Electric Field Induced Associations in the Double Layer of Salt-in-Ionic-Liquid Electrolytes
Pith reviewed 2026-05-24 04:00 UTC · model grok-4.3
The pith
A theory of salt-in-ionic-liquid electrolytes predicts increased ion aggregation at positive voltages in the electrical double layer.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
The theory predicts that the SiIL becomes more aggregated at positive voltages through the formation of large, negatively charged aggregates that screen the electrode charge, while at large negative voltages the alkali metal cations displace the IL cations in the double layer due to their higher charge density after breaking associations.
What carries the argument
Cayley tree aggregates formed by thermoreversible ion associations, incorporated into electrical double layer theory to determine voltage-dependent screening and composition.
If this is right
- IL cations populate the EDL at negative voltages because they are not strongly bound to anions.
- At sufficiently large negative voltages, alkali metal cations replace IL cations due to higher charge density.
- Positive voltages induce formation of large negatively charged aggregates that screen the electrode.
- The SiIL appears more associated in certain electric fields than expected from conventional intuition.
Where Pith is reading between the lines
- Capacitance or conductivity near the electrode may show non-monotonic dependence on voltage polarity.
- The model could be used to predict local transference numbers or ion mobility in the double layer.
- Surface-sensitive experiments could detect changes in aggregate populations by comparing positive and negative electrode polarities.
Load-bearing premise
The electrical double layer theory can incorporate thermoreversible association of ions into Cayley tree aggregates without additional approximations that break at high voltages or concentrations.
What would settle it
Molecular dynamics simulations measuring the size distribution or charge of ion aggregates in the double layer at positive electrode voltages, compared against the bulk, would confirm or refute whether aggregation increases.
Figures
read the original abstract
Ionic liquids (ILs) are an extremely exciting class of electrolytes for energy storage applications because of their unique combination of properties. Upon dissolving alkali metal salts, such as Li or Na based salts, with the same anion as the IL, an intrinsically asymmetric electrolyte can be created for use in batteries, known as a salt-in-ionic liquid (SiIL). These SiILs have been well studied in the bulk, where negative transference numbers of the alkali metal cation have been observed from the formation of small, negatively charged clusters. The properties of these SiILs at electrified interfaces, however, have received little to no attention. Here, we develop a theory for the electrical double layer (EDL) of SiILs where we consistently account for the thermoreversible association of ions into Cayley tree aggregates. The theory predicts that the IL cations first populate the EDL at negative voltages, as they are not strongly bound to the anions. However at large negative voltages which are strong enough to break the alkali metal cation-anion associations, these IL cations are exchanged for the alkali metal cation because of their higher charge density. At positive voltages, we find that the SiIL actually becomes $\textit{more aggregated while screening the electrode charge}$ from the formation of large, negatively charged aggregates. Therefore, in contrast to conventional intuition of associations in the EDL, SiILs appear to become more associated in certain electric fields. We present these theoretical predictions to be verified by molecular dynamics simulations and experimental measurements.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript develops a theory for the electrical double layer (EDL) in salt-in-ionic-liquid (SiIL) electrolytes that consistently incorporates thermoreversible association of ions into Cayley-tree aggregates. Key predictions are that IL cations populate the EDL first at moderate negative voltages, alkali-metal cations replace them at large negative voltages due to higher charge density, and at positive voltages the system becomes more aggregated overall via formation of large, negatively charged aggregates that screen electrode charge.
Significance. If the central predictions hold, the work supplies a self-consistent framework for interface behavior in associated asymmetric electrolytes relevant to battery applications. The explicit use of the same bulk association constants in the EDL is a methodological strength that distinguishes it from ad-hoc treatments.
major comments (1)
- [Model derivation (likely §2–3)] Model derivation (likely §2–3): the position-dependent chemical potentials for monomers and aggregates are closed using the same thermoreversible association constants as in the bulk. The central claim of increased aggregation at positive voltages is load-bearing on this closure remaining valid under steep potential gradients; the manuscript must demonstrate that no field-dependent shift in binding energies or non-local corrections for large trees are required, or else quantify the regime where the local-density approximation breaks.
minor comments (2)
- [Abstract] Abstract: the phrase 'more aggregated while screening the electrode charge' is clear but would benefit from a one-sentence qualifier on the voltage range over which the effect is predicted.
- [Discussion/Conclusion] The manuscript states that predictions should be verified by MD and experiment; adding a short paragraph outlining the most direct falsifiable signature (e.g., voltage-dependent cluster-size distribution from simulation) would strengthen the presentation.
