Preparation Of Gel Polymer Electrolyte With Ionic Conductivity Enhancement And Its Application In Lithium Oxygen Battery

Nowadays,a shortage of fossil fuels is creating a demand for renewable energy.Therefore,energy storage devices have attracted much attention.The theoretical energy density of lithium-ion batteries?LIBs?,which have been widely used in the past two decades,cannot meet that of emerging industries such as electric cars.Therefore,lithium-oxygen?Li-O₂?batteries with a theoretical energy density close to that of gasoline have attracted extensive attention.A typical rechargeable Li-O₂batterie consists of a lithium metal negative electrode,a porous air positive electrode and a lithium ion conductive electrolyte.Li₂O₂is deposited on the cathode surface by oxygen reduction reaction?ORR?during discharge and removed from the cathode surface by oxygen evolution reaction?OER?during charging.However,lithium oxygen battery also has its limitations.During the charge-discharge cycle,lithium metal will react with electrolyte to reduce the cycling capacity,and the oxygen crossing of the positive electrode will also corrode the electrode.To solve the above problems,researchers protect the lithium metal anode and optimize the air cathode material to ensure the battery efficiency.But recent reports suggest that electrolytes are still the main cause of lithium-oxygen battery failure.For example,ether-based electrolytes react with superoxide radicals formed in the solvent during charge and discharge,resulting in the formation of insulating lithium carbonate,resulting in high electrical polarization and reduced recyclability.Some scholars proposed the use of functionalized ether-based solvents?such as DMDMB and 3-glycol?to increase the concentration of lithium ions in electrolyte,but this method is not ideal for slowing down the decomposition of electrolyte.Therefore,the development and application of new electrolytes are crucial to the development of lithium oxygen batteries.In this paper,polyvinylidene fluoride-hexafluoropropylene?PVDF-HFP?,polyethylene?PEO?,polymethyl methacrylate?PMMA?and other polymers were used to prepare gel polymer electrolyte by crosslinking polymerization.At the same time,different fillers were added to the polymer electrolyte to improve the ionic conductivity and the migration quantity of lithium ions?Li⁺?.The microstructure,element composition and chemical functional groups of electrolyte were characterized by X-ray diffraction?XRD?,FTIR Spectrometer,X-ray photoelectron spectroscopy?XPS?,scanning electron microscopy?SEM?and transmission electron microscopy?TEM?.The electrolyte was used to assemble the buckle Li-O₂batteries,and the electrochemical properties of the battery were studied by cyclic voltammetry and constant current charge-discharge method.The work done is as follows:?1?We report a flexible gel electrolyte with a 3D porous structure.By combining PVDF-HFP with PEO,the polymer was crosslinked to effectively inhibit the crystallization of PEO at room temperature,thus improving the ionic conductivity.At the same time,GO was inserted into the cross-linked polymer and jointly constructed a high-speed transport pathway for Li⁺.GO can also improve the porosity of the electrolyte membrane and shorten the pore diameter while ensuring high ionic conductivity,so as to obtain high liquid absorption rate and liquid retention rate.^[48]Based on the deliberately design,the ionic conductivity of the as-prepared PHPG is as high as 3.4?10^-4S cm^-1at 25?and the achieved Li⁺transference number is close to0.58.In terms of electrochemical properties,the Li|PHPG|Li symmetric cell can run stably for over 400 h at 0.5 m A cm^-2.The Li-O₂battery based on the PHPG can operate for more than 300 times at high current density of 1 m A cm^-2with low polarization,showing excellent cycling performance.Besides,the large-scale Li-O₂battery with PHPG can work continuously under extreme conditions.This study provides a valuable approach for the advance electrolytes in Li-O₂batteries.?2?We report an organic-inorganic interaction reinforced composite gel polymer electrolyte?CGPE?.The synthesized PPSE-15 shows a double complexation structure caused by the organic-inorganic interaction.The crosslinking of PVDF-HFP and PMMA polymers increased the amorphous region in PPSE-15,and the movement of polymer chain was enhanced,resulting in a Li⁺migration quantity of up to 0.62.The polar functional groups of Si-O on Si O₂filler in PPSE-15 can enhance electrolyte-electrode interface stability and induce uniform deposition of Li⁺,thus inhibiting the growth of lithium dendrite.The Li/PPSE-15/Li symmetric batteries can run stably for more than 750 hours under the current density of 0.5 m A cm^-2and no dendrite was observed on the surface of lithium metal after cycling.The typical charge and discharge curves of PPSE-15 based Li-O₂battery show that the capacity retention reaches 48.3%after 5 cycles of full charge and discharge.As expected,the Li-O₂battery assembled with PPSE shows stable cycle of more than 300 cycles at room temperature and different current densities.?3?In this work,Ti₃C₂T_xMXene as a 2D filler was appended to crosslinked PVDF-HFP and PEO for the first time to fabricate PPM electrolyte for Li-O₂battery.Two-dimensional parallel arrangement of Ti₃C₂T_xMXene can effectively regulate the growth direction of lithium dendrites and lead to the horizontal growth.In addition,the inherent fluorine terminations?-F?in Ti₃C₂T_xMXene are favorable for forming uniform and durable solid electrolyte interface?SEI?at the lithium electrode-electrolyte interface,which is beneficial to the electromigration of lithium ions.Moreover,the EO group in PEO demonstrates a weak trapping reaction with anions in lithium salt,which inhibits the migration of anions and thus increases the migration amount of lithium ions.Based on the above merits,the PPM-5 based lithium symmetrical battery shows a marginal overpotential within 700 hours and the PPM-5 based Li-O₂battery can run stably for more than 200 cycles at room temperature.This study is of great significance for improving the safety and cycleability of Li-O₂batteries and promoting its commercial application.?4?We present a layered gel polymer electrolyte with crosslinked polymers based on PVDF-HFP/PEO and h-BN as fillers?PPH-5?.Under the combined action of cross-linked polymer and h-BN,the ionic conductivity of PPH-5 prepared by us reached 5.35?10^-4S cm^-1s cm^-1at room temperature.In addition,a large number of defects in h-BN enable lithium ions to permeate the film rapidly,which effectively promotes the dissociation of lithium salts.As a result,PPH-5 has a high lithium ion migration number of 0.58.The lithium symmetrical battery based on PPH-5 can stabilize the circulation above 800 hours at the current density of 0.05 m A cm^-2.The Li-O₂battery assembled with PPH-5 can maintain more than 250 cycles at room temperature with a low overpotential.