Preparation And Research Of Gel Electrolytes For Lithium-ion Battery

Rechargeable lithium-ion battery（LIBs）has been widely studied and applied in electronic portable products,electric vehicles and other new fields of energy storage devices.Today,most commercial power batteries use electrolyte systems composed of liquid electrolytes and polyolefin diaphragms.The use of liquid electrolytes（LEs）may lead to safety issues,including solvent leakage,flammability of organic solvents,etc.Solid electrolytes（SPEs）greatly improve the thermal stability and safety performance of LEs.However,due to its harsh temperature requirements,high cost and difficult to large-scale production,it still hinders its practical application.In recent years,studies have shown that gel electrolytes（GPEs）not only have the safety performance required for lithium-ion batteries,but also have good electrochemical performance,which is considered as a promising research direction.Several different GPEs were studied and prepared in this paper.（1）At present,Polyvinyl oxide（PEO）-based gel electrolytes are rarely reported,because pure PEO will dissolve in commercial electrolytes and have poor mechanical properties,which makes it difficult to inhibit the growth of lithium dendrites during the battery charge-discharge cycle.In this paper,polyacrylic acid（PAA）was introduced to form hydrogen bond with PEO to prepare PEO/PAA composite membrane,which was used as gel electrolyte（GPE）after absorbing liquid electrolyte.The infrared spectrum and tensile test show that the composite film has strong hydrogen bonding and good mechanical properties.The X-ray diffraction（XRD）and differential scanning calorimetry（DSC）tests show that the crystallinity decreases with the increase of PAA content.Thermogravimetric（TG）test shows that the thermal stability of the composite film is better than that of pure PEO.When the amount of PAA was 30 wt%,the tensile strength of PEO/PAA composite film reached about 6.60 MPa,much higher than that of pure PEO film,and the tensile strength of GPE reached about 0.1MPa.At the same time,GPE also showed excellent electrochemical performance,with ionic conductivity of 1.66×10^-3S/cm at room temperature.Lithium ion migration number reached0.58.The battery charge-discharge test shows that the specific discharge capacity and capacity retention rate are still as high as 134.1 m A·h/g and 88.7%after 500 charge-discharge cycles with a rate of 0.5C.Under high rate charge and discharge（2C）,the maximum specific discharge capacity can reach 138.2 m A·h/g.This kind of environmental protection,low cost,high electrochemical performance PEO/PAA based gel electrolyte GPE has industrial advantages.（2）At present,most in situ polymer gel electrolyte precursors need to be induced by light and heat.These initiators and catalysts may adversely affect battery performance.In this paper,for the first time,an in situ polymerized gel electrolyte of polyurea system was presented.The precursor was prepared by the reaction of isophorone diisocyanate（IPDI）,polyether amine（D2000）and tri（2-aminoethyl）amine（TREN）.The gel was formed within 72 hours at room temperature.After igniting the gel electrolyte with a flamegun,it will be extinguished naturally within 0.5 seconds,indicating that it has good flame-retardant property.When the IPDI equivalent is 1.3,the gel electrolyte has the best electrochemical performance.The ionic conductivity at room temperature is 4.46×10^-4 S/cm,and the electrochemical window is about4.3 V.At the same time,it also has good charge-discharge cycle performance.After 300charge-discharge cycles at a rate of 0.5C,the specific discharge capacity can still reach 136.77m A·h/g.The volume retention rate reached 90.44%,and the interface impedance after circulation was less than that of commercial liquid electrolytes.The results show that polyurea system is a potential choice of gel electrolyte materials.（3）Currently,in situ polymergel electrolyte precursors generally need to be loaded with diaphragms to support their mechanical properties and ensure the normal operation of batteries.In this paper,a mixture of hydroxyethyl acrylate and commercial liquid electrolyte（LB-001）was cured with ultraviolet light（UV）to form an in situ polymerized gel electrolyte without a loaded diaphragm.After a series of electrochemical tests,the electrochemical window reached 5.2 V.At room temperature,the battery with Li Fe PO₄/Li electrode has a high ionic conductivity(2.29×10^-3 S/cm).In addition,the battery with Life PO₄/Li electrode has a high specific discharge capacity of 137.70 m A·h/g and a high capacity retention rate of 85.76%after 200 charging-discharge cycles at 0.5C.It shows that it is a promising gel electrolyte material.