Simulated Author's Rebuttal
We thank the referee for the positive evaluation of the work's significance and for the detailed comment on the model. We respond to the major comment below.
read point-by-point responses
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Referee: Model derivation (likely §2–3): the position-dependent chemical potentials for monomers and aggregates are closed using the same thermoreversible association constants as in the bulk. The central claim of increased aggregation at positive voltages is load-bearing on this closure remaining valid under steep potential gradients; the manuscript must demonstrate that no field-dependent shift in binding energies or non-local corrections for large trees are required, or else quantify the regime where the local-density approximation breaks.
Authors: The closure with bulk association constants follows directly from the mean-field treatment in which short-range binding is governed by local monomer densities while the electrostatic potential enters the chemical potentials separately. This is the same approximation used in the bulk theory and is standard in density-functional treatments of associating electrolytes. We agree, however, that the central prediction of increased aggregation at positive voltages would benefit from an explicit statement of the approximation's regime of validity. In the revised manuscript we will add a short paragraph estimating the length scale on which the potential varies relative to aggregate size and binding energy, thereby quantifying where the local-density approximation is expected to remain reasonable. revision: yes
Circularity Check
No significant circularity; model extends bulk association to EDL self-consistently
full rationale
The paper develops an explicit theoretical framework for the EDL that incorporates thermoreversible Cayley-tree association via position-dependent chemical potentials and a free-energy functional closed under the same association constants used in bulk. No quoted step reduces a prediction to a fitted parameter on the target EDL data, a self-citation chain, or a renaming of an input. The central claim (increased aggregation at positive voltages) follows from solving the model equations rather than being imposed by definition. External benchmarks (MD, experiment) are invoked for verification, confirming the derivation remains independent of its own outputs.
Axiom & Free-Parameter Ledger
free parameters (1)
- association equilibrium constants
axioms (2)
- domain assumption Ions form thermoreversible Cayley tree aggregates
- domain assumption Standard electrical double layer framework applies with aggregate modifications
Reference graph
Works this paper leans on
-
[1]
First, we solve the our system of equations numerically for ¯φ +, ¯φ − , ¯φ ⊕ ,¯p+− ,¯p− +,Λ over a range of electrostatic potential values. This is done for a spectr um of dimensionless electrostatic potential (Φ /eβ ) to allow one to create a sufficiently fine grid, for our purposes here a spacing of 0.001 was used. Here to simplify further steps, we store...
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[2]
Using the dimensionless ρe map, we can then numerically solve the Poisson-Boltzmann equation to get a solution for the electrostatic potential profile in t he EDL
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[3]
The electrostatic potential profile in the EDL along with our interp olation maps al- lows us to predict profiles of the various quantities of interest in the EDL: dimen- sionless charge density, total volume fractions of each species, v olume fraction of free cations, volume fraction free anions, volume fraction aggregates (could additional ex- tract indiv...
-
[4]
In order to obtain our differential capacitance predictions, we fi rst solved for the po- tential at the interface using a wide range of surface charge dens ities for our boundary conditions, for our work here we used a fine grid spacing of 0.0001 C/ m2. From this map, we constructed splines to calculate how the surface charge d ensity depends on 38 the pote...
-
[5]
To obtain the screening length, we next applied a ± 0.0001 V electrostatic potential boundary condition at the surface. From the electrostatic poten tial profile in the EDL, we obtained the screening length by fitting the exponential decay f or the profile, thus extracting the exponential decay constant for a range of mole fr actions of alkali metal salt sho...
-
[6]
The cation exchange voltage and maximum aggregation voltage ut ilized the method in step 1. For these plots, we utilize the values for the case displayed in the main text but individually varied the values of the mole fraction of alkali metal sa lt (xs), regular solution interaction strength ( χ ), and “bare” association constant ( λ 0) extracting the cat...
