Investigation by XPS of the Interface between Graphite Electrode and Ionic Liquid Based Electrolyte

Li-ion batteries are used as the power source for nomad electronics due to their high energy density. To increase their safety and stability, the development of electrolytes based on ionic liquids (ILs) instead of flammable, organic carbonates gained a lot of attention. With graphite (Cgr) electrode, addition of organic carbonates (e.g. vinylene carbonate, VC) is essential to improve their performance due to the creation of interfacial compatibility between the Cgr and IL. Simulations of [Li+] doped ILs showed the transport of the [Li+], coordinated by four [NTf2-] is realized through [NTf2-] exchange in the first coordination shell. But the carbonate additive can destroy these ion pairs. Consequently, in IL//VC media, could VC contribute both in SEI formation and in the coordination [Li+]? In this work, in C1C6ImNTf2//VC(5% vol.)//LiNTf2 (1mol.L-1), we determine through {1H-7Li}, {1H-19F} (HOESY), pulsed field gradient spin-echo (PGSE) NMR experiments and simulation study, the coordination shell of [Li+], and its interface with graphite electrode through surface techniques: XPS, SEM-FIB, impedance. Then, it was tested in safe and low-cost full cell based on Cgr//LiFePO4 (LFP) at 60A°C. Results demonstrates: i) the vicinity of [Li+] and VC in solution, ii) on Cgr surface, the presence of IL layers (<5nm), LiF peak from anion and from the relative ratio of Ncation/Nanion a higher degradation of cation. Keywords: ionic liquid, vinylene carbonate Li-ion batteries, interface, NMR, XPS