| Liquid-electrolyte lithium-ion batteries(LIBs)have been large-scale application in mobile phones,power tools and other portable electronic devices,as well as vehicles for short-range travelling.However,limited by the electrolyte,it is difficult to be used as power batteries for electric vehicles(EVs)and plug-in hybrid electric vehicles(P-HEVs).Due to the liquid electrolyte is flammable and incompatible with high-voltage cathode materials,it cannot meet the requirements of a higher safety and longer range travelling of new energy vehicles.Compared with their liquid counterparts,solid-state lithium batteries(SSLi Bs)have shown superior safety,cycling performance,energy density and lower requirements for battery packaging.Garnet lithium lanthanum zirconium oxygen(Li7La3Zr2O12,LLZO)has received much attention since it was reported in 2007 due to the high lithium ionic conductivity at room temperature(>10-4S·cm-1),negligible electronic transport(10-8 S·cm-1),the stability to lithium metal and a wide electrochemical window(0-6 V).This dissertation discusses the effect of sintering process and the additives on the lithium ionic conductivity of LLZO.The interface impedance between solid electrolyte and lithium metal was also investigated.The interface stability of solid electrolyte/liquid electrolyte with different compositions was studied.The main conclusions are as follows:(1)The Al-doped cubic garnet Li6.4Al0.2La3Zr2O12(LLZALO)with lithium ionic conductivity at room temperature of 2.10×10-4 S·cm-1 was synthetized by a solid-state method.The relative density is 91.2%.The cubic garnet structure of LLZO was stabilized and the lithium ionic conductivity was improved by added Al2O3.Different sintering conditions affect the lithium volatilization and relative density.The results show that the synergistic effect of Li concentration and relative density determine the Li+ionic conductivity.Compared to the relative density,Li concentration plays a major role in determining the ionic conductivity via the solid-state method.(2)Although the lithium ionic conductivity of LLZALO was increased to2.10×10-4 S·cm-1,compared with Ta and Nb-doped LLZO,LLZALO has lower lithium ion conductivity except Al co-doped with Si.In order to play a better role of Al during sintering,the Li Al Si O4(LAS)as the sintering aid was added into the LLZMO(M=Ta,Nb)for the first time.Surprisingly,the results show that the lithium ionic conductivity of Li6.4La3Zr1.4Ta0.6O12(LLZTO)and Li6.75La3Zr1.75Nb0.25O12(LLZNO)is increased obviously after 1wt.%LAS added.The lithium ionic conductivities of 1wt.%LAS-LLZTO and 1wt.%LAS-LLZNO ceramic reach to 5.14×10-4 S·cm-1 and 6.93×10-4S·cm-1 at room temperature(25?),respectively,which are much higher than pure LLZTO and LLZNO.(3)The reason of increased lithium ionic conductivity of LLZTO and LLZNO after added LAS was investigated.The addition of LAS enhances the contact between LLZTO grains,which enables greater enhancement of the mobility of lithium ions.The content of LAS has a significant influence on lithium ionic conductivity and it exists an optimum content of 1wt.%.When the content is higher than 1wt.%,a glass-like phase of LAS at grain boundaries was found,which becomes obvious at 2wt.%.The lithium ion migration can be blocked by these glass-like phases.The addition of LAS improved the relative density of LLZNO greatly.LAS enhanced the grains connectivity and reduced the grain boundary resistance.With the sintering time and sintering temperature increased,LLZNO reacted with LAS during sintering.During this process,LAS increased grains connectivity and densified the LLZNO ceramic pellets,and then incorporated into LLZNO,which limited the Li+movement and the deteriorated lithium volatilization,correspondingly,decreased the lithium ionic conductivity.LAS has different effects on LLZTO and LLZNO.The relative density of LLZTO is unchanged after LAS added,while that of LLZNO increases from 80%to 94%.These results indicated that Ta can act as a sintering additive to improve the sintering performance,but LLZNO needs the sintering additives,such as LAS.(4)The lithium ionic conductivity of LAS-LLZNO prepared by a simple solid state method is increased to 6.93×10-4 S·cm-1,so the compatibility with the cathode materials and lithium metal must be optimized to improve the performance of SSLi Bs.LAS reduced the interface resistance of Li/LLZNO and enhanced the stability between the Li and LLZNO.Due to the uneven dissolution and deposition of lithium metal during the circulation process of Li/LAS-LLZNO/Li,the contact area between lithium and solid electrolyte becomes smaller and smaller,gradually forming point contact,so that the short circuit was caused by the dendrite of lithium passed through the grain boundary or pores of solid electrolyte.These results verified the feasibility of using LAS-LLZNO as the electrolyte in SSLi Bs.Trace electrolyte can improve the interface contact performance between cathode and solid state electrolyte,furthermore,enhanced the cycle performance of SSLi Bs.The compatibility of LLZNO with different liquid electrolytes is different.The stability of LLZNO/LiTFSI+DOL+DME is higher than that of LLZNO/LiPF6+EC+DMC.The addition of LiNO3 can improve the interface performance of SE/LiPF6+EC+DMC. |
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