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Supplementary Fig. 9a. The dominant final solid product can be determined as Li2S by corresponding fitted peaks of S2p37, 38 as shown in Supplementary Fig. 9b and quantitative calculation with an accuracy of 96.5 % in Supplementary Table 3. Application of the Li2S4-enabled solution pathway In this work, advantages of Li2S4 content in bulk electrolytes have been demonstrated (Fig. 1d and Supplementary Fig. 4), in terms of steering the synergetic chemical/electrochemical reactions (Fig. 4) and high compatibility with Li metal (discussed in the Supplementary Notes and shown in Supplementary Figs. 10–14. Li-S cells with high sulfur loadings and under lean electrolyte conditions are expected to deliver a high specific capacity and long cycle life if the cell were just cycled between Li2S4 and Li2S, with a theoretical capacity of 1254 mAh gs−1, by the Li2S4-retaining solution pathway. ACFC was initially used as a model material to study the electrolyte-dependent sulfur reduction pathway under lean electrolyte conditions. Surprisingly, even Li-S cells using ACFC are fully charged at 2.8 V, where the retained Li2S4 might be oxidized into Li2S8 or migrate toward the Li metal anode with Li+ owing to the stronger interaction between Li+ and short-order LiPS41, high sulfur utilization and CE are still maintained as observed in Fig. 1. This indicates that Li2S4, not the intermediate Li2S6, is always generated from the reduction of Li2S8 in G2 to continuously prevail as the solution pathway in ACFC. Supplementary Figs. 15 shows the cycling performance of LiǀE’_G2_1.5 M Li2S4ǀACFC with a high sulfur loading of 7.6 mg cm−2 at 1/30C (i.e., 0.42 mA cm−2) for discharging, and 1/7.5C for charging in the range 1.80–2.80 V. The E/S ratio was determined to be 5.20. The potential dip is greatly alleviated, even though the cell was charged to 2.8 V. Moreover, both excellent cycling retention and high CE can be achieved with a stable areal capacity of ca. 6 mAh cm−2. Impact of the charge cut-off potential of LiǀEꞌ_G2_1.5 M Li2S4ǀACFC on the middle discharge potential (i.e. an indication of cell cycling retention and the sustainability of solution-mediated pathway) was investigated for 20 cycles with the same charge/discharge rate of 1/30 C at 30 oC as shown in Supplementary Fig. 16. Two different charge cut-off potentials of 2.63 V (i.e. oxidation to Li2S8) and 2.48 V (i.e. roughly oxidation to Li2S4) were selected based on CV measurement of this cell (Supplementary Fig. 16a). The cell terminated at 2.48 V showed a stable middle discharge potential but the cell terminated at 2.63 V showed a rapid decrease in the middle discharge potential, indicating the solution pathway is highly sustained when the charge cut-off potential is carefully tuned to terminate the cell charge to the stage of Li2S4. To demonstrate the general application of the Li2S4-enabled solution pathway, Electrospun carbon fiber (ECF) has been used to replace ACFC as sulfur-loading substrate because ECF is a typical macroporous carbon matrix which cannot prevent diffusion of high- order LiPS (i.e., soluble S62− in reaction [6]) into electrolytes, and thus can be used to verify the Li2S4-enabled solution pathway. The Liǀ E_G2_0.5 M Li2S4ǀECF_Li2S8 cell was cycled with a high sulfur loading of 6.2 mg cm−2 at an E/Scathode ratio of 3.0, 0.20 mA cm−2, and 30 °C in the range 1.80–2.45 V, as shown in Fig. 5. The low charge cutoff potential of 2.45 V was deliberately chosen to prevent the oxