Weakly Coordinating Solvents with FSI-Inspired Structures for Highly Efficient Lithium Metal Batteries
Authors: Nishimura Shoma , Kondo Yasuyuki , Takeuchi Kazuki , Katayama Yu , Oka Kouki , Yamada Yuki 
Published: December 2024

１．Introduction

Low Coulombic efficiency (CE) of lithium (Li) plating/stripping due to electrolyte decomposition on Li metal negative electrodes is a critical issue in the practical application of Li metal batteries. Our previous study[1] revealed that increasing the concentration of lithium bis(fluorosulfonyl)imide (LiFSI) in organic electrolytes leads to a weakly coordinated environment of Li+ with FSI−, which increases Li+ chemical potential (µ
Li
+) and Li electrode potential (E
Li), thus minimizing electrolyte reductive decomposition and achieving high CEs. However, concentrated electrolytes have disadvantages such as high viscosity and high cost.

Herein we focused on FSI-inspired solvents with a weakly Li+-coordinating structure of -NSO2F.[2][3] This class of solvents have been studied with a focus on their solid electrolyte interphase (SEI) forming ability similar to LiFSI,[2][3] but there has been no understanding on the µ
Li
+ and E
Li in such solvents. We studied the E
Li and Li plating/stripping CE in various FSI-inspired solvents and discuss their correlation.

２．Experimental

The electrolytes were prepared by dissolving LiFSI in FSI-inspired solvents (diMeFSI; N,N-dimethylsulfamoyl fluoride, EtMeFSI; N-ethyl, N-methylsulfamoyl fluoride, diEtFSI; N,N-diethylsulfamoyl fluoride), FEC; fluoroethylene carbonate, and DME; 1,2-dimethoxyethane at a LiFSI:solvent molar ratio of 1/8 (Figure 1.). A three-electrode cell was used for cyclic voltammetry (CV) with a Pt plate as the working electrode and Li foil as the counter and reference electrodes. Ferrocene (Fc, 1 mM) was dissolved in the electrolytes. E
Li was evaluated with reference to the redox potential of Fc as an internal standard. Cu|Li coin cells were used to evaluate Li plating/stripping CEs. Li was plated on Cu for 1 hour and stripped up to 0.5 V at a current density of 0.5 mA cm-
2.

３．Results & Discussion

Figure 2. shows E
Li in various electrolytes with reference to Fc/Fc+. The E
Li values of FSI-inspired solvents were notably higher (-2.9 V vs. Fc/Fc+) than FEC or DME even at the same LiFSI:solvent molar ratios (1/8). This is due to the weakly Li+-solvating nature of the FSI--inspired solvents, which was also supported by DFT calculations.

Figure 3. shows the average Li plating/stripping CEs of 2~20 cycles in various electrolytes. For diMeFSI, EtMeFSI, diEtFSI, DME, and FEC, the LiFSI/solvent ratios were 1/8. The data of other various electrolytes (limited to salt concentrations below 2 M) are reproduced from our previous publication[1] and shown as black dots. Among such dilute electrolytes, the highest E
Li (-2.92 V) and the highest CE can be both achieved in diMeFSI electrolytes. This suggests that introducing such an FSI structure into solvents is a promising way to not only forming a better FSI-derived SEI but also upshifting the E
Li for highly efficient Li metal anodes.

4
．References

[1] S. Ko et al., Nat. Energy,
7, 1217 (2022).

[2] W. Xue et al., Energy Environ. Sci., 13, 212 (2020).

[3] K. Jiang et al., ACS Energy Lett.
7, 3378 (2022).

5.Acknowledgement

This work was partially supported by JSPS KAKENHI Grant-in-Aid for Transformative Research Areas (B) (23H03824, 23H03827).

Figure 1