Ion Dynamics in the Bulk of Water-in-salt Electrolytes and Under Confinement in Multiporous Carbon

Description

Typical organic electrolytes used in electrochemical energy stroage applications suffer from several limitations such as instability, flammability, and reduced conductivity at low temperatures. These drawbacks can lead to decreased battery performance, safety concerns, and environmental issues. In contrast, water-in-salt electrolytes can meet these challenges by providing improved stability, non-flammability, higher conductivity at low temperatures, and a wide electrochemical potential window.[1] However, the higher viscosity of water-in-salt electrolytes results in slower ion transport and inadequate wetting of subnanometer-sized pores in carbon electrodes, necessitating a balance between voltage, power density, energy density, and stability [2] Hence, understanding the ion transport in the bulk of water-in-salt electrolytes and under confinement in carbon pores is the key to widen their applications. In this regard, in-situ PFG-NMR[3], small-angle X-ray scattering,[4] electrochemical quartz crystal microbalance,[5] and theoretical simulation[6] techniques have been employed for accurate elucidation of ion transport and charging behavior in carbon pores with typical organic electrolytes. However, those methods are often sophisticated, expensive, and may not differentiate the ion transport behavior in pores of different sizes in a multiporous carbon electrode.

In addressing this limitation, we introduce a facile and reliable approach to quantify and differentiate the diffusivity of lithium bis(trifluoromethanesulfonyl)imide and choline chloride based water-in-salt electrolytes in the micro and meso-pores of a multiporous carbon. We determined the in-pore ion diffusivity from the diffusivity values of the bulk electrolyte, along with the porosity and tortuosity of the carbon elctrode.[7] The average bulk ion diffusivity was obtained from PFG-NMR data in literature, while the porosity was measured with N2 gas adsorption. The electrode's tortuosity was determined from electrochemical impedance spectroscopy (EIS), considering the maximum capacitance, knee frequency and in-pore ionic resistance.[7] Our approach considers these parameters and efficiently differentiates between the ion diffusivity within micro- and mesopores taking into account their respective pore volume and surface area fractions. The trend of in-pore diffusivity values was found to be comparable with that of in-situ PFG-NMR investigations reported in literature. Micropores were found to have a fifty times higher tortuousity than mesopores, leading to nearly nine times greater ionic resistance and reduced ion diffusivity compared to mesopores. The combined ionic resistance of micro and mesopores equals the total in-pore ionic resistance as evidenced by EIS. Additionally, the contribution of the inverse micro and mesopore capacitances matches the full cell capacitance obtained from EIS, affirming the accuracy and validity of our method. In summary, our approach offers a promising facile method to differentiate ion diffusivity in micro- and mesopores of a multiporous carbon, thereby fundamentally advancing the development of electrochemical capacitors based on water-in-salt electrolytes and porous carbon electrodes.


References:
[1]. L. Suo, O. Borodin, T. Gao, M. Olguin, J. Ho, X. Fan, C. Luo, C. Wang, K. Xu, Science 350. 6263 (2015) 938-943.
[2]. P. Lannelongue, R. Bouchal, E. Mourad, C. Bodin, M. Olarte, S. le Vot, F. Favier, O. Fontaine, J. Electrochem. Soc. 165 (2018) A657–A663.
[3]. L. Borchardt, D. Leistenschneider, J. Haase, M. Dvoyashkin, Adv. Energy. Mater. 8 (2018) 1800892
[4]. S. Boukhalfa, D. Gordon, L. He, Y.B. Melnichenko, N. Nitta, A. Magasinski, G. Yushin, , ACS Nano. 8 (2014) 2495–2503.
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[7]. J.H. Xu, T. Schoetz, J.R. McManus, V.R. Subramanian, P.W. Fields, R.J. Messinger, J. Electrochem. Soc. 168 (2021) 060514.

Period	9 Jun 2024 → 12 Jun 2024
Event title	37th Topical Meeting of the International Society of Electrochemistry
Event type	Conference
Location	Stresa, ItalyShow on map
Degree of Recognition	International