Sintering Technology Of High Performance Garnet Ceramic Electrolyte And Its Application In Solid State Li-S Battery

Solid-state electrolyte（SSE）is expected to fundamentally solve the safety problems of lithium batteries.It can also combine with lithium metal and high-specific-energy cathode materials to form Li-metal solid-state batteries,which is expected to further improve the energy density of lithium batteries.Among various solid electrolytes,garnet-type Li₇La₃Zr₂O₁₂（LLZO）ceramic electrolytes have become a research hotspot because of their advantages of high ionic conductivity at room temperature,stable contact with lithium metal,high chemical and electrochemical stability.However,this system still faces serious challenges,including the hardship of sintering densification causing large grain boundary impedance,the solid-solid-point contact between LLZO with lithium metal and the cathode materials leading to poor interfaces,the battery assembly process needs to be developed due to the big brittleness of ceramic electrolyte,critical current density（CCD）needs to be increased in order to improve the ability of inhibiting lithium dendrite growth,etc.In view of the key problems faced in the sintering and preparation of garnet ceramic solid electrolyte,in this paper the sintering process was optimized,and the garnet ceramic electrolyte with high relative density and high conductivity was stably prepared.Meanwhile,the problems such as large interface impedance and complex solid-state battery assembly process were overcame,and solid-state lithium sulfur battery was constructed.The main work in this paper is as follow:1.Aiming at solving the key problems in the process of preparing garnet-type LLZO ceramic electrolyte by atmospheric sintering,such as the difficulty of densification,serious lithium volatilization and abnormal grain growth caused by long-term high-temperature sintering,a rapid ultra-high temperature sintering（RUHTS）method has been proposed.It was found that the relative density and conductivity of LLZO ceramics prepared by RUHTS method were higher than those prepared by conventional long-time sintering,and the grain size was smaller and the grain boundary was tighter.For li_6.5La₃Zr_1.5Ta_0.5O₁₂（LLZTO）electrolyte,the conductivity and relative density of LLZTO ceramic electrolyte prepared by RUHTS method at 1360°C×10 min were up to 8.5×10^-4S cm^-1and 96.9%,respectively.This method can not only produce high-quality LLZO ceramic electrolyte,but also greatly shorten the sintering time,improve the preparation efficiency and reduce production energy consumption.2.In order to further solve the problem that lithium volatilizes easily and lithium atmosphere is difficult to be controlled during the preparation of LLZO ceramic electrolyte during high temperature sintering,an efficient"mutual-compensating Lithium-loss"（MCLL）method was developed to control lithium atmosphere.The LLZTO ceramics prepared by this method had high relative density（96%）,high lithium content（5.54%）,high conductivity(7.19×10^-4S cm^-1),large room temperature CCD value(0.85 m A cm^-2),and the comprehensive performance was obviously better than the traditional mother-powder method and the self-compensating lithium loss method.The resulting LLZTO ceramic was then used to successfully assemble a Li-metal solid-state battery.It is found that in addition to relative density and pure cubic phase structure,the lithium content in the LLZO ceramic electrolyte is also an important factor affecting the conductivity of LLZO.The MCLL method can provide sufficient lithium atmosphere for LLZO sintering and avoid the use of mother powder,which can effectively reduce the raw material cost of preparing LLZO ceramic SSE.3.Add sintering aids to reduce LLZO sintering temperature,use low-cost Nb⁵⁺instead of Ta⁵⁺to stabilize LLZO cubic phase,and study the influence of sintering aids on the properties of LLZO ceramic electrolyte.Li_6.5La₃zr_1.5Nb_0.5O₁₂（LLZNO）ceramic electrolyte was prepared with 1 wt%Li₃PO₄（LPO）as sintering aids combined with RUHTS method and its properties were tested.The experimental results show that Nb⁵⁺doping successfully stabilizes the cubic phase,and LPO does not affect the crystal structure of LLZO.The conductivity and density of LLZNO-LPO ceramics sintered at1340°C for 8 min are 4.28×10^-4S cm^-1and 92.5%,respectively,which are better than that of pure LLZNO.The obtained LLZNO-LPO was assembled into a Li-Li symmetric battery,and the critical current density was tested to be 1.2 m A cm^-2.The results show that LPO can reduce the sintering temperature and improve the properties of LLZO ceramics.4.LLZNO ceramic electrolyte was applied to quasi-solid lithium-sulfur battery.Firstly,the stability of LLZNO to lithium metal was tested by Li-Li symmetric cell.One Li-Li symmetric battery was constructed by melting lithium and let ceramics contact with lithium directly.The initial single-side interfacial impedance was 1674Ωcm²,which increased to 3314Ωcm²after 96 h.The interfacial impedance increases rapidly because Nb⁵⁺would be reduced when Nb-doped LLZO contacts with lithium metal.Another Li-Li symmetric cell with trace amount of liquid electrolyte wetting the interface showed an initial single-side interface impedance of 150Ωcm²,which increased slightly to 179Ωcm²after 96 h.Therefore,the quasi-solid lithium-sulfur batteries were assembled by wetting the two sides of the ceramic with trace amount of liquid electrolyte.At the rate of 0.1C,the specific discharge capacities of the battery in the 1^stand 60^thcircles are 943 and 602 m Ah g^-1respectively.The average coulombic efficiency for all the cycles is up to 99.4%,indicating that polysulfide shuttle can be effectively inhibited.5.A solid-state lithium-sulfur battery with high safety and no shuttling effect was constructed by wetting the cathode side of LLZTO ceramic with modified ionic liquid.Taking advantage of the stability of LLZTO ceramic electrolyte to Li metal,LLZTO ceramic electrolyte was directly contacted with the lithium anode,and only a small amount of modified ionic liquid was needed to wet the cathode electrode.The dense LLZTO ceramic electrolyte can effectively block polysulfide shuttling and protect the lithium anode.The interfacial impedance between LLZTO and lithium anode is 16Ω,and the galvanostatic cycling performance tested at current densities of 0.3 m A cm^-2delivers a stable cycling.The total impedance of the solid-state lithium-sulfur battery is147Ω,and the specific discharge capacity of the first cycle is 1185 m Ah g^-1,the coulombic efficiency of the first cycle is 96.6%,and the specific discharge capacity of the battery is 526 m Ah g^-1after 100 cycles.SEM-EDS surface scanning showed that the ceramic electrolyte successfully blocked polysulfide shuttling.It is proved that solid-state lithium-sulfur battery composed of LLZTO ceramic electrolyte has significant advantages in safety and the inhibition of shuttling effect.