The Impact of Fluorination/Iodization of the Li Metal|Single Ion Conducting Polymer Electrolyte Interface
Authors: Owensby Kyra D'Yana , Lehmann Michelle , Sacci Robert L , Lin Yan-Ru , Tsai Wan-Yu , Chen Xi Chelsea 
Published: December 2024

Lithium (Li) metal is one of the most promising anodes for energy-dense batteries as we move towards an electrified economy because of its high theoretical specific capacity (3860 mAh/g). However, Li is highly reactive, leading to continuous consumption of Li and electrolytes and non-uniform Li stripping and plating, which raises safety concerns for traditional liquid electrolytes. Solid polymer electrolytes help to mitigate some of these safety concerns, such as flammability and leaking, as they do not contain flammable organic liquids.1,2 These electrolytes, however, experience concentration polarization, leading to low Li transference numbers (t+).3,4 Single-ion conducting (SIC) polymers are able to overcome this limitation and provide a Li t+ close to unity by immobilizing the anion. However, immobilizing the anion can decrease the ionic conductivity of the polymer. LiF has been shown to interface with other crystalline materials in the solid electrolyte interface (SEI), improving the ionic conductivity of other classes of electrolytes.5 This suggests fluorinating an SIC could improve its ionic conductivity and overall battery performance.

In this work, the impact of fluorination on a previously developed SIC polymer membrane with a poly(ethylene oxide) (PEO) backbone was investigated. We compared the effects on the membrane of two different fluorinated ethers: Fluoroethylene carbonate (FEC) and 1,1,2,2-Tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE). This evaluation has thus far comprised a series of techniques, including electrochemical impedance measurements, chronoamperometry, and charge/discharge cycling tests. Preliminary results in Li symmetric cells have shown that although neither fluorinated ether significantly impacts the Li t+, the introduction of both decreases the overall resistance of the cell. Additionally, TTE increased the cycling duration for the cell (at 1 mA/cm2 @70°C), while FEC does not seem to affect cycling duration.

References:

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2 Owensby, K. D., Sahore, R., Tsai, W.-Y. & Chen, C. X. Understanding and controlling lithium morphology in solid polymer and gel polymer systems: mechanisms, strategies, and gaps. Material Advances
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3 Stolz, L. et al. Single-Ion versus Dual-Ion Conducting Electrolytes: The Relevance of Concentration Polarization in Solid-State Batteries. ACS Applied Materials and Interfaces
14, 11559-11566, doi:10.1021/ACSAMI.2C00084/ASSET/IMAGES/LARGE/AM2C00084_0009.JPEG (2022).

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