In situ Observation of Evolving H₂ and Solid Electrolyte Interphase Development at Potassium Insertion Materials within Highly Concentrated Aqueous Electrolytes

Abstract

Solid‐electrolyte interphases (SEI) are key to stable, high voltage lithium‐ion batteries (LIBs) as a protective barrier that prevents electrolyte decomposition. SEI are thought to play a similar role in highly concentrated water‐in‐salt electrolytes (WISE) for emerging aqueous batteries, but their properties remain unknown. In this work, we utilized advanced scanning electrochemical microscopy (SECM) and operando electrochemical mass spectrometry (OEMS) techniques to gain deeper insight into the SEI that occur within highly concentrated WISE. As a model, we focus on a 55 mol/kg K(FSA)0.6(OTf)0.4 electrolyte and a 3,4,9,10‐perylenetetracarboxylic diimide negative electrode. For the first time, our work showed distinctly passivating structures with slow apparent electron transfer rates alike to SEI found in LIBs. In‐situ analyses indicated stable passivating structures when PTCDI was stepped to low potentials (~‐1.3 V vs. Ag/AgCl). However, the observed SEI was discontinuous at the surface and H2 evolution occurred as the electrode reached more extreme potentials. OEMS measurements further confirmed a shift in the evolution of detectable H2 from ‐0.9 V to <‐1.4 V vs. Ag/AgCl when changing from dilute to concentrated electrolytes. In all, our work shows a combined approach of traditional battery measurements with in‐situ analyses for improving characterization of other unknown SEI.

A High‐Entropy Prussian Blue Analog for Aqueous Potassium‐Ion Batteries

“…This is likely attributed to the discontinuity and incomplete coverage of the solid electrolyte interphase (SEI) on the PTCDI anode surface, as recently revealed by Komaba et al. [ 28 ] through advanced electroanalytical methods. Hence, it is inferred that the diminished performance of the anode material primarily contributes to the capacity decline in the full cell.…”

Quantifying Interfacial Ion Transfer at Operating Potassium-Insertion Battery Electrodes within Highly Concentrated Aqueous Solutions

“…For scanning electrochemical (SECM) measurements, we used an in-lab-constructed SECM setup and a modified three-electrode cell (SB1a; EC Frontier) as described in our previous report. 19 The cell was covered with Parafilm© and bubbled with N 2 , then left above the solution during SECM measurements to minimize O 2 entering the cell. We positioned the PtUME by applying a constant potential (0.6 V vs Ag/AgCl) to continuously oxidize [Fe(CN 6 )] 4– while moving it toward the PTCDI surface.…”

“…For scanning electrochemical (SECM) measurements, we used an in-lab-constructed SECM setup and a modified three-electrode cell (SB1a; EC Frontier) as described in our previous report . The cell was covered with Parafilm© and bubbled with N 2 , then left above the solution during SECM measurements to minimize O 2 entering the cell.…”

“…Electrode Preparation. Platinum ultramicroelectrodes (PtUMEs) were prepared as reported elsewhere 19 using Pt microwires (20 μm diameter, Nilaco). Before each experiment, the PtUME was polished using 1.0 μm alumina (MicroPolish Alumina, Buehler) on a felt pad.…”

“…In recent years, scanning electrochemical microscopy (SECM) has proven to be a powerful instrument for gaining localized dynamic information and mapping of various electrochemical processes within energy storage. This includes quantification of ion transfer, , as well as insights on the interphase formation process, ,− single particle measurements, gas evolution, among others. ,, Although SECM is well suited for measuring localized ion transfer, most of the available ion sensing methods tend to show instability at typical battery electrolyte concentrations of ∼1 mol dm –3 (≡1 M) and above. There are some alternative methods, including scanning ion conductance microscopy, though they also show some limitations.…”