Article Cite This: J. Phys. Chem. C 2018, 122, 4144−4149
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Diffusion of Lithium Cation in Low-Melting Lithium Molten Salts Keigo Kubota,*,† Zyun Siroma,† Hikaru Sano,† Susumu Kuwabata,†,‡ and Hajime Matsumoto*,†,‡ †
Department of Energy and Environment, Research Institute of Electrochemical Energy, National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan ‡ Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan ABSTRACT: The self-diffusion coefficients of the lithium cation (D(Li+)) and counteranion (D(anion)) in the molten lithium amide, such as lithium bis(fluorosulfonyl)amide (Li[FSA]) and lithium fluorosulfonyl(trifluoromethylsulfonyl)amide (Li[FTA]), were measured by a pulsed-gradient spin− echo nuclear magnetic resonance method at 150 °C. In the relationships between viscosity and the resulting self-diffusion coefficient (the D(Li+) is 1.4 × 10−11 m2·s−1 and the D(anion) is 5.5 × 10−12 m2·s−1 for the Li[FSA] and the D(Li+) is 1.7 × 10−12 m2·s−1 and the D(anion) is 6.6 × 10−13 m2·s−1 for the Li[FTA]), the Li[FTA] and Li[FSA] have a stronger intercalation between ions than the high-temperature lithium molten salts. However, their ionic conductivities are higher than those estimated by D(Li+) and D(anion); specifically, that of the Li[FTA] is three times higher. Therefore, the Li[FTA] would have special conductive behavior, which leads to the superionic nature of the Walden plot and the high rate performance against much higher viscosity in lithium secondary battery. Such unique conduction and battery behaviors using the Li[FTA] might be due to the robust and asymmetric structure of the FTA−.
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INTRODUCTION The liquid electrolyte of a lithium secondary battery is usually composed of a lithium salt and an organic solvent.1 In general, the organic solvent must be able to dissolve the lithium salts, most of which have melting points above 200 °C, but the organic solvent has some problems, such as flammability and high vapor pressure. Recently, a solid electrolyte has been developed to improve the safety and durability of the battery.2 However, the liquid electrolytes leave a margin for improvement considering their productivity and compatibility with composite electrodes, which cannot be combined with the solid electrolytes. To improve the safety of the liquid electrolytes, room-temperature ionic liquids (RTILs) have attracted much interest due to their nonflammability and low vapor pressure.3 On the basis of such a background, we have attempted to develop a low-melting lithium molten salt, which melts below the melting point of the lithium metal (180 °C), as the electrolyte without any organic solvent or organic cation of the RTIL. It is ultimately a simple constructions like a liquid electrolyte, which is composed of only a lithium cation (Li+) and counteranion. We reported that a lithium (fluorosulfonyl)(trifluoromethylsulfonyl)amide with a structural asymmetry (Li[FTA], Li[FSO2(CF3SO2)N]) formed a molten salt electrolyte above the melting point of 100 °C, which might be the lowest value of all of the reported robust amide anions.4,5 It showed a stable charge and discharge with the lithium metal or composite electrodes without any organic solvents, RTILs, and the other additive salts at 110 °C, and its rate performance was better than that of an organic electrolyte at 25 °C.5 This result © 2018 American Chemical Society
is contrary to the high viscosity and poor ionic conductivity of the Li[FTA] (17 000 mPa·s and 0.2 mS·cm−1 at 110 °C) based on the organic solvents at 25 °C (usually four orders of magnitude lower viscosity and three orders of magnitude higher conductivity than the Li[FTA]). The Li[FTA] can be used at 45 °C by the addition of a cesium salt with the same FTA anion.6 The cation mixture showed a better rate performance than the organic electrolyte at the same temperature (65 °C).6 These molten salt electrolytes are expected to be the electrolytes of novel molten salt batteries operating from around room temperature to over 100 °C. To elucidate the unique rate performance versus the poor transport property, the self-diffusion coefficients (D) of the Li+ (D(Li+)) and counteranion (D(Li+)) for the Li[FTA] are required because it is a significant parameter for the mobility of ions and intimately interrelated with the viscosity and ionic conductivity. The D values for the various electrolytes of the lithium secondary battery, which are a mixture of the lithium salts and solvents, such as organic solvent or RTILs, were reported in many papers7−10 by a pulsed-gradient spin−echo nuclear magnetic resonance (PGSE-NMR) method at room temperature. In contrast, the D values of the conventional lithium molten salts at high temperature (above 300 °C), such as halides or nitrates used in a thermal battery,11 were mostly measured by a capitally method12 or molecular dynamics Received: November 15, 2017 Revised: February 1, 2018 Published: February 1, 2018 4144
DOI: 10.1021/acs.jpcc.7b11281 J. Phys. Chem. C 2018, 122, 4144−4149
Article
The Journal of Physical Chemistry C
Figure 1. 7Li NMR and 19F-NMR spectra of (a) 7Li of Li[FSA], (b) 19F of Li[FSA], (c) 7Li of Li[FTA], and (d) 19F of Li[FTA].
