An SEI-Forming Fluorophosphate Solvent for High-Temperature Lithium-Ion Batteries
Authors: Takeuchi Kazuki , Kondo Yasuyuki , Katayama Yu , Oka Kouki , Yamada Yuki 
Published: December 2024

Introduction

Recently, there has been an increasing demand for high-energy-density lithium-ion batteries (LIBs) that operate under broad temperature conditions (–40 to 70°C). As an electrolyte solvent, ethylene carbonate (EC) has been essential to form a solid electrolyte interphase (SEI), which effectively suppresses side reactions such as graphite exfoliation and continuous electrolyte decomposition on negative electrodes(1). However, EC is not only flammable but also susceptible to severe oxidative and reductive decomposition at high temperatures, thereby limiting the operational temperature of LIBs to around 55°C(2). There has been no alternative solvent that has (i) SEI-forming ability, (ii) nonflammability, and (iii) high-temperature stability. Hence, it has been challenging to realize the high-temperature and safe operation of LIBs.
In this study, we designed and synthesized a novel phosphate solvent, FDPO, which is nonflammable and can form stable SEI on graphite. FDPO has a structural feature similar to LiPO2F2, which can improve SEI properties and high-temperature performance of batteries(3-4). We investigated the charge-discharge performances of negative and positive electrodes for LIBs with FDPO-based electrolytes at high temperature.

Experimental methods

Electrolytes were prepared by mixing LiPF6, FDPO, and methyl 2,2,2-trifluoroethyl carbonate (FEMC), which is known as a low viscous and flame-retardant solvent. Charge-discharge tests were conducted using coin cells with the natural graphite|Li cell configuration. The current density of 186 mA g-1 (2C) was applied from the 4th cycle after pre-conditioning three cycles at a current density of 37.2 mA g-1 (10C). The cut-off voltage range was set to 2.5 V – 0.01 V.

Results and discussion

Figure 1 shows the charge-discharge performances of natural graphite|Li coin cells at 70°C. The cell with the commercial electrolyte exhibited severe capacity decay to ca. 142 mAh g-1 after 50th cycles. On the other hand, the cell with FDPO-based electrolyte retained a high reversible capacity of ca. 343 mAh g-1. In addition, the charge-discharge efficiency was over 99.6% in the FDPO-based electrolyte, indicating suppressed electrolyte decomposition even at 70°C. Based on these results, the FDPO itself is thermally stable and also forms a thermally stable SEI on graphite, thereby suppressing capacity degradation under high-temperature conditions. In the presentation, the results of X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry analyses for the SEI derived from FDPO-based electrolytes, along with the charge-discharge performances of positive electrodes, will be discussed.

References

(1) Wang, A. et al., npj Computational Materials, 2018, 4, 15.
(2) Lu, L. et al., J. Power Sources, 2013, 226, 272.
(3) Yang, G. et al., RSC Adv, 2017, 7, 26052.
(4) Kuang, S. et al., ACS Appl. Mater. Interfaces, 2022, 14, 19056-19066.

Acknowledgment

A part of this work was supported by JST GteX (JPMJGX23S3).

Figure 1