Surface chemistry and electrochemistry of an ionic liquid and lithium on Li4Ti5O12(111)—A model study of the anode|electrolyte interface

Formation of an Artificial Mg2+-Permeable Interphase on Mg Anodes Compatible with Ether and Carbonate Electrolytes

Rechargeable Mg-ion batteries typically suffer from either rapid passivation of the Mg anode or severe corrosion of the current collectors by halogens within the electrolyte, limiting their practical implementation. Here, we demonstrate the broadly applicable strategy of forming an artificial solid electrolyte interphase (a-SEI) layer on Mg to address these challenges. The a-SEI layer is formed by simply soaking Mg foil in a tetraethylene glycol dimethyl ether solution containing LiTFSI and AlCl₃, with Fourier transform infrared and ultraviolet-visible spectroscopy measurements revealing spontaneous reaction with the Mg foil. The a-SEI is found to mitigate Mg passivation in Mg(TFSI)₂/DME electrolytes with symmetric cells exhibiting overpotentials that are 2 V lower compared to when the a-SEI is not present. This approach is extended to Mg(ClO₄)₂/DME and Mg(TFSI)₂/PC electrolytes to achieve reversible Mg plating and stripping, which is not achieved with bare electrodes. The interfacial resistance of the cells with a-SEI protected Mg is found to be two orders of magnitude lower than that with bare Mg in all three of the electrolytes, indicating the formation of an effective Mg-ion transporting interfacial structure. X-ray absorption and photoemission spectroscopy measurements show that the a-SEI contains minimal MgCO₃, MgO, Mg(OH)₂, and TFSI^-, while being rich in MgCl₂, MgF₂, and MgS, when compared to the passivation layer formed on bare Mg in Mg(TFSI)₂/DME.

Surface Science and Electrochemical Model Studies on the Interaction of Graphite and Li-Containing Ionic Liquids

The process of solid-electrolyte interphase (SEI) formation is systematically investigated along with its chemical composition on carbon electrodes in an ionic liquid-based, Li-containing electrolyte in a combined surface science and electrochemical model study using highly oriented pyrolytic graphite (HOPG) and binder-free graphite powder electrodes (Mage) as model systems. The chemical decomposition process is explored by deposition of Li on a pre-deposited multilayer film of 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP][TFSI]) under ultrahigh vacuum conditions. Electrochemical SEI formation is induced by and monitored during potential cycling in [BMP][TFSI]+0.1 m LiTFSI. The chemical composition of the resulting layers is characterized by X-ray photoelectron spectroscopy (XPS), both at the surface and in deeper layers, closer to the electrode|SEI interface, after partial removal of the film by Ar⁺ ion sputtering. Clear differences between chemical and electrochemical SEI formation, and also between SEI formation on HOPG and Mage electrodes, are observed and discussed.