Phase Diagram, Conductivity, and Glass Transition of LiTFSI–H2O

Article Cite This: J. Phys. Chem. C 2018, 122, 16624−16629

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Phase Diagram, Conductivity, and Glass Transition of LiTFSI−H2O Binary Electrolytes Michael S. Ding* and Kang Xu

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U. S. Army Research Laboratory, Adelphi, Maryland 20854, United States ABSTRACT: The aqueous electrolyte system of lithium bis(trifluoromethanesulfonyl)imide, LiTFSI−H2O, was systematically and accurately measured for a complete liquid− solid phase diagram and an extensive set of data on electrolytic conductivity and glasstransition temperature. The conductivity data set was fitted with a VFT-based function (VFT: Vogel−Fulcher−Tammann), of which the three parameters were set to Laurent polynomial functions of composition. The fitting results were correlated with other quantities, and comparisons were made between the results of this study and those of other aqueous and carbonate electrolyte systems. The results of these measurements, correlations, and comparisons strongly suggest a decoupling of cationic conduction from the movement of the bulk solution, in sharp contrast to what has been observed in any nonaqueous electrolytes so far.

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INTRODUCTION This work is a follow-on to the one published earlier,1 where electrolytic conductivity and glass transition were studied for water solutions of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium trifluoromethanesulfonate (LiTf) in a 3:1 mol ratio, denoted as Li(TFSI0.75Tf0.25)−H2O hereafter. That work was motivated by the unusual and promising electrochemical properties of the superconcentrated aqueous solutions of these salts and their potential use for a moisturetolerant and nonflammable lithium-ion battery.2−5 The work also revealed signs of decoupling between the electrolytic conductivity and the flow of electrolyte liquid by correlating the glass-transition temperature, Tg, and the vanishing mobility temperature, T0, of the electrolytes at varying concentrations. This follow-on work focused on the LiTFSI−H2O binary system as one of the end-members of the LiTFSI−LiTf−H2O ternary system. In addition to the systematic measurements of conductivity and glass transition, a liquid−solid phase diagram was experimentally mapped using differential scanning calorimeters in the entire concentration range, thus significantly extending and refining the phase diagram published earlier on the same binary.6 Furthermore, the question of whether and to what degree the electrolytic conductivity is decoupled from the movement of the electrolyte liquid was addressed by studying the conductivity and the correlation between Tg and T0 of this binary system and by comparing them with those of Li(TFSI0.75Tf0.25)−H2O and other carbonate solutions of lithium salts.

by adding water or concentrated by adding salt successively to obtain solutions of desired concentrations. When necessary, small ovens were used to provide a heated environment to accelerate the dissolution of salt or homogenization of solution. All of these processes were carried out in a dry room, and all samples were prepared and stored in tightly capped glass vials to prevent water loss. For more concentrated solutions, special vials with high-temperature liner and cap were used. Measurement of Phase Transition and Glass Transition. Phase transition points and glass-transition temperature of a sample were determined at a slow heating rate of 2 °C/ min using two differential scanning calorimeters (DSC250 or MDSC 2920, both by TA Instruments). A liquid nitrogen cooler was used for low-temperature control, and calibration was performed using the standards of cyclohexane (−87.06 °C for a solid−solid transition and 6.45 °C for melting), indium (156.60 °C for melting), and tin (231.93 for melting). For differential scanning calorimetry (DSC) samples, about 10 mg of electrolyte liquid was enclosed in a pair of aluminum pan and lid (0219-0062, PerkinElmer Instruments) and hermetically sealed with a crimper (0219-0061, PerkinElmer). Vitrification of a sample was achieved by predipping the sample into liquid nitrogen and subsequently scanning it up through its glass transition. Crystallization of a sample that was otherwise hard to crystalize was assisted by adding a small amount of mesocarbon microbeads (MCMBs) into the DSC sample as a nucleating agent to induce the desired crystallization. Measurement of Electrolytic Conductivity. Electrolytic conductivity, κ, was measured using an in-house-designed computerized measurement system, which consisted of an Agilent E4980A precision LCR meter for impedance scan, a

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EXPERIMENTAL SECTION Sample Preparation. LiTFSI was purchased from the electrolyte branch of BASF (later known as Gotion) and used as received. The salt was dissolved in distilled water to first prepare a few starting solutions, which were then either diluted © 2018 American Chemical Society

