The physicochemical properties of a [DEME][TFSI] ionic liquid-based electrolyte

2019-03-27 09:05:53 adman 1

3.1. Physicochemical properties of the electrolyte mixtures

To understand the basics behind the characteristics of the tested electrolytes, we first measured the physicochemical properties of the binary mixtures without added salt. For this, eight different mixtures of [DEME][TFSI] and DOL were prepared evenly on a molar scale. The conductivity of the binary mixtures without added salt first increases on increasing the ionic liquid content due to adding more charge carriers. This continues until a decline is caused due to high viscosity (Fig. 1a). According to expectations, higher temperatures lead to higher conductivities, but peak conductivity remains at the same composition regardless of the temperature. A maximum conductivity is reached around the molar ratio X[DEME][TFSI] = 0.2.

Fig. 1

Fig. 1. The physicochemical properties of [DEME][TFSI]:DOL mixtures without LiTFSI salt at various molar ratios and temperatures between 278.15 K and 313.15 K: a) conductivity, b) dynamic viscosity, c) density.

Viscosity is a function of temperature and ionic liquid content and increases from around 0.5 mPa·s to over 200 mPa·s for the higher ratios of ionic liquid measured at low temperatures. At high molar ratios of [DEME][TFSI], a temperature increase causes a greater increase of viscosity than at lower molar ratios (Fig. 1b). The density has a steeper increment for low ionic liquid contents, which levels out with a low dioxolane content. Heating the mixtures decreases the density by approximately the same amount in all the different molar ratios tested (Fig. 1c). Data for these measurements is included in the Supporting information file in Tables S1-S3.

For further tests, mixtures around the maximum conductivity were chosen (X = 0.101, 0.199, 0.401) and three different concentrations of LiTFSI salt were prepared (0.1 M, 0.5 M, 1.0 M). The physicochemical properties of these nine electrolytes were tested in the same way as described above and the data can be found in Tables S4-S6, while for clarity only the values at room temperatures (298.15 K) are shown in Fig. 2. For comparison, the values for electrolytes with the same molar ratios without LiTFSI addition are also depicted.

Fig. 2

Fig. 2. The physicochemical properties of [DEME][TFSI]:DOL:LiTFSI mixtures at 298.15 K: a) conductivity, b) dynamic viscosity, c) density.

The conductivity change due to an increase in ionic liquid content at different salt concentrations exhibits a different trend (Fig. 2a). This result is coherent with the conductivity dependence on ionic liquid content (Fig. 1a). At low LiTFSI salt concentrations, the conductivity maximum is still X = 0.199. A further increase in LiTFSI molarity exhibits the same change as increasing the molar ratio of the ionic liquid − a decrease in the conductivity of the electrolyte. Consequently, the electrolyte with high LiTFSI concentrations has the highest conductivity at the lowest ratio of ionic liquid. Here we should emphasize that the conductivity values represent both anion and cation migration.

The viscosity (Fig. 2b) and density (Fig. 2c) of all the electrolyte solutions increase with high ionic liquid ratios and high LiTFSI salt concentrations. If we compare the change influenced by increasing the lithium salt concentration while maintaining the same molar content of the ionic liquid, we see that with density, the absolute increase is approximately the same with all the different ionic liquid contents tested (Fig. 2c). On the contrary, with viscosity, the absolute difference in the same scenario increases on increasing the ionic liquid content.

3.2. Polysulfide solubility determination with impedance spectroscopy

Polysulfides dissolved during battery operation change the physicochemical properties of the electrolyte. To test how the difference in polysulfide solubility influences the electrochemical behavior, we performed electrochemical impedance spectroscopy measurements on freshly assembled cells and on charged cells after 10 cycles. The resistive intercept values were obtained and their inverse values (conductance) are plotted in Fig. 3.

Fig. 3

Fig. 3. The conductance of different electrolyte mixtures determined from resistive intercept measurements with EIS: a) freshly assembled battery cells with no polysufides dissolved, b) after 10th charge.

