Probing the thermodynamics and kinetics of ethylene carbonate reduction at the electrode–electrolyte interface with molecular simulations

Abstract

Understanding the formation of the solid–electrolyte interphase (SEI) in lithium-ion batteries is an ongoing area of research due to its high degree of complexity and the difficulties encountered by experimental studies. Herein, we investigate the initial stage of SEI growth, the reduction reaction of ethylene carbonate (EC), from both a thermodynamic and a kinetic approach with theory and molecular simulations. We employed both the potential distribution theorem and the Solvation Method based on Density (SMD) to EC solvation for the estimation of reduction potentials of Li+, EC, and Li+-solvating EC (s-EC) as well as reduction rate constants of EC and s-EC. We find that solvation effects greatly influence these quantities of interest, particularly the Li+/Li reference electrode potential in EC solvent. Furthermore, we also compute the inner- and outer-sphere reorganization energies for both EC and s-EC at the interface of liquid EC and a hydroxyl-terminated graphite surface, where total reorganization energies are predicted to be 76.6 and 88.9 kcal/mol, respectively. With the computed reorganization energies, we estimate reduction rate constants across a range of overpotentials and show that EC has a larger electron transfer rate constant than s-EC at equilibrium, despite s-EC being more thermodynamically favorable. Overall, this manuscript demonstrates how ion solvation effects largely govern the prediction of reduction potentials and electron transfer rate constants at the electrode–electrolyte interface.

Switching of Redox Levels Leads to High Reductive Stability in Water-in-Salt Electrolytes

“…The redox potentials in electrolytes can not be computed with classical force fields, and concentration effects are hard to be accounted for with implicit solvation models. ,− AIMD-based free energy calculations are used in this work to compute the redox potentials of TFSI – and HER potentials of water (Figure c). One-electron reduction potentials of TFSI – (BETI – in HME) and HER potentials of water are calculated with free energy perturbation (FEP) theory and thermodynamic integration (TI) method, in which 30–100 ps of AIMD trajectories are generated to achieve statistical convergence of thermodynamic integrals. − In FEP theory, a fictitious mapping Hamiltonian H η = (1 – η) H R + ηH P is constructed by linear combination of Hamiltonian of reactant H R and product H P through the Kirkwood coupling parameter η. − The free energy difference Δ A can be rigorously obtained by TI, normalΔ A = prefix∫ 0 1 false⟨ normalΔ E false⟩ η nobreak0em0.25em⁡ normald η , in which the ⟨Δ E ⟩ η is the ensemble average of vertical energy gaps along AIMD trajectories.…”

Unraveling the origin of reductive stability of super-concentrated electrolytes from first principles and unsupervised machine learning

“…67,69 However, the absolute potentials of both the Li reference electrode and SHE can change considerably from aqueous to non-aqueous electrolytes, which would lead to about a 1 V shift in computed redox potentials by using the implicit solvation model. 70–73…”

“…67,69 However, the absolute potentials of both the Li reference electrode and SHE can change considerably from aqueous to non-aqueous electrolytes, which would lead to about a 1 V shi in 22,23 The results are computed by using the HSE06 functional and the corresponding values are listed in Table 1 and S2 computed redox potentials by using the implicit solvation model. [70][71][72][73] For PC solutions containing TFSI − , localization of inserted electron on TFSI − would lead to spontaneous decomposition of TFSI − , and thus for computing the one electron reduction potential a restraining potential is applied to help maintain the integrity of the TFSI − structure. Note that this restraint should not be regarded as a computational artefact but a matter of chemical denition and the congurations with and without restraining potential do not show much difference.…”

Electrochemical Properties and Local Structure of the TEMPO/TEMPO+ Redox Pair in Ionic Liquids

“…A crucial impact of local environment of the solute on thermodynamics and kinetics of the electron transfer is known for relatively simple systems, such as metal ion redox couples in water or organic liquids. − In the case of a more complex solvent, ionic liquids, a dramatic role is played by their microheterogeneity and electrostrictive effects, as well as by viscosity, − though the latter can influence the kinetics of the electron transfer process not in a straightforward way …”

“…Indeed, a redox pair that accepts or donates an electron to the system of interest can have a drastic effect on the reaction rate, especially if it enters the solvation shell of the main solute. For instance, during an investigation of a solid−electrolyte interphase formation in Li-ion batteries, Mundy et al 20 have demonstrated that electron transfer can be reduced by about 15% if an ethylene carbonate molecule accepting an electron approaches the metal species. The latter can be explained by a greater energy required to rearrange the solvent in proximity to the solute, even despite being a more favorable process from the thermodynamic point of view.…”