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GC/MS As a Practical Tool for Monitoring the Evolution of the Negative Electrode/Electrolyte Interface in Lithium-Ion Battery
In the past two decades, many research efforts have been dedicated to SEI layer characterization at the interface between the negative electrode and the electrolyte, but so far the relationship between its composition/structure and protective property remains unclear.
In this study, we propose a direct and practical method to monitor the evolution of this interphase property during cycle life. The principle of this method is to analyze the chemical evolution of the electrolyte upon cycling through liquid GC/MS. The technique gives access to the presence of ethylene glycol bis-(alkyl carbonate), hereafter ethylene bis-carbonate (ROCO2CH2CH2OCO2R, with R = alkyl, reported as 111 or alkyl dicarbonates1–3). These soluble compounds are formed at the very beginning of battery cycle life; linear carbonate such as DMC, DEC or EMC gives a lithium alkoxide (ROLi) after electrochemical reduction. This compound can be partly dissolved in electrolyte and initiate chemical degradation of electrolyte through nucleophilic attack, thus yielding the ethylene bis-carbonate compounds. Their formation is responsible for the early change in electrolyte composition, and emphasizes the degradation by forming oligomers4. Ethylene bis-carbonate formation can be one of the reasons for battery performance decay, since it reduces the ionic conductivity of electrolytes.
A thorough study on the generation profile of ethylene bis-carbonates shows that it depends on potential and on temperature. With our in-depth knowledge on the influence of the chemical nature of linear carbonates, we clearly show that the soluble compounds are probes for SEI protective property evolution.
From this study, we discern several major advantages of this approach using liquid GC/MS:
a) Easy and practical to apply, it does not require any sample washing.
b) Quantification of the electrolyte degradation and estimation of the amount of electrolyte consumed for lithium alkoxide generation.
So far, direct study of SEI composition and morphology through XPS, TOF-SIMS and TEM could give information on the SEI thickness, but would not give any information on the amount of electrolyte that was consumed to form each component.
c) Indicator of SEI dissolution, especially when more soluble compounds are detected while electrolyte reduction is not possible. Monitoring the ethylene bis-carbonate generation profile can give information on SEI layer modifications upon first charge/discharge as well as aging.
d) Probe for SEI-layer protective quality, especially the ethylene bis-carbonate compound.
We successfully tested the efficiency of SEI-reinforcing additives such as VC, FEC, VEC and 1,3-PS and this technique is currently applied to test other new additives on Li and Na-ion batteries.
This technique can be applied to all energy storage systems using carbonate-based electrolyte. Moreover, it gives complementary information to the direct analysis of SEI layer through XPS, TOF-SIMS or FTIR. Combining the two of them helps to better understand the relationship between morphology/composition and protective property.
1. Gachot, G. et al. Deciphering the multi-step degradation mechanisms of carbonate-based electrolyte in Li batteries. J. Power Sources 178,409–421 (2008).
2. Sasaki, T., Abe, T., Iriyama, Y., Inaba, M. & Ogumi, Z. Formation mechanism of alkyl dicarbonates in Li-ion cells. J. Power Sources 150,208–215 (2005).
3. Sasaki, T. et al. Effect of an Alkyl Dicarbonate on Li-Ion Cell Performance. J. Electrochem. Soc. 152,A1963–A1968 (2005).
4. Gachot, G. et al. Gas Chromatography/Mass Spectrometry As a Suitable Tool for the Li-Ion Battery Electrolyte Degradation Mechanisms Study. Anal. Chem. 83, 478–485 (2011).