Cl^-/Cl₃^- Redox Voltammetry to Recognize the Interfacial Layer on Positively Electrified Carbon in "Water-in-Salt"...

A "Water-in-Salt" electrolyte solution (WiSE) is a promising aqueous medium for lithium-ion batteries containing highly concentrated electrolytes. For the increased kinetic overpotential of water oxidation in WiSE, the formation of an interfacial layer (IFL) on a positively electrified electrode is crucial. Nonetheless, most related studies have been restricted to theoretical approaches. In this Article, we voltammetrically study the Cl^-/Cl₃^-/Cl₂ redox reaction on Pt and glassy carbon (GC) electrodes in WiSE containing LiTFSI (WiSE_LiTFSI) and demonstrate that careful monitoring of Cl^-/Cl₃^- redox voltammetry can allow recognition of an IFL formed on a positively electrified electrode. The voltammetric wave attributed to the electro-oxidation of Cl^- on a GC electrode was negatively more shifted as the molal concentration of LiTFSI was increased from 0.5 to 6 m, while there was no shift on Pt. Also, there was voltammetric resolution into two peaks associated with Cl^-/Cl₃^- and Cl₃^-/Cl₂ on the GC electrode in WiSE_LiTFSI, while only unresolved, one redox-paired voltammograms were observed on Pt, regardless of the molal concentration of LiTFSI. These two main voltammetric features indicate the LiTFSI-induced IFL coupled with Cl^- and Cl₃^- on a GC electrode induced by an applied potential of ∼2 V versus the point of zero charge (PZC). We found other halide/halogen redox reactions did not show differentiated voltammetric behaviors in WiSE_LiTFSI, which demonstrates the uniqueness of the Cl^-/Cl₃^- redox reaction for recognizing the IFL formed on a positively charged electrode surface. Lastly, a strong interaction between the IFL and Cl species was also confirmed by XPS measurements.