The resistive intercepts of fresh batteries were compared to the conductivity measurements of the electrolytes (Fig. 2a). The conductance trends are in good agreement with the obtained conductivities, although the values cannot be directly compared since the “cell constant” was not determined and the pouch cell setup is not sufficiently advanced to eliminate the measurement disturbance effects [26]. No attempt was made to differentiate between the resistance of the electrolyte in the cathode and separator pores. We attribute the small discrepancies with the data from electrolytes with X[DEME][TFSI] = 0.2 to not regulating the temperature conditions for battery cell measurement.

After 10 cycles of discharge and charge accompanied by polysulfides dissolution and diffusion, the resistive intercept was measured again. Large change can be observed in the resistance of the electrolytes with low LiTFSI salt concentration and for electrolytes with high dioxolane content (Fig. 3b). For these electrolyte compositions, higher polysulfide solubility is proposed. Comparison of the conductivity values obtained from the resistive intercept between fresh and cycled cells show the least difference when 1.0 M electrolyte was used. This is in agreement with papers showing that highly concentrated electrolytes can effectively prevent polysulfides dissolution [27], [28]. The least change is observed with high [DEME][TFSI] contents, which supports the claim of lower polysulfide solubility in ionic liquid electrolytes [8].

3.3. The connection between the physicochemical properties and electrochemical performance:

The comparison of the electrochemical performance of the nine different mixtures of electrolytes was done on two different bases. We compared the change due to the difference in ionic liquid content at the same concentration of LiTFSI salt and the opposite, the influence of changing the concentration of the lithium salt in the same [DEME][TFSI]:DOL mixture. The Coulombic efficiencies, specific capacities and overpotentials are compared.

Coulombic efficiencies range from close to 90 % for electrolytes with low LiTFSI salt concentrations to almost 100 % for 1.0 M LiTFSI as evident from Fig. 4a–c. A uniform trend is also visible in Figures 4d-4f, from which it can be concluded that a higher dioxolane content also equals a lower Coulombic efficiency. These results can be linked to the polysulfide solubility (Fig. 3b). The electrolytes, which poorly solvate polysulfides, exhibit a smaller extent of the polysulfide shuttle, which in turn results in better charging efficiencies. In this study, the difference in the passivation nature of lithium by using two different solvents has a minor impact as can be deduced from the observed results.

Fig. 4

Fig. 4. Coulombic efficiencies during 100 cycles at C/10 current rate: a-c show a comparison of electrolytes with the same ionic liquid molar content and different concentrations of LiTFSI salt with a) X = 0.101, b) X = 0.199, c) X = 0.401. d-f show a comparison of the performance of electrolytes with the same LiTFSI concentration with d) 0.1 M LiTFSI, e) 0.5 M LiTFSI, f) 1.0 M LiTFSI.

The overpotential contains the contributions of the ohmic, activation and concentration polarization. [29] The values were calculated as half the difference of the potentials difference between the charge and discharge curves at approximately 50 % DOD for a given battery cell experiment. They range from 50 mV to 170 mV. The trend is different to the one for cycling efficiencies. The largest overpotentials are exhibited for electrolytes with low lithium salt concentrations (Fig. 5a–c) and with high ionic liquid content (Fig. 5d–f). For the latter, we attribute this difference to the difference in viscosity.

Fig. 5

Fig. 5. Overpotentials over 100 cycles at C/10 current rate: a-c show a comparison of electrolytes with the same ionic liquid molar content and different concentrations of LiTFSI salt with a) X = 0.101, b) X = 0.199, c) X = 0.401; d-f show a comparison of the performance of electrolytes with the same LiTFSI concentration with d) 0.1 M LiTFSI, e) 0.5 M LiTFSI, f) 1.0 M LiTFSI.

For a better understanding of the reasons behind the overpotential value trends for electrolytes with the same ionic liquid content and different Li+ salt concentrations, we also compared the impedance spectra of charged cells after 10 cycles (Fig. 6a–c) measured at OCV. Although the measured values cannot be directly compared, the impedance contributions still serve as a good indicator of the contributions. Here, smaller ohmic resistances are seen for lower salt concentrations (Fig. 6 − insets). This elucidated that the concentration polarization contribution should be taken into account when assessing the overpotentials for electrolytes with lower lithium salt concentrations.

