Preparation And Research On The Performance Of Ionic Liquid-Based Solid Electrolytes

Lithium metal is of great interest as anode material for secondary rechargeable batteries due to its light weight(0.53 g cm^-3),low standard reduction potential（-3.04 V&Li/Li⁺）and high theoretic al storage capacity(3860 m Ah g^-1).The electrolyte,as the carrier of ion transport in the battery,has a profound impact on the electrochemical performance of the battery.Conventional organic-based li quid electrolyte have the advantages of high ionic conductivity and great contact with electrodes,bu t their inherent problems of flammability,leakiness and volatility seriously threaten the safe operatio n of batteries.In addition,due to the uneven distribution of Li ions on the Lithium metal surface,wh ich cause the formation of Li dendrites during Li plating/striping processes,leading to shortened cyc le life or even short circuit.Therefore,the use of solid-state electrolytes with non-flammable,excell ent encapsulation and good mechanical properties is expected to fundamentally solve the safety prob lems associated with conventional electrolytes.Compared with conventional organic-based liquid electrolyte,solid-state electrolytes have the a dvantages of good heat resistance and high electrochemical stability.In addition,its inherent mecha nical modulus can effectively inhibit lithium dendrite puncture.Compared with inorganic solid-state electrolytes,polymer electrolytes have the advantages of high flexibility and easy processability,an d are compatible with the deformation during electrode cycling.Nevertheless,its own high crystalli nity,low ionic conductivity at room temperature and low mechanical properties hinder the applicatio n of polymer electrolytes.In order to solve the above problems,this thesis introduces inorganic fast ionic conductors,ionic liquids and MOF（metal-organic frameworks）,respectively,to construct soli d electrolyte diaphragms with both high ionic conductivity and mechanical modulus.The specific re search work is as follows.（1）Polyvinylidene fluoride（PVDF）was combined with rigid ceramic particles Li_1.5Al_0.5Ge_1.5（P O₄）₃（LAGP）,and ionic liquid Pyr_{1（2O1）}TFSI with high flame retardancy and high ionic conductivity was introduced to alleviate the problems of poor interfacial contact and low ionic conductivity of sol id-state electrolytes.The solid-state electrolyte（PIL-1）inherits the flexibility of PVDF and the high shear modulus of LAGP.After experimental tests and theoretical calculations,it is found that Pyr_{1（2O）⁺1}ions are more easily adsorbed on the LAGP surface than TFSI^-ions,facilitating the release of flu orine-containing anions from the ionic liquid,and TFSI^-generates Li F-rich SEI layers in situ by red uctive decomposition on the lithium anode surface.The solid state electrolytes show great performa nce in lithium metal batteries.At a current density of 0.1 m A cm^-2,the Li|PIL-1|Li symmetric battery exhibits long-term cyclic stability for 2700 h with a polarization voltage of 70 m V;at a current dens ity of 0.5 C,the Li|PIL-1|Li Fe PO₄ battery exhibits long-term cyclic stability for 196 cycles with a ca pacity decay of 9.3%.（2）In the previous work of the group,we attached the ionic liquid EMIMTFSI to the PMMA b ackbone by covalent bonding,and the resulting poly（ionic liquid）（PIL）retains the mechanical prop erties of the polymer and inherits the advantages of high ionic conductivity,thermal stability and no n-flammability of the ionic liquid.However,as the ionic groups are immobilized on the polymer cha ins,the ionic mobility decreases and the glass transition temperature increases,resulting in an increa se in viscosity and a corresponding decrease in conductivity.In this chapter,we prepared a composit e solid-state electrolyte with excellent performance by regulating the lithium salt concentration and i ncreasing the carrier concentration;and adding MOF（UIO-66-NH₂）with Li-ion selection function.Compared with the pure poly（ionic liquid）electrolyte without MOF,the room temperature ionic con ductivity of the composite solid-state electrolyte can reach 7.79×10^-4 S cm^-1 and the Li-ion transfer n umber is 0.538.Li|Ui O-66-NH₂@PIL|Li Fe PO₄ battery exhibits long-term cyclic stability for 200 cy cles at a current density of 0.5 C with a capacity loss rate of only 6.9%.