| Li ionic solid state electrolyte are the important component in all solid state lithium battery.Garnet solid state electrolyte Li7La3Zr2O12 is one of the most promising li ion solid state electrolyte due to its high stability with lithium metal anode,high ionic conductivity at room temperature,high mechanical properties and so on.However,the ionic conductivity of garnet is still lower enough compared with liquid electrolyte.And due to its thermodynamics are unstable at room temperature,element doping is need to acquire high ionic conductivity cubic Li7La3Zr2O12.In this paper,Sol-gel synthesis,doping modifactions and the application in all solid state battery of Li7La3Zr2O12 were investigated.An simplified Sol-gel route was chosen to synthesis the Li7La3Zr2O12.Li2CO3,La?NO3?3·6H2O,ZrO?NO3?2·xH2O was chosen to as raw material.Citric acid was the chelating agent in it.Then we have synthesis the Li7La3Zr2O12 garnet-like solid state electrolyte successfully.The influence of calcining and sintering craft,concentration of the sol on the structure and properties of Li7La3Zr2O12 were investigated.The optimal calcining temperature and holding time were 850?and 6 h,respectively.The optimal sintering temperature and holding time were 1200?and 10 h,respectively.The optimal concentration of sol was0.06 M.Under the optimal craft,the Li7La3Zr2O12 pellets sintering in aluminum crucible exhibited ionic conductivity about 5.2×10-5 S cm-1.The relatively density was 89.7%.Which was far from the reported average value of 5.0×10-4 S cm-1.The main reason is that the Al doping process is difficult to control,and Al contained phase was easy to enrich at the grain boundary,which was unfavorable to obtain high purity cubic LLZO.We have prepared garnet-like solid state electrolyte Li7-3xFexLa3Zr2O12 samples by substitution of Li site by aliovalent cations via Sol-gel route firstly.The solution behavior of iron in Li7La3Zr2O12 and its influence on physical,chemical and electrochemical performance on Li7La3Zr2O12 were investigated systematically.We can conclude that the cubic phase of Li7La3Zr2O12 can be acquired at room temperature by iron doping from the XRD and Raman spectra.The higest solubility of iron in Li7La3Zr2O12 was 30mol%due to the difference in radius between the doped ions and intrinsic ions.We can conclude that iron was beneficial to grain refinement of Li7La3Zr2O12 from SEM,which was cause by the pinning effect of the Fe atom that are not support the movement of the boundary.The relatively density of Li6.4Fe0.2La3Zr2O12was 91.5%.The sample of Li6.4Fe0.2La3Zr2O12 exhibited highest ionic conductivity of1.23×10-3 S cm-1 and lowest activation energy of 0.26 eV,which was comparable with the commercial liquid electrolyte of 1.0×10-2 S cm-1.It is mainly attributed to high ionic conductivity cubic phase,preferred lithium ion concentration and tightly connection between grains.From XPS results,element in Li6.4Fe0.2La3Zr2O12 are exist in the form of Li+,Fe3+,La3+,Zr4+.We have investigated the application prospect of Li6.4Fe0.2La3Zr2O12 in all solid state battery for the first time,which reveals perfect prosperity in this field.We have tested the electron conductivity of Li6.4Fe0.2La3Zr2O12.The result of ion migration number are almost1,which means Li6.4Fe0.2La3Zr2O12 are an electronic insulator and a good ionic conductor.Because of high density and high ionic conductivity that was benficial to ease the dendritic of short circuit,the critical current density of Li6.4Fe0.2La3Zr2O12 was approximately to1.0 mA cm-2.Which was almost equal to the reported value.DC polarization cycling performance was conducted at current density of 0.5 mA cm-2 up to 600 h and no signs of absolutely short circuit,which indicates that Li6.4Fe0.2La3Zr2O12 has well cycle stability.It is mainly benefits from high ionic conductivity and stable ion conduction among the electrode-electrolyte interface. |
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