The Study About Modification Of Grain-boundary Of Lithium Titanate Titanate Solid Electrolyte And The Interface On Lithium

The lithium-conducting ability of the grain boundary and the interface problem matching with the lithium metal are the problems that must be solved in the application process of the solid electrolyte.The perovskite-structured oxide solid electrolyte lithium lanthanum titanate（LLTO）is particularly affected by these two aspects.This paper is devoted to the study of the grain boundary of LLTO solid electrolyte materials and the interface with lithium.In the application process of LLTO,there are the following problems:LLTO’s grain boundary lithium-ion conductivity is relatively low,resulting in poor overall lithium conductivity;secondly,when matched with a lithium metal anode,Ti⁴⁺in LLTO will be reduced to a low price by lithium State,the electronic conductivity is increased,and a battery short circuit occurs.In addition,the interface contact resistance between the solid electrolyte and the lithium metal anode is very large,which affects the performance of the solid-state battery.In this paper,based on the above two key interface problems,the following measures are taken for research:1.The low melting point anti-perovskite-type lithium-rich material Li3OCl and LLTO are doped and sintered to supplement lithium at the grain boundary of LLTO to optimize the grain boundary conductivity.2.Using magnetron sputtering to deposit a thin film on the surface of the electrolyte sheet,through the in-situ reaction of the thin film and lithium metal to improve the interface contact and avoid the problem of reduction of Ti4+in LLTO by lithium metal.By solid-phase sintering method（1350℃）,LLTO was doped and sintered with different mass percentages of Li₃OCl（denoted as LLTO-X,X is the mass percentage of doped Li₃OCl）,and different mass ratios of doping to ion conductivity The impact of the rate.The LLTO-2 sample showed the highest grain boundary conductivity(1.52×10^-4 S/cm)and total conductivity(1.43×10^-4 S/cm).The diffusion activation energy was calculated by the variable temperature AC impedance test.It was found that the diffusion activation energy of the unmodified LLTO grains was 0.31 e V,and the diffusion activation energy of the grain boundaries was 0.46 e V.After the original sample was doped and sintered with 2%Li₃OCl,the diffusion activation energy of the LLTO-2 crystal is reduced to 0.24 e V,and the diffusion activation energy of the grain boundaries is reduced to 0.36 e V.In addition,Li₂O and Li Cl raw materials Li₂O and Li Cl were mixed and sintered with LLTO at a doping ratio of 2%（1350 ^oC）,and it was found that the conductivity optimization function is not as good as Li₃OCl,indicating the uniqueness of Li₃OCl material selection.Through the XRD test,it was found that after doping with Li₃OCl,the crystal form of LLTO changed from tetragonal phase to cubic phase,and the cubic phase had a higher ionic conductivity,which was the reason for the increase in grain conductivity.Through the comparison of XPS,it was found that with the increase of the doping amount,the ratio of O/Ti in the sample increased,indicating that the cations（lithium ions）contained in the system increased,which confirmed that the lithium supplementation effect of doping and sintering does exist.Through the analysis of TEM and EELS,it was found that the LLTO grains have lithium loss during the sintering process,so that partially ordered cations appear inside the grains.After the composite Li₃OCl is sintered,due to the compensation effect of lithium ions,a grain boundary influence zone appears in the boundary area of the grain and the grain boundary.This region has a higher lithium-ion content,and the disorder degree of cation arrangement is higher,which is conducive to ion transfer.Assemble the solid-state battery by double-layer coating.By comparing the EIS data and the cycle data of the battery,it is found that the conductivity of the solid electrolyte has a greater influence on the internal resistance of the battery,which proves that the composite sintered Li₃OCl improves the conductivity.Another big problem that plagues LLTO applications is the stability problem when matching with lithium metal.The symmetrical battery assembled with LLTO and lithium metal is short-circuited after 18 hours of cycling.At this time,Ti⁴⁺in LLTO has been reduced by lithium.The surface of the LLTO electrolyte sheet was modified by magnetron sputtering,and the improvement effect of thin-film layers of different elements on the interface of LLTO electrolyte was explored.In this paper,a layer of Sn,Ti,Si,and C films were sputtered on the surface of LLTO solid electrolyte using magnetron sputtering technology,respectively.It was found that the deposition of tin and titanium films cannot effectively prevent Ti⁴⁺in LLTO electrolyte from being reduced by lithium metal.The deposited silicon thin film and carbon thin film can effectively inhibit the reduction of Ti⁴⁺.Among them,the Li|Si-LLTO-Si|Li symmetric battery assembled with silicon thin film as the interface modification layer can stabilize the circulation for 700 hours under the current condition of 0.1 m A.The Li|C-LLTO-C|Li symmetric battery assembled with the thin film as the interface modification layer can be stably cycled for 900 hours.In addition,the effect of different thicknesses of silicon and carbon thin films on improving the interface performance of the solid electrolyte was investigated,and it was found that the thickness had little effect on the optimization of its electrochemical performance.After disassembling the symmetrical cell after cycling,the interface reaction products were analyzed using XPS,and it was found that the chemical composition of the thin film modification layer changed with the increase of the cycle number,and the silicon film,carbon film and lithium metal were detected at the same time.The reaction process:（1）The silicon film on the surface of LLTO reacts with the metal lithium electrode during the symmetrical cell cycle to form Lix Si and Li_xSi O_y;（2）The carbon film on the surface of LLTO reacts with the metal lithium electrode during the symmetrical cell cycle Generate Li C₆.