Toward Sustainable Li–S Battery Using Scalable Cathode and Safe Glyme-Based Electrolyte

The search for safe electrolytes to promote the application of lithium–sulfur (Li–S) batteries may be supported by the investigation of viscous glyme solvents. Hence, electrolytes using nonflammable tetraethylene glycol dimethyl ether added by lowly viscous 1,3-dioxolane (DOL) are herein thoroughly investigated for sustainable Li–S cells. The electrolytes are characterized by low flammability, a thermal stability of ∼200 °C, ionic conductivity exceeding 10–3 S cm–1 at 25 °C, a Li+ transference number of ∼0.5, electrochemical stability window from 0 to ∼4.4 V vs Li+/Li, and a Li stripping-deposition overpotential of ∼0.02 V. The progressive increase of the DOL content from 5 to 15 wt % raises the activation energy for Li+ motion, lowers the transference number, slightly limits the anodic stability, and decreases the Li/electrolyte resistance. The electrolytes are used in Li–S cells with a composite consisting of sulfur and multiwalled carbon nanotubes mixed in the 90:10 weight ratio, exploiting an optimized current collector. The cathode is preliminarily studied in terms of structure, thermal behavior, and morphology and exploited in a cell using standard electrolyte. This cell performs over 200 cycles, with sulfur loading increased to 5.2 mg cm–2 and the electrolyte/sulfur (E/S) ratio decreased to 6 μL mg–1. The above sulfur cathode and the glyme-based electrolytes are subsequently combined in safe Li–S batteries, which exhibit cycle life and delivered capacity relevantly influenced by the DOL content within the studied concentration range.


Figures number: 6
Tables number: 5 Figure S1 reports the Nyquist plots obtained via EIS at various temperatures (Fig. S1a-c) in SS|electrolyte|SS cells to calculate the ionic conductivity of the TE-5% (Fig. S1a), TE-10% (Fig. S1b) and TE-15% (Fig. S1c) electrolytes, as well as the chronoamperometric curves (Fig. S1d-f) alongside Nyquist plots (insets in Fig. S1d-f) recorded on Li|Li cells used to calculate the Li + transference number (t + ) of the TE-5% (Fig. S1d), TE-10% (Fig. S1e) and TE-15% (Fig. S1f) solutions through Bruce-Vincent-Evans method (see eq. 2 in the Manuscript). 1 The Nyquist plots are analyzed through NLLS fitting method using the Boukamp software 2,3 to obtain the electrolyte resistance (Re) from the spectra of Figure S1a-c and the electrode/electrolyte interphase resistance (Ri) form the spectra of Figure S1d-f.See Experimental section in the Manuscript for details on NLLS method, Figure 1c for the ionic conductivity trends, Figure 1f for graphical representation of the t + values and Table 2 for additional parameters used in eq. 2.
The results suggest crucial role of the selected electrolyte in stabilizing the SEI on lithium surface and mitigate the resistance of Li + transport.See Experimental section in the Manuscript for details on NLLS method, and Figure 1c for the ionic conductivity trends, and Figure 2a for the resistance trends.Table S2.NLLS analyses 2,3 performed on the Nyquist plots displayed in Figure S2b and e recorded by EIS upon aging of a Li|TE-10%|Li cell; frequency range: 500 kHz -100 mHz; alternate voltage signal: 10 mV.See Figure 2a in the Manuscript for corresponding trend and Table 1 for electrolyte acronyms.
Table S3.NLLS analyses 2,3 performed on the Nyquist plots displayed in Figure S2c and f recorded by EIS upon aging of a Li|TE-15%|Li cell; frequency range: 500 kHz -100 mHz; alternate voltage signal: 10 mV.See Figure 2a in the Manuscript for corresponding trend and Table 1 for electrolyte acronyms.

