Topic 01-18 : Characterization and understanding of the SEI in Li-ion batteries based on silicon and ionic liquid

This PhD project aims at understanding the formation and evolution of the surface passivation layers on both silicon particles and positive electrode material (NMC?) during aging and cycling of full Li-ion batteries with ionic liquids, essentially based on pyrrolidinium or phosphonium combined with bis(trifluoromethanesulfonyl)imide (LiTFSI), bis(fluorosulfon)imide (LiFSI) anions or mixes of both anions, and evaluating novel formulations and compositions of ionic liquids which may include passivation layer promoting additives. Although it plays a major role in electrochemical performance, the interface between silicon and electrolyte has rarely been characterized in depth and systematically, especially in those cycling conditions.

The impact of innovating types of binder, such as plastic crystals that can be also used as an electrolyte component, as well as the interaction between ionic liquid and binder will be investigated. Full cells will be cycled and stopped after different numbers of cycles, at the end of charging and at the end of discharge. Specific samples stopped during charging or in the course of discharge may also be studied. Different characterization techniques (MAS NMR, XPS, impedance measurements, microscopy and electrochemical measurements) will be combined in order to obtain qualitative and quantitative information on the chemical composition and physicochemical properties of the passivation layer. This approach will allow a better understanding of the end-of-life mechanism of full Li- ion cells using silicon or silicon-graphite electrodes with electrolytes based on ionic liquids and plastic crystals. The results will make it possible to direct the research towards new types of ionic liquids having both good thermal and electrochemical stabilities or leading to a control of the passivation layer while guaranteeing good ionic conduction properties.

The student will spend 3  months in Cidetec, Spain in order to develop all solid state batteries test cells using plastic crystals as the electrolyte on the pilot line.