Enhancement of Ionic Conductivity of Composite Membranes for All-Solid-State Lithium Secondary Batteries
Authors: Park Jun-Woo , Park Jae-Yeong , Choi Jeong-Hee , Lee Sang-Min , Doh Chil-Hoon 
Published: February 2020

Recently, significant attention has been paid to conventional lithium ion battery (LIBs) with large-scale energy storage and longer cycle properties because new applications, such as electric vehicles (EVs) and energy storage systems (ESSs), are gradually emerging onto the market. Moreover, large-scale battery systems are needed to reduce the system costs of these new applications. Safety issues are of even greater concern for such batteries than for existing smaller batteries. As a promising solution to these issues, the use of a solid electrolyte (SE, based on a polymer and a ceramic) has been suggested [1-2].  In contrast to organic-based liquid electrolytes, all-solid-state lithium ion batteries with a solid electrolyte are considered safe because of their non-flammability. Various solid electrolytes based on high molecular weight dielectric polymer hosts have been tested, and in particular dry electrolyte membranes based on polyethylene oxide (PEO) have been widely studied. PEO-based membranes can be used in all-solid battery designs in a free standing form without modifying current battery fabrication processes[3]. However, the ionic conductivities of PEO-based membranes are very low (10-
6-10-
8 S/cm) at the room temperature because of their low effective carrier mobilities. The carrier mobility can be enhanced by stabilizing the highly conductive amorphous matrix with an inorganic filler (nano-sized SiO2, Al2O3, etc.) [4-5]. However, high ionic conductivities (>10-
4S/cm) can only be obtained in these composites when they have very high filler contents (e.g. >30 wt.%), which result in poor process ability and mechanical properties [6]. In this study, we propose the new concept of a free-standing composite membrane with high ionic conductivity and good process ability, and report our implementation of this concept through the incorporation of garnet-type LLZO solid electrolyte powder into a PEO polymer matrix. Further, we discuss whether the ion conductivity of the composite membrane is improved by adding a small amount of Ionic liquids (ILs) with LLZO solid electrolyte powder because of synergetic effects, and whether the mechanical flexibility of the composite membrane is maintained. Finally, we assess the feasibility of all-solid electrode designs that use oxide solid electrolyte powders.

 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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Figure 1