Researchers at Tokyo Metropolitan University have developed a new practical method to make a flexible composite Al-doped LLZO (Al-LLZO) sheet electrolyte (75 μm in thickness) for Li-metal batteries, which can be mass-produced at room temperature.

This ceramic-based flexible sheet electrolyte, with an ionic conductivity of close to 1.0 × 10^-4 S cm^-1 at 25 °C, enables Li-metal batteries to operate at both 60 and 30 °C, demonstrating its potential application for developing practical Li-metal batteries. A paper on their work is published in ACS Applied Materials & Interfaces.

The mechanical robustness and operability of the flexible LLZO composite sheet at a wide range of temperatures makes it a promising electrolyte for Li-metal batteries. Credit: Tokyo Metropolitan University

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Li-ion batteries for electric vehicles (EVs) still require substantial improvements in capacity and safety. This has led to a renaissance of research interest in lithium metal batteries: lithium metal anodes have a much higher theoretical capacity than the graphite anodes in commercial use now. However, there are major technological hurdles associated with lithium metal anodes.

In liquid-based batteries, for example, lithium dendrites can grow which might short-circuit the battery and even lead to fires and explosions.

That’s where solid-state inorganic electrolytes have come in: they are significantly safer, and a garnet-type ceramic Li₇La₃Zr₂O₁₂, better known as LLZO, is now widely regarded as a promising solid-state electrolyte material for its high ionic conductivity and compatibility with Li metal. However, producing high-density LLZO electrolytes requires very high sintering temperatures, as high as 1200 °C. This is both energy inefficient and time-consuming, making large-scale production of LLZO electrolytes difficult.

In addition, the poor physical contact between brittle LLZO electrolytes and the electrode materials usually results in high interfacial resistance, greatly limiting their application in all-solid-state Li-metal batteries.

Inorganic solid-state electrolytes not only can stabilize the Li-electrolyte interface, but also can reduce the risk of fires and explosions from flammable liquid electrolytes.

Compared with sulfides (e.g., Li₂S-P₂S₅ and Li₁₀GeP₂S₁₂), ceramic oxides (e.g., (La,Li)NbO₃) and Li_1.3Al_0.3Ti_1.7(PO₄)₃) are generally more stable against ambient air and thus can be more easily handled during processing. In particular, the garnet-type LLZO has attracted tremendous interest as a promising solid electrolyte due to its high ionic conductivity (about 10^-3 S cm^-1 at 25 ˚C) and high chemical stability against metallic Li.

However, high-temperature sintering (e.g., 1200 ˚C), which is very energy inefficient and time-consuming, is generally necessary to densify LLZO powders. Meanwhile, Li loss and various side reactions are likely to occur at high temperatures. Therefore, considerable efforts have been put into lowering the sintering temperature of LLZO powders. … Besides the high sintering temperature, the poor physical contact between LLZO pellets and cathode materials usually results in high interfacial resistance. To solve this issue, co-sintering of cathode materials, e.g., LiCoO₂ (LCO), and LLZO powders was proposed. However, LCO would decompose at sintering temperature higher than 900 °C. Therefore, the concept of IL-contained quasi-solid-state electrolytes was explored.

…Here we report a room-temperature synthesis of a flexible composite Al-LLZO sheet electrolyte by tape-casting and IL-impregnation (IL: Li(G4)FSI, a lithium bis(fluorosulfonyl)imide (LiFSI) and tetraglyme (G4) equimolar complex, regarded as a solvate IL.

—Cheng et al.

The researchers combined a garnet-type ceramic, a polymer binder, and an ionic liquid, producing a quasi-solid-state sheet electrolyte. The synthesis is carried out at room temperature, requiring significantly less energy than existing high-temperature (> 1000°C) processes.

The team led by Professor Kiyoshi Kanamura at Tokyo Metropolitan University first cast a LLZO ceramic slurry onto a thin polymer substrate. After drying in a vacuum oven, the 75-micron thick sheet electrolyte was soaked in an ionic liquid (IL) to improve its ionic conductivity. Inside the sheets, the IL successfully filled the microscopic gaps in the structure and bridged the LLZO particles, forming an efficient pathway for Li-ions. They also effectively reduced interfacial resistance at the cathode.

On further investigation, the researchers found that Li-ions diffused through both the IL and the LLZO particles in the structure, highlighting the role played by both. The synthesis is simple and suitable for industrial production: the whole process is carried out at room temperature without any need for high-temperature sintering.

Though challenges remain—such as improving the ionic conductivity—the team says that the mechanical robustness and operability of the flexible composite sheet at a wide range of temperatures makes it a promising electrolyte for Li-metal batteries.

This work was supported by the Advanced Low Carbon Technology Research and Development Program of Specially Promoted Research for Innovative Next Generation Batteries (ALCA-SPRING) (Grant No. JPMJAL1301) from the Japan Science and Technology Agency (JST).

Resources

• Eric Jianfeng Cheng, Takeshi Kimura, Mao Shoji, Hiroshi Ueda, Hirokazu Munakata, and Kiyoshi Kanamura (2020) “Ceramic-Based Flexible Sheet Electrolyte for Li Batteries” ACS Applied Materials & Interfaces 12 (9), 10382-10388 doi: 10.1021/acsami.9b21251