Received: May 30, 2018 Published: June 28, 2018 16624

DOI: 10.1021/acs.jpcc.8b05193 J. Phys. Chem. C 2018, 122, 16624−16629

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transition, not the onset temperature as is commonly practiced for pure substances. The reason for this is to maintain consistency between the melting of a pure component and the liquidus transition of an adjacent solution. For the latter, the only reliable and workable way to evaluate its liquidus point is to use the peak temperature on a usually slow-changing prepeak thermal absorption curve without an onset point.7 This peak temperature, when the solution approaches the component in composition, transitions into the peak temperature of the melting of the component, not the onset point. The same is true for other processes toward which the liquidus point approaches, such as the eutectic and peritectic meltings. Therefore, all phase transition points plotted in Figure 1 are the peak temperatures of the associated processes, and the more important ones are numerically marked on the diagram, with the associated onset points given in parentheses for reference. It is also worth noting that by adding MCMB particles in the DSC samples, those compositions located in the “glass zone”6 near the eutectic composition were all successfully induced to crystallize and their phase transition temperatures thus accurately determined, as marked with the corresponding open dots in the phase diagram. The effectiveness of MCMB particles as a nucleating agent was first demonstrated and studied in the electrolytes of LiPF6 in a binary carbonate solvent8 and has since been found effective in nucleating crystals in other nonaqueous electrolytes9 as well as many additional carbonate electrolytes.10,11 The phase diagram in Figure 1 shows two distinct entities in the solid state: LiTFSI·(H2O)4 and LiTFSI·H2O. As the small lithium ions have a strong tendency to form stable solvates with small solvent molecules of high polarity and donicity, such as ethylene carbonate,12−14 acetonitrile,15 and water,16 we have assumed these two distinct entities to be ionic compounds between the TFSI− anion and a solvated Li+ cation and expressed them as (H2O)4Li·TFSI and H2OLi·TFSI in the phase diagram. However, as distinct as (H2O)4Li·TFSI seems to be in the solid state, the existence of this entity in the liquid state was evidenced by neither the κ values nor the Tg values (viscosity) measured and plotted across this composition, which were completely smooth and absent of any anomalies. To facilitate the use of the phase diagram in Figure 1 in the formulation and preparation of LiTFSI−H2O electrolytes, the part from 0 to 0.6 of mole fraction of LiTFSI is replotted in Figure 2 with only the phase lines and with an additional axis for molal concentrations in the units of mol kg−1. Electrolytic Conductivity and Glass Transition: Correlation, Comparison, and Discussion. Electrolytic conductivity, κ, of LiTFSI−H2O electrolytes of x mole fraction in salt, denoted here as xLiTFSI−(1 − x)H2O, was measured in the ranges of (0.001, 0.34) for x and (−15, 90) °C for temperature θ. The results are plotted as open dots in Figure 3, for a selected set of measurements for visual clarity, along with their fitting function plotted as curves, to demonstrate the range of their values and the closeness of the functional fit. This fitting function was developed by fitting to the entire set of measured κ values (excluding samples with any sign of precipitation) a VFT-based equation (VFT: Vogel−Fulcher− Tammann):17

Tenney Jr. environmental chamber for temperature control, and a set of conductivity cells for holding the electrolyte samples. These cells were made of a Pyrex cell body sealable with a ground-glass stopper and had a nominal cell constant of 0.1 cm−1. They were individually calibrated with a standard KCl solution of 100 mS cm−1 at room temperature. The measurement temperature, θ, ranged from 90 to −15 °C in 5 K decrement, stopping at each for an hour of thermal equilibration before a measurement. After the measurement at each set temperature, readings from five thermocouples placed near the conductivity cells were recorded and averaged to give the actual temperature for the conductivity values. Each conductivity measurement consisted of an impedance scan from 20 Hz to 2 MHz with an amplitude of 10 mV, yielding a Z′Z″ plot for the determination of the κ value.

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RESULTS Liquid−Solid Phase Diagram. The measured phase transition points of samples of varying concentrations are summarized as open dots in the phase diagram of Figure 1. These points have been identified on a DSC scan with the peak temperature of the thermal event associated with the phase

Figure 1. Liquid−solid phase diagram of the LiTFSI−H2O binary system, with the open dots plotting the measured phase transition points and solid line segments delineating the different phase fields. The nonvertical line segments are polynomial fits to the open dots in the appropriate segments. Phase fields are labeled in red with the two end compositions in equilibrium. Numbers for the associated phase transition temperature are given both as peak temperatures on the DSC scan and as onset points in parentheses, and numbers for composition are preceded with an x. The horizontal phase line at 148.9 °C signals a solid−solid phase transition in LiTFSI.

ln κ = A − 16625

E 1 1 ln T − a 2 R T − T0

(1)