-
[7]
M. Watanabe, M. L. Thomas, S. Zhang, K. Ueno, T. Yasuda, an d K. Dokko, Chem. Rev. 117, 7190 (2017)
work page 2017
-
[8]
A. Lewandowski and A. ´Swiderska-Mocek, J. Power Sources 194, 601 (2009)
work page 2009
-
[9]
A. Balducci, U. Bardi, S. Caporali, M. Mastragostino, an d F. Soavi, Electrochem. commun. 6, 566 (2004)
work page 2004
- [10]
- [11]
-
[12]
L. G. Chagas, S. Jeong, I. Hasa, and S. Passerini, ACS Appl . Mater. Interfaces 11, 22278 (2019)
work page 2019
-
[13]
Q. Zhou, P. D. Boyle, L. Malpezzi, A. Mele, J.-H. Shin, S. P asserini, and W. A. Henderson, Chem. Mater. 23, 4331 (2011)
work page 2011
- [14]
- [15]
-
[16]
K. Matsumoto, J. Hwang, S. Kaushik, C.-Y. Chen, and R. Ha giwara, Energy & Environmental Science 12, 3247 (2019)
work page 2019
-
[17]
T. C. Mendes, F. Zhou, A. J. Barlow, M. Forsyth, P. C. Howl ett, and D. R. MacFarlane, Sustainable Energy & Fuels 2, 763 (2018)
work page 2018
-
[18]
J. Zhao, G. Gorbatovski, O. Oll, T. Thomberg, and E. Lust , Electrochimica Acta 319, 82 (2019)
work page 2019
- [19]
-
[20]
Q. Guo, P. Fan, Y. Zhang, L. Guan, H. Wang, A. Croft, and G. Z. Chen, RSC Sustainability (2024)
work page 2024
- [21]
-
[22]
D. R. MacFarlane, N. Tachikawa, M. Forsyth, J. M. Pringl e, P. C. Howlett, G. D. Elliott, J. H. Davis, M. Watanabe, P. Simon, and C. A. Angell, Energy En viron. Sci. 7, 232 (2014)
work page 2014
-
[23]
D. R. MacFarlane, M. Forsyth, P. C. Howlett, M. Kar, S. Pa sserini, J. M. Pringle, H. Ohno, M. Watanabe, F. Yan, W. Zheng, et al. , Nature Reviews Materials 1, 1 (2016)
work page 2016
- [24]
- [25]
-
[26]
J. P. Hallett and T. Welton, Chem. Rev. 111, 3508–3576 (2011)
work page 2011
-
[27]
M. V. Fedorov and A. A. Kornyshev, Chem. Rev. 114, 2978 (2014)
work page 2014
-
[28]
M. McEldrew, Z. A. H. Goodwin, N. Molinari, B. Kozinsky, A. A. Kornyshev, and M. Z. Bazant, J. Phys. Chem. B 125, 13752 (2021)
work page 2021
-
[29]
B. Qiao, G. M. Leverick, W. Zhao, A. H. Flood, J. A. Johnso n, and Y. Shao-Horn, J. Am. Chem. Soc. 140, 10932 (2018)
work page 2018
-
[30]
M. Gouverneur, J. Kopp, L. van W¨ ullen, and M. Sch¨ onhoff, Phys. Chem. Chem. Phys. 17, 40 30680 (2015)
work page 2015
-
[31]
M. Gouverneur, F. Schmidt, and M. Sch¨ onhoff, Phys. Chem. Chem. Phys. 20, 7470 (2018)
work page 2018
-
[32]
M. Brinkk¨ otter, A. Mariani, S. Jeong, S. Passerini, an d M. Sch¨ onhoff, Adv. Energy Sustain- ability Res. 2, 2000078 (2021)
work page 2021
- [33]
-
[34]
N. Molinari, J. P. Mailoa, N. Craig, J. Christensen, and B. Kozinsky, J. Power Sources 428, 27 (2019)
work page 2019
-
[35]
N. Molinari, J. P. Mailoa, and B. Kozinsky, J. Phys. Chem . Lett. 10, 2313 (2019)
work page 2019
- [36]
-
[37]
P. Kubisiak, P. Wr` obel, and A. Eilmes, J. Phys. Chem. B 124, 413 (2020)
work page 2020
-
[38]
T. C. Lourenco, L. G. Dias, and J. L. Da Silva, ACS Appl. En ergy Mater. (2021)
work page 2021
- [39]
-
[40]
J. B. Haskins, W. R. Bennett, J. J. Wu, D. M. Herna´ andez, O. Borodin, J. D. Monk, J. Charles W. Bauschlicher, and J. W. Lawson, J. Phys. Chem. B 118, 11295 (2014)
work page 2014
- [41]
-
[42]
Z. A. H. Goodwin, M. McEldrew, B. Kozinsky, and M. Z. Baza nt, PRX Energy 2, 013007 (2023)
work page 2023
-
[43]
M. McEldrew, Z. A. Goodwin, S. Bi, M. Z. Bazant, and A. A. K ornyshev, J. Chem. Phys. 152, 234506 (2020)
work page 2020
-
[44]
M. McEldrew, Z. A. H. Goodwin, H. Zhao, M. Z. Bazant, and A . A. Kornyshev, J. Phys. Chem B 125, 2677–2689 (2021)
work page 2021
-
[45]
M. McEldrew, Z. A. Goodwin, S. Bi, A. Kornyshev, and M. Z. Bazant, J. Electrochem. Soc. 168, 050514 (2021)
work page 2021
- [46]
-
[47]
J. B. Haskins, J. J. Wu, and J. W. Lawson, J. Phys. Chem. C 120, 11993 (2016)
