Influence of the Ion-Specific Electric Double Layer at the Air–Aqueous Phosphazene-Based Polymer Electrolyte Interface

Polymer electrolytes offer promising opportunities for lithium-ion batteries, supercapacitors, organic solar cells, and fuel cells. However, molecular-level insights into ion distribution and their influence on the molecular structure of interfacial water and polymer side chains at the air-aqueous polymer-electrolyte interface remain limited, and the fundamental understanding could help design improved device materials. In the present study, we employed interface-sensitive sum frequency generation (SFG) vibrational spectroscopy, surface tensiometry, and zeta potential measurements to probe ion-specific effects at the aqueous polymer electrolyte interface. Using a range of chloride (LiCl, NaCl, and MgCl₂) and lithium (LiPF₆, LiClO₄, LiBF₄, and LiCl) salts, we explored how the ion-induced impact affects the water and polymer molecules at the air-methoxyethoxyethoxyphosphazene (MEEP)-aqueous interface. The qualitative analysis based on salt-concentration-dependent studies suggests that the changes in broad convoluted OH-oscillator strength of water molecules are governed by the ion-specific electric double layer (EDL) structure under the polymer polar field. The spectroscopic observations indicate a two-plane EDL structure: a compact layer of tightly bound ions near the top surface and a diffuse layer of loosely distributed ions extending deeper into the interface. The SFG-extracted OH-oscillator intensity trend (Li⁺ > Na⁺ > Mg²⁺) for chloride salts indicates that the interface is susceptible to the presence of cation-specific ions, with Mg²⁺ lies closest to the interface, while Li⁺ resides deeper within the interfacial depth. This cation-specific surface propensity is further supported by CH-spectral data and surface tension measurements. At ultralow concentrations, SFG studies show that lithium salts modulate interface molecules through an EDL structure influenced by polymer-cation interactions and the field effect generated by anions. Notably, LiPF₆ and LiClO₄ exhibit extreme field effects by significantly reorienting the water dipoles. A deeper understanding of cation-specific effects, spatial ion distribution coupled with insights from water dipole alignment, provides crucial insights for predicting the surface chemistry of EDL structures and optimizing advanced electrochemical devices' performance, efficiency, and longevity.