Solubilization of SEI lithium salts in alkylcarbonate solvents

The SEI (Solid Electrolyte Interphase) at the surface of electrodes in lithium-ion batteries is composed of various lithium compounds, organic or mineral, which have a direct impact on cycling performance. The main lithium species constituting the SEI and selected in this study are lithium fluoride LiF, lithium carbonate Li2CO3, lithium hydroxide LiOH, lithium oxide Li2O, lithium methoxide LiOCH3 and lithium ethoxide LiOC2H5. Their solubilities were determined in ethylene, propylene, dimethyl, diethyl and vinylene carbonates (EC, PC, DMC, DEC and VC) and in one of their mixtures commonly used in lithium-ion batteries (EC/PC/3DMC) by mean of atomic absorption spectroscopy (AAS). These solutions were also investigated by EIS (Electrochemical Impedance Spectroscopy) and conductimetry measurements. Results show that while solubilization properties differ between LiF and other lithium compounds considered, their association pattern in solution is identical and solutions are mainly constituted of quadrupolar aggregates.

► SEI lithium compounds are weakly soluble in alkylcarbonate solvents.
► Lithium oxide Li2O is almost insoluble in alkylcarbonates.
► Ethylene carbonate EC is highly solubilizing towards lithium salts.
► SEI lithium compounds aggregate as apolar and non-conductive species in alkylcarbonate solvents.