Sintering And Optimization Of Garnet-type Li~+ion Solid Electrolytes

The state-of-art commercial lithium ion batteries?LIBs?use organic electrolytes,which have potential safety hazards such as leakage,burning,and explosion in unconventional conditions.Solid-state batteries based on inorganic solid electrolytes are of great significance for solving the safety problems existing in traditional liquid batteries.In addition,the solid-state lithium battery has the advantages of high specific energy,long cycle life,low self-discharge,and strong designability,and has been widely used in microelectronic systems and portable electronic devices.The key to the development of solid-state batteries lies in the development of high-performance solid electrolytes.In general,the electrolyte should have high lithium ion conductivity,a wide electro-chemical window and good chemical stability to the electrode material.The garnet-structured Li-Garnet solid electrolyte?Li7La3Zr2O12,LLZO?has high Li⁺ion conductivity(near 10^-3 S cm^-1),stability to metallic lithium and a wide electrochemical window?0⁴ V?.It is one of the candidate electrolyte materials for next generation high-safety solid-state lithium batteries.Since its first report at 2007,researchers have focused on the doping and structural studies of such ceramics.Metal cations such as Al,Ga,Nb,and Ta were attempted to be doped into the LLZO crystal lattice to stabilize the cubic phase and increase the conductivity.Structural analysis of the tetragonal and cubic phases,calibration of the lithium ion position,and understanding of the lithium ion transport have been studied in detail.However,the sintering mechanism and corresponding optimization of such ceramics have rarely been reported.LLZO contains a large amount of alkali metal lithium?29 mol%?.This is similar to???-Al2O3,which also contains a large amount of alkali metal sodium.There are two problems in the sintering process:volatilization of alkali metal element and abnormal grain growth.In the published work,the complex and expensive hot-pressing method,plasma sintering method and field assisted hot press method were used to sinter LLZO to obtain high density?>97%?ceramics.However,low-cost ambient air sintering method is difficult to obtain ceramics with a density exceeding 95%,which needs further exploration and research.This paper aims to study the ambient air sintering mechanism and corresponding optimization of LLZO,to explore low-cost preparation and sintering method to obtain high-quality ceramic electrolytes.The detailed research content are as follows:?1?Sintering behavior and mechanism of cubic LLZO:developing low-cost sintering method.The existence of volatile lithium compounds?VLC?was confirmed and the influence of VLC on sintering results and ceramic properties was explored.A time-gradient sintering experiment was designed to observe the evolution of ceramic grains under high and low VLC atmospheres.The microstructure and properties of ceramics are characterized and analyzed to infer the possible sintering mechanism and determine the optimized sintering condition.Thereafter,low-cost LixZrOy compounds were designed to replace high-cost LLZO bed powders to compensate for Li-loss during high-temperature sintering.Non-platinum and no additional bed powder sintering method is researched and developed to further reduce the LLZO sintering costs.?2?Sintering aid La2Zr2O7?LZO?:decreasing sintering temperature and increasing conductivity.The VLC in the sintering process reacts with the inert LZO,and the products are sintering aids in LLZO ceramics:1.5La₂Zr₂O₇+4.5Li₂O?g??Li₇La₃Zr₂O₁₂+Li₂ZrO₃During sintering,LZO at the grain boundaries reacts in situ with VLC to form highly active LLZO and Li2ZrO3.On the one hand,it promotes inter-grain mass transfer sintering;on the other hand,it improves grain boundary bonding to increase the Li⁺ion conductivity.In addition,small-scale laboratory mass production of high conductive LLZO ceramics can be realized based on this component.?3?Sintering inhibitor MgO:inhibiting grain growth and improving mechanical strengthIt is difficult for Mg²⁺to enter the LLZO lattice and MgO is chemically stable to LLZO.MgO can be used as the second phase material to hinder the surface transport of LLZO grains in the sintering process and control the grain growth,thereby preparing high-strength LLZO/MgO composite ceramics.The gradient sintering experiment was designed to observe the grain growth evolution.TEM methods were applied to observe the triple grain boundary of composite ceramic.The existence of Mg and the inhibition mechanism of MgO were analyzed.In addition,a two-step sintering method?high temperature-short time sintering,and low temperature-long time incubation?was designed to further improve the bending strength of LLZO/MgO composite ceramics.?4?Study on Doping and Sintering of Nb-LLZO SystemThe phase,sintering behavior,ceramic microstructure and properties of Nb-LLZO doped with Nb=0.2⁰.7 pfu were studied.The best Li vacancy number was determined and the preparation process and sintering conditions of high-quality Nb-LLZO ceramics were explored.At the same time,the Nb-LLZO-MgO composite ceramics with different Nb doping content were prepared by combining the sintering inhibitor MgO,and the micro-morphology and properties of the Nb-LLZO-MgO composite ceramics were studied to find the optimal doping amount of Nb.?5?Further exploration of powder preparation and ceramic sintering processFor mass production,a set of water-based attrition milling and spray drying methods was designed and explored to avoid the risks associated with the application of organic solvents,and to reduce the cost of powder processing equipment and materials.