RESEARCH / ■ THEME III · ONGOING
Electrolytes, confinement and ion transport
I study how ions are solvated and transported in battery electrolytes, and how that changes when the electrolyte is confined inside a separator. Using the electrolyte's own carbonyl and anion stretches as reporters, I resolve ion speciation with FTIR, two-dimensional correlation analysis and polarisation- resolved pump–probe spectroscopy, and relate it to macroscopic transport through impedance measurements.
A separator is usually drawn as a passive porous sheet whose only job is to keep the electrodes apart while letting ions through. The spectroscopy says otherwise. Put the same electrolyte inside the pores and its speciation changes: contact ion pairs become far more prominent, a new carbonyl environment appears, and the conductivity drops by nearly an order of magnitude at concentrations where the bulk liquid is perfectly well behaved.
That makes the separator a chemical component with a designable surface rather than a purely mechanical one. It also raises the question I most want to answer next. These measurements show what the ion’s environment looks like inside the pore, but not how fast the ion sheds that environment when it reaches the electrode. Desolvation is the rate-limiting step in interfacial charge transfer, and it is a dynamical quantity. Voltage-dependent surface-enhanced 2D IR offers a way to measure it directly.
Methods
- Polarisation-resolved mid-IR pump–probe spectroscopy
- Two-dimensional correlation spectroscopy
- ATR-FTIR of confined electrolytes
- Electrochemical impedance spectroscopy
- Quantum-chemical band assignment
Publications in this theme
3 records → publicationsIn preparationIn preparation · 2026
Electrolyte Swelling Dynamics on Polymer Separator Membrane Revealed by ATR-FTIR, 2D-correlation Spectroscopy and Quantum Chemical Calculations
In preparationIn preparation · 2026
Battery Separators as Active Regulators of Ion Solvation and Transport
PublishedBull. Korean Chem. Soc. · 2022 · 43 (2), 215–221
Solvation structure of phosphonium ionic liquid/CH3SCN mixture as electrolytes for Li-ion batteries: Infrared pump-probe spectroscopic studies
Projects in this theme
- Why it matters
- Battery performance is decided at molecular scale: how ions are solvated, how they shed that solvation at an interface, and how the charge carriers move. Vibrational spectroscopy resolves the speciation (free solvent, ion pairs and aggregates), and 2D IR time-resolves the picosecond exchange among them. Two gaps stand out: desolvation is a dynamical quantity that bulk studies cannot capture, and the separator has hardly been studied by molecular spectroscopy at all.
- My contribution
- I showed that the separator is an active participant. For LiPF₆ in diethyl carbonate, combining FTIR, 2D correlation analysis, IR pump–probe spectroscopy, quantum-chemical calculation and impedance measurements, I found that nanoconfinement inside a polypropylene separator restructures lithium solvation. I have also characterised lithium solvation and transport in ionic liquid–cosolvent electrolytes by mid-IR pump–probe spectroscopy.
- What we found
- Confinement in roughly 43 nm separator pores produces a new carbonyl signature near 1730 cm⁻¹ and markedly enhanced contact-ion-pair features in the PF₆⁻ stretching region, indicating increased ion pairing and clustering even at moderate 1–2 M salt concentrations, with a corresponding eightfold to ninefold suppression of ionic conductivity. The separator regulates electrolyte speciation rather than merely permitting transport through it.
- Where it goes next
- Measure lithium solvation-shell exchange directly at a working electrode by voltage-dependent surface-enhanced 2D IR, against a bulk 2D IR reference, to test whether the desolvation barrier shows up as a potential-dependent change in exchange rate. In parallel, vary separator surface chemistry and pore size, and extend to sodium, zinc and water-in-salt electrolytes, toward design rules that maximise free charge carriers.