The Interface Regulation Of Solid-state Li Metal Battery Based On Li₇La₃Zr₂O₁₂ Electrolyte

With the rapid development of low-carbon economy and rising energy demand,finding energy storage devices to meet the development needs is a hot spot in the field of new energy.Lithium-ion batteries（LIBs）have been widely used as high-efficiency energy storage power sources in information technology,electric vehicles,aerospace and other fields.At this stage,higher requirements are placed on both energy density and safety performance of LIBs.Solid-state lithium metal batteries（SSLMBs）using high-energy-density lithium metal as the negative electrode,matching non-flammable solid-state electrolytes and new cathode materials,are expected to be compatible with the needs of both high energy density and high safety.Among many electrolytes,Li₇La₃Zr₂O₁₂（LLZO）has attracted much attention owing to its high ionic conductivity,wide electrochemical window,high mechanical strength and good thermal stability.However,the growth of lithium dendrites in the LLZO and large interfacial resistance with the positive electrode limit the application of LLZO in SSLMBs.This thesis priory takes tantalum-doped LLZO electrolyte Li_6.5La₃Zr_1.5Ta_0.5O₁₂（LLZTO）as the research object.Aiming at suppressing the growth of lithium dendrites in the electrolyte and reducing the excessive resistance of the side of positive electrode,the high performance LLZTO electrolyte was prepared and functional layers were constructed at the Li/LLZTO and LLZTO/cathode.The main work includes:Improving the structural density of LLZTO electrolytes is first used to surpress the lithium dendrite,which caused by the defect of structure,such as holes and crack in the bulk electrolyte.Specially,a new method combimed secondary ball milling and granulation process was used to fabricate the high-relative-density LLZTO electrolyte.With the improved specific surface area,the surface free energy was improved to sinter dense-structure electrolyte at relative low temperature.The granulation process was used to reduce the adsorption of gas between particles and increase the bulk density of the green body,and eventually benefit the densification process.As a result,the prepared cubic phase Li_6.5La₃Zr_1.5Ta_0.5O₁₂（LLZTO）with high ionic conductivity(3.2×10^–4 S/cm)and dense structure（density～98%）was experimentally prepared.The as-prepared LLZTO electrolyte can achieve a critical current density of 1.0 m A cm^–2 in a lithium metal symmetric battery.Aiming at the growth of lithium dendrite caused by the uneven deposition of Li⁺at the Li/LLZTO interface,based on the dense LLZTO structure,the Li/LLZTO interface was modified by Zn-Cu composite.With the alloying reaction,Li₃Zn with high lithium ion migration can be formed,resulting the interface represented a"lithiophilic"properties and a low resistance.The Li₃Zn alloy can be stabilized at the interface as a Li⁺transport framework but with little effect on the uniform deposition of Li⁺.With a small amount of Cu doping（the Zn-Cu ratio is 20:1 and 10:1）,the good wettability of lithium metal on the surface can be kept.Specially,the good electronic conductivity of Cu can homogenize the electric field,thereby promoting the uniform deposition of Li⁺.When the Zn-Cu ratio is 10:1,the Li/Zn-Cu-LLZTO-Cu-Zn/Li symmetric cell can achieve the highest critical current density of 2.8 m A cm^–2 and a good lithium deposition/stripping(more than 450 hours at a current density of 0.8 m A cm^–2,more than 50 hours at 2 m A cm^–2).The Li/Zn-Cu-LLZTO/LFP full cell can be cycled for 50 cycles at room temperature with a capacity of 130 m Ah g^–1.For lithium dendrites caused by Li⁺reduced by electtrons at the grain boundaries in LLZTO,a low electron transport layer was designed by an in-situ reaction between Co₃O₄ and lithium on the Li/LLZTO interface.The obtained Li₂O/Co interface can reduce electron injection into LLZTO to restrain the lithium dendrite growth on the grian boundaries.The First-principles calculations show that the Co₃O₄ layer can form a phase-equilibrium layer with LLZO through chemical bonds.The low Gibbs reaction free energy between Co₃O₄ and Li ensures the contact stability with Li metal anode,resulting in a small contact resistance,but the interfacial resistance increases with the thickness of Co₃O₄.At the same time,Li/Co₃O₄-LLZTO can be passivated to form a stable interface during the first lithium deposition process to ensure the stability of the interface,and finally Co₃O₄-LLZTO has a good inhibitory effect on the growth of lithium dendrites.With a thickness of 100 nm for Co₃O₄ layer,the interfacial impedance of Li/Co₃O₄-LLZTO is about 75Ω?cm^–2,and the critical current density of Li/Co₃O₄-LLZTO-Co₃O₄/Li symmetric cell can reach 4.8 m A?cm^–2,and realize an excellent lithium deposition/stripping performance(more than 600 hours at a current density of1 m A?cm^–2).The Li/Co₃O₄-LLZTO/Li Fe PO₄ full cell can be stably cycled for 100cycles at room temperature with a capacity of 114 m Ah g^–1.To solve the high interfacial impedance of solid-state LLZTO/cathode side,an in-situ solidified polymer electrolyte（SPE）was designed to modify the interface of LLZTO/cathode.The SPE is cross-linked with glycidyl ether oxypropyl cyclotetrasiloxane（CTS）and polyethylene glycol diglycidyl ether（PEGDE）as the backbone,and lithium bis（trifluoromethanesulfonyl）imide（Li TFSI）is provided Li⁺for dissociation.With the increase of CTS,the curing ability,thermodynamic properties,and the voltage window of the SPE is enhanced,the ionic conductivity.is decreased.When the CTS content is 10%,the electrolyte can be effectively cured,and the ionic conductivity of the formed SPE is 3.7×10^–4 S/cm at room temperature,the ion mobility number is 0.45,and the voltage window is 5.02V.Benefiting from the good fluidity of the electrolyte precursor and the Lewis acid-based interaction between LLZO and PEGDE,a low interface resistance is obtained between SPE and LLZTO.Both ionic and electronic conductivity can be affected by the SPE in the cathode.The in-situ polymerized SPE has an influence on the ionic conductivity and electronic conductivity of cathode electrode,thereby affecting the charge-discharge specific capacity of the battery.When the SPE is 20%w.t.,the electrolyte,electronic conductor and cathode material have good contact,and the all-solid-state lithium metal battery Li/LLZTO/poly-LCO has the highest first charge-discharge specific capacity.Finally,combined with the in-situ cured SPE-modified cathode,an all-solid-state battery was assembled with Co₃O₄-modified Li/LLZTO,and its performance was evaluated.The all-solid-state battery Li/Co₃O₄-LLZTO/poly-LCO maintained a capacity of 127.2m Ahg^–1 after 100 cycles at a current density of 0.05 C.