Conventional lithium-ion batteries(LIBs)that are mainly developed based on combustible organic liquid electrolytes,and common cathode-anode systems currently have suffered from high safety risks and low energy density.Solid-state lithium batteries(SSLBs)are considered to be one of the most feasible technologies that can fundamentally solve the safety problems of LIBs and offer an opportunity to greatly increase energy density.As the core component of SSLBs,solid-state electrolytes(SSEs)dominate the development and application of SSLBs.Garnet-type Li7La3Zr2O12(LLZO)is widely regarded as one of the candidate SSEs with great application potential,owing to the advantages of high ionic conductivity,wide electrochemical stability window,and good stability to lithium metal.The wet chemical technique is considered to be a suitable route for the large-scale preparation of LLZO.However,so far,the ionic conductivity of LLZO SSEs obtained via wet chemical techniques in most of the reported literatures is generally not high,and the process of preparing LLZO by wet chemical techniques is still immature.Moreover,LLZO usually has poor air-stability,and some related scientific issues still need to be studied.In addition,most of the LLZO obtained so far are powders,sheets or blocks,etc.,and LLZO is essentially a ceramic with high density and poor machinability,which will result in great challenges to the application of LLZO in SSLBs.Based on the current main problems of LLZO and challenges faced in the application process of LLZO,the following research works mainly done in this paper are as follows:(1)The key factors influencing the preparation of LLZO ceramic materials via the wet chemical technique are explored,and a reasonable preparation route(the modified wet chemical technique)is proposed.The factors such as precursors,sintering system,crucible material,high-temperature lithium volatilization,raw materials and liquid medium system,etc.involved in the preparation of LLZO by the wet chemical technique are studied in detail.Considering the addition sequence and timing of raw materials,it is easy to obtain high-quality precursors by first obtaining a homogeneous mixture containing Zr and Li elements and then adding La2O3.The sintering system of "the low-temperature short-time pre-calcination+the high-temperature long-time calcination+the secondary higher-temperature sintering"for the preparation of LLZO by the wet chemical technique is formulated.The effects of Al2O3 and MgO crucibles on LLZO ceramic samples calcined at high temperature(925℃)and secondarily sintered at higher temperature(1200℃)are specially investigated,and it is demonstrated that the MgO crucible can be well used to prepare LLZO via the wet chemical technique designed in this paper.It is demonstrated that using 20 wt%excess lithium can well compensate for the high-temperature lithium volatilization for the sintering system formulated in this paper.It is confirmed that the system of Li2CO3,La2O3,Zr(CH3COO)4 solution,and CH3COOH solution is suitable for the preparation of LLZO by the wet chemical technique.(2)The Li-site and Zr-site doping effects of LLZO based on the wet chemical technique are studied.The most famous Al,and Ga doped at the Li sites and Ta doped at the Zr sites are selected separately.The most representative Li6.25Al0.25La3Zr2O12,Li6.4Ga0.2La3Zr2O12,and Li6.4La3Zr1.4Ta0.6O12 are prepared by the modified wet chemical technique and the effects of LLZO doping Al and Ga at the Li sites as well as doping Ta in at Zr sites are discussed in many ways.The phase structure and evolution mechanism of LLZO ceramic mother powders and ceramic electrolyte pellets modified by Li-site Al,Ga,and Zr-site Ta doping are revealed.It is demonstrated that the introduction of Ta5+at the Zr4+sites is easier to stabilize the cubic LLZO phase than the introduction of Al3+as well as Ga3+at the Li+sites.After the secondary sintering at a higher temperature(1200℃),the modified LLZO doped with Al at the Li sites is a single LLZO cubic phase(Ia-3d space group),while the modified LLZO doped with Ga at the Li sites shows the characteristic of cubic phase with the space group I-43d.The Rietveld refinement reveals the roles of Al3+substitution at the Li+sites and Ta5+substitution at the Zr4+sites to adjust crystal structure.The electrochemical performance of Li-site Al-,Ga-and Zr-site Ta-doped LLZO are compared.It is found that Li6.4La3Zr1.4Ta0.6O12 doped with Ta at the Zr sites can achieve the greatest enhancement for ionic conductivity based on the modified wet chemical technique in this paper and delivers a high ionic conductivity of 6.88 × 10-4 S cm-1 at room temperature.(3)The preparation and properties of the modified LLZO doped with Nb at the Zr sites based on the wet chemical technique are studied.The Zr-site doping of LLZO is realized by replacing Ta with low-cost Nb,and the typical Nb-doped Li6.5La3Zr1.5Nb0.5O12(Nb-LLZO)is prepared by the modified wet chemical technique.The Nb-LLZO ceramic electrolyte pellets show a single cubic LLZO phase with good crystallinity,and the internal grain size of Nb-LLZO differs.BVEL(bond valence energy landscape)shows that the introduction of Nb5+ into the LLZO lattice to replace Zr4+is beneficial to improve the lithium-ion conductivity.The Nb-LLZO ceramic electrolyte prepared by the modified wet chemical technique in this paper delivers a high room-temperature ionic conductivity(7.00×10-4 Scm-1)and an extremely low room-temperature electronic conductivity(9.94×10-8 S cm-1).The effects of atmosphere exposure on Nb-LLZO ceramic electrolytes are specifically investigated,and the related mechanisms are discussed.The cell parameters of Nb-LLZO increase with extending exposure time,and atmosphere exposure can change the distribution of Li in the Nb-LLZO lattice.It is found that there is a linearly increasing relationship between the content of Li2CO3 and the exposure days.The performance of the Li|Nb-LLZO|S cells based on the Nb-LLZO ceramic pellets demonstrates that Nb-LLZO ceramic pellets can be used as the solid-state electrolyte for solid-state lithium batteries(SSLB s).The first discharge capacity of the Li|Nb-LLZO|S battery is 843.5 mAh g-1(0.02C,25℃);when the current density increases to 0.1C,the discharge capacity decreases to 369.7 mAh g-1.(4)The preparation and properties of Nb-LLZO-based composite electrolytes are studied.The necessity of developing the composite electrolytes based on LLZO is analyzed by designing mechanical experiments.The Nb-LLZO/PVDF-HFP/LiTFSI composite electrolyte membranes are prepared with Nb-LLZO obtained by the modified wet chemical technique in this paper as the fillers combined with PVDF-HFP and LITFSI.The appearance,structure,thermal stability,micromorphology,and solid-state lithium battery application are mainly investigated.The Nb-LLZO/PVDF-HFP/LiTFSI composite electrolyte membrane shows a very flat appearance,excellent bending ability,the obvious LLZO cubic phase characteristics,and typical α-phase and β-phase characteristics of PVDF-HFP,as well as good thermal stability.The charge-discharge tests of Li|LiFePO4 cells and Li|S/AC(activated carbon)/CNT(carbon nanotube)cells based on Nb-LLZO/PVDF-HFP composite electrolyte membranes show that the Nb-LLZO/PVDF-HFP/LiTFSI composite electrolytes have good electrochemical stability.The Li|Nb-LLZO/PVDF-HFP/LiTFSI|LiFePO4 cell still has a high discharge capacity(146.7 mAh g-1)after 20 cycles under the test conditions of 25℃ and 100 μA cm-2.The Li|Nb-LLZO/PVDFHFP/LiTFSI|S/AC/CNT cell delivers an initial discharge capacity of 828.6 mAh g"1 under the test conditions of 25℃ and 50 μA cm-2,and when the current density is switched to 250 μA cm-2,the discharge capacity is 519.4 mAh mAhg-1. |