Thermodynamic effects of solid electrolyte interphase formation from solvation and ionic association in water-in-salt electrolytes
Pith reviewed 2026-05-15 19:02 UTC · model grok-4.3
The pith
A thermodynamic theory of the electrical double layer explains how solvation and ionic associations expand the stability window of water-in-salt electrolytes.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
We develop a thermodynamic theory of hydration and ionic associations in the electrical double layer of water-in-salt electrolytes, parameterized from bulk molecular dynamics simulations and validated against electrical double layer simulations. Using this theory, changes in the electrochemical stability window are rationalized through modifications to bulk electrolyte activity via the Nernst equation and to reaction kinetics via the Butler-Volmer equation and coupled ion-electron transfer, both arising from the concentration of reactant species in the Helmholtz layer.
What carries the argument
thermodynamic theory of hydration and ionic associations in the electrical double layer (EDL)
If this is right
- Bulk electrolyte activity changes directly impact stability as described by the Nernst equation.
- Reactant concentrations in the Helmholtz layer thermodynamically alter reaction kinetics according to the Butler-Volmer equation.
- Inorganic solid electrolyte interphase forms to passivate the anode due to these interface effects.
- The model applies to understanding stability in graphite and lithium-metal anodes as well as other battery technologies.
Where Pith is reading between the lines
- The theory could be used to predict optimal salt concentrations for maximizing stability windows in new electrolyte formulations.
- Similar approaches might apply to other concentrated electrolytes beyond water-in-salt systems.
- Experimental measurements of double-layer structure could directly test the model's predictions for specific salt-water mixtures.
Load-bearing premise
The thermodynamic theory parameterized from bulk molecular dynamics simulations accurately captures the reactant distribution and solvation environments near the electrode interface.
What would settle it
Direct comparison of the predicted electrical double layer concentrations from the theory against independent molecular dynamics simulations of the interface or experimental measurements would falsify the approach if they disagree qualitatively.
Figures
read the original abstract
Water-in-Salt-Electrolytes (WiSEs) are a promising class of next-generation electrolytes. Unlike classical dilute electrolytes or more conventional battery electrolytes, WiSEs are characterised by their super-concentrated salt concentration with only a small amount of water, which gives rise to their expanded electrochemical stability window (ESW). The expansion of the ESW is, in part, due to the formation of an inorganic solid electrolyte interphase (SEI) that passivates the anode; this principle is also important in graphite and Li-metal anodes, and beyond Li-ion technologies. The solvation and ionic associations are key descriptors in understanding the expansion of the ESW. Specifically, as reactions which lead to the SEI (or cathode electrolyte interphase, CEI) must occur at the electrode-electrolyte interface, the distribution of reactants and their various solvation environments are critical. This distribution near the interface is referred to as the electrical double layer (EDL), in the absence of reactions. Here we further develop and analyse a recently proposed thermodynamic theory of hydration and ionic associations in the EDL of WiSEs. We parameterize this theory from bulk molecular dynamics simulations and benchmark it against EDL simulations, finding good qualitative agreement. Using this thermodynamic theory, we rationalise changes in the ESW through: changes in the activity in the bulk electrolyte through the Nernst equation, which directly changes the stability of the electrolytes; and thermodynamic changes to the kinetics of these reactions, from the Butler-Volmer equation and coupled ion electron transfer kinetics, through the concentration of reactant species in the Helmholtz layer.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript develops and analyzes a thermodynamic theory of hydration and ionic associations in the electrical double layer (EDL) of water-in-salt electrolytes (WiSEs). The theory is parameterized from bulk molecular dynamics simulations, benchmarked with qualitative agreement against separate EDL simulations, and applied to rationalize expansion of the electrochemical stability window (ESW) via shifts in bulk electrolyte activity (Nernst equation) and changes in reaction kinetics through reactant concentrations in the Helmholtz layer (Butler-Volmer equation with coupled ion-electron transfer).
Significance. If the bulk-parameterized theory transfers quantitatively to the interface, the work supplies a physically grounded route to predict how solvation and ion pairing control SEI/CEI formation and ESW limits in super-concentrated electrolytes, directly relevant to next-generation battery anodes.
major comments (1)
- [Benchmarking and application sections (abstract and main text)] The central rationalization of ESW changes requires accurate absolute concentrations and activities of reactant species in the Helmholtz layer (for both the Nernst shift and the Butler-Volmer rate). The only reported support is 'good qualitative agreement' with EDL simulations; no quantitative error metrics, sensitivity analysis, or direct comparison of predicted versus simulated Helmholtz populations are provided. Because interfacial fields can alter solvation shells and pairing relative to bulk, qualitative shape agreement does not establish that the logarithms of concentration (which enter the thermodynamic expressions) are accurate to the precision needed for the claimed effect sizes.
minor comments (1)
- [Abstract] The abstract states that the theory is 'further develop[ed]' but does not specify the functional form of the thermodynamic free-energy terms or the exact set of fitted parameters obtained from bulk MD.
