High Rate and Stable Cycling of Lithium Metal Anode

Lithium (Li) metal is an ideal anode material for Li batteries due to its extremely high theoretical specific capacity (3860 mAh g^-1), low density (0.59 g cm^-3) and the lowest negative electrochemical potential (−3.040 V vs. standard hydrogen electrode).¹ However, dendritic Li growth and limited Coulombic efficiency (CE) during Li deposition/stripping have prevented their applications in rechargeable Li batteries, especially at high current densities.^2,3 Electrolyte is one of the most critical elements that affect the cycling stability of Li metal anode, since Li is thermodynamically unstable with any kinds of organic solvents and salts.⁴ The interactions between organic electrolytes and Li metal results in significant side reactions that not only lead to low CE but also consume Li metal and/or electrolyte materials. This phenomena becomes especially serious at high current densities.⁵Therefore, extensive studies have been conducted to understand how electrolyte formulations affect the cycling of Li metal electrodes. 

   Here, we demonstrate that the use of a highly concentrated electrolyte composed of 1,2-dimethoxyethane (DME) solvent and the salt lithium bis(fluorosulfonyl)imide (LiFSI) results in the nondendritic plating of Li metal at high rates and with high efficiency. The electrochemical performance of Li metal plating/stripping with a concentrated LiFSI-DME electrolyte is shown in Figure 1. Figure 1a gives the typical voltage profile of Li plating/stripping on Cu substrate using different current densities varying from 0.2 to 10 mA/cm². A pair of well-defined charge/discharge plateaus can be distinguished for all the voltage curves with applied current densities up to 10 mA/cm², and almost all the Li deposition capacity can be recovered during the stripping process as seen from the charge capacity. The voltage polarization at 0.2 mA/cm² is as low as 13 mV, and just slightly increases to 122 mV at 4 mA/cm². Even when the current density increases to 10 mA/cm², the voltage polarization is still only 270 mV. The average Coulombic efficiency of the cycling is > 99% (0.2, 0.5, and 1 mA/cm²), 98% (4 mA/cm²), 97% (8 mA/cm²), 96.7% (10 mA/cm²), respectively. Long term cycle tests as shown in Figure 1b indicate the Coulombic efficiency keeps stable without any decrease up to 400 cycles, manifesting an extraordinary cycling stability of Li metal in this electrolyte. These results provide a route for future efforts to optimize electrolytes for the safe and highly efficient utilization of Li metal electrodes for advanced energy storage applications. Detailed electrochemical characterization and study of the fundamental mechanisms behind the high rate Li cycling and stability of the electrolytes will be discussed in the presentation.

Figure 1. Electrochemical performance of Li metal plating/stripping with a concentrated LiFSI-DME electrolyte: (a) Voltage profile of Li plating/stripping on Cu substrate using different current densities. (b) Coulombic efficiency of Cu|Li cells at different current densities. 

Acknowledgements

This work was supported by the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the Basic Energy Sciences, Office of Science of the U.S. DOE.

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