Garnet-Based Flexible Composite Sheet Electrolyte
Authors: Cheng Eric Jianfeng , Kimura Takeshi , Munakata Hirokazu , Kanamura Kiyoshi 
Published: February 2020

Li-ion batteries are widely used in portable electronics, such as laptops and smartphones, since their first commercialization by Sony in 1991.1 However, Li-ion batteries are approaching the theoretical limit because of the relatively low theoretical specific capacity of the graphite anode material (372 mA h g-1). Larger scale applications, such as electric vehicles (EVs) with a range of 800 km, require substantial improvements in capacity, and safety of the state-of-the-art Li-ion battery technology. Recently, Li metal batteries have regained tremendous interest as promising candidates for energy-storage devices, because the metallic Li is an ideal anode material with a high theoretical specific capacity (3860 mA h g-1), and the lowest negative electrode potential (-3.04 V vs. Standard hydrogen electrode (SHE)).2 Solid state electrolytes could not only enable stable cycling of metallic Li anode, but could also reduce the risk of fires and explosions of combustible organic liquid electrolytes.
Garnet-type Li7La3Zr2O12 (LLZO) has been widely regarded as the most promising solid oxide electrolyte because it has a high ionic conductivity (of the order of 10-4 S cm-1 25 ºC) and can also act as a physical barrier to prevent Li dendrite penetration.3 However, the high resistance between LLZO and electrode materials greatly limits the rate performance of all-solid-state batteries with LLZO electrolytes and Li metal anodes.4 Moreover, high-temperature sintering for large scale production of thin ceramic electrolytes is impractical, and the device integration of brittle ceramic electrolytes would also be difficult.
Here, we report a room-temperature synthesis of a flexible composite Al-LLZO sheet electrolyte by tape-casting and ionic liquid (IL-impregnation. The IL was LiFSI/EMI-FSI, lithium dissolved in 1-ethyl-3- (EMI-FSI). The sheet electrolyte was highly flexible, mechanically robust, and highly conductive (as high as 6.5 ×10-4 S cm-1 25 ℃, higher than all those of sintered dense Al-LLZO pellets and LLZO/PEO flexible sheet electrolytes reported to date). The non-flammable and non-volatile IL impregnated in the sheet electrolyte (about 12 %) can not only increase the ionic conductivity of the sheet electrolyte, but can also effectively reduce the resistance. The synthesis requires no high-temperature sintering, and is easy to carry out and scale up, and would be beneficial to the development of practical Li metal batteries.

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