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electrolyte conditions at room temperature. In particular, diglyme (G2) is employed to prepare an optimized high concentration of Li2S4 with the aid of NO3−. This Li2S4-retaining strategy is feasible to decouple the sulfur reduction pathway from both the bulk electrolyte and the electroactive surface area, to circumvent conventional solid-solid charge transfer in the lower discharge plateau, and to make reversible Li stripping/plating. Results and discussion A new Li2S4-dictated pathway under lean electrolyte conditions In a Li-S cell with a critical electrolyte-to-sulfur (E/S) ratio (i.e., ≤4.0 mLE gs−1 when the maximum solubility of LiPS reaches 8 M [S]; where mLE and gs represent volume of electrolyte and weight of sulfur in Li-S cell, respectively, Supplementary Fig. 1), sulfur is first reduced at the cathode during discharge to form quasi-solid Li2S4 by reactions [1-2] as shown below 30. In the subsequent reaction represented by the lower discharge plateau [3], nucleation and growth barriers of insulating solid Li2S present a tremendous hurdle for Li+ diffusion and solid-solid charge-transfer process. S8 (s) + 2Li+ + 2e− → Li2S8 (s or l) Li2S8 (s or l) + 2Li+ + 2e− → 2Li2S4 (s) Li2S4 (s) + 6Li++ 6e− → 4 Li2S (s) [1] [2] [3]; EΘ=2.1 V In order to investigate electrolyte dependence of sulfur utilization in the lower discharge plateau under lean electrolyte conditions, selection of a carbonaceous model matrix is important to eliminate effects of the diffusion/migration of high-order LiPS in the bulk electrolytes from cathodes. A microporous sulfur-loading matrix of activated carbon fiber cloth (ACFC) was used as a model substrate (Supplementary Fig. 2) because it can confine high-order LiPS in micropores.11 Despite different electrolytes and sulfur sources, no diffusion/migration of high- order LiPS from micropores of ACFC was observed in cyclic voltammograms (Supplementary Fig. 2). Moreover, the relatively high pore volume of ACFC allows high sulfur loadings in micropores with limited solvent permeation 11, akin to where the Li-S redox reactions would take place in practical applications. A Li-S cell was assembled with a sulfur containing electrolyte E_G2_0.5 M Li2S4 (E_G2 represents 1 M LiTFSI_0.3 M LiNO3 in G2) as the sulfur source and ACFC as the carbon matrix. Sulfur loading is 1.58 mg cm−2 calculated by total amount of sulfur in the electrolyte divided by the area of ACFC electrode. A typical discharge/charge curve is shown in Fig. 1a, which shows a discharge capacity of 1590 mAh gs−1 and a coulombic efficiency (CE) of 99.1 %, suggesting that 95.1 % of the sulfur is reduced to solid Li2S. To determine the sulfur reduction mechanism in E_G2_0.5 M Li2S4, potentiodynamic cycling with galvanostatic acceleration (PCGA) was performed in the LiǀE_G2_0.5 M Li2S4ǀACFC; results are shown in Fig. 1b. PCGA is a quasi- equilibrium technique that can provide very useful detailed information on electrochemical Page 3 of 24PDF Image | A lithium-sulfur battery with a solution-mediated pathway
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