DOI: 10.1021/acs.jpcc.8b05193 J. Phys. Chem. C 2018, 122, 16624−16629

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The Journal of Physical Chemistry C A = 9.45536 + 12.0879x − 99.353x 2 + 322.155x 3 − 293.051x 4

(2)

Ea 0.00122304 2.29676 = 255.59 − + + 934.038x 2 R x x + 14463.8x 3 T0 = 200.544 +

(3)

0.000233526 0.382306 − − 203.419x x x2 (4)

where the gas constant R = 8.314 J mol−1 K−1 and Ea/R and T0 are in the unit of K. The average fitting error was 0.014, equivalent to 0.34% of the data range. With these expressions, eq 1 is plotted as a surface in Figure 4 in the full measurement

Figure 2. Partial liquid−solid phase diagram of LiTFSI−H2O, constructed of only the phase lines in Figure 1 in the range of (0, 0.6) for mole fraction of LiTFSI, with an additional axis on top for the molal concentration readings in the units of mol kg−1. Figure 4. Electrolytic conductivity, κ, of xLiTFSI−(1 − x)H2O solutions, plotted as a surface against mole fraction x and temperature θ. The spheres are the measured values, intersected by their fitting surface of eq 1, with the parts above colored blue and parts below colored brown.

ranges of x and θ, along with all measured κ values used in the generation of the function, to demonstrate the overall closeness of the fit and the change of κ with simultaneous changes in x and θ. The surface shows the general feature of a declining ridge: κ decreases as θ lowers and it first increases and then decreases as x increases. This can be explained in general terms by the rise of electrolyte viscosity with lowering θ and rising x, in combination with the rising number of charge carriers but also ion pairs with rising x.18 To compare LiTFSI−H2O with Li(TFSI0.75Tf0.25)−H2O for their conductivities, the κ surface in Figure 4 is replotted with that of the latter system in Figure 5,1 which shows that the κ values of the two systems are quite close to each other, those of Li(TFSI0.75Tf0.25)−H2O being only slightly higher. This closeness was quite unexpected considering the significantly larger size of TFSI− than Tf− and the quarter portion of substitution of TFSI− by Tf− in the latter electrolyte. On account of the smaller Tf− either moving faster as a part of major current carriers or pairing more strongly with Li+ causing a fall in viscosity,19 one would expect to see significantly higher values of κ in the Li(TFSI0.75Tf0.25)−H2O system. It therefore follows that none of these two mechanisms was operative in the aqueous systems.

Figure 3. Electrolytic conductivity κ vs temperature θ for selected electrolytes of xLiTFSI−(1 − x)H2O of x mole fraction. The open dots represent the measured data, and the curves plot a fitting function.

where the three parameters, A, Ea, and T0 had been set to Laurent polynomial functions of x.10 This function was to serve as an accurate and efficient summary of the measured κ values, a ready source for κ-value lookups, and a functional representation of κ in the space of (x, T) for observation of changes in κ with simultaneous changes of x and T and for overall comparison of κ with other systems. With the aim of a good fit and a relatively simple representation, the fitting yielded the following parametric functions: 16626

DOI: 10.1021/acs.jpcc.8b05193 J. Phys. Chem. C 2018, 122, 16624−16629

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electrolyte of the same concentration in mole fraction as a PC electrolyte has a much higher molal concentration. As an example, at x = 0.19, the molality of a salt−H2O electrolyte is 13.02 mol kg−1, whereas that of a salt−PC is only 2.30. Another interesting observation on these aqueous electrolytes is that the ridgelines on the κ(x, θ) surface of Figure 5 stay quite constant in composition as θ lowers, in sharp contrast to the carbonate electrolytes of lithium salts. To make this more clearly, the κ surface in Figure 4 is replotted in part as a contour plot in Figure 7, along with its ridgeline (thick

Figure 5. Conductivity surfaces of κ for the electrolyte systems of xLiTFSI−(1 − x)H2O (yellow) and xLi(TFSI0.75Tf0.25)−(1 − x)H2O (gray), plotted together for comparison, in the space of mole fraction x and temperature θ.