work page 2016
-
[48]
Z. A. Goodwin, M. McEldrew, J. P. de Souza, M. Z. Bazant, a nd A. A. Kornyshev, J. Chem. Phys. 157, 094106 (2022)
work page 2022
-
[49]
Z. A. Goodwin and A. A. Kornyshev, Electrochim. Acta 434, 141163 (2022)
work page 2022
-
[50]
Z. A. Goodwin, G. Feng, and A. A. Kornyshev, Electrochim . Acta 225, 190 (2017)
work page 2017
-
[51]
M. Chen, Z. A. H. Goodwin, G. Feng, and A. A. Kornyshev, 41 J. Electroanal. Chem. 819, 347 (2018)
work page 2018
-
[52]
Z. A. H. Goodwin and A. A. Kornyshev, Electrochem. Commu n. 82, 129 (2017)
work page 2017
- [53]
-
[54]
G. Feng, M. Chen, S. Bi, Z. A. Goodwin, E. B. Postnikov, N. Brilliantov, M. Urbakh, and A. A. Kornyshev, Phys. Rev. X 9, 021024 (2019)
work page 2019
- [55]
-
[56]
E. Gongadze, U. van Rienen, V. Kralj-Igliˇ c, and A. Igli ˇ c, Computer methods in biomechanics and biomedical engineering 16, 463 (2013)
work page 2013
-
[57]
W. H. Stockmayer, The Journal of chemical physics 11, 45 (1943)
work page 1943
-
[58]
M. S. Wertheim, Journal of statistical physics 35, 19 (1984)
work page 1984
-
[59]
M. S. Wertheim, Journal of statistical physics 35, 35 (1984)
work page 1984
-
[60]
Wertheim, Journal of statistical physics 42, 459 (1986)
M. Wertheim, Journal of statistical physics 42, 459 (1986)
work page 1986
-
[61]
Wertheim, Journal of statistical physics 42, 477 (1986)
M. Wertheim, Journal of statistical physics 42, 477 (1986)
work page 1986
-
[62]
D. Lar ´ ıa, H. R. Corti, and R. Fern´ andez-Prini, Journa l of the Chemical Society, Faraday Transactions 86, 1051 (1990)
work page 1990
- [63]
-
[64]
J.-P. Simonin, O. Bernard, and L. Blum, The Journal of Ph ysical Chemistry B 103, 699 (1999)
work page 1999
-
[65]
F. Sciortino, E. Bianchi, J. F. Douglas, and P. Tartagli a, The Journal of chemical physics 126 (2007)
work page 2007
-
[66]
L. Guldbrand, B. J¨ onsson, H. Wennerstr¨ om, and P. Lins e, The Journal of chemical physics 80, 2221 (1984)
work page 1984
-
[67]
R. Kjellander and S. Marˇ celja, Chemical physics lette rs 127, 402 (1986)
work page 1986
-
[68]
P. Grochowski and J. Trylska, Biopolymers: Original Re search on Biomolecules 89, 93 (2008)
work page 2008
-
[69]
R. R. Netz, The European Physical Journal E 5, 557 (2001)
work page 2001
-
[70]
M. Z. Bazant, M. S. Kilic, B. D. Storey, and A. Ajdari, Adv ances in colloid and interface science 152, 48 (2009)
work page 2009
-
[71]
M. Z. Bazant, B. D. Storey, and A. A. Kornyshev, Phys. Rev . Lett. 106, 046102 (2011)
work page 2011
-
[72]
J. P. de Souza, Z. A. Goodwin, M. McEldrew, A. A. Kornyshe v, and M. Z. Bazant, Phys. Rev. Lett. 125, 116001 (2020). 42
work page 2020
-
[73]
J. P. de Souza, A. A. Kornyshev, and M. Z. Bazant, The Jour nal of Chemical Physics 156 (2022)
work page 2022
-
[74]
Y. Avni, R. M. Adar, and D. Andelman, Physical Review E 101, 010601 (2020)
work page 2020
-
[75]
R. M. Adar, S. A. Safran, H. Diamant, and D. Andelman, Phy sical Review E 100, 042615 (2019)
work page 2019
-
[76]
A. Levy, M. McEldrew, and M. Z. Bazant, Physical Review M aterials 3, 055606 (2019)
work page 2019
-
[77]
Z. Yu, N. P. Balsara, O. Borodin, A. A. Gewirth, N. T. Hahn , E. J. Maginn, K. A. Persson, V. Srinivasan, M. F. Toney, K. Xu, K. R. Zavadil, L. A. Curtiss , and L. Cheng, ACS Energy Lett. 7, 461 (2022)
work page 2022
-
[78]
A. A. Kornyshev, J. Phys. Chem. B 111, 5545 (2007)
work page 2007
-
[79]
A. Ahmadiparidari, S. Fuladi, L. Majidi, S. Plunkett, E . Sarnello, H. Gholivand, Z. Hemmat, S. Rastegar, S. N. Misal, N. Jimenez, et al. , Journal of Power Sources 491, 229506 (2021)
work page 2021
- [80]
discussion (0)
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