The Effect Of Doping On LLZO And The Optimization Of The Li/LLZO Interface

As the fast development of energy storage industry,Li-ion batteries have been focused on by researchers due to its great potential of future application in electric vehicles.Solid electrolytes represent an important trend because they have not only good ion conduction properties,but also longer services lives and better security properties compared to liquid electrolytes widely used nowadays.Through all types of solid electrolytes,garnet type electrolytes such as LLZO are payed great attentions to by researchers due to its superior performance.One of the main problems for LLZO’s industrial application is the interface resistance between LLZO and lithium metal electrode.In this work,we investigated in the improvements of Li-LLZO interfacial resistance by doping.LLZO with different dopants was synthesized and their performances were tested.The XRD analyses of LLZO sintered at different temperatures showed that the best sintering temperature for LLZO is 1200℃.Adding Al into LLZO with a proportion of 0.2mol%would effectively decrease the sintering temperature to 1000℃.The experiment included multi-step temperature sintering method for preparation of samples.Four samples with different components were gained and were called LLZTaO,LLZNbO,LLZSbO,LLZTeO for short.We conducted densification calculations,XRD analyses,SEM characterization analyses and EIS tests for conductivity comparison.By assembling of Li-LLZO-Li symmetric batteries,we conducted EIS tests for Li-LLZO interfacial resistance performances of different samples.By densification calculations,XRD analyses and SEM analyses,doping samples all had high densification ratios and crystallinity degrees.Cubic phases which contribute to high conduction performances were found in all samples with density over 90%.This proved the feasibility of multi-step temperature sintering method for the preparation of LLZO doping samples.The results show that the LLZO with different dopants maintained the ionic conductivity of at least 5×10^-4S/cm and Ta>Nb>Sb≈Te.All ionic conductivities are slightly higher than that of undoped LLZO,or at least equivalent.The LLZO doped with Ta had the best performance in terms of the interfacial resistance with lithium metal and Ta>Sb>Te>Nb.This result was in accordance with the theoretical calculation results by DFT methods.The EIS results also showed that the interfacial resistance was decreasing with the increasing temperature,and the interfacial resistance of LLZO doped with Ta at 100℃was only one third of that at 25℃.PEO-LLZTaO composite electrolyte was made for symmetric batteries,which had the overall resistance of 1.3×10⁵Ω,while the symmetric batteries made of LLZTaO electrolyte without PEO had the overall resistance of 3×10⁵Ω.This result proves the potential of polymer-LLZO composite electrolyte materials.