Design And Application Of Ionic Liquid-based Solid Electrolyte For Lithium Battery

Energy storage devices use random and intermittent renewable energy sources to store energy to reduce human dependence on fossil fuels and achieve the goal of carbon neutrality,and lithium metal secondary batteries are one of the most important energy storage devices.The disadvantages of conventional liquid organic electrolytes used in conventional lithium metal batteries,such as volatility,low boiling point,and easy leakage,are prone to triggering safety accidents consequently.In recent years,scientists have successfully developed solid-state electrolytes with higher safety,but most solid-state electrolytes suffer from problems such as low ionic conductivity at room temperature and high interfacial impedance.Based on this,from the perspective of improving the safety,ionic conductivity,and compatibility of the electrode/electrolyte interface of the solid-state electrolytes,a series of ionic liquid-based solid-state electrolytes were designed and prepared by selecting suitable matrix materials and ionic liquids.The specific research contents are as follows:（1）Three-dimensional porous graphitic phase carbon nitride nanosheets（PCN）were prepared by a simple thermal oxidative etching method using graphitic phase carbon nitride（g-C3N4）as the precursor.Compared with the conventional g-C3N4,the PCN obtained by thermal exfoliation is not only simple to prepare and has good compatibility with liquid electrolytes,but also has a large specific surface area and abundant pores.The solid electrolyte（PCNIL）prepared by mixing PCN and 1-ethyl-3-methylimidazolium bis（trifluoromethanesulfonyl）imide ionic liquid（[EMIM][TFSI]）has high mechanical strength（energy storage modulus of 5 MPa）,large ionic conductivity(0.75 m S cm^-1),and excellent thermal stability.Li|PCNIL|LFP cells are capable of stable cycles over a wide temperature range of 25°C to 150°C.（2）Although the solid electrolyte has high mechanical and electrochemical properties,its poor flexibility impedes its large-scale application.In this chapter,a high-performance flexible solid-state electrolyte（PIG）was successfully developed by co-doping poly（vinylidene fluoride-co-hexafluoropropylene）（PVDF-HFP）with GO（graphene oxide）and[EMIM][TFSI].In PIG,GO and ionic liquid reduces the crystallinity of PVDF-HFP and improve the ionic conductivity of the electrolyte.Moreover,GO can effectively bind with anions,enhance the ionization of lithium salts,promote the uniform distribution of charge and lithium ions in the electrolyte,and suppress the formation of lithium dendrites.The obtained PIG electrolyte has a high lithium ion transfer number of 0.55,and the Li|PIG|Li symmetric cell also has a lower voltage polarization and operates stably for 1000 h at a current density of 0.1 m A cm^-2,while Li|PIG|Li Fe PO₄ cells also exhibit excellent rate performance and cycling stability.（3）The conventional ionic liquids have higher viscosity and higher cost,and the coordination of Li⁺with TFSI^-anions tend to form negatively charged[Li（TFSI）₂]^-or[Li（TFSI）₄]^3-clusters,resulting in slow lithium ions transport.In this chapter,UIO-66particles and solvated ionic liquid（SIL）were uniformly mixed by ball milling and pressed into solid electrolyte sheets（SSE）.In SSE,SIL was encapsulated in the 3D porous channel of UIO-66 as an ionophore.Since the metal ions of UIO-66 interact with TFSI^-,weakening the coordination environment with lithium ions.And the microporous structure of UIO-66 further restricts the movement of TFSI^-,increasing ionic conductivity and lithium ions migration number.In addition,according to the Lewis acid-base theory,TFSI^-is a Lewis base,while Zr⁴⁺in UIO-66 is a Lewis acid.The Lewis acid-base effect of UIO-66 can inhibit TFSI^-in SIL and release more free lithium ions to participate in the ion transport process.Eventually,a uniform lithium ions channel is formed,which suppresses the formation of Li dendrites and achieves stable lithium deposition.Owing to this unique structure,the Li|SSE|LFP battery constructed with SIL/UIO-66 as the solid electrolyte not only exhibits excellent cycling stability at room temperature but also achieves 600 cycles of stability at 90°C.（4）In response to the problem of the currently prepared ionic liquid-based solid electrolyte processing methods are complicated and not conducive to industrial production.In this chapter,solid electrolyte slurry（SIL/PCN）was prepared by a simple and scalable ball milling method with porous g-C₃N₄（PCN）as the backbone,solvated ionic liquid（SIL）as the ionic conductor,and DMF as the solvent.The slurry was coated on the LFP cathode sheet by the scraping method to construct the solid electrolyte-integrated LFP electrode（SIL/PCN|LFP）.In the integrated cathode,the thickness of SIL/PCN is 50μm,satisfying the industrial requirements for the high energy density of solid-state lithium-ion batteries.In addition,the integrated electrode can reduce the interfacial impedance between the solid-state electrolyte and LFP,and suppress the growth of lithium dendrites.These excellent properties enable the Li|SIL/PCN|LFP solid-state battery assembled at room temperature with excellent electrochemical performance.