(Digital Presentation) Effects of the Morphological Changes Associated with a Li Metal Anode on Deposition and Dissolution Cycle in Bis(fluorosulfonyl)Amide-Based Ionic Liquids
Authors: Nishikawa Mika , Thomas Morgan L. , Serizawa Nobuyuki , Katayama Yasushi 
Published: December 2024

Lithium metal has been considered as a promising candidate for the negative electrode of secondary batteries for several decades because its capacity (3860 mAh g–1) is higher than those of currently used electrode materials. However, the lithium metal anode has been known to exhibit poor cycle performance or give rise to short circuits due to the dendritic or whisker-like morphology of lithium formed during deposition and dissolution. Analysis of the morphology of lithium deposits, especially the cross-section, is considered to be important for contributing to research on improving the charge-discharge ability of lithium metal anode. The formation of a uniform and highly Li+conductive solid electrolyte interphase (SEI) on the lithium metal surface leading to dense and low porosity lithium deposits and a reversible deposition-dissolution reaction of lithium is a significant challenge. It has been suggested that the bis(fluorosulfonyl)amide (FSA–)-derived SEI in ionic liquid (IL) electrolyte containing a high concentration of LiFSA can improve cycle performance and suppress dendritic growth [1,2]. We have reported the cycle performance of lithium and the formation of a SEI for electrolytes comprised of LiFSA in MOEMPFSA (MOEMP+: 1-methyl-1-methoxyethylpyrrolidinium) or BMPFSA (BMP+: 1-methyl-1-butylpyrrolidinium) [3]. The resistance of the SEI in LiFSA-MOEMPFSA (1:1 molar ratio) was lower than that in LiFSA-BMPFSA (1:1 molar ratio), and the deposition and dissolution performance of a lithium metal anode was improved in LiFSA-MOEMPFSA compared to LiFSA-BMPFSA. This is thought to be due to the variation in SEI composition with IL cation structure. Previous studies have demonstrated the cycle performance at a capacity of 1 or 4 mAh cm–2 [3,4] for lithium metal. However, these values are rather low when considering the proposed application as a negative electrode material for next-generation batteries, and it is essential to study a larger capacity. In the present study, the morphological changes of Li during deposition and dissolution cycle at 10 mAh cm–2 and the relationship between cycle performance and the morphology of deposited Li was investigated.

The electrolytes were prepared by mixing MOEMPFSA or BMPFSA and LiFSA at 50.0-50.0 mol%. The water contents in the ionic liquid electrolytes were less than 50 ppm, as determined by Karl Fischer titration. The preparation of 10 mAh cm–2 deposited Li on Cu was conducted using a Cu|Li planar electrode cell (laminate-like) at 0.1, 0.5 and 1.0 mA cm–2. Celgard 3501 was used as the separator. The cross-sections of the deposited lithium on Cu were revealed by processing with Ar ion milling at –80 °C. The observation of the cross-section of deposited lithium on Cu was carried out with a scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDX) and without exposure to air.

The SEM images of the cross-sections of a 10 mAh cm–2 lithium deposit on Cu at 0.1 or 0.5 mA cm–2 in LiFSA-MOEMPFSA reveal a denser and lower porosity than that at 1.0 mA cm–2. It is suggested that columnar growth of lithium was observed due to particle growth rather than nucleation at lower current density. The morphology of the 10 mAh cm–2 lithium deposits on Cu at 0.5 mA cm–2 in LiFSA-MOEMPFSA was denser than that in LiFSA-BMPFSA. This is thought to be due to the differences in the resistance of the SEI derived from MOEMPFSA and BMPFSA. The denser and lower porosity deposited lithium is considered to be promising for operation at as high a capacity as 10 mAh cm–2, and is expected to contribute to the improvement of the reversibility of the lithium anode with suppressing dendritic growth.

Acknowledgments

This study was supported by the Green Technologies of Excellence (GteX, JPMJGX23S0) program of the Japan Science and Technology Agency (JST). MOEMPCl was supplied by Nisshinbo Holdings Inc.

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