references were a LiCl aqueous solution (1 mol·dm−3) for 7Li at 0 ppm and hexafluorobenzene (C6F6) for 19F at −164.9 ppm. The values of the parameters for the pulse sequence of the Li[FSA] are G = 0.1 to 0.8 T·m−1 for the strength of the gradient pulse, δ = 5−10 ms for the pulse width, and Δ = 600− 1500 ms for the diffusion time corresponding to the interval between the two gradient pulses. For the Li[FTA], the values of the parameters are G = 0.1 to 0.43 T·m−1, δ = 5−10 ms, and Δ = 200−1000 ms. The diffusion coefficients were obtained by a regression analysis of the signal attenuation curve according to the Stejskal equation.20 The 7Li NMR and 19F NMR spectra of the Li[FSA] and Li[FTA] are shown in Figure 1. For the 7Li NMR, a single peak of the Li+ at 1.57 ppm was observed in each salt. For the 19F NMR, a single peak of the FSO2 side chain at 56.6 ppm was observed in the Li[FSA]. On the contrary, the same peak of the FSO2 side chain at 59.8 ppm and another peak at −73.7 ppm, attributed to the CF3SO2 side chain, were observed for the Li[FTA]. These results well agreed with those of ionic liquids containing the FTA anion.
simulation.13,14 Recently, the PGSE-NMR method was performed at high temperature for a few alkali metal molten salts.15−17 We report the D of the Li+ (D(Li+)) and counteranion (D(Li+)) for the Li[FTA] measured by PGSE-NMR at 150 °C. Furthermore, we also measured another robust and small amide such as a lithium bis(fluorosulfonyl)amide (Li[FSA], Li[(FSO2)2N]) with a relatively low-melting lithium salt (m.p.: 140 °C),18 to investigate the effect of the structural asymmetry of amide anions on the transport property of the lithium singlemolten salt system. We discuss these results and the other transport properties, such as the viscosity and ionic conductivity, using the Stokes− Einstein equation and Nernst−Einstein equation. These results of the Li[FSA] and Li[FTA] were then compared with those of the high-temperature lithium molten salts, the other lowmelting lithium molten salts (lithium ionic liquids10) synthesized from LiBH4, oligoethylene glycol monomethyl ether (n = 7.2), and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) (Li[(CH3(OCH2CH2)7.2O)2(H(CF3)2CO)2B]) or pentafluorophenol (PFP) (Li[(CH3(OCH2CH2)7.2O)2(C6F5O)2B]), and mixtures of the RTILs or organic solvents.
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RESULTS AND DISCUSSION Diffusion Coefficient of Lithium Molten Salts. For the measurement of the D by the PGSE-NMR at high temperature, the first point to confirm is that no convection occurred in the sample during the measurement.16 The dependence of the selfdiffusion coefficient of Li+ (D(Li+)) and counteranions (D(anion)) in Li[FSA] and Li[FTA] on Δ at δ = 7 ms is shown in Figure 2. If the convection occurs, then D may decrease with the increase in Δ. On the basis of the result, the D(Li+) and D(anion) are independent of Δ. Therefore, there was no effect of convection that occurred in this measurement. The D(Li+) is 1.4 × 10−11 m2·s−1 and the D(anion) is 5.5 × 10−12 m2·s−1 for the Li[FSA], and the D(Li+) is 1.7 × 10−12 m2· s−1 and the D(anion) is 6.6 × 10−13 m2·s−1 for the Li[FTA]. They are average values of the NMR results for the various parameters in which the relative uncertainty was
