Development of Organic-Inorganic Hybrid Electrolytes Based on Ionic Liquid-Nanoparticles-Composite
Authors: Sun Yan , Matsuo Takahiro , Gambe Yoshiyuki , Honma Itaru 
Published: February 2020

[Introduction]
The development of high-performance all-solid-state Li-ion batteries are strongly required for high safety and reliability. The properties of solid-state electrolytes are crucial to the performance of all-solid-state Li-ion batteries. However, the conventional solid-state electrolytes with sufficient ion conductivity and high stabilities during battery operation are limited to few cases. Here, we propose a new idea to fabricate new type of solid-state hybrid electrolytes using mixture of room temperature ionic liquids (RTILs)-Li-salts and oxide nanoparticles. Presently, RTILs-Li salts mixture are considered as a potential candidate electrolyte material because of its promising properties, such as high ion conductivity, high stability and wide potential window. Recently, some researches revealed that RTILs-Li-salts are quasi-solidified incorporating with oxide particles attributed to the strong interaction between the oxide particles surfaces and RTILs-Li-salts. These composite materials maintain the liquid-like properties of RTILs-Li-salts.
Our group have reported about a favorable soft sheet hybrid electrolyte based on RTILs-Li-salt and 7 nm-SiO2 nanoparticles. The liquid-like properties of hybrid electrolytes was proved to be beneficial. However, the insufficient power density of hybrid electrolytes became the problem. To solve this problem, we seek to sufficiently increase the power density of hybrid electrolytes incorporating with different nanoparticles. Therefore, the outstanding point of this study is that various commercial oxide nanoparticles were utilized to fabricate quasi-solid-state hybrid electrolytes with RTILs-Li-salt.
[Experimental]
The quasi-solid-state powders were prepared by mixing nanoparticles (CeO2: 15-30 nm, ZrO2: 30-60 nm and gamma-Al2O3: 5 nm) with RTIL-Li-salt mixture. Nanoparticles were preheated to 80℃ for 24h to rule out effects due to physicsorbed water. RTIL-Li-salt mixture was prepared by mixing equimolar concentration of tetraethylene glycol dimethyl ether (tetraglyme, G4) and lithium bis(trifluoromethanesulfonyl)amide (Li-TFSA) powder. The resultant solution was mixed with the above-mentioned nanoparticles in methanol for 3h. The quasi-solid-state powders were achieved by evaporating the mixture on a hot plate. The volume ration of nanoparticles reached 75, 60 and 75 vol%, respectively. Self-assembled hybrid electrolyte sheets were obtained by further mixing the powder of polytetrafluoroethylene (PTFE) with the quasi-solid-state powder at 5 wt%. All of the raw materials were used without any further treatment, but stored in the argon-filled glove box. The ion conductivities of quasi-solid-state hybrid electrolytes were measured by the ac impedance method over the frequency range from 1 × 106 to 1 Hz.
[Results and discussion]
White quasi-solid-state powders were successfully achieved incorporating various nanoparticles with G4/LiTFSA solution. Self-assembled electrolyte sheets were obtained mixing quasi-solid-state powders with 5 wt.% PTFE as final samples.
The ionic conductivity of CeO2, Al2O3, ZrO2 – x vol% G4/LiTFSA composites were measured as a function of inverse temperature, respectively. Considered with the ionic conductivity of SiO2, it was found that all the samples had liquid-like high ion conductivities although the conduction paths, i.e. RTIL-Li-salts, are quasi-solidified. Additionally, CeO2, Al2O3 and ZrO2-based quasi-solid-state electrolytes displayed higher ion conductivities than that of SiO2-based one. The reason can be attributed to the enhancement of interaction between RTIL and nanoparticle surfaces. Thus, the higher performance of the Li-ion battery composed of the above-mentioned quasi-solid-state electrolytes can be expected. From applications point of view, the hybrid electrolytes might be potentially applied to various electrochemical devices, such as bipolar all-solid-state Li-ion batteries with wider electrical potential ranges.