Investigation of the Interface Between Graphite Electrode and Ionic Liquid Based Electrolyte

Lithium ion batteries are extensively used as the power source for the consumer electronics, such as mobile phones and laptops because of their high voltage and high energy density (smaller and smatter technologies) (1). They are the most promising option for the next generation of the hybrid and electric vehicles. Extensive study is focus to increase their safety cause of the use of flammable organic carbonate electrolyte (2) and their capacity loss resulting from the reactions of the electrolyte onto the electrode surface (3). The electrolytes derived from ionic liquids (ILs) have gained a lot of attention due to their thermal and electrochemical stability, flame retardant performance and high ionic conductivity (4). Nevertheless, the poor cycling observed with neat IL-electrolytes in devices using graphite electrode (Cgr) as anode is a significant drawback for implementation of these systems in industrial production.

From the literature it is known, that for IL-electrolytes the addition of organic carbonates is crucial for improving the performance of the carbon based batteries (5). Their role is associated to the creation of interfacial compatibility between the Cgr/IL and formation of Solid Electrolyte Interphase (SEI) (6). Several studies concern the SEI characterization on Cgr surface with organic carbonates (7) or neat IL based electrolytes (8) but few with IL based electrolytes doped by carbonate. Recently, we have tested safe and low-cost full cell configuration based on Cgr//LiFePO₄ (LFP) with as electrolyte a mixture of (1‑hexyl-3-methylimidazolium (bis(trifluoromethanesulfonyl) imide) C₁C₆ImNTf₂ with LiNTf₂ (1mol.L^-1) and vinylene carbonate, VC (5% vol.) (5).

In this work, we report the evolution of Cgr electrode, before and after one and 400 cycling (at 60°C) by FIB-SEM and XPS techniques. FIB-SEM method shows the presence IL components inside the graphite particles and formation of non-homogenous film. The post mortem XPS analyses indicated that the surfaces were covered by an IL layer. The appearance of peak associated to LiF on the graphite surface after the first  cycle is clearly observed, nonetheless its intensity does not change after 400 cycles as the ratio F / N, Table 1. In the N 1s region the peaks due to both the cation and anion, in pure ILs show a relative ratio of N_cation/N_anion close to stoichiometry (2:1). This ratio changes with the number of cycling from 0,8 to 0,6; suggesting a higher degradation of cation. Further results will be discussed during the poster presentation.

References:

1. J.-M. Tarascon, M. Armand, Nature, 2001, 414

2. S.-Y. Bae, E-G. Shim, D-W. Kim, J. Power Sources, 2013, 1-6

3. J. T. Lee, N. Nitta, J. Benson, et al., CARBON, 2013, 52, 388-397

4. J.-K. Park, Wiley‑VCH, Weinheim, 2012

5. H. Srour, H. Rouault, C. Santini, J. Electrochem Soc, 2013, 160, 781-785

6. M. Holzapfel, P. Novak et al., Chem. Commun., 2004, 2098-2099

7. Y. Zhang et al., American Journal of Analytical Chemistry, 4, 2013, 7-12

8. E. Markevich et al., Journal of The Electrochemical Society, 155,2, 2008, A132-A137