Rechargeable Aqueous Lithium-Air Battery with Lithium Conducting Polymer Electrolyte Separator
Authors: Zakria Hazirah Syahirah binti , Mizutani Tomoya , Mori Daisuke , Taminato Sou , Eimura Hiroki , Ikoma Kei , Sawamoto Atsushi , Yamamoto Osamu , Takeda Yasuo , Imanishi Nobuyuki 
Published: December 2024

Rechargeable lithium-air batteries are a promising high energy alternative to lithium-ion batteries. Two main types of lithium-air batteries are under development: non-aqueous lithium-air batteries and aqueous lithium-air batteries. The non-aqueous lithium-air batteries consist of a lithium anode and a carbon-based air electrode in a non-aqueous electrolyte, and offer high theoretical energy density of 3505 Wh kg-1 and 3436 Wh L-1. Over the past two decades, many researchers have developed non-aqueous lithium-air batteries. However, no technological breakthrough has been found to achieve these high energy densities. Aqueous lithium-air batteries consist of a lithium metal anode, a non-aqueous electrolyte, a lithium-ion conductive solid electrolyte that is stable in water, an aqueous electrolyte, and a carbon-air electrode. The cell reaction of this system is as follows1;
4 Li + O2 + H2O ⇌ 4 (LiOH‧H2O) (1)
The theoretical energy densities of this system based on equation (1) are 1910 Wh kg-1 and 2004 Wh L-1. Several problems in non-aqueous systems, such as electrolyte decomposition by reactive oxygen species and ingress of atmospheric moisture, do not occur in aqueous systems. In addition, LiOH, the discharge product of the air electrode, dissolves in the catholyte and does not interfere with the electrode reaction.
NASICON-type lithium ion conductor of Li1+xAlxTi2-x(PO4)3 (LATP) has been mainly used for lithium ion conductive solid electrolyte separators, which is the key material for aqueous lithium-air batteries. Its ionic conductivity ranges from 10-3 to 10-4 S cm-1 at room temperature. The heavy weight of the solid electrolyte reduces the gravimetric energy density, but the degree of reduction is determined by the amount of electricity generated.2 The weight ratio of the LATP (100 μm thick) to the total cell weight (excluding the container) decreases from 0.55 at 10 mAh cm-2 to 0.42 at 20 mAh cm-2 and 0.25 at 50 mAh cm-2. The calculated energy densities are 551, 830, and 1190 Wh kg-1 for capacities of 10, 20, and 50 mAh cm-2, respectively. The area capacities reported in the literature are usually less than 10 mAh cm-2.3 Therefore, to improve the energy density of aqueous lithium-air batteries, the weight of the solid electrolyte must be reduced. However, LATP thin films less than 50 μm thick are not mechanically tough, making it difficult to use them as separators for these batteries.
Recently, Toray developed a water-impermeable lithium-ion conductive polymer film.4 The film is less than 10 μm thick and the ionic conductivity is 3×10-5 S cm-1 at 25 °C. Using an H-type cell, pure water was placed in one chamber and a LiCl solution in the other, with this film placed in between. Since no chloride ions were observed in the pure water chamber for 200 hours, no water permeation was considered to have occurred.
The test cell system consisted of a lithium metal anode, 4.5 M LiN(CF3SO2)2 in 1,2-diethoxyethane (DEE) anode, this separator, 1M LiOH-10 M LiCl, and a carbon air electrode. All experiments were conducted at 25 °C. A stable open circuit voltage (OCV) of 3.05 V was observed. This OCV is comparable to calculated values and those reported for aqueous lithium-air batteries with LATP separators.5 The total cell resistance was 350 Ω and the cell area was 1 cm2. The battery was successfully operated for 100 cycles at 0.2 mA cm-2 and 0.2 mAh cm-2.
References

N. Imanishi, Y. Takeda, O. Yamamoto, Electrochemistry, 80, 706 (2012)

M.S. Park, S.B. Ma, D.J. Lee, D. Im, S.-G. Doo, O. Yamamoto, Scientific Reports, 4, Article number: 3815 (2014)

N. Imanishi, O. Yamamoto, Materials Today Advances, 4, Article number: 100031 (2019)

https://www.toray.co.jp/news/details/20220530165632.html

S. Sunahiro, M. Matsui, Y. Takeda, O. Yamamoto, N. Imanishi, J. Power Sources, 262, 338 (2014)

Acknowledgments
This work was supported by the program to Develop and Promote the Commercialization of Energy Conservation Technologies to Realize a Decarbonized Society of the New Energy and Industrial Technology Development Organization (NEDO; Japan) Grant Number JPNP 21005.