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Fluoroethylene carbonate effects as an electrolyte additive on the initial cathode electrolyte interface

Donakowski, Anthony
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Steirer, K. Xerxes
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Date
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2022
Date Submitted
Keywords
electrolyte decomposition
 FEC
 lithium ion battery
 NMC
 XPS
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2022 - Mines Theses & Dissertations
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https://hdl.handle.net/11124/15446
Embargo Expires
2023-09-30
Abstract
Lithium ion battery technology has revolutionized how and where we use energy. However, improvements are still needed to increase lifespan, safety, and energy density. Electrolyte additives can provide a safe and economical route towards fast charging applications for layered mixed transition metal oxides. One such system of interest is the LiNiMnCoO (NMC) cathode paired with a graphite anode and the standard ethylene carbonate:ethylmethylene carbonate (EC):(EMC) Gen2 electrolyte. Common electrolyte additives include fluoroethylene carbonate (FEC), vinylene carbonate (VC), and Lithium difluoro (oxalate) borate (LiDFOB). Each of which are meant to stabilize the electrolyte and electrode surfaces by several different mechanisms. FEC was choosen for this study due to it having been shown to suppress parasitic side reactions between the electrolyte and cathode by promoting the formation of a solid electrolyte interface layer (SEI). Specifically, the cathode electrolyte interface (CEI) has been shown to form Li2CO3, LiF, LiOH, and other hydrocarbon species. Furthermore, the decomposition products of FEC will react with these surface species to stabilize the CEI while the decomposition products of FEC, VC will suppress continuous electrolyte cathode reactions. FEC further promotes cell stability by scrubbing HF that may have formed form any moisture contamination in the electrolyte. The ideal CEI would be a uniform layer of ionically conducting material that is also electronically insulating. An ideal candidate for an economical process for uniform CEI formation is the promotion of LiF using FEC. To understand the material level process in which the FEC concentration controls the growth rate of LiF and how LiF will promote cell stability, NMC cathodes were closely investigated using X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD), Electrical Impedance Spectroscopy (EIS), and Scanning Electron Microscopy (SEM). Adding 1-2% by volume of FEC to the standard Gen2 electrolyte recipe facilitated the growth of LiF-rich CEI as shown by XPS. Moreover, the LiF layer created a measurable, polarizing effect between transition metal cations and the anion containing electrolyte. Although, the LiF-rich CEI reduced discharge capacity, the composition of the CEI was directly affect by the FEC concentration and LiPF6 electrolyte decomposition products decreased. Data suggests that adding 5% vol. and above caused the FEC to become a co-solvent and increases the charge transfer resistance of NMC cathode as shown by EIS. Any changes in performance that could be attributed to the structural change of the NMC or cation mixing were ruled out using XRD and SEM. The concentration of LiF within the CEI layer increased steadily from 10.74% to 42.05% for washed electrodes, while the LiF concentration for cycled electrodes remained above 30% for all except the 2% FEC concentration
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