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Li2S4 was reported to be as low as 0.0625 M in DOL/DME.28The low solubility of Li2S4 and the poor stability of S3*− in DME and DOL (i.e., low concentration ratio of S32−/ S62−) 24, 27 means the synergetic chemical/electrochemical reactions are not likely to take place in the DOL/DME- based electrolytes. This is evidenced by the lack of elevation of the lower discharge plateau and a much lower discharge specific capacity, as shown in Fig. 2b. Reaction [6] will not take place in a low concentration ratio of S32−/S62− (CS32-/CS62-) if the bulk electrolytes still contain S62− when the cell discharges beyond reaction [2], as depicted in Fig. 3b and demonstrated in Fig. 2c, and if the bulk electrolytes contains low concentration of CS32− originating from low concentration of CS42- in Fig. 2a. Further, a higher concentration of Li2S4 is found to result in a substantial decrease of lithium cation diffusion coefficient by 7Li PFG-NMR measurements (Supplementary Table 1) to maintain the charge neutrality and then decrease the local supply of S32−. To investigate the solvent-dependent of S3∙− in electrolytes, non-invasive EPR measurements were performed for electrolytes of E_G2, E_G2_0.5 M Li2S4 and E_DOL/DME_saturated Li2S4 (images shown in Supplementary Fig. 6) in Fig. 3c. EPR spectrum at 125 K of E_G2_0.5 M Li2S4 clearly reveals the pattern of S3∙− radical at g=2.03415, 18, and the EPR spectrum also shows the persistent presence of the radical at room temperature (T=298 K). Similar pattern was not observed for E_G2 and greatly weakened for E_DOL/DME_saturated Li2S4. The concentration of S3∙− radical is calculated to be 0, 0.6 mM, and 11.2 mM for E_G2, E_DOL/DME_saturated Li2S4 and E_G2_0.5 M Li2S4 respectively; the large difference in the concentration of S3∙− clearly highlights the dramatic impact of solvents on the presence of S3∙− and the resultant CS32− that steers distinctive sulfur reduction pathway. Raman results further confirm the presence of S3∙− in E_G2_0.5 M Li2S4 as shown in Supplementary Fig. 7, where a dominant and sharp peak of 534.6 cm-1 stemming from S3∙− was observed 34. Experimental verification of the Li2S4-enabled solution pathway Since Li2S4 retention in the bulk electrolyte can drastically enhance the sulfur utilization under lean electrolyte conditions, it is very important to directly demonstrate whether or not Li2S4 plays a role in the proposed solution pathway. To demonstrate the occurrence of synergetic chemical/electrochemical reactions, operando sulfur K-edge X-ray absorption spectroscopy (XAS) was employed to probe sulfur speciation in a Li-S cell using electrospun carbon fiber (ECF) and Li2S8, with E_G2_0.5 M Li2S4 as the bulk electrolyte. Identification of S3∙− (i.e. a kick-off agent of reaction [6]) through XAS can be used as a criteria for verification of Li2S4-enabled solution pathway. Based on the above discussions, strong S3∙− signal should only occur in a Li-S cell using E_G2 and E_G2_0.5 M Li2S4 electrolyte, not in a Li-S cell with no pre-added Li2S4 electrolyte (such as E_G2). This means the occurrence of S3∙− depends on the composition of electrolytes, not on the selection of carbon host. Considering the very week signal obtained in our initial experiment when ACFC host is used, we replaced ACFC by ECF in the final experiment because the cells with ECF host is much thinner than ACFC host (with the similar capacity) and exhibit a much stronger XAS signal for S3∙−. Fingerprints of Li2S4, Li2S3, Li2S6, and Li2S reported in previous articles are summarized in Page 7 of 24PDF Image | A lithium-sulfur battery with a solution-mediated pathway
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