Preparation And Electrochemical Performance Of Imidazolium Ionic Liquid Modified PEO-based Electrolyte

Booming popularity of electric vehicles requires batteries to increase the energy density to more than 500 Wh kg^-1.However,the low theoretical capacity of graphite anode limits the energy density of the battery.The theoretical capacity of Li metal anode(3860 m Ah g^-1)is ten times higher than the value of graphite(372 m Ah g^-1).Therefore,Li metal is regarded as the anode for next-generation high energy density.However,some problems severely limit the large-scale use of lithium metal batteries,such as the uncontrollable growth of lithium dendrites,many side reactions at the interface,large volume changes and low coulomb efficiency.At the same time,the current commercial liquid electrolyte has brought huge safety risks,such as leakage,burning,and explosion.Polymer solid electrolytes have received extensive attention because of their high energy density,ability to be processed into arbitrary shapes,and relatively safe and reliable.Polymer solid electrolyte replaces liquid electrolyte:on the one hand,it can avoid the risk of liquid leakage,and improve the safety of the battery;on the other hand,due to the higher mechanical strength,it can effectively suppress lithium dendrites.However,before commercialization,there are still many practical problems that need to be solved urgently,such as the interface stability between the solid electrolyte and the electrode,and the ion migration number and ion conductivity at room temperature.Graphene oxide?GO?has a large specific surface area,good electrical insulation properties and excellent mechanical properties.When it is introduced into the polymer electrolyte as a carbon-based filler,it can enhance the mechanical strength of the polymer electrolyte,which has attracted the attention of researchers.However,GO tends to restack owing to strong interlayer interaction,thus resulting in dramatical capacity fading.As the representative of green chemistry,ionic liquid?IL?is the hotspot and frontier of current chemistry research.GO swells easily in polar organic solvents.IL can be embedded in GO during this process,thereby inhibiting the reunion of GO.IL and GO are introduced into the polymer system at the same time to make a composite polymer electrolyte.The electrolyte has both the advantages of GO and IL,so it has good application prospects.In this paper,poly?ethylene oxide??PEO?polymer is used as the matrix to study the polymer electrolyte modified by ionic liquid and graphene oxide.The main contents and results are as follows:?1?Using the interaction between GO,ionic liquid and PEO?PEO as the matrix,GO as the filler,and ionic liquid as the plasticizer?,an anion-immobilized polymer electrolyte?PGI?was prepared.The tightly cross-linked framework of fixed anions in PGI can not only reduce concentration polarization,but also inhibit the growth of lithium dendrites.The results show that when the GO content is 1%and the ionic liquid content is 30%,the PGI electrochemical performance is optimal:the room temperature conductivity is as high as 10^-4 S cm^-1,the electrochemical stability window is 4.7V,and the lithium ion migration number is 0.52.And the PGI shows outstanding interfacial stability toward lithium metal electrodes.In addition,the full battery assembled into Li Fe PO₄/PGI/Li showed excellent cycle performance.In an assembled cell?Li Fe PO₄ cathode,Li metal anode,PGI electrolyte?,the discharge capacity can reach to 154 m Ah g^-1 after 100cycles at 60? for 0.1 C,120.2 m Ah g^-1 at room temperature at rate of 0.2C and 110 m Ah g^-1 at 0.5 C?60??.?2?In order to improve the problems of poor mechanical properties and low electrical conductivity of PEO electrolyte,the strategy of adding GO filler was used to modify and optimize it.First,IL was grafted to GO by epoxide ring-opening reaction,and then a new filler?GO-IL?for IL modified GO was prepared by anion exchange reaction.The GO-IL filler not only overcomes the difficulty of GO agglomeration,but also introduces IL into the polymer electrolyte.Finally,PEO/GO-IL electrolyte was prepared by solution casting.The results show that the room temperature ion conductivity is 1.3×10^-5 S cm^-1and the electrochemical window is4.48V.The cells with PEO/GO-IL electrolyte could be well charged/discharged sequentially for more than 300 h with negligible cell voltage loss at 0.1 m A cm^-2.In addition,the full battery assembled into Li Fe PO₄/?PEO/GO-IL?/Li showed better cycle performance.After 100 cycles of the battery,the capacity retention rate is as high as 88%.