Study On Preparation And Performance Of The Garnet Type Solid Electrolyte Li₇La₃Zr₂O₁₂ And All-solid-state Lithium Battery

In recent years,the lithium-ion battery has been developing quickly and widely used in many industries.But the present lithium-ion battery mainly uses organic electrolyte or gel polymer electrolyte,which is flammable and explosive.Such safety problems have caused people's attention.The solid electrolyte with high ionic conductivity,good chemical stability,safety and reliability,has become the focus of the current research.In this thesis,Li₇La₃Zr₂O₁₂?LLZ?solid electrolyte was the research object.It studied the influence of different synthesis techniques,methods,metal cations doping and sintering additive on the structure and properties of LLZ.The phase transformation process and structural stability of LLZ were also researched.The prepared LLZ samples were measured by thermogravimetric analysis and differentialscanningcalorimetry?TG-DSC?,X-raydiffraction?XRD?,microstructure analysis?SEM and FESEM?,energy dispersive X-ray spectroscopy?EDX?,Raman spectra?Raman?and Inductively Coupled Plasma?ICP?.What is more,the performances of the solid-state lithium battery using LLZ as solid electrolyte were tested and characterized.The LLZ solid electrolytes have been prepared via the solid-state reaction methods,the sintering temperature and time of the green body would affect the phase structure of LLZ.The sintering temperature of the simplex structure cubic phase LLZ should be not lower than 1180?and the sintering time should be at least 15 h.However,when the sintering temperature is higher than 1250?,the product will decompose.The crystallinity of the obtained LLZ samples at 1180?for 20 h is well and the crystal structure is integrated.The ionic conductivity is2.85×10^-44 S cm^-11 at 30?and the activation energy is 0.50 eV.Solid electrolyte LLZ powder was successfully synthesized by co-precipitation method.This method can reduce the synthesis temperature of LLZ powder.However,the grain size of the ceramic solid electrolyte sample is smaller,which will increase the content of grain boundary,so that the total ion conductivity decreases and the activation energy increases.Al doping can cause the phase of Li_7-3xAl_xLa₃Zr₂O₁₂2 transformed from tetragonal phase to cubic phase.Al³⁺substituted Li⁺could reduce the Li content in the lattice and form the lithium vacancy,which were beneficial to enhance the ionic conductivity of the solid electrolyte.When the content of A1 is x=0.3,the ion conductivity of the sample can reach 3.73×10^-4S cm^-1at 30?,and the activation energy is 0.42 eV.Ti⁴⁺doping improves the sintering density of LLZ solid electrolyte and the high valence Ti⁴⁺may enter the position of Al and Li,which leads to the precipitation of Li₂CO₃.The sample of Li₇La₃Zr_1.7Ti_0.3O₁₂2 shows the highest total ionic conductivity of about 4.16×10^-44 S cm^-11 at 30°C with the activation energy of 0.48 eV.Ti-substituted LLZ is stable when in contact with Li metal.Excessive H₃BO₃ sintering additive would prevent Al³⁺from entering the LLZ lattice,which affects the transformation of phase structure.Based on this,the tetragonal phase LLZ must be must be transformed into the cubic phase LLZ in the first place with the hot treatment,and then added the H₃BO₃.The sintering additive H₃BO₃ can promote the sintering densification of LLZ ceramics,however,too much H₃BO₃ sintering additive would produce excessive grain boundary glass phase,thereby reducing the ion conductivity of the sample.The lithium vacancy formed in the low temperature cubic phase is helpful for the Al³⁺to enter into the lattice easily,and promote the formation of the high temperature cubic phase.The transformation process of the garnet type LLZ from tetragonal phase to cubic phase is the result of cationic doping and Li content in lattice.The lithium vacancy is formed and the tetragonal lattice is distorted when Al³⁺substituted Li⁺,Ce⁴⁺doping instead of La³⁺and Nb⁵⁺doping to replace Zr⁴⁺.However,the lattice distortion transformed into cubic phase structure is a continuous process,Therefore,It's necessary to control the doping content of metal cations and the amount of Li element in the raw materials for the preparation of cubic phase garnet LLZ.Cyclic voltammetry tests demonstrate that LLZ solid electrolyte is in good contact with metal Li and can be applied to all solid-state lithium batteries.In LiFePO₄/LLZ/Li and LiCoO₂/LLZ/Li all solid-state lithium batteries,the first discharge capacities are 107 mAh g^-11 and 75 mAh g^-1,respectively,with the current density of 10?A cm^-2at 50?.And that showed a good performance.At higher current density,the polarization of the battery increases,the redox peak broadens and deviates,the charge and discharge capacity obviously attenuates,so the performance is deteriorated.