Tracking the Li+ concentration on the surface of lithium metal electrodes upon cycling

Lithium-metal batteries are expected to allow for much higher energy and power densities. High-concentration Li-salt electrolytes appear as a promising approach to enable such batteries, as they show higher Coulombic efficiencies and stability towards lithium metal.1,2
Herein, we introduced fiber optic evanescent wave (FOEW) sensors that can be easily implemented in pouch cells to gain a better understanding of the origin of such key advantages. As electrolyte, we chose N-butyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide (PYR14FSI) comprising lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) with a molar ratio of 9:1, 8:2, 7:3, and 6:4. The introduction of the FOEW sensors into Li‖NCM622 cells enabled the quantitative operando tracking of the Li+ concentration at the lithium metal surface, revealing significant differences between the different compositions, which we correlated with other physicochemical properties of the electrolytes such as the viscosity, conductivity, and limiting current density as well as the overall cell performance. The results show that there is an optimum concentration for achieving stable cycling and high specific capacities. Moreover, the findings suggest that the charge transport mechanism changes from a vehicular-type transport to structural diffusion.

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