Preparation and Characterization of Inorganic Li7La3Zr2O12 (LLZO)/Polyether-Based Polymer Composite Solid Electrolytes.
Authors: Kato Masaki , Hiraoka Koji , Hakamada Yuta , Hirose Ryo , Suzuki Naamo , Hayano Tatsuya , Seki Shiro 
Published: February 2020

Li-ion battery has high capacity, good cycle ability, wide operating voltage and has applied to various of electric devices. However, organic solvents with high flammability are used for most industrial Li-ion battery and there are many risks such as leakage, fire and explosion. To solve these problems, all-solid-state Li battery applying to inflammable solid electrolyte has many attract attention in the world. All-solid-state battery has many favorable characteristics such as high safety, high energy density by stacking, wide operating temperature and voltage. Inorganic solid electrolyte such as garnet structure, NASICON-type are vigorously studied due to high ionic conductivity and high cation transport number. However, mechanical properties of thin pellets are extremely weak and interfacial formation between electrolyte and various of electrodes are also difficult. In contrast, polymer electrolytes are easy to form stable interface between electrolyte and electrodes and possible to form thin electrolyte due to high flexibility and mechanical property. Polyether based solid polymer electrolyte is one of the potential candidate and shows 10-4～10-5 [S cm-1] at room temperature.
In this study, we prepared to inorganic/polymer composite solid electrolyte which has high flexibility and mechanical property by using of cubic-Li7La3Zr2O12(LLZO) with garnet structure and polyether based polymer electrolyte and investigated to ionic conduction property and thermal property, interfacial stability and resistance between Li metal and electrolyte.

Experimental

Preparation and characterization of hybrid solid electrolyte film

Preparation of sample and cell preparation were examined in Ar-filled grove box. LiN(SO2CF3)2 and DMPA (photo initiator) were dissolved in polyether-based macromonomer solution ([Li] / [O] =0.1, amount of O was based on oxide unit from polyether and DMPA is 1000ppm based on weight of macromonomer). Cubic-LLZO were mixed in weight ratio of macromonomer : LLZO = 1: x (x = 0.1, 0.2, 0.25, 0.5, 0.75, 1, 2) and a little acetonitrile were added to the solution to obtain homogenerous solution. The solutions were dried over 12h. The solutions were casted on glass plate and covered by two glass plate and 0.5mm teflon spacer. composite solid electrolyte films were fabricated by radical polymerization under UV at 5 min.

Characterization of hybrid solid electrolyte

Ionic conductivity of composite solid electrolyte were measured by AC impedance method. Measurement samples were cut out 12mm diameter disk. Frequency range is from 200kHz to 10mHz with a 100mV amplitude. Temperature range is -5 to 80 and all samples were thermally equilibrated at each temperature at least 1.5 h prior to the measurement.
Thermal property of solid electrolytes were measured by differential scanning calorimetry measurement. Temperature range were -100～200℃ and conducted with the samples cooling to -100 , followed by heating to 200 . Heating rate was 10 /min.
Interfacial stability of composite solid electrolyte were measured by AC impedance method. The composite solid electrolyte films were cut into circle of 19mm diameter and then were sandwiched between Li metal. These samples were encapsulated into 2032-type coin cell completely . Coin cells were maintained at 60 over 100h in a constant temperature chamber and measured interfacial resistance each 5h.

Result

Fig. 1 shows appearance of composite solid electrolyte. Composite solid electrolyte has high flexibility and mechanical property at room temperature and this is one of valid method to obtain pelletized inorganic-based solid electrolyte without sintering process.
Fig. 2 shows temperature dependence of ionic conductivity of composite solid electrolytes. Ionic conductivity showed convex decreasing tendency with temperature, and showed
10-4 [S cm-1] at 60℃. Fig. 3 shows Nyquist plots of LLZO-free system and x = 1 electrolytes at 5℃. Semicircular arc of composite solid electrolyte changed asymmetry by adding of LLZO compared with LLZO free system. We tried to fit these Nyquist plot by each equivalent circuit. Experimental data and calculated values were well agreed. This result indicated two Li conduction pass of polymer phase and LLZO particle to particle (grain boundary) in LLZO/polyether composite system. Fig. 4 shows time dependence of interfacial resistance between Li metal and composite solid electrolyte at 60℃. Interfacial resistance between Li metal and composite solid electrolyte decreased with time passage and became almost constant value about 50h. Values of interfacial resistance were stabilized even after 100h passed. This result suggested extremely stable interfacial formation between LLZO/polyether composite solid electrolyte and Li metal.

Figure 1