S8
Figure S3 displays higher magnification for the anodic stability LSV curves (Fig. S3a, see Figure 2bd in the Manuscript) and Li stripping-deposition tests (Fig. S3b-d, see Figure 2e in the Manuscript).
The raise of DOL concentration causes the expected decrease in anodic stability due to the DOL-ring cleavage on Li surface promoting SEI formation (Fig. S3a), while, at the same time, lowers the polarization as likely ascribed to viscosity reduction of the TEGDME-based electrolyte (Fig. S3b-d).Figure S4 shows the galvanostatic voltage profiles related to Li-S cells using the S:MWCNTs 90:10 w/w electrode and the DOL:DME-control electrolyte cycled either at C/5 (Fig. S4a) or C/3 (Fig. S4b).
Both the cells exhibit reversible conversion of Li and S to Li2Sx species during discharge through two plateaus between 1.9 and 2.3 V and their reverse oxidation during charge between 2.3 and 2.5 V, leading to a cycle life of 200 cycles.The respective cycling trends in Figure 4 in the Manuscript reveal satisfactory capacity retention and Coulombic efficiency approaching 100% for the whole test.Table S5.Comparison of the electrochemical performance obtained from the Li-S cell using TE-10% (see Figure 5 in the Manuscript for voltage profiles and cycling trend) with previous literature works reported in the Manuscript.See Table 1 in the Manuscript for electrolyte acronym.
Figure S6 shows the rate capability tests performed on Li-S cells using the TE-5%, TE-10% and TE-15% electrolytes.The voltage profiles (Fig. S6a-c) show a satisfactory performance from C/20 to C/8 for all the solutions, while only TE-10% and TE-15% display suitable cycling at C/5.In addition, all the solutions present limited response at C/3, and almost total deactivation of the conversion process at C/2.The corresponding cycling trend (Fig. S6d) further evidences the superior rate capability of TE-15%, which shows stable cycling at C/5 with steady state capacity of 640 mAh g -1 , followed by TE-10% delivering 500 mAh g -1 despite a decreasing trend.

Figure S2 .
Figure S2.Nyquist plots achieved through EIS upon aging of Li|Li cells using either (a, d) TE-5%.(b, e) TE-10% or (c, f) TE-15%.The Nyquist plots are analyzed through NLLS method 2,3 and the results are reported in Tables S1, S2 and S3, while the corresponding interphase resistance trends are displayed in Figure 2a in the Manuscript.Frequency range: 500 kHz -100 mHz; alternate voltage signal: 10 mV.SeeTable 1 in the Manuscript for electrolyte acronyms.

Figure S3 .
Figure S3.Magnifications of (a) anodic stability curves determined in Figure 2b-d of the Manuscript by LSV and (b, c) lithium-stripping deposition tests performed on Li|Li cells displayed in Figure 2e of the Manuscript at the (b) 1 st day and (c) 8 th day for the TE-5%, TE-10% and TE-15% electrolytes; (d) histogram representation of the overvoltage polarization related to the lithium-stripping deposition tests at the 1 st and 8 th day of measurement.LSV range: from OCV to 5.0 V vs. Li + /Li; scan rate: 0.1 mV s -1.See Table 1 in the Manuscript for electrolyte acronyms.

Figure S4 .Figure
Figure S4.Voltage profiles of Li cells using the DOL:DME-control electrolyte coupled with the S:MWCNTs 90:10 w/w electrode tested at either (a) C/5 or (b) C/3 between 1.7 and 2.8 V. E/S ratio: 10 µL mg -1 .See corresponding capacity and Coulombic efficiency trends in Figure 4 of the Manuscript.

Table 2
in the Manuscript for the parameters used to evaluate t + and Figure1ffor a histogram representation of the obtained values.See Table1in the Manuscript for electrolyte acronyms.

Table 1
in the Manuscript for electrolyte acronyms.

Table
. See Table1in the Manuscript for electrolyte acronyms.S4 compares the chemical-physical properties of TE-5%, TE-10% and TE-15% with other electrolytes used in Li-S battery according to previous literature reported in the Manuscript.The results displayed in this work demonstrate the achievement of a compromise between DOL:DME and TEGDME-based solutions for the properties of TE-5%, TE-10% and TE-15%.

Table S4 .
Comparison of the physical-chemical properties of TE-5%, TE-10% and TE-15% with other electrolytes employed in Li-S cell according to previous literature papers reported in the Manuscript.See Table1in the Manuscript for electrolyte acronyms.

Table S5
reports a comparison of the results achieved in this work with literature papers reported in the Manuscript where glyme-based electrolytes are coupled with a sulfur electrode in Li-S batteries.The comparison is carried out by taking in consideration operative conditions similar to those adopted herein, that is, an electrolyte without addition of lithium polysulfides (Li2Sx) and with aluminumbased current collector.The outcomes show that the performance achieved by our Li-S batteries are in line with previous works, with additional bonus of enhanced sulfur loading and the safety content.