Preparation And Properties Of Polyethylene Glycol-based Gel Polymer Electrolytes

As an essential part of the energy storage and conversion system,the non-aqueous secondary battery has the outstanding advantages of long cycle life and high energy density,which can meet the requirement of the rapidly expanding electric vehicle,energy storage and portable electronic devices market.As far as batteries are concerned,the electrolyte is an integral part,known as the "blood" of the battery,which has been extensively and deeply studied by researchers.Liquid electrolytes used in the conventional secondary Lithium ion based batteries have a risk of leakage and flammation,especially it will result in the catastrophic consequence once the battery is damaged.In addition,the insufficient impregnation will lead to uneven distribution of the electrolyte during the battery assembly process,as well as the decomposition of electrolyte during the formation,further deteriorating the performance of batteries due to the formation of air bubbles inside.Therefore,researchers began to resort to other types of electrolytes,in which polymer electrolytes are considered to be the most promising candidates.However,compared with liquid electrolytes and solid electrolytes,polymer electrolytes are trapped in obvious shortcomings such as low ionic conductivity and narrow electrochemical window,so it is particularly urgent to improve these performances of polymer electrolytes.Gel polymer electrolytes(GPE)combine several advantages of liquid and solid electrolytes,such as acceptable ionic conductivity,low reactivity with electrode materials,and low risk of leakage,which are favored by researchers,therefore the extensive research has been carried out.In this paper,two methods were employed to design and synthesize different gel polymer electrolytes by chemical cross-linking and physical blending respectively.By means of a series of physical and chemical characterizations,the corresponding electrochemical parameters were obtained in details and both of electrolytes showed favorable performance.The content of this dissertation is delivered as follows:(1)A novel polyethylene glycol-based gel polymer electrolyte was prepared by a chemical cross-linking route.The schematic diagram of the synthesis and possible structure of GPE is shown in depth,and the changes of active functional groups were characterized by Fourier transform infrared spectroscopy(FTIR)to verify the reaction process.SEM showed the as prepared GPE was tightly and thoroughly impregnated into the glass fibre separator,forming the composite structure.At the same time,the mechanical intensity of GPE film was also enhanced due to the supporting of separator.The more than 400 h cycling was acquired with the assembled Li/GPE/Li,originating from the excellent interfacial stability.(2)The relevant electrochemical tests were also carried out on the as prepared GPE,and the linear potential scan showed that the electrochemical window of the asprepared GPE reached 5.2 V.Comparing the CV curves of GPE and LE batteries,the results show that for the same electrode materials,the CV curves are basically identical to each other.The conductivity test shows that the conductivity of the prepared GPE reaches 3.10×10-3 S cm-1 at 25℃.Using the ionic conductivity data at different temperatures,the corresponding activation energy is calculated to be 9.68 kJ mol-1.GPE also shows excellent compatibility when applied to sodium-ion battery systems and some conventional anode material systems.(3)Another GPE was prepared by physical blending PVDF-HFP,PMMA and PEO firstly,and then being fully immersed in the liquid electrolyte.The further investigation revealed the crystallinity of the polymer decreased owing to the amorphous region increased,resulting in the enhanced chain segment movement.The ionic conductivity of GPE was detected at different temperature to be 2.05×10-3 S cm-1 at room temperature.Additionally,there was no obvious redox peak at 0～5.0 V in LSV curve.The mechanism of the improved ionic conductivity was analyzed furthermore.The Na3V2(PO4)3/GPE/Na battery exhibited the accepatable rate and cycling performance,implying that GPE was possible to be applied in rechargeable Na-ion batteries.