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

Abstract

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.

Figure 1

(a) TGA and (b) the corresponding DTG curves of the electrolytes acquired under N₂ flow between 25 and 800 °C at the 5 °C min^–1 rate; (c) ionic conductivity trends of the electrolytes reporting, in addition, the linear fit for each electrolyte; see Nyquist plots in Figure S1a–c in the Supporting Information; (d) histogram representation of the activation energy values calculated using the Arrhenius equation (eq 1) on the ionic conductivity trends in (c); (e) histogram representation of the weighted average dielectric constants (ε_w) calculated considering the electrolyte solvents ratio and the ε values of pure DOL (7.1) and TEGDME (7.8); (f) histogram representation of the Li⁺ transference number (t⁺) of the electrolytes determined through the Bruce–Vincent–Evans method (eq 2); see chronoamperometric curves and related Nyquist plots in Figure S1d–f in the Supporting Information; (g) FT-IR spectra of the TE-5%, TE-10%, and TE-15% solutions. See Table 1 for electrolyte acronyms.

Figure 2

(a) Interphase resistance trends related to Li|Li cells using either TE-5%, TE-10%, or TE-15% aged for 18 days; see the corresponding Nyquist plots in Figure S2 and NLLS analyses in Tables S1–S3 in the Supporting Information; (b–d) ESW evaluation of the (b) TE-5%, (c) TE-10%, and (d) TE-15% electrolytes performed via CV in the cathodic region (0.01–2.0 V vs Li⁺/Li) and LSV in the anodic one (from OCV to 5.0 V vs Li⁺/Li) at a scan rate of 0.1 mV s^–1; (e) lithium stripping-deposition tests performed on Li|Li cells using either TE-5%, TE-10%, or TE-15%. Figure S3 in the Supporting Information reports magnifications of the anodic stability curves and lithium stripping-deposition tests. See Table 1 for electrolyte acronyms.

Figure 3

Physical–chemical characterization of the S:MWCNTs 90:10 w/w composite. In detail: (a) XRD of the composite and bare MWCNTs; reference data for sulfur (ICSD #27840) and graphite (ICSD #76767) are reported for comparison; (b) TGA and the corresponding DTG (right y-axis) performed under N₂ flow between 25 and 1000 °C at the 5 °C min^–1 rate; (c,d) SEM images at various magnification recorded in secondary electron mode; (e) SEM picture acquired in backscattered electrons mode and (f,g) corresponding elemental maps of (f) sulfur and (g) carbon; (h,i) TEM images at various magnifications.

Figure 4

Galvanostatic cycling of Li cells using the S:MWCNTs 90:10 w/w electrode and the DOL:DME-control electrolyte with sulfur loading of 2.2–2.3 mg cm^–2 and E/S ratio of 10 μL mg^–1. In particular: (a) voltage profiles and the (b) corresponding capacity trend related to the rate capability test carried out at increasing scan rates from C/10 to C/2 between 1.8 and 2.8 V from C/10 to C/3 and in the 1.7–2.8 V voltage range for C/2; current rate was lowered back to C/10 after 25 cycles; (c,d) capacity trends (right y-axis reports Coulombic efficiency) recorded at constant current rates of either (c) C/5 or (d) C/3 between 1.7 and 2.8 V (see voltage profiles in Figure S4 of the Supporting Information); (e) voltage profiles and the (f) corresponding capacity trends (right y-axis reports Coulombic efficiency) acquired at C/10 between 1.7 and 2.8 V using sulfur loading increased to 5.2 mg cm^–2 and E/S ratio limited to 6 μL mg^–1.

Figure 5

(a–c) Voltage profiles and (d) cycling trends (right y-axis shows Coulombic efficiency) of Li–S cells galvanostatically cycled at C/5 constant rate using either the (a) TE-5%, (b) TE-10%, or (c) TE-15% electrolyte and the S:MWCNTs 90:10 w/w electrode. Sulfur loading: 1.7–2.1 mg cm^–2; E/S ratio: 15 μL mg^–1; voltage range: 1.7–2.8 V.