Direct determination of ionic transference numbers in ionic liquids by electrophoretic NMR

Gouverneur, Martin; Kopp, Jakob; Wüllen, Leo van; Schönhoff, Monika

doi:10.1039/c5cp05753a

Cited by 115 publications

(146 citation statements)

References 46 publications

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“…The findings will facilitate modelling of full supercapacitor devices, such that the interplay between structure and ion transport can be understood across the full range of lengthscales. Future measurements will be made to establish how ionic migration in an electric field differs from equilibrium self-diffusion, [46][47][48] such that the non-equilibrium states reached at the fastest charging/discharging rates can be probed. Only then will we begin to fully unravel the relationships between materials properties and performance in supercapacitors.…”

Section: Resultsmentioning

confidence: 99%

Direct observation of ion dynamics in supercapacitor electrodes using in situ diffusion NMR spectroscopy

et al. 2017

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Ionic transport inside porous carbon electrodes underpins the storage of energy in supercapacitors and the rate at which they can charge and discharge, yet few studies have elucidated the materials properties that influence ion dynamics. Here we use in situ pulsed field gradient NMR spectroscopy to measure ionic diffusion in supercapacitors directly. We find that confinement in the nanoporous electrode structures decreases the effective self-diffusion coefficients of ions by over two orders of magnitude compared to neat electrolyte, and in-pore diffusion is modulated by changes in ion populations at the electrode-electrolyte interface during charging.Electrolyte concentration and carbon pore size distributions also affect in-pore diffusion and the movement of ions in and out of the nanopores. In light of our findings we propose that controlling the charging mechanism may allow the tuning of the energy and power performances of supercapacitors for a range of different applications.As renewable energy and green technologies such as electric vehicles become prevalent, we must develop new ways to store and release energy on a range of timescales. Rechargeable batteries are ideal for timescales of minutes or hours (electric cars, portable electronic devices, grid storage etc.), while supercapacitors are more promising for second or sub-second timescales and are increasingly being used for transport applications where rapid charging and discharging are required. The superior power handling and cycle lifetime of supercapacitors comes at the expense of energy density, with recent materials-driven research aiming to address this issue by fine-tuning the nanoporous structure of the carbon electrodes, 1,2 and by using ionic liquid electrolytes that are stable at higher voltages. 3,4 Both approaches have afforded some increases in energy density, though not without sacrificing power density. The delicate balance between energy and power density must be understood if supercapacitors are to be used in a wide range of applications.Fundamental studies based on spectroscopic, 5-14 and theoretical, 15-18 methods have recently revealed the complex nature of charging in supercapacitors. Prior to charging, the electrode pores contain a large number of electrolyte ions, 15,19,20 and as a result charge storage is generally more complex than simple counter-ion adsorption (counter-ions are defined as having charge opposite to the electrode in which they are located). [5][6][7]15 A range of different charging mechanisms can operate

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Section: Resultsmentioning

confidence: 99%

Direct observation of ion dynamics in supercapacitor electrodes using in situ diffusion NMR spectroscopy

et al. 2017

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“…Thedifferences in transference numbers obtained by electrochemical and NMR methods may be related to the presence of ion pairs, large ion clusters,oraggregates. [17] Nevertheless,both results strongly suggest that the intramolecular and intermolecular interactions of salt anions and salt-anion-polymer matrices via hydrogen bonding impedes the migration of anions under an electric field and the replacement of Fatoms in TFSI anion with hydrogen could effectively improve the T Li þ values.A s depicted in Figure 3b,d Theh igh anodic stability of lithium salts has long been treated as one of the most critical parameters in screening suitable anions,s ince the operating voltages of conventional LIBs are higher than 4Vvs.Li/Li + . [7] To evaluate the anodic stability of those H-containing anions,adiluted solution of LiX/propylene carbonate (PC) was chosen for the following reasons:1 )the high dielectric constant of PC (e = 64.9 at 25 8 8C [7a] ), which prompts the dissolution of lithium salts;2)the good electrochemical stability of PC towards oxidation, which helps to determine the stability of the anions without the interference of solvent oxidation; [7a] and 3) the low concentration of lithium salts could minimize side effects that result from the increased viscosity upon their addition (for example, al imited diffusion coefficient and sluggish electrochemical kinetics).…”

