Anode Solid Electrolyte Interphase Formation in the presence of FEC: an In-Situ Infrared Spectroscopic Study

• Main Author: Yonas Beyene Yohannes
• Other Authors: Shawn D. Lin
• Format: Others
• Language: en_US
• Published: 2018
• Online Access: http://ndltd.ncl.edu.tw/handle/m93kv2

博士 === 國立臺灣科技大學 === 化學工程系 === 106 === The Solid Electrolyte Interphase (SEI) forms on electrodes of most Li-ion batteries (LiBs), but its formation mechanism and the properties that may govern the performance of LiBs are a mystery. The goal of this dissertation is to understand SEI growth by examining the formation of interface layer using in situ infrared spectroscopy. We focus on the role of fluoroethylene carbonate (FEC) on both silicon-based and carbon anodes. At first, the effect of FEC additive on the formation of SEI over Si-based anode is studied using in-situ DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy). SEI species were observed at an onset potential of 1.4 V in the first lithiation cycle using an electrolyte containing 2 wt% vinylene carbonate (VC) + 10 wt% FEC and at 1.1 V in an electrolyte without FEC additive. With blended VC and FEC, high carbon containing species including poly (FEC), poly (VC), and polycarbonates were identified, while poly (VC) and polycarbonates formed in the absence of FEC. The FEC additive also lead to a higher content of organic phosphorous fluorides as compared to the electrolyte containing no FEC. Electrochemical analyses indicated that the combination of 2 wt% VC and 10 wt% FEC resulted in lower impedances and improved the stability of the Si-electrode through cycling as compared to that without FEC. DRIFTS provided evidence that similar SEI species formed continuously after the initiation in the first cycle, and this formation was recorded for five cycles. For the second part, the effect of VC and FEC on SEI formation on MCMB anodes in Li-ion batteries is studied. Incorporation of 2 wt% VC into standard electrolyte (1 M LiPF6 in ethylene carbonate (EC)/ ethyl methyl carbonate (EMC), (1:2)) results in the generation of poly (VC), polycarbonates and Li2CO3 at an onset of 1.0 V. Incorporation of VC inhibits the generation of lithium alkyl carbonates. Lithium alkyl carbonate, poly (FEC), poly (VC), Li2CO3 and organic phosphorous fluorides species are formed when 5 wt% FEC is incorporated with standard electrolyte. The reduction of FEC is responsible for the formation of a surface film resulting in a lower impedance compared with the electrolyte containing 2 wt% VC. The continuous formation of SEI is also observed over MCMB when with 2% VC or with 5% FEC additive is included in the electrolyte. We propose a chain transfer mechanism leading to the continuous SEI formation after its initiation in the first cycle. A radical trapping reagent, 2,2,6,6-tetramethylpiperinyl-oxide (TEMPO), is included to test the proposed model. Using this new additive with 1 M LiPF6/EC:EMC + 5 wt% FEC base electrolyte, the continuous SEI formation on MCMB-electrode can be stopped. The insight towards the nature of the SEI formation by FEC on anode electrodes is discussed.