Fig. 6

Fig. 6. Impedance spectra for charged battery cells after 10 cycles of discharge/charge, comparison of different lithium salt concentrations in the same compositions of dioxolane and [DEME][TFSI]: a) X = 0.101, b) X = 0.199, c) X = 0.401 (Insets show the high-frequency regions of the spectra).

The specific capacities fading trend during 100 cycles (Fig. S1) follows a reverse trend of overpotentials (Fig. 5).

The highest capacities reached were for the 1.0 M LiTFSI in X[DEME][TFSI] = 0.101. One would expect an additional capacity fade for electrolytes with higher dioxolane contents, which is somewhat visible in the data comparison for electrolytes with the same LiTFSI salt concentration (Table 1, Fig. S1d-S1f). Higher dioxolane content equals larger capacity fades, which would likely result in lower capacities of the currently best performing electrolyte if the cells are cycled further.

Table 1. Initial specific discharge capacity, average capacity and the capacity fade over 100 cycles at C/10 for the different electrolyte mixtures.


c(LiTFSI)0.1 Minitial capacity [mAh/gS]710.0590.5321.1
average capacity [mAh/gS]615.0404.0296.3
capacity fade over 100 cycles [mAh/gS]135.9186.549.6
0.5 Minitial capacity [mAh/gS]925.3513.2344.7
average capacity [mAh/gS]667.4490.0353.7
capacity fade over 100 cycles [mAh/gS]392.123.211.3
1.0 Minitial capacity [mAh/gS]1007.1826.0527.8
average capacity [mAh/gS]668.1634.0487.2
capacity fade over 100 cycles [mAh/gS]339.0192.052.3

From the gathered data, we can draw various conclusions about the influence of the measured electrolyte's physicochemical parameters on the Li–S battery performance.

The electrolyte's conductivity determined in our case does not have a direct connection to any of the battery system properties measured. It is worth noting that since an ionic liquid electrolyte was used, an array of charged species can migrate and contribute to the overall determined conductivity, while only the charge transferred by the Li+ ion is important for battery cell function. This is further complicated by the fact that polysulphides are dissolved in the electrolyte during battery operation. Similarly, no influence of the density of the electrolytes was observed.

When comparing electrolytes with the same lithium salt concentration, a parallel can be drawn between the viscosity trend (Fig. 2b) and the overpotential, ohmic resistance values (Fig. 6) and specific discharge capacities achieved (Fig. 5d–f, S1d–f). This can be explained by the hindered diffusion of electroactive species towards the electrode surface, which increases the low-frequency contributions in the EIS spectra (Fig. 6). For electrolytes with different lithium salt concentrations, the connection is more complicated, since the overpotential in an electrochemical system is influenced by both the ohmic resistance and the concentration polarization. [29] When using low concentrations of the dissolved Li+ salt, the concentration polarization becomes an important contribution to the overpotential, which consequently reduces the specific capacity (Fig. 5a–c, S1a–c).

The polysulfide solubility is in our case a function of the amount of ionic species dissolved in the electrolyte (Fig. 3). Higher lithium salt concentrations and higher molar ratios of the ionic liquid impede the dissolution of polysulfide species. In a binary electrolyte system, with two solvents in different ratios, this correlation would of course depend upon the solubility of polysulfides in the chosen components. The solubility of polysulfides has an influence on the Coulombic efficiencies (Fig. 4) and can to some extent be linked to the increased capacity fade (Table 1, Fig. S1).

The best electrochemical performance in terms of specific capacity retention and stability is obtained in the 1.0 M LiTFSI in X[DEME][TFSI] = 0.199. The slightly lower capacity compared to the 1.0 M LiTFSI in X[DEME][TFSI] = 0.101 in the formation cycles could be ascribed to the higher viscosity of the electrolyte with a higher ionic liquid content. The average Coulombic efficiency during 100 cycles is 97.0 %. Higher Coulombic efficiencies can be obtained with increased ionic liquid content but, due to increased viscosity and polarization, the obtained specific capacities are lower and not favorable for practical application. Further improvements with the introduction of a third co-solvent are probably possible and will be explored by our group in the future.