Section: Communicationsmentioning

confidence: 96%

Enhanced Lithium‐Ion Conductivity of Polymer Electrolytes by Selective Introduction of Hydrogen into the Anion

et al. 2019

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The anion chemistry of lithium salts plays a pivotal role in dictating the physicochemical and electrochemical performance of solid polymer electrolytes (SPEs), thus affecting the cyclability of all‐solid‐state lithium metal batteries (ASSLMBs). The bis(trifluoromethanesulfonyl)imide anion (TFSI−) has long been studied as the most promising candidate for SPEs; however, the Li‐ion conductivities of the TFSI‐based SPEs still remain low (Li‐ion transference number: ca. 0.2). In this work, we report new hydrogen‐containing anions, conceived based on theoretical considerations, as an electrolyte salt for SPEs. SPEs comprising hydrogen‐containing anions achieve higher Li‐ion conductivities than TFSI‐based ones, and those anions are electrochemically stable for various kinds of ASSLMBs (Li–LiFePO4, Li–S, and Li–O2 batteries). This opens up a new avenue for designing safe and high‐performance ASSLMBs in the future.

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“…[31][32][33][34][35] If t eNMR þ ¼ 0:5, the diffusion potential is zero. Recently, Schönhoff and coworkers have applied eNMR to determine the transference number of cations and anions in various pure ionic liquids [36] and of lithium ions in a variety of electrolytes. [37][38] Since electrophoretic NMR is not a very common technique, pulsed-field gradient NMR is often used to determine the selfdiffusion coefficients of the ions and to calculate a related parameter, the transport number t PFG þ .…”

Section: Liquid Electrolytes With Two Types Of Mobile Ions: Transportmentioning

confidence: 99%

Dynamic Ion Correlations in Solid and Liquid Electrolytes: How Do They Affect Charge and Mass Transport?

2019

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Solid and liquid electrolytes for electrochemical energy storage and conversion cells, such as batteries, supercapacitors and fuel cells, contain often high concentrations of mobile ions. Therefore, the ion dynamics in these electrolytes is characterized by pronounced directional correlations between successive ion movements, which exert a strong influence on charge and mass transport. In this manuscript, we review the relevant transport properties of (i) single-ion conducting solid electrolytes and of (ii) liquid electrolytes with a single type of cations and a single type of anions. All transport quantities are based on Onsager's linear irreversible thermodynamics and are defined in the laboratory frame of reference, so that they can be easily related to correlations functions of the equilibrium ion dynamics by means of linear response theory. In the case of single-cation conducting solid electrolytes, we discuss how the complex interplay between cation-cation and cation-lattice interactions leads to a competition between cation self-correlations and distinct-cation correlations. In the case of liquid electrolytes, we describe how cation-cation, anion-anion, and cation-anions correlations influence the various transport quantities, such as total ionic conductivity, Haven ratio, salt diffusion coefficient and cation transference numbers. Moreover, we discuss how, in dilute liquid electrolytes, ion correlations can be governed by ion pair formation. Finally, the competition between cation/ anion and cation/polymer chain interactions can lead to negative cation transference numbers in polymer electrolytes, i. e. to cations migrating towards the positive electrode. Taken together, these case studies showcase how ion correlations in electrolyte systems can strongly influence the overall efficiency of energy storage and conversion devices due to transport limitations.[a] Dr.

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Direct determination of ionic transference numbers in ionic liquids by electrophoretic NMR

Cited by 115 publications

References 46 publications

Direct observation of ion dynamics in supercapacitor electrodes using in situ diffusion NMR spectroscopy

Direct observation of ion dynamics in supercapacitor electrodes using in situ diffusion NMR spectroscopy

Enhanced Lithium‐Ion Conductivity of Polymer Electrolytes by Selective Introduction of Hydrogen into the Anion

Dynamic Ion Correlations in Solid and Liquid Electrolytes: How Do They Affect Charge and Mass Transport?

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