To compare the κ values of the aqueous systems with those of carbonate electrolytes previously measured and to demonstrate how much more conductive the aqueous systems are, especially at higher x and lower θ, ratios κ of LiTFSI−H2O over those of Li(TFSI0.75Tf0.25)−H2O (green),1 LiPF6−PC (red),18 and LiBF4−PC (blue)20 are plotted against θ in Figure 6, at the compositions of x = 0.19 (solid), 0.17 (long dash), Figure 7. Contour plot of κ of xLiTFSI−(1 − x)H2O in the space of mole fraction x and temperature θ, with the legends in the units of mS cm−1, constructed out of a portion of the κ-surface in Figure 4, together with its ridgeline plotted in a thick gray line. For comparison, corresponding ridgelines of Li(TFSI0.75Tf0.25)−H2O, LiPF6−PC, LiBF4−PC, and LiBOB−PC are plotted in the lines of green, red, blue, and purple, respectively.

gray line). For comparison, the corresponding ridgeline of Li(TFSI0.75Tf0.25)−H2O is plotted in green,1 and those of PC solutions of LiPF6,18 LiBF4,20 and lithium bis(oxalate)borate (LiBOB)21 are plotted in red, blue, and purple, respectively. It can be seen that the ridgelines of the two aqueous systems drop down quite vertically until the low-θ end, where they actually turn slightly toward higher x (more so for LiFTSI− H2O). In contrast, those of the carbonate electrolytes of lithium salts turn much more sharply in the lower x direction. As the latter behavior is likely due to the increasing dominance of viscosity over the number of charge carriers at lower θ,18 this contrast seems to indicate that under higher viscosity conditions, the ionic conduction in the aqueous systems is not as dependent on the viscosity of the electrolyte liquid as in the carbonate systems. Further evidence for this higher degree of separation between the ionic conductivity and the flow of electrolyte liquid in the aqueous systems can be obtained from correlating their Tg with their T0 and by comparing the correlations with those of the carbonate systems. As can be seen in Figure 8, where measured Tg values are plotted with the corresponding T0 values obtained from fitting to the measured κ data, T0 diverges in the direction opposite to that of Tg as x rises in the aqueous systems, which is particularly pronounced with the LiTFSI−H2O system. In contrast, in all carbonate electrolytes,

Figure 6. Ratios of conductivity κ of xLiTFSI−(1 − x)H2O over those of xLi(TFSI0.75Tf0.25)−(1 − x)H2O (green), xLiPF6−(1 − x)PC (red), and xLiBF4−(1 − x)PC (blue), with x being 0.19 (solid), 0.17 (long dash), 0.15 (medium dash), and 0.13 (short dash). Complete legends for the three lower concentrations are shown only for red but implied for green and blue.

0.15 (medium dash), and 0.13 (short dash). Here, LiPF6, LiBF4, and PC stand for lithium hexafluorophosphate, lithium tetrafluoroborate, and propylene carbonate, respectively. It can be seen that the two aqueous systems have comparable conductivities, consistent with Figure 5, but are, for the most part, an order of magnitude more conductive than the carbonates. Furthermore, this ratio grows with higher x and lower θ (conditions of higher viscosity) and reaches over 100 for LiPF6−PC at x = 0.19 and θ = −10 °C. Incidentally, because of the much lower weight of H2O than PC, an aqueous 16627

DOI: 10.1021/acs.jpcc.8b05193 J. Phys. Chem. C 2018, 122, 16624−16629

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suggest that while relatively stable hydrated Li+ (mostly quadruply hydrated) largely determined the electrolytic conductivity, pairing of Li+ by either TFSI− or Tf− was probably too weak to be of any influence. As a consequence, the Li+ contribution to the overall ion conductivity should prevail, as evidenced by the high Li+-transference number (0.73) measured in a previous work.16 These observations of (1) a greatly enhanced κ in the aqueous systems relative to the nonaqueous, (2) the enhancement becoming more pronounced under the conditions of higher viscosity, (3) an anion independency of κ in the aqueous systems, and (4) a decoupling of κ from the flow of electrolyte liquid in the aqueous systems can be consistently explained by a recently proposed nanoheterogeneity model for these concentrated aqueous electrolytes, in which molecular dynamics simulation and other means were used to gather evidential support for the formation in the concentrated LiTFSI−H2O electrolytes a nanoheterogeneous liquid structure of two interpenetrating, dynamic nanodomain networks: one of clusters of highly H2O-solvated Li+ providing a 3D percolating channel for fast Li+ transports resulting in a high Li+ transference number and the other of TFSI-rich domains largely immobilizing the anion movement and thus hindering the flow of the electrolyte liquid.16 The existence of such a Li+conducting channel in this model could account for observations (1) and (3), whereas observations (2) and (4) could be explained by the coexistence of the channel with the TFSI-rich network, imparting rigidity (viscosity) to the electrolyte liquid. What remain to be seen are how the system of LiTf−H2O with a significantly smaller anion would fit into this scheme and how the model would be affected by the replacement of a portion of TFSI− by Tf−, in order to account for the proximity in Tg for the two aqueous systems.

