Li7La3Zr2O12 powders were purchased from Nikkiso Co. Ltd. and used as received. The LiClO4/PEO molar ratio was fixed at 1:6, and the amount of LLZO was varied from 37.5 to 47.5 wt.% with respect to the total PEO+LLZO weight of the composite membrane. PEO, LiClO4, and LLZO were simply mixed in acetonitrile (ACN), and the ACN component of the slurry was slowly evaporated at room temperature in a dry-room, resulting in composite solid electrolyte membranes with various LLZO contents in a PEO matrix. Also, optimum condition of the composite membrane with a small amount of ILs was prepared. The ionic conductivity of the electrolytes was determined through complex impedance method using an AC impedance analyzer. The cathode electrode with the optimum composition was prepared as follows. LiNi0.6Co0.2Mn0.2O2/PEO/ LLZO/LiClO4/carbon black (60:15:17.5:2.5:5 wt%) were mixed to create the cathode slurry. A coin cell (2032 type) was fabricated in a dry-room using a composite cathode electrode, solid electrolyte membranes and Li foil anode. The charge-discharge measurements were carried out within 2.5-4.3V.
Fig.1a shows the variations of the ionic conductivity of the composite membranes with LLZO content. Increasing the LLZO content from 37.5% to 47.5% enhances the ionic conductivity slightly, but further increases in the LLZO content result in decreases in the ionic conductivity. The composite membrane with the maximum ionic conductivity was prepared under optimum conditions with 42.5% LLZO and 52.5% PEO-LiX. Thus the incorporation of the inorganic phase (LLZO) into the organic matrix (PEO-LiClO4) has a synergetic effect on the Li ionic conductivity. The ionic conductivity of the as-synthesized composite membrane was found to be significantly enhanced with respect to those of PEO-only and LLZO-only membranes, and the optimum composition was determined. Fig.1b shows the variations of the ionic conductivity of the composite membranes with ILs amount. In our approach to enhancing ionic conductivity while retaining a free standing design, we combined the organic matrix (PEO-LiX) and an inorganic solid electrolyte (LLZO) with small amount of ILs to form a composite membrane. The ionic conductivity is increased with adding ILs because the composite membrane of ILs to assist in the formation of ionic contacts.
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