Synergistic Performance of Lithium Difluoro(oxalato)borate and Fluoroethylene Carbonate in Carbonate Electrolytes for Lithium Metal Anodes

Brett L Lucht,Zachary L Brown

doi:10.1149/2.0181903jes

Abstract

There is significant interest in the development of rechargeable high-energy density batteries which utilize lithium metal anodes. Recently, fluoroethylene carbonate (FEC) and lithium difluoro(oxalato)borate (LiDFOB) have been reported to significantly improve the electrochemical performance of lithium metal anodes. This investigation focuses on exploring the synergy between LiDFOB and FEC in carbonate electrolytes for lithium metal anodes. In ethylene carbonate (EC) electrolytes, LiDFOB is optimal when used in high salt concentrations, such as 1.0 M, to improve the electrochemistry of the lithium metal anode in Cu||LiFePO4 cells. However, in FEC electrolytes, LiDFOB is optimal when used in lower concentrations, such as 0.05–0.10 M. From surface analysis, LiDFOB is observed to favorably react on the surface of lithium metal to improve the performance of the lithium metal anode, in both EC and FEC-based electrolytes. This research demonstrates progress toward developing feasible high-energy density lithium-based batteries.

Highlights

The development of energy storage technology is an important topic for facilitating the employment of renewable energy in society
Fluoroethylene carbonate (FEC) containing electrolytes have been reported to improve the performance of lithium metal electrodes via the generation of polymeric species and LiF within the Solid Electrolyte Intephase (SEI),[5] similar to that reported for silicon anodes, which may contribute to the improved cycling performance of lithium metal anodes.[6,7,8,9]
The intensity of the P2p peak at ∼135.2 eV, characteristic of LixPFy and LixPFyOz,[18,19] is similar for both the 1.0 M LiPF6 FEC and 0.10 M Lithium difluoro(oxalato)borate (LiDFOB) FEC electrolytes, yet the intensity and peak position of the peaks the B1s spectra are different supporting the presence of boron decomposition products on the surface of lithium metal plated from the 0.10 M LiDFOB FEC electrolyte

Summary

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Lithium difluoro(oxalato)borate (LiDFOB) has been reported to generate nano-structured LiF for lithium metal electrodes, thereby improving the electrochemical performance of the lithium metal anode.[10] the optimal amount of LiDFOB to use in carbonate electrolytes for the lithium metal anode has not been explored. Given the reported improvement in plating/stripping of the lithium metal anode with FEC and LiDFOB containing electrolytes, exploring their synergy can assist researchers in developing high performance electrolytes for the lithium metal anode. Several carbonate electrolyte compositions containing FEC and LiDFOB have been investigated via a combination of electrochemical analysis with Cu||LiFePO4 cells and ex-situ surface analysis of the cycled electrodes. Electrodes were washed with 4 × 500 μL battery grade DMC and dried under vacuum for 20 minutes, stored overnight in an argon-filled glove box. The binding energy was corrected based on the F1s spectrum, assigning LiF to 685 eV

Results

FEC:DMC

Conclusions