Study On Reactive Flash Sintering Behavior Of Ta-doped Garnet-Type Electrolytes

Garnet solid electrolyte Li₇La₃Zr₂O₁₂（LLZO）is a solid oxide electrolyte material with high electrical conductivity.The structure can be stabilized to the cubic phase by doping with Ta element,thereby further improving the performance of the lithium lanthanum zirconium oxide material.In addition,in order to ensure the electrical properties of the LLZO solid electrolyte,in the traditional normal pressure sintering process,the material usually needs to undergo a long-term heat treatment,which leads to the volatilization of lithium element,so it is difficult to obtain an electrolyte with a fully dense structure,which limits the performance of the LLZO electrolyte.application.At present,some new sintering techniques have been applied to the preparation of LLZO materials.In this thesis,the"reactive flash sintering"was used to obtain the Ta-LLZO solid electrolyte.This process is characterized by the sintering temperature and the short sintering time.At the same time,the phasing and densification of the material was achieved in a flash sintering process,and the flash sintering behavior of the material under DC/AC electric field and its effect on the microstructure were studied.The Ta-doped LLZO precursor powder was prepared by the solid-phase method,and the phase composition of the initial powder at different calcination temperatures and the effect of DC/AC electric field on the flash parameters were analyzed.First,the effect of the initial powder properties on the DC flash ignition behavior was studied by using the applied electric field as a DC electric field.The results show that for the uncalcined samples,starting with mixed solid oxides,when the DC electric field strength increases from 50 V/cm to 100 V/cm,the corresponding flashover temperature decreases from695°C to 615°C,that is,the flash sintering point decreases gradually with the increase of the electric field strength.Compared with the above powders without calcination,under the same field strength of 50 V/cm,the pretreatment time is 4 h,the calcination temperature is 750°C and 900°C,and the furnace temperature is 668°C and 623°C,respectively.From the analysis of the phase composition,it can be considered that with the increase of the high-conductivity cubic LLZO composition,the flash-sintering phenomenon appears in the lower temperature range.At the same time,the actual temperature of the sample during the flash ignition process was estimated by using the blackbody radiation theory,and the dynamic temperature characteristics of the sample under the action of a DC electric field were analyzed.With the increase of electric field strength,the flash sintering point of the sample gradually decreased,and the peak temperature and stable temperature increased.Secondly,the flash sintering behavior of solid-state lithium ion electrolyte Li_6.25La₃Zr_1.5Ta_0.5O₁₂（Ta-LLZO）prepared with different multiphase precursor powders under AC electric field was investigated.Among them,under different pretreatment temperatures,the influence of powder composition on the flash sintering point is consistent with the DC electric field.For the samples pretreated at 900°C for 4 h,when the applied AC electric field strength was 50 V/cm and the frequency was 800 Hz,the flash sintering phenomenon appeared at 590°C,the current limiting density was 150m A/mm²,and the sintering was 30 s.After sintering,the crystal structure was transformed into a pure cubic phase;when the sintering time was increased to 80 s,the density could reach 95%,and the sample showed a good and uniform microstructure.In addition,the particle size of the alternating electric field is regionally uniform,and the particle size is refined with increasing frequency.The blackbody radiation theory can be applied in an AC electric field,and the results obtained are similar to those in a DC electric field.The electrolyte prepared by AC reactive flash sintering achieves a room temperature conductivity of 0.15 m S/cm and a conductivity activation energy of 0.21 e V.In conclusion,the in situ reactive flash sintering method can be used as one of the optimized methods for the preparation of garnet-type solid electrolytes.