Design,preparation,and Properties Of Lithium-rich Garnet Structure Solid-state Electrolytes

Because of the advantages of all-solid-state lithium batteries in terms of safety,energy density,packaging,and operational temperature range,it is very likely to"subvert"the current power battery industry pattern.The inorganic oxide lithium-ion solid-state electrolyte has attracted a great deal of attention from researchers as a crucial component of the all-solid-state battery.Li₇La₃Zr₂O₁₂（LLZO）garnet-based solid-state electrolytes with high ionic conductivity(10^-3to 10^-4S/cm)and superior chemical stability over lithium metal anodes are driving the development of solid-state lithium-ion batteries.LLZO ionic conductors exist in two phase forms and exhibit excellent electrical conductivity only in the cubic crystal rather than the more thermodynamically stable tetragonal crystal structure.This thesis takes the preparation of pure and dense garnet solid electrolyte as the starting point,and deeply explores the guiding role of specific component reaction intermediates for the final product,laying the foundation for accurate and rapid preparation of oxide ceramic electrolyte.Based on this,the framework structure of lithium-rich garnet-based oxide superionic conductors is further clarified,and the functional motifs in the structure are tailored by manipulating atoms to improve the intrinsic chemical instability of the electrolyte.On the basis of our comprehensive understanding of the interaction between ion transport properties and chemical stability in lithium-rich garnet-based oxide superionic conductors from a structural perspective,we designed the incorporation of external atoms into the ceramic bulk phase and grain boundaries to optimize both important properties of garnet-structured superionic conductors.The main research of this thesis is divided into three specific parts as follows:（1）Garnet-structured solid-state electrolytes with cubic phase have shown significant promise for the development of high-energy-density Li-ion batteries.However,preparing pure-phase lithium-rich garnet oxides remains a prominent problem due to uncontrollable solid diffusion at high temperatures that results in multiple products.Herein,we focused our efforts on identifying and regulating intermediates involved in the reaction process in order to effectively and precisely prepare cubic pure-phase garnets.Intermediate 2 consisting of cubic Li_6.4Ga_0.2La₃Zr₂O₁₂（LGLZO）and 10.99%pyrochlore La₂Zr₂O₇stands out from the four intermediates,which exhibits high sintering activity in a wide temperature range of 1100～1240°C to yield ceramics with pure phases.Ceramic specimens obtained by sintering Intermediate 2 at a temperature of 1200°C possess a high ionic conductivity of 8.8×10^-4S cm^-1with a low activation energy of 0.314 e V,suggesting their potential for lithium-ion battery membranes.We demonstrate the ability to synthesize phase-pure LGLZO with high ionic conductivity that can be achieved through sintering a specific component intermediate containing cubic LGLZO main phase and La₂Zr₂O₇spurious phase,in which a small amount of pyrochlore can serve as a sintering agent to facilitate ceramic densification.The present study has laid the foundation for screening effective components for the rapid preparation of dense garnet solid electrolytes in cubic phase.（2）Garnet-based superionic conductors hold great promise in next-generation lithium-ion batteries because of their favorable ionic conductivity and unique stability against Li-metal anodes;however,they still face the challenge of air stability for mass production/practical application.Herein,we identify their structural features and tailor the functional elements to enhance the chemical stability via synergistic control of doping and non-stoichiometry.The optimal composition of Li_6.25Ga_0.2La₃Zr₂O_11.85F_0.15（LGLZO-0.15F）garnet exhibits great air durability without noticeable impurity accumulation even in a continuous air exposure for 60 days,owing to the high affinity of the fluorine dopant for lithium occupying the octahedral site.Meanwhile,LGLZO-0.15F possesses an appreciable Li-ion conductivity of 8.4×10^-4S cm^-1with an activation energy of 0.29 e V.Benefiting from these properties,hybrid and all-solid-state lithium batteries constructed with LGLZO-0.15F electrolytes demonstrate low overpotential,high Coulombic efficiency,and stable cycling performance.（3）Since the discovery of the superionic conductor LLZO in 2007,the garnet-structure electrolyte has been considered one of the most promising and significant solid-state electrolytes for Li-ion batteries with potential benefits in energy density,electrochemical stability,high-temperature stability,and safety.However,achieving a combination of high ionic conductivity and excellent chemical stability in a solid electrolyte simultaneously remains a significant challenge in light of their competitive nature.Hence,intelligently manipulating the chlorine atoms to precisely modulate the structural framework of the garnet-based oxide superionic conductor,further rationalizing the physical and chemical behavior of the electroactive material and achieving the"dream linkage"between component-structure-property.The obtained Li_6.3Ga_0.2La₃Zr₂O_11.9Cl_0.1（LGLZO-0.10Cl）possesses a high lithium ion conductivity of 1.12×10^-3S·cm^-1at room temperature and a low activation energy of0.23 e V.NPD reveals that the substitution of heterovalent Cl^-for O^2-in garnet sublattices creates more available lithium vacancies in the interstitial space and enlarges the triangular"bottleneck"shared by tetrahedral and distorted octahedral sites through which the ions must cross.In addition,the incorporation of chloride ions densifies the garnet-based ceramic pellet,rendering the material excellent stability in ambient air,thermal environments containing H₂O and CO₂vapor,and even in water.Li Fe PO₄/Li all-solid-state battery based on LGLZO-0.10Cl solid electrolyte shows excellent rate performance and long cycle performance.