Saptarsi Mondal Ultrafast & multidimensional IR spectroscopy
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Electrolytes, confinement & transport

Battery separators as active regulators of ion transport

Testing whether the porous separator in a lithium-ion cell — normally treated as an inert spacer — changes the chemistry of the electrolyte inside it, using vibrational spectroscopy to resolve ion speciation under nanoconfinement.

Ongoing Collaborative Korea University
The same electrolyte, inside and outside the separator. Confinement promotes contact ion pairing and clustering, and ionic conductivity falls by close to an order of magnitude.

The result that started this project was a discrepancy. Conductivity measured through a separator was far lower than the bulk electrolyte’s conductivity corrected for porosity and tortuosity — a gap usually absorbed into an empirical factor. Rather than fit the gap, we asked whether the electrolyte inside the pore is simply a different liquid.

It is. The carbonyl stretching region gains a new environment absent from the bulk, and the anion stretching region shows contact ion pairing at concentrations where the bulk liquid is well dissociated. Confinement is doing chemistry: with less room and a polymer surface nearby, ions that would be freely solvated instead find each other.

The open question, and the reason this connects directly to my interfacial work, is dynamics. Everything measured so far describes what the confined environment looks like. It does not describe how quickly an ion sheds that environment when it reaches the electrode — which is the step that actually limits charge transfer. Voltage-dependent surface-enhanced 2D IR is the route to measuring it.

Scientific motivation
Separator research has been dominated by macroscopic transport measurement and imaging. Neither can say what the electrolyte looks like at molecular scale inside a pore. If confinement shifts the balance between free ions, solvent-separated ion pairs and contact ion pairs, then the separator is a chemical component with designable surface properties rather than a mechanical one — and the conductivity losses attributed to tortuosity may have a molecular origin instead.
The research question
Does nanoconfinement within a polymer separator change lithium solvation speciation, and can any resulting change account for the measured suppression of ionic conductivity?
My contribution
I ran the spectroscopic programme: ATR-FTIR of the confined electrolyte, two-dimensional correlation analysis to track how bands evolve with concentration, ultrafast IR pump–probe spectroscopy for the dynamics, and quantum-chemical calculation to assign the marker bands. I connected these to impedance measurements so the molecular picture could be tested against macroscopic transport.
Results and current status
Lithium hexafluorophosphate in diethyl carbonate, confined within a polypropylene separator with roughly 43 nm pores, shows a new carbonyl signature near 1730 cm⁻¹ together with markedly enhanced contact-ion-pair features in the PF₆⁻ stretching region. Both indicate increased ion pairing and clustering even at moderate 1–2 M salt concentrations, and they are accompanied by an eightfold to ninefold suppression of ionic conductivity. The separator is an active regulator of electrolyte speciation.
Future direction
Vary separator surface chemistry — polar versus hydrophobic functionalisation, oxide and ceramic coatings — and pore architecture systematically, and extend beyond lithium to sodium, zinc and aqueous water-in-salt electrolytes where interfacial water is itself central. The goal is design rules connecting surface chemistry to the balance between solvent-separated and contact ion pairs, and hence to the population and mobility of free charge carriers.
Collaboration With Prof. Kyungwon Kwak and Prof. Minhaeng Cho at the Center for Molecular Spectroscopy and Dynamics, Korea University, and with Sourav Palchowdhury and Kwanghee Park. I led the spectroscopic measurement and analysis.

Publications from this project

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