Performance of Lithium–Sulfur Battery with Ionic Liquids for Deep Space Environment under High Radiation Dose
Authors: Uehara Atsushi , Sone Yoshitsugu , Ishikawa Masashi 
Published: December 2024

INTRODUCTION

Recently, lithium–ion batteries (LIBs) about to be applied to satellites for deep space exploration with a long range because of their long cycle life and high energy density. However, the energy density of LIBs is not high enough to be used as batteries for deep space exploration. Therefore, lithium–sulfur batteries (Li–S batteries), which are expected to have a higher energy density than LIBs, seem attractive for deep space exploration. In addition, batteries for the deep space exploration must be made of materials that are resistant to radiation and vacuum. In our previous study, N–methyl–N–propylpyrrolidinium bis(fluorosulfonyl)imide (MPPyFSI) of the ionic liquid was found to be resistant to the radiation (12 krad), which is expected in Earth orbit, and be applicable as an electrolyte for LIBs1). However, it was reported that the specific capacity of the Li–S batteries with MPPyFSI decreased after irradiation to the dose (55 Mrad) expected in the deep space environment, and that it could not be cycled2). In this study, to develop an electrolyte for stable operation of Li–S batteries under deep space environment, we applied a new electrolyte using an ionic liquid to Li–S batteries and investigated the effect of irradiation dose (e.g., 55 Mrad) on the battery and battery materials in deep space environment.

Experiment

The active material was mixed with microporous activated carbon loaded with 36 wt. % sulfur, acetylene black, and magnesium alginate in a weight ratio of 90 : 3 : 7. The resulting slurry was applied on an Al foil and used as the sulfur cathode. A microporous polyimide membrane was used as the separator, 1.2 mol kg−1 lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) / MPPyFSI as the electrolyte, and 1.0 mol kg−1 lithium hexafluorophosphate (LiPF6) / MPPyFSI were prepared and used. A two–electrode laminated cell was fabricated with the above configuration, and constant–current charge–discharge tests were conducted. The voltage range was 1 – 3 V and the current density was 0.1 C. After 15 cycles of charge–discharge, irradiation tests were conducted on the fully charged cells. The irradiation tests were carried out at Laboratory for Advanced Nuclear Energy, Tokyo Institute of Technology, and 60Co was used as the γ–ray source. The irradiation dose was 20 Mrad. After the irradiation tests, constant–current charge–discharge tests were performed again under the same conditions as above.

Results and Discussion

Constant–current charge–discharge tests were carried out to investigate the cycling characteristics of Li–S batteries with 1.0 mol kg−1 LiPF6 / MPPyFSI before and after irradiation (irradiation dose 20 Mrad). In the 30th cycle after irradiation, the discharge capacity was 285.5 mAh g−1 and the capacity retention was 23.6% (= discharge capacity of the 30th cycle / discharge capacity of the 15th cycle × 100 [%]). Although the polarization increased and the capacity decreased after irradiation, the Li–S batteries with 1.0 mol kg−1 LiPF6 / MPPyFSI showed stable cycle characteristics, confirming that the Li–S batteries with 1.0 mol kg−1 LiPF6 / MPPyFSI can maintain its electrochemical performance even after irradiation. This indicates that the LiPF6 / MPPyFSI system is resistant to high intensity radiation (γ–rays). The authors would like to express their sincere appreciation to Mr. Isao Yoda, Senior Technical Specialist, Laboratory for Advanced Nuclear Energy, Tokyo Institute of Technology, for his cooperation in the irradiation experiments in this study.

Reference

1) M. Yamagata, Y. Sone, S. Nakasuka, M. Kono, M. Ishikawa et al., Electrochemistry, 83 (2015) 918.
2) A. Matsushima et al., the 61st Battery Symposium Japan, 1H02 (2020).