained membrane has a maximum tensile strength of 8.4MPa, an electrolyte absorption ratio of 244% and an electrolyte remaining ratio of 77%. The resultant FGPE has a room temperature ionic conductivity of 1.81×10-3 S cm-1 and apparent activation energy for ions transportation of 6.48 kJ mol-1. The assembled Li/LiFePO4 cells could deliver an initial capacity of 151 mAh g-1 at the 0.1C rate, and maintained 97% of the initial capacity after 50 cycles. At the rates of 0.2, 0.5, 1 and 2C, the discharge capacities are 148, 144, 140 and 135 mAh g-1, respectively. Compared with the FGPE from pristine PVdF/PMMA/PVdF fibrous membrane, the(PVdF/PMMA/PVdF)-b-PMMA derived FGPE exhibits the improved performance cycling and rate capability.3. The SiO2/PVdF nanocomposite fibrous membrane was prepared by electrospinning method through alkane treatment of the PVdF powders previously. PMMA component was grafted to the nanocomposite fibers to form the SiO2/PVdF-g-PMMA membranes. They were characterized by SEM, DSC, stress-strain test, LSV, EIS and galvanostatic charge-discharge. The results indicated that the SiO2/PVdF-g-PMMA fibrous membranes had excellent electrolyte absorption ratio and electrolyte remaining ratio, and improved tensile strength and elongation. The fibrous SiO2/PVdF-g-PMMA membrane grafted with 20 wt% PMMA possesses a high ionic conductivity up to 2.31×10-3 S cm-1 at room temperature, and a high tensile strength of 8.2MPa and elongation ratio of 86%. The assembled Li/LiFePO4 cells with this FGPE could deliver 157 mAh g-1 at the 0.1C rate, and maintained 97% of the initial capacity after 30 cycles. At the rates of 0.2, 0.5, 1 and 2C, the discharge capacities are 153, 147, 139 and 130 mAh g-1, respectively. The results suggest the FGPE possesses good cycling performance and rate capability.The research results showed that the methods of in-situ blending the PVdF/PMMA/PVdF sandwich fibrous membrane with PMMA and grafting PMMA to the SiO2/PVdF nanofibers could solve the problem of electrolyte leakage. The assembled lithium-ion batteries exhibited excellent cycling performance and rate capability.