Structural Design And Performance Regulation Of Organic-Inorganic Composite Solid-State Electrolytes Towards Lithium Batteries

It is of great significance for social development to explore new sustainable energy resources and safe high-energy storage system.Lithium batteries can well relieve the excessive dependence of society on non-renewable mineral energy,balance supply and demand of electric energy,thereby alleviating the energy and environmental crisis.As an essential energy storage medium,they are extensively used in multifarious portal electronic devices,smart wearable equipment,new energy vehicle,and giant power grids.However,they hitherto remain drawbacks such as heavy body,dissatisfied energy density,poor safety and stability,and limited operational conditions,which still confine their potentials beyond.The use of lithium metal（Li）as anode is expected to further increase the energy density of lithium batteries and enable them to cope with the energy demands of future society.The high chemical activity but weak stability,flammability,unconstraint for Li dendrites and encapsulation issues of the currently used liquid organic electrolytes make them incompatible with Li anodes,increasing the difficulty of developing safe and stable high energy density Li batteries.Solid-state electrolytes are regarded as a critical way to realize the practical application of high-energy metallic lithium batteries,owing to their better safety,electrochemical stability,ambient adaptability,and mechanical robustness compared with liquid electrolyte.But the ionic conductivity of solid-state electrolytes is low at conventional using temperature,meanwhile,the transport efficiency of ions is not satisfied yet.In addition,due to the solid-solid contact between solid-state electrolytes and electrodes,the carrier transport at the interface is normally limited,and it is also difficult to regulate the chemical/electrochemical balance,which greatly restricts the highly efficient transfer between the chemical energy and electric energy of the electrode materials.This paper aims to,based on the organic-inorganic composite solid electrolytes,simultaneously improve the ionic conductive performance,the electrochemical stability and interface compatibility versus metallic lithium anode of electrolytes,through in-situ construction of nanocomposite skeleton,doping modification,fabrication of multi-functional interface layer,and hierarchical heterostructure design,etc.,and combined with various characterization techniques,the mechanism of ionic conduction and interface modification is explored and discussed.The specific research contents and main results are as follows:（1）In situ fabrication of a highly surface-interface active mesoporous framework to construct the solid-state nanocomposite ionic gel electrolytes（n-CIE）with high Li⁺conduction performance.A solid-state n-CIE with excellent electrochemical stability and capability of regulating the ionic fluxes was prepared by facile in situ liquid-liquid solidification and post-treatments.The results show that the ion coordination environments in this electrolyte is altered due to the internal mesoporous scaffold containing abundant active groups and unsaturated sites.The pores within scaffold gradually evolve into functional ionic channels that can partially restrict the transport of giant ions,thus significantly enhancing the Li⁺conduction efficiency of the electrolyte.The ionic conductivity of the electrolyte can reach 7.58×10^-4 S cm^-1,and the Li⁺transference number is 0.48.In addition,the n-CIE promotes the formation of inorganic interfacial layers between the electrolyte and Li electrode.The Li||Li symmetrical cells assembled with n-CIE can achieve stable Li stripping/plating for more than 1000 h at a current density of 0.25 m A cm^-2.The assembled Li Co O₂||Li and Li Ni_0.8Mn_0.1Co_0.1O₂||Li batteries exhibit high-capacity retention of 86.2%and 99.3%after 350 and 75 cycles,respectively.（2）Li_6.75La₃Zr_1.75Nb_0.25O₁₂ nanowires（LLZN NWs）doping to construct flexible polymethylmethacrylate（PMMA）-based composite solid-state electrolytes with decent intrinsic electrochemical performances.Given the stiffness and brittleness,high mass ratio within batteries stuff,and lower electrochemically stable window of the electrolyte prepared in the previous research part,in this section,the LLZN NWs with high aspect ratio were first prepared by electrospinning and rational heat treatments,and it was then compounded with PMMA and Li Cl O₄to obtain a flexible and light-weight PMMA based composite solid electrolyte.The results show that the introduction of LLZN NWs can improve the degree of freedom of the amorphous region in the composite electrolyte,and promote the dissociation of free Li⁺.At room temperature（25^oC）,compared with the pure PMMA-based solid polymer electrolyte(5.98×10^-7 S cm^-1,0.22),the ionic conductivity and lithium-ion transference number（0.45）of PMMA-based composite solid electrolyte(2.20×10^-5 S cm^-1)are significantly increased.In addition,benefits from the superior electrochemical stability of LLZN components,the composite solid electrolyte also shows a higher electrochemical window（～4.90 V）and better stability versus metallic lithium.The Li Co O₂/Li battery assembled with this composite solid electrolyte can realize a capacity retention of 92.3%after 80 cycles at 60^oC.（3）Fabrication of bilayer heterogeneous composite solid-state electrolyte with highly interfacial compatibility by using metal organic framework（MOF）composite gel.Based on the concept of constructing composite solid state electrolytes and the enhancement on electrolytes performance from porous framework materials in previous parts,this section first constructed a polyvinylidene fluoride hexafluoropropylene（PVDF-HFP）based composite solid-state electrolyte matrix which includes LLZN NWs filler in second part,and then fabricated a gel interface layer of MOF fillers and polymer composite on one side of it,and finally obtained a bilayer heterogeneous composite solid state electrolyte.The results indicate that the bilayer heterogeneous composite solid electrolyte exhibits higher ionic conductivity(2.00×10^-4 S cm^-1),lithium-ion transference number（0.62）and ultimate oxidation potential（4.92 V）.The MOF functional composite gel layer provides good interfacial wettability and simultaneously promotes the construction of composite solid-electrolyte-interface layer（SEI）as well as the uniform deposition of Li⁺,the Li||Li symmetrical cell based on this electrolyte achieves a stable cycle of more than 1700 h at a current density of 0.25 m A cm^-2.The assembled Li Co O₂（Li Fe O₄）||Li batteries can deliver decent capacity and better cycle stability.The pouch cells assembled with this electrolyte perform excellent safety and durability under extreme conditions.（4）Designing well-integrated layered heterogeneous composite solid-state electrolytes to achieve high electrochemical and interfacial stability of batteries.In order to improve the overall Li⁺conduction in the bilayer heterogeneous composite solid-state electrolyte and to reduce the influence of the built-in interface,this section prepared a well-structured gradient heterogeneous composite solid-state electrolyte by vacuum filtration,casting electrolyte precursor,curing and subsequent compaction.The results show that the ionic conductivity of this electrolyte can reach2.73×10^-4 S cm^-1.The embedded polyvinylidene fluoride（PVDF）framework and Li_6.4La₃Zr_1.4Ta_0.6O₁₂（LLZT）layer can limit the migration of large anions（Li⁺transference number is 0.65）,and can effectively enhance the electrochemical stability of the whole electrolyte bulk（the ultimate oxidation potential is 4.77 V）.Time of flight secondary ion mass spectrometry,X-ray photoelectron spectroscopy,and COMSOL simulation results show that PVDF framework with high dielectric constant and LLZT layer with single-ion conduction property can promote the uniform distribution of electric field within electrolyte,and ensure the formation of thin and dense composite solid-electrolyte-interphase layer,finally realizing the chemical-electrochemical balance at the interface.The Li Co O₂||Li and Li Ni_0.6Co_0.2Mn_0.2O₂||Li batteries assembled with this electrolyte present capacity retention of 79.0 and 85.4%after 1000 and 270 cycles,respectively.