Study On Composite Design And Interface Modification Of Li₇La₃Zr₂O₁₂ Solid Electrolyte

Traditional Li-ion batteries are limited by the flammable liquid electrolytes and the low theoretical capacity of the graphite anode,which can no longer meet the needs of high safety and high energy density.Lithium metal batteries employing solid-state electrolytes（SSEs）are promising to solve these problems.Among them,garnet-type electrolyte hold great application potential due to their excellent ionic conductivity,good thermal stability,and stability to lithium metal anode.However,the application of garnet-based solid electrolyte is still restricted by the following two aspects:（1）The spontaneous formation of Li₂CO₃ on the surface of garnet electrolyte hinders Li⁺transport;（2）Inefficient electrode/electrolyte solid-solid interface contact leads to interfacial dendrite growth and charge transfer obstruction.In this dissertation,the Li₇La₃Zr₂O₁₂（LLZO）garnet solid electrolyte is employed as a typical representative,proposing plasma processing and chemical surface modulation strategies to remove the Li₂CO₃ impurity phase.In addition,the problems related to solid-solid contact and dendrite growth are solved by polymer recombination,as well as construction of lithiophilic coatings and a flexible mixed conductive interface layer.Then,LLZO-based solid-state lithium metal batteries（SSLMBs）with excellent electrochemical performance were realized.The main research contents are as follows:（1）A vertically aligned structure composed of clean surface LLZO particles are fabricated by plasma bombardment treatment with ion directional transport structure design,which composite with a flexible polymer matrix to obtain PLLZO_V/PPL solid electrolyte.Clean and efficient plasma processing has been proposed to remove the Li₂CO₃ impurity phase on the surface of LLZO and inhibit its regeneration.On this basis,the LLZO particles are arranged vertically to construct a rapid Li⁺diffusion path.Then,the PLLZO_V/PPL obtained by compounding with flexible polymer achieving sufficient contact at the electrode/electrolyte interface.Based on the above advantages,the Li Fe PO₄（LFP）/Li solid-state battery assembled by PLLZO_V/PPL exhibited a high specific discharge capacity of128.4 m A h g^-1 at 0.5 C and 109.7 m A h g^-1 at 1.0 C,and good cycle stability.（2）A LLZO-enhanced composite electrolyte（LPPL）with high ionic conductivity and mechanical properties is developed with a thin and toughening design.Aiming at the problems of the composite electrolytes with low strength,large thickness（>100μm）,and insufficient ionic conductivity,electrospinning was used to construct LPPL solid electrolytes with a thickness of only 20μm.The thin-layer design shortens the Li⁺transmission path,causing a enhanced ionic conductivity with 1.58×10^-4 S cm^-1 at 60°C.In addition,the obtained LPPL possesses high tensile strength with 8.2 MPa,resulting in an effective inhibition of the puncture of lithium dendrites.Ultimately,the LPPL-based SSLMBs maintain a reversible capacity of 149.1 m A h g^-1 after 250 cycles at a current density of 0.5 C;It delivers a reversible capacity of 100.5 m A h g^-1 after 430 cycles at the current density of 1.0C.The soft-packaged LFP/LPPL/Li battery demonstrates high safety under destructive conditions such as bending and cutting.Moreover,the LPPL is also suitable for the SSLMBs with high-energy NCM811 or high-capacity sulfur cathodes.（3）A LLZO with high intrinsic conductivity,low interfacial impedance,and lithiophilicity is constructed by using the dual strategy of bulk doping and surface sulfide modification.We design a Ta doping strategy to obtain a Li_6.4La₃Zr_1.6Ta_0.5O₁₂（LLZTO）pellet with cubic phase structure and high ionic conductivity(1.38×10^-4 S cm^-1,25℃).The surface layer of Li₂CO₃is converted into Li₂CO₃ sulfide through rapid surface vulcanization,which makes LLZTO ceramic sheet possess excellent lithium wettability and low interfacial resistance.The lithiophobic Li₂CO₃ surface layer is converted into a lithiophilic sulfide through rapid surface vulcanization,enabling the LLZTO pellets to have excellent lithium wettability and low interfacial resistance.The lithium symmetric cells exhibit stable plating/stripping cyclability without lithium dendrite growth for 200 h at the current densities of 0.05 m A cm^-2,0.1 m A cm^-2,and 0.2 m A cm^-2,respectively.The solid-state LFP/Li cells composed of modified LLZTO pellets show excellent rate and cycling performance.It exhibits a discharge-specific capacity of 137.3 m A h g^–1 and 92.6%capacity retention after 100 cycles at the current density of 0.1 C.Moreover,it still shows a high specific discharge capacity of 114.3 m A h g^–1after 150 cycles of 0.5 C.（4）A flexible ionic/electronic mixed conductive interface layer（MCI）is constructed based on conductive two-dimensional materials to suppress the dendrite growth on the LLZTO/Li interface.Two-dimensional MXene sheets and zero-dimensional LLZTO particles dispersed in a flexible polymer network（PEO/Li TFSI）to form continuous electronic and ionic conductive network structures,respectively,which homogenize the electric-field distribution inside the MCI and induce uniform deposition of Li⁺.In addition,the flexible polymer matrix effectively improves the solid-solid contact of LLZTO/Li.The lithium symmetric cell assembled with MCI modified LLZTO exhibits low interfacial resistance and polarization voltage,and is cycled at the current density of 0.2 m A cm^–2without dendrite growth.The LFP/LLZTO-MCI/Li solid-state cell demonstrates excellent rate performance up to 2.0 C.It exhibits a discharge capacity of 124.2 m A h g^–1 after 100 cycles at a current density of 0.1 C;it can also be stably cycled dozens of times at 0.5 C,showing good cycling performance.