Figure 8. Comparison of Tg and T0 trend lines in salt concentration for the electrolyte systems of LiTFSI−H2O (black dots and curves), Li(TFSI0.75Tf0.25)−H2O (green), LiBOB−PC (purple), LiPF6−PC (red), and LiBF4−PC (blue). All Tg values have been measured, and the T0 values have been obtained by fitting the VFT equation to the corresponding measured κ values, except LiTFSI−H2O, for which the T0 and Ea curves plot eqs 4 and 3, respectively, derived from fitting a VFT-based equation to the entire set of measured κ values.

the T0 curves trend with the Tg curves as x rises, even though the two sets still diverge. This opposite trending of T0 from Tg seems to be compelling for the support of a decoupling of ionic movement from the movement of bulk solution in the aqueous systems. The Tg curve of LiTFSI−H2O in Figure 8 shows a steady decline as x increases from about 0.08 to 0.15, deviating from the general trends of Tg rising with increasing x as seen in the carbonate electrolytes in the figure. This anomaly likely reflects a sustained period of change in the structure of the electrolyte liquid as more salt is added, accompanied by a corresponding decline in viscosity. In fact, the Tg of LiTFSI−H2O starts out in the company of the carbonates but, after the period of decline, ends up much below those of the carbonates. The corresponding fall in viscosity most likely contributed to the high values of κ in the concentrated aqueous electrolytes. Also worth noticing in Figure 8 is the closeness in values between the two sets of Tg points for the aqueous systems. In the carbonate electrolytes of lithium salts, Tg usually rises with larger anions, as demonstrated in the upshifting of the Tg curves from LiBF4 to LiPF6 to LiBOB plotted in the figure. In contrast, the Tg curve of the LiTFSI−H2O system with a larger anion is slightly below that of Li(TFSI0.75Tf0.25)−H2O with an effective smaller anion. In the carbonate electrolytes, the increase in viscosity, which scales with Tg, with larger anions as seen in Figure 8 was partially accounted for by the weaker pairing between the larger anions and lithium cations, which leaves more “free” cations to compete for solvation by the solvent molecules, thus effectively increasing the salt concentration.19 The fact that neither the conductivity (Figure 5) nor the viscosity (Tg in Figure 8) of the aqueous systems was much affected by the type of anions present seemed to

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CONCLUSIONS The electrolyte system of xLiTFSI−(1 − x)H2O (x in mole fraction) was experimentally studied for its phase behavior, glass transition, and electrolytic conduction at various temperatures θ and compositions x, resulting in a complete liquid−solid binary phase diagram, a set of glass-transition points Tg in the range of (0.04, 0.26) for x, and a set of accurately determined conductivities κ in the ranges of (0.001, 0.34) for x and (−15, 90) °C for θ. The latter set was further fitted in its entirety with a VFT-based equation, of which the three parameters were set to Laurent polynomial functions of x. The fitting yielded a function κ(x, θ), an accurate and efficient representation of the measured κ data, and T0(x), the vanishing mobility temperature T0 as a function of x. Compared to Li(TFSI0.75Tf0.25)−H2O, the LiTFSI−H2O system had similar κ(x, θ) surface and Tg values. Compared to carbonate electrolytes, its κ values were much higher, especially at higher x and lower θ, and with increasing x, its T0 diverged from its Tg much more drastically, all pointing to a certain degree of decoupling between the electrolytic conductivity and the flow of electrolyte liquid. Most of these observations can be consistently accounted for with the use of a recently proposed model of nanoheterogeneity structure formed in the concentrated LiTFSI−H2O electrolyte liquids.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 16628

DOI: 10.1021/acs.jpcc.8b05193 J. Phys. Chem. C 2018, 122, 16624−16629

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(18) Ding, M. S.; Jow, T. R. Conductivity and Viscosity of PC-DEC and PC-EC Solutions of LiPF6. J. Electrochem. Soc. 2003, 150, A620− A628. (19) Ding, M. S.; Jow, T. R. How Conductivities and Viscosities of PC-DEC and PC-EC Solutions of LiBF4, LiPF6, LiBOB, Et4NBF4, and Et4NPF6 Differ and Why. J. Electrochem. Soc. 2004, 151, A2007− A2015. (20) Ding, M. S.; Jow, T. R. Conductivity and Viscosity of PC-DEC and PC-EC Solutions of LiBF4. J. Electrochem. Soc. 2004, 151, A40− A47. (21) Ding, M. S.; Xu, K.; Jow, T. R. Conductivity and Viscosity of PC-DEC and PC-EC Solutions of LiBOB. J. Electrochem. Soc. 2005, 152, A132−A140.

Michael S. Ding: 0000-0002-9302-1032 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding from U. S. Army Research Laboratory is gratefully acknowledged. REFERENCES

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