Spectroscopic Studies of Cation Structure Dependence on Lithium-Ion Conductivity in Phosphonium Ionic Liquid Electrolytes
Authors: Nawate Shoki , Alshehabi Abbas , Matsumoto Mitsuhiro , Inoue Yohtaro , Yoshii Kazuki , Katakura Katsumi , Tsunashima Katsuhiko , Yamada Hirohisa 
Published: December 2024

1. Introduction

In recent years, there has been active progress towards transitioning to electric vehicles (EVs) with the aim of realizing a carbon-neutral society, resulting in intensified development of lithium-ion secondary batteries (LIBs) with higher energy density. However, commercially available electrolytes for LIBs, while having a wide electrochemical window, often exhibit high volatility, posing a risk of fire hazard during thermal runaway. Ionic liquids (ILs) are promising materials as alternative electrolytes because of their excellent properties such as flame retardance, volatility resistance, and wide potential window.
In our laboratory, we have focused on quaternary phosphonium-based ILs, which show higher electrochemical stability compared to common ILs such as imidazolium, pyrrolidinium, and ammonium, and evaluated them as electrolytes for LIBs.1) As a result, by optimizing the cation structure, we have found that ILs derived from trimethylphosphine can achieve relatively high-rate charge-discharge capability.2) In this study, we aimed to clarify the effect of the phosphonium cationic structure of ionic liquids on discharge rate characteristics.

2. Experimental

Bis(fluorosulfonyl)amide (FSA)-based phosphonium ILs, triethyl(buthyl)phosphonium FSA (P2224-FSA) and triethyl(ethoxymethyl)phosphonium FSA (P222(2O1)-FSA), were dried by heating and stirring at over 80 °C for 24 h under vacuum. Then lithium salt (LiFSA) was added to these ILs (0.2–1.5 mol dm–3).
Electrochemical measurements were conducted using a two electrodes half-cell. The cell was assembled using Li metal and graphite as the positive and negative electrodes, respectively, with a separator between them. The charge–discharge tests were conducted at 303 K in a constant current (CC) mode between 0 and 1.5 V (vs. Li metal). For the discharge rate test condition was followings, the charge condition was fixed at 0.05 C, and the discharge conditions were set to 0.05 C, 0.5 C, 1 C, 2 C, and 5 C. Self-diffusion coefficients of Li+, phosphonium and FSA– were determined by PGSTE-NMR. The conductivities were determined by an ac impedance measurement using a two Au-electrode cell. Raman spectra of the ILs were measured by micro-Raman spectroscopy featuring a continuous-wave green laser at room temperature (293 K).

3. Results and discussion

Fig. 1 shows the results of the discharge rate test with phosphonium ILs electrolytes. Although the discharge capacity of P2224-FSA decreased with increasing the C-rate, P222(2O1)-FSA, which includes an ether structure, maintained high discharge capacity even at high-rate conditions In this study, we investigated the spectroscopic behavior of phosuphonium ILs for clarifying their ionic conduction mechanism.
The self-diffusion coefficients, ionic transfusion numbers and conductivities of each C
Li
+ = 1 M ILs are listed in Table 1. Despite the higher self-diffusion coefficients of the respective ions in P222(2O1)-FSA than in P2224-FSA, there was no significant difference between each IL’s ionic conductivity.
Raman spectroscopy was used to evaluate the interaction between Li+ and FSA–. Fig. 2 shows the Raman spectra of C
Li
+ = 1.5 M P2224-FSA and P222(2O1)-FSA. Two peaks appear around 1,220 cm–1 and 1,230 cm–1, which originate fromfree FSA– and Li+-boundFSA, respectively.3) The intensity ratio of these bands tended to change with IL cation species. The ratio derived from free FSA– was greater for P222(2O1)-FSA than for P2224-FSA. These results suggested that the structure of phosphonium affected Li+ solvation of phosphonium ILs.

4. Conclusions

We found that the phosphonium cation with an ether chain weakens the FSA– solvation to Li+. This suggests that desolvation of part of the FSA– might occur due to the interaction of the ether chain with Li+. In ILs based on phosphoniums with ether chains, the ILs cationic structure might more influence on the Li+ conduction mechanism than in phosphonium ILs whiout ether chain.

Acknowledgments

This study was supported by a Grant-in-Aid for Scientific Research (B) from JSPS, Japan. This work was partly supported by a project, Gear of Education and Advanced Resourced (Gear 5.0).

References

1) Tsunashima and M. Sugiya, Electrochem. commun., 9, 2353–2358 (2007).
2) Matsumoto et al., SSRN Electron. J., (2023).
3) Fujii, et al., J. Phys. Chem. C, 117, 19314−19324 (2013).

Figure 1