Investigation of the Interphase Between Graphite Electrode/ Ionic Liquid Electrolyte during the First Discharge
Authors: Bolimowska Ewelina , Santini Catherine C. , Rouault Helene , Benayad Anass , Santos Pena Jesus 
Published: February 2020

Lithium ion batteries directly convert electrical energy and chemical energy reversibly by use of lithium ions in a chemical redox reaction. However, with respect to their high energy density, there are still challenges regarding safety, high voltage applications, and long-term stability (1). For this purpose, extensive study is focused to increase their safety due to the use of flammable organic carbonate electrolyte (2) and their capacity loss resulting from the reactions of the electrolyte onto the electrode surface (3). Major interest focuses in the study of additives to the electrolytes which are applied for a specific and selective purpose. Currently, ionic liquids (IL) are described as an alternative class of electrolyte solvent compared to standard carbonate based systems due to their thermal and electrochemical stability, flame retardant performance and high ionic conductivity (4). The solvation of lithium salts in IL leads to the creation of a lithium ion carrying species quite different from those found in traditional nonaqueous lithium battery electrolytes which has impact on the total ion conductivity, the lithium ion mobility, and also the lithium ion delivery at the electrode. A challenge of their application is overcoming their limitations implying low cycling and power delivery for IL-based batteries (5).  For Li-ion batteries based on graphite (Cgr) electrode, the addition of organic carbonates (e.g. vinylene carbonate,VC) is essential to improve their performance (6). In the literature, the role of the VC additive is linked to the SEI formation and creation of interfacial compatibility between the Cgr electrode and the ionic liquid (7). However, the exact nature of the interface between graphite electrode and ionic liquid based electrolyte is still under investigation.

The aim of this work is focused on the understanding of the formation ionic liquid based electrolyte/graphite interphase during the first discharge. For this purpose, a three electrode Swagelok cell with WE-graphite (Cgr), CE and RE-lithium electrodes, glass fiber separator Whatman, along with electrolyte based on the mixture of (1-hexyl-3-methylimidazolium (bis (trifluoromethane-sulfonyl) imide) C1C6ImNTf2 with LiNTf2 (1mol.L-1) and 5% vol. VC (vinylene carbonate) has been built and the electrode/electrolyte interfacial properties examined by the electrochemical impedance spectroscopy (EIS) at 60°C. This in situ method allowed to measure the interphase changes at different steps of the discharge (potentials 0.8 V; 0.6 V; 0.4 V ; 0.2 V ; 0.035 V; 0.01 V vs Li+/Li). According to the electrical circuits employed for fitting the corresponding Nyquist plots the formation of two films starting at 0.2V is proved.

In order to understand the formed films structures, Cgr electrodes have been prepared for complementary XPS analyses in Li/Cgr coin cells discharged at C/50 rate at 60°C and stopped during the first reduction step at the same potentials as in the impedance experiment. The electrodes were removed from the coin cells and washed with DMC. The XPS analyses showed the formation of LiF, Li2S, Li2O, polyoxysulfurs and Li2NSO2CF3 at different stages of discharge. The full evolution of those films in function of applied potential is under progress and will be discussed during the communication.

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