Mechanism of ion transport in PEO/LiTFSI complexes: Effect of temperature, molecular weight and end groups

Devaux, Didier; Bouchet, Renaud; Glé, David; Denoyel, Renaud

doi:10.1016/j.ssi.2012.09.020

Cited by 222 publications

(255 citation statements)

References 63 publications

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“…This is still much higher than PEO-based electrolytes wherein t + ss values are reported to be in the range of 0.1-0.4. 30 Furthermore, t + ss values for C8-DMC electrolytes are higher than those for C6-DMC electrolytes. In previous work, it was shown that t + ss for a 9.1 wt% LiTFSI in PFPE with DMC endgroups (MW = 1.1 kg/mol) was 0.9, 8 which is higher than that of 10 wt% LiTFSI in C8-DMC.…”

Section: Resultsmentioning

confidence: 92%

“…In contrast, t + ss of PEObased electrolytes decrease with increasing molecular weight. 30 The presence of specific interactions between PEO and Li cations is wellestablished, and this effect is used to describe the dependence of t + ss and molecular weight. 31 Our observations in PFPE electrolytes thus suggest the presence of specific interactions between the anions and the fluorinated backbone.…”

Section: Resultsmentioning

confidence: 99%

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Effect of Anion Size on Conductivity and Transference Number of Perfluoroether Electrolytes with Lithium Salts

Shah

Olson

Karny

et al. 2017

J. Electrochem. Soc.

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Mixtures of perfluoropolyethers (PFPE) and lithium salts with fluorinated anions are a new class of electrolytes for lithium batteries. Unlike conventional electrolytes wherein electron-donating oxygen groups interact primarily with the lithium cations, the properties of PFPE-based electrolytes appear to be dependent on interactions between the fluorinated anions and the fluorinated backbones. We study these interactions by examining a family of lithium salts wherein the size of the fluorinated anion is systematically increased: lithium bis(fluorosulfonyl)imide (LiFSI), bis(trifluoromethanesulfonyl)imide (LiTFSI) salts and lithium bis(pentafluoroethanesulfonyl)imide (LiBETI). Two short chain perfluoroethers (PFE), one with three repeat units, C6-DMC, and another with four repeat units, C8-DMC were studied; both systems have dimethyl carbonate end groups. We find that LiFSI provides the highest conductivity in both C6-DMC and C8-DMC. These systems also present the lowest interfacial resistance against lithium metal electrodes. The steady-state transference number (t + ss ) was above 0.6 for all of the electrolytes and was an increasing function of anion size. The product of conductivity and the steady-state transference number, a convenient measure of the efficacy of the electrolytes for lithium battery applications, exhibited a maximum at about 20 wt% salt in all electrolytes. Amongst the systems studied, LiFSI/PFE mixtures were the most efficacious electrolytes.

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Section: Resultsmentioning

confidence: 92%

Section: Resultsmentioning

confidence: 99%

Effect of Anion Size on Conductivity and Transference Number of Perfluoroether Electrolytes with Lithium Salts

Shah

Olson

Karny

et al. 2017

J. Electrochem. Soc.

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“…This is in agreement with literature and the reason for this observation is well-established. 2,3,64,65 At low salt concentrations, conductivity increases with increasing salt concentration due to an increase in the concentration of charged species. However, ion transport in polymer electrolytes is coupled to segmental motion, which slows down in the presence of salt due to interactions between ether oxygen atoms and lithium ions.…”

