Effect of Salt Concentration in Sulfolane-Based Electrolyte on Long-Term Li Plating/Stripping Behavior
Authors: Maeyoshi Yuta , Kubota Masaaki , Abe Hidetoshi , Abe Koji , Kanamura Kiyoshi 
Published: February 2020

Lithium metal is considered as the ultimate anode material for rechargeable batteries because it has the highest theoretical specific capacity (3,860 mAh g−1) and the lowest electrochemical potential (−3.04 V versus standard hydrogen electrode) of all possible candidates. However, dendritic Li growth and low Li Coulombic efficiency (CE) during Li plating/stripping hinder the practical applications of rechargeable lithium-metal batteries. In recent years, highly stable Li plating/stripping cycling has been achieved in the concentrated electrolytes of lithium bis(fluorosulfonyl) imide (LiFSI) in various solvents.1
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2 Among the reported electrolytes, sulfolane (SL) is a promising solvent for high energy lithium metal batteries, since it has high anodic stability and low flammability. The concentrated electrolyte composed of LiFSI and SL shows high CE of ca. 98% and stable cycling of Li metal anode, however, the effect of LiFSI concentration on the long-term cyclability is unclear.
Concentrated LiFSI/SL electrolytes have high viscosity and thus poor wettability toward conventional polyolefin separators, preventing stable Li plating/stripping cycling. We have previously reported that three dimensionally ordered macroporous polyimide (3DOM PI) membrane has high electrolyte wettability owing to its high porosity of ca. 70% and the relatively polar constituent of imide structure.3 Furthermore, uniform distribution of pores in a hexagonal close-packed arrangement of 3DOM PI membrane provides even current distribution during Li plating/stripping processes, preventing the dendritic Li growth. Therefore, 3DOM PI separator is expected to stabilize Li plating–stripping cycling.
Here we studied the effect of LiFSI concentration in SL-based electrolyte on long-term Li plating/stripping behavior by using 3DOM PI separator. 3DOM PI separator exhibits good wettability toward concentrated LiFSI/SL electrolyte and highly stable Li plating/stripping than surfactant-coated polypropylene separator, enabling the evaluation of the long-term cyclability. Higher concentrated LiFSI/SL electrolyte realizes superior cyclability with low overpotential and high CEs during cycling. The ratio of F and N is higher, while that of C is much lower in the SEI on the Li metal cycled in the higher concentrated electrolyte. These results suggest that the FSI-derived SEI in the highly concentrated electrolyte suppresses decomposition of SL and subsequent increase in interfacial resistance, leading to long-term stable Li plating–stripping cycling.
Acknowledgement
This work is supported by The Furukawa Battery Co., Ltd. and 3DOM Inc.
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