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Potentiometric MRI of a Superconcentrated Lithium Electrolyte: Testing the Irreversible Thermodynamics Approach.

ABSTRACT: Superconcentrated electrolytes, being highly thermodynamically nonideal, provide a stringent proving ground for continuum transport theories. Herein, we test an ostensibly complete model of LiPF₆ in ethyl-methyl carbonate (EMC) based on the Onsager-Stefan-Maxwell theory from irreversible thermodynamics. We perform synchronous magnetic resonance imaging (MRI) and chronopotentiometry to examine how superconcentrated LiPF₆:EMC responds to galvanostatic polarization and open-circuit relaxation. We simulate this experiment using an independently parametrized model with six composition-dependent electrolyte properties, quantified up to saturation. Spectroscopy reveals increasing ion association and solvent coordination with salt concentration. The potentiometric MRI data agree closely with the predicted ion distributions and overpotentials, providing a completely independent validation of the theory. Superconcentrated electrolytes exhibit strong cation-anion interactions and extreme solute-volume effects that mimic elevated lithium transference. Our simulations allow surface overpotentials to be extracted from cell-voltage data to track lithium interfaces. Potentiometric MRI is a powerful tool to illuminate electrolytic transport phenomena.

SUBMITTER: Wang AA

PROVIDER: S-EPMC8438662 | biostudies-literature | 2021 Sep

REPOSITORIES: biostudies-literature

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Potentiometric MRI of a Superconcentrated Lithium Electrolyte: Testing the Irreversible Thermodynamics Approach.

Wang Andrew A AA Gunnarsdóttir Anna B AB Fawdon Jack J Pasta Mauro M Grey Clare P CP Monroe Charles W CW

ACS energy letters 20210815 9

Superconcentrated electrolytes, being highly thermodynamically nonideal, provide a stringent proving ground for continuum transport theories. Herein, we test an ostensibly complete model of LiPF<sub>6</sub> in ethyl-methyl carbonate (EMC) based on the Onsager-Stefan-Maxwell theory from irreversible thermodynamics. We perform synchronous magnetic resonance imaging (MRI) and chronopotentiometry to examine how superconcentrated LiPF<sub>6</sub>:EMC responds to galvanostatic polarization and open-ci ...[more]

PMID: 34541321

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Project description:The formation of a solid electrolyte interphase (SEI) between the anode surface and the electrolyte of lithium-ion batteries (LIBs) has been considered to be the most important yet the least understood issue of LIBs. To further our understanding in this regard, the density functional theory (DFT) B3PW91/6-311++G(3df,3pd) together with the implicit solvent model and the transition state theory were used for the first time to comprehensively explore the electroreduction mechanism of a novel additive, 4-chloromethyl-1,3,2-dioxathiolane-2-oxide (CMDO), and a few other solvents and additives, such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), fluoroethylene carbonate (FEC), and even ethylene sulfite (ES), for comparison. The one-electron reduction potential of Li+-coordinated compounds Li+(X) for forming decomposition precursors [c-Li+(X˙-)] decreases in the following sequence: CMDO (1.9-2.2 V vs. Li+/Li) ∼ ES(1.9 V) > FEC (0.7 V) > EC (0.47 V) > PC (0.45 V) > DMC (0.38 V); this implies that CMDO is reduced prior to other solvents or additives in the mixture. Although the ring opening of [c-Li+(CMDO˙-)] is the least kinetically favorable, as reflected by the highest energy barrier (E a), i.e., CMDO (18.8-22.9 kcal mol-1) ∼ ES (23.4) > FEC (16.2) > PC (12.5) > EC (11.2) > DMC (8.0), CMDO still shows the highest overall reaction rate constant (∼1053 s-1) for forming an open ring radical [o-Li+(CMDO˙)-]. In addition, the termination reaction of [o-Li+(CMDO˙)-] for forming LiCl is thermodynamically more favorable than that of Li2SO3 or organic disulfite (LiSO3)2-R, which supports the experimental observation that the halogen-containing LiF or LiCl additives are predominant over all other halogen-containing species in the SEI layer. Moreover, the hybrid model by including the second solvation shell of Li+ via a supercluster [(CMDO)Li+(PC)2](PC)9 and the implicit solvent model (SMD) can result in a reduction potential (∼1.7 V) that is in excellent agreement with the experimental reduction peak.

Dataset Information

Potentiometric MRI of a Superconcentrated Lithium Electrolyte: Testing the Irreversible Thermodynamics Approach.

Publications

Potentiometric MRI of a Superconcentrated Lithium Electrolyte: Testing the Irreversible Thermodynamics Approach.

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