Section: Resultsmentioning

confidence: 99%

Negative Transference Numbers in Poly(ethylene oxide)-Based Electrolytes

Pesko

Timachova

Bhattacharya

et al. 2017

J. Electrochem. Soc.

185

367

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The performance of battery electrolytes depends on three independent transport properties: ionic conductivity, diffusion coefficient, and transference number. While rigorous experimental techniques for measuring conductivity and diffusion coefficients are well-established, popular techniques for measuring the transference number rely on the assumption of ideal solutions. We employ three independent techniques for measuring transference number, t + , in mixtures of polyethylene oxide (PEO) and lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) salt. Transference numbers obtained using the steady-state current method pioneered by Bruce and Vincent, t +,SS , and those obtained by pulsed-field gradient NMR, t +,NMR , are compared against a new approach detailed by Newman and coworkers, t +,Ne , for a range of salt concentrations. The latter approach is rigorous and based on concentrated solution theory, while the other two approaches only yield the true transference number in ideal solutions. Not surprisingly, we find that t +,SS and t +,NMR are positive throughout the entire salt concentration range, and decrease monotonically with increasing salt concentration. In contrast, t +,Ne has a non-monotonic dependence on salt concentration and is negative in the highly-concentrated regime. Our work implies that ion transport in PEO/LiTFSI electrolytes at high salt concentrations is dominated by the transport of ionic clusters. Energy density and safety of conventional lithium-ion batteries is limited by the use of liquid electrolytes comprising mixtures of flammable organic solvents and lithium salts. Polymer electrolytes have the potential to address both limitations. However, the power and lifetime of batteries containing solvent-free polymer electrolytes remain inadequate for most applications. The performance of electrolytes in batteries depends on three independent transport properties: ionic conductivity, σ, salt diffusion coefficient, D, and cation transference number, t + .1 The poor performance of batteries with polymer electrolytes is generally attributed to low conductivity, which is on the order of 10 −3 S/cm at 90• C for mixtures of polyethylene oxide (PEO) and lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) salt, 2,3 compared to that of liquid electrolytes which is 10 −2 S/cm at ambient temperatures. 4 Much of the literature in this field has been devoted to increasing the ionic conductivity of these materials.5-32 The purpose of our work is to shed light on another transport property of polymer electrolytes, the transference number.In a pioneering study, Ma and coworkers showed that the transference number of a mixture of PEO and a sodium salt is negative. 33Following this approach, others have obtained t + <0 in polymers containing lithium or sodium salts. [34][35][36] Nevertheless, the majority of reports for t + in polymer electrolytes fall between zero and one. [37][38][39][40][41][42][43][44][45][46][47][48][49][50][51] In contrast, all reports of t + in non-aqueous liquid electrolytes containi...

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“…Thus, our choice of 110 K as the reduced temperature is arbitrary; any value of T − T g would lead to a qualitatively similar dependence of σ r on r. Figure 5b shows σ r in PEO and P(2EO-MO) electrolytes as a function of r. At low salt concentrations (r < 0.06) PEO has a higher reduced conductivity compared to that of P(2EO-MO), while above r = 0.06 σ r in P(2EO-MO) electrolytes surpasses that of PEO. Reduced conductivity has been studied for a number of different electrolytes, 10,60,72,68 and P(2EO-MO)/ LiTFSI is the first system to exhibit a σ r greater than that of PEO/LiTFSI. A greater reduced conductivity could be attributed to improved lithium mobility, improved anion mobility, or a larger degree of dissociation between ions, leading to a greater number of effective charge carriers in the system.…”

Section: ■ Experimental Sectionmentioning

confidence: 99%

Optimizing Ion Transport in Polyether-Based Electrolytes for Lithium Batteries

et al. 2018

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ABSTRACT:We report on the synthesis of poly(diethylene oxidealt-oxymethylene), P(2EO-MO), via cationic ring-opening polymerization of the cyclic ether monomer, 1,3,6-trioxocane. We use a combined experimental and computational approach to study ion transport in electrolytes comprising mixtures of P(2EO-MO) and lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) salt. Mixtures of poly(ethylene oxide) (PEO) and LiTFSI are used as a baseline. The maximum ionic conductivities, σ, of P(2EO-MO) and PEO electrolytes at 90°C are 1.1 × 10 −3 and 1.5 × 10 −3 S/cm, respectively. This difference is attributed to the T g of P(2EO-MO)/ LiTFSI (−12°C), which is significantly higher than that of PEO/LiTFSI (−44°C) at the same salt concentration. Self-diffusion coefficients measured using pulsed-field gradient NMR (PFG-NMR) show that both Li + and TFSI − ions diffuse more rapidly in PEO than in P(2EO-MO). However, the NMR-based cation transference number in P(2EO-MO) (0.36) is approximately twice that in PEO (0.19). The transference number measured by the steady-state current technique, t +,ss , in P(2EO-MO) (0.20) is higher than in PEO (0.08) by a similar factor. We find that the product σt +,ss is greater in P(2-EO-MO) electrolytes; thus, P(2EO-MO) is expected to sustain higher steady-state currents under dc polarization, making it a more efficacious electrolyte for battery applications. Molecular-level insight into the factors that govern ion transport in our electrolytes was obtained using MD simulations. These simulations show that the solvation structures around Li + are similar in both polymers. The same is true for TFSI − . However, the density of Li + solvation sites in P(2EO-MO) is double that in PEO. We posit that this is responsible for the observed differences in the experimentally determined transport properties of P(2EO-MO) and PEO electrolytes.

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Mechanism of ion transport in PEO/LiTFSI complexes: Effect of temperature, molecular weight and end groups

Cited by 222 publications

References 63 publications

Effect of Anion Size on Conductivity and Transference Number of Perfluoroether Electrolytes with Lithium Salts

Effect of Anion Size on Conductivity and Transference Number of Perfluoroether Electrolytes with Lithium Salts

Negative Transference Numbers in Poly(ethylene oxide)-Based Electrolytes

Optimizing Ion Transport in Polyether-Based Electrolytes for Lithium Batteries

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