Preparation And Characterization Of Ionic Liquid Based Electrolyte For Lithium Batteries

In recent years,lithium-ion batteries?LIBs?have been widely used in the field of electronic equipment due to their high energy density,long cycle life and low self-discharge rate.They also show broad application prospects and huge potential economic benefits in electric vehicles,energy storage and space technology.As the energy density of the battery increases,the safety issues have become increasingly prominent.As a main component of LIBs,electrolytes greatly affect the related performance of batteries,especially safety performance.Therefore,the development of safety electrolytes for LIBs has become a hot research spot.Among them,ionic liquids?ILs?is an appealing and promising solvent with non-volatility,non-flammability,high thermal stability and good electrochemical stability.In order to improve the electrochemical performance of electrolytes and the safety of batteries,this paper focuses on the design and preparation of IL-based hybrid electrolytes,IL-based gel polymer electrolytes and solid composite polymer electrolytes,which contains ILs as main body or modification solvent.In this work,we present a systematic study the effects of the content of ILs,the types and contents of inorganic fillers on electrolyte state,electrochemical performance and safety performance.The interaction and composition relationship between components in different electrolyte systems were investigated.Thus,laying the theoretical foundation to develop the reliable and comprehensive performance of lithium batteries and providing technical support for the high-energy density battery system to solidify.?1?The ILs electrolyte was modified by the co-solvent forming IL-based hybrid electrolytes.The results showed that EC and PC could partially replace TFSI^-associated with Li⁺,which weakened the association between Li⁺and TFSI^-and changed the thermal phase transition behavior of the ILs.IL-based hybrid electrolytes(0.8PYR₁₄TFSI+0.2LiTFSI+5%EC/PC)showed an improved ionic conductivity and ion migration number.By further adding 3wt%additive VC,the composition and morphology of SEI film on the graphite surface could be changed,thus,the anion oxidation reaction and the cation intercalation problems of ILs can be effectively suppressed.Furthermore,the compatibility of lithium metal electrodes and Li/LiFePO₄batteries with the modified IL-based hybrid electrolytes was studied.The results showed that the additive VC can improve the stability of lithium metal electrode in ILs-based electrolytes.The XPS spectrum showed that VC participated in the formation of interfacial film and effectively inhibited the decomposition reaction of ILs at high current density.The Li/LiFePO₄ battery exhibited an improved first discharge specific capacity(134.1 mAh g^-1)and a better cycling stability.The capacity retention could reach 87.1%after 100 cycles.On the basis of these studies,the thermal stability and safety performance of the modified ILs-based electrolyte and related battery components were studied by ARC.The results showed that the IL-based hybrid electrolytes had the higher initial exothermic temperature?330.36°C?and the smaller temperature rise rate compared to the organic electrolyte.And it was noteworthy to mention that the IL-based hybrid electrolytes could increase the thermal runaway temperature of the battery to 270°C,showing the better thermal stability and thermal safety.?2?ILs gel polymer electrolytes?MGPE-ILs?based on a micro-porous polymer membrane were designed to improve both the battery safety and maintain rapid migration channels for Li⁺,which possessed a high ionic conductivity of 1.11 mS cm^-1and a low ion transport activation energy(Ea,4.623 J mol^-1)at 25°C with an electrochemical stability window up to 5 V versus Li/Li⁺.Furthermore,LFP/MGPE-90%IL/Li also demonstrated excellent cycling stability with 94.1%capacity retention up to 100 cycles and a high columbic efficiency of 99%at 25°C.However,for the IL-based electrolyte and MGPE-ILs,Li⁺ionic transference number(t _Li+)was also lower.In order to resolve the aboved problems and eliminate the limitation of the organic solvent component to further improve the thermal stability of the electrolyte,the IL-based composite gel polymer electrolyte?CGPE-ILE?was further prepared.The results showed that the ionic conductivity of CGPE-ILE could be improved through organic/inorganic SiO₂ nano-particles composite.On basis of these,the surface of SiO₂ was further modified to design and prepare SiO₂-PAA-Li?SPL?fillers with uniform nano-size and active shell-core structure.After the modification of SiO₂ surface,the compatibility of SPL with polymer was better,which promoted the formation of high-conductivity phase interfaces and increased the ion transport channel.CGPE-ILE-SPL?5:0.75?exhibited improved ionic conductivity(25°C,0.74 mS cm^-1)and t _Li+?0.48?.In addition,the LiFePO₄/Li battery exhibited better cycle stability in the CGPE-ILE-SPL electrolyte than the CGPE-ILE-SiO₂ electrolyte.?3?Replacing inorganic inert filler with active inorganic filler Li_1+xAl_xGe_2-x?PO₄?₃?LAGP?,CGPE-ILE-LAGP was prepared.The different effects of SiO₂,SPL and LAGP on related electrolyte performance were compared.The results showed that LAGP had single lithium ion conductivity and played a significant role in improving the Li⁺transport in CGPE.Compared with CGPE-ILE-SiO₂?5:1?and CGPE-ILE-SPL?5:0.75?,CGPE-ILE-LAGP?5:0.5?had a higher ionic conductivity of 0.76 mS cm^-1,and the improved t _Li+of 0.54.To address these concerns,we put forward a strategy to prepare “firmness and flexibility”solid composite electrolyte?SCE-LAGP?via further increase of the content of inorganic ceramic particles LAGP on the basis of gel polymer electrolyte.The results showed that the polymer matrix and the inorganic filler LAGP could form interfacial transition layer.Compared with the inorganic all-solid electrolyte LAGP,the interface impedance value of the composite interface layer was significantly smaller than that of the grain interface impedance of inorganic solid electrolyte,which was more conducive to the efficient and rapid transmission of Li⁺.When the m_PVDF-HFP:m_LAGP was at 5:5,SCE-LAGP still maintained a high ionic conductivity of0.439 mS cm^-1,which showed a higher t _Li+?0.64?,a better electrochemical stability?ESW,5 V?and the stability to lithium.However,the interface problems of battery with the solidified electrolyte led to the poor battery cycling.In responsed to the above problem,the chelate copolymer?PVDF-GMA-IDA,PGI?was further designed to modify the composite electrolyte.The results showed that there had a covalent anchoring effect in the two-phase interface between the inorganic ceramic and the polymer matrix,which effectively increased the volume fraction of the continuous interface layer between the active inorganic ceramic particles and the gel polymer layers.Thus,the modified composite electrolyte exhibited an improved ionic conductivity of0.579 m S cm^-1,optimized lithium-interface with smaller interfacial impedance and interface stability.The solid LiFePO₄/Li battery exhibited improved cycle stability and coulombic efficiency,and the battery capacity remained at 135.2 mAh g^-11 after 100cycles.?4?The chelating copolymer?PGI?was used as the interfacial modifier of the LiNi_0.8Co_0.15Al_0.05O₂ cathode.The results showed that the accurate interfacial modification via synergistic coupling between NCA cathode materials and chelating copolymer was an effective protection with uniform distribution of the coating on the electrode surface,which effectively inhibited the occurrence of side reactions between the NCA and the electrolyte.Importantly,the three dimensional Li⁺ion conductive pathways in the cathode are constructed through the gel polymer coating layers on the surface of the active material and amount of electrolyte ingredient between cathode particles.As a result,the solid-state battery reflected lower and stable charge transfer resistance,meanwhile,the effective ion channels in the composite cathode improves cycling stability of the NCA/Li solid-state battery.