Simulated Author's Rebuttal
We thank the referee for their careful reading and constructive critique. The concern about quantitative validation of Helmholtz-layer concentrations is well taken, and we have revised the manuscript to include additional metrics and analysis as detailed below.
read point-by-point responses
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Referee: [Benchmarking and application sections (abstract and main text)] The central rationalization of ESW changes requires accurate absolute concentrations and activities of reactant species in the Helmholtz layer (for both the Nernst shift and the Butler-Volmer rate). The only reported support is 'good qualitative agreement' with EDL simulations; no quantitative error metrics, sensitivity analysis, or direct comparison of predicted versus simulated Helmholtz populations are provided. Because interfacial fields can alter solvation shells and pairing relative to bulk, qualitative shape agreement does not establish that the logarithms of concentration (which enter the thermodynamic expressions) are accurate to the precision needed for the claimed effect sizes.
Authors: We agree that the original manuscript provided only qualitative benchmarking and that this is insufficient to fully substantiate the precision of the logarithmic concentration terms used in the Nernst and Butler-Volmer expressions. In the revised manuscript we now report quantitative error metrics (mean absolute percentage error and root-mean-square deviation) between the thermodynamic model predictions and the EDL simulation populations for the key reactant species (free water, Li+-water complexes, and ion pairs) within the first 5 Å of the electrode. We have also added a sensitivity analysis that propagates ±10 % variations in the bulk-derived association and hydration free energies through to the predicted ESW shifts, showing that the direction and approximate magnitude of the effects remain robust. While interfacial fields can indeed modify solvation relative to bulk, the model is constructed from position-dependent chemical potentials extracted from bulk MD; the observed qualitative agreement in the EDL density profiles indicates that the dominant thermodynamic contributions are captured. We acknowledge that a fully quantitative transferability test would ultimately require interface-specific reparameterization, which lies beyond the present scope. revision: yes
Circularity Check
No significant circularity; parameterization from independent bulk MD and standard equations remain self-contained
full rationale
The paper parameterizes its thermodynamic theory of hydration and ionic associations directly from bulk molecular dynamics simulations and applies the standard Nernst equation for bulk activity shifts plus Butler-Volmer kinetics for Helmholtz-layer concentrations. Benchmarking against separate EDL simulations is reported only as qualitative agreement, with no indication that target ESW rationalizations are fitted or redefined to match the inputs by construction. No self-definitional loops, fitted inputs renamed as predictions, or load-bearing self-citations that reduce the central claim to unverified prior results are present in the provided text. The derivation chain uses external simulation data and classical electrochemical relations without circular reduction.
Axiom & Free-Parameter Ledger
free parameters (1)
- thermodynamic parameters for hydration and association
axioms (2)
- standard math Nernst equation links bulk activity changes directly to electrolyte stability
- standard math Butler-Volmer equation and coupled ion-electron transfer kinetics govern reaction rates at the interface
Lean theorems connected to this paper
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IndisputableMonolith/Cost/FunctionalEquation.leanwashburn_uniqueness_aczel unclear?
unclearRelation between the paper passage and the cited Recognition theorem.
thermoreversible ionic aggregation and solvation theory... free energy functional (F) ... Cayley tree clusters ... sticky-cation approximation ... modified Poisson-Boltzmann equation
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IndisputableMonolith/Foundation/RealityFromDistinction.leanreality_from_one_distinction unclear?
unclearRelation between the paper passage and the cited Recognition theorem.
parameterized from bulk molecular dynamics simulations ... α = 0.3
What do these tags mean?
- matches
- The paper's claim is directly supported by a theorem in the formal canon.
- supports
- The theorem supports part of the paper's argument, but the paper may add assumptions or extra steps.
- extends
- The paper goes beyond the formal theorem; the theorem is a base layer rather than the whole result.
- uses
- The paper appears to rely on the theorem as machinery.
- contradicts
- The paper's claim conflicts with a theorem or certificate in the canon.
- unclear
- Pith found a possible connection, but the passage is too broad, indirect, or ambiguous to say the theorem truly supports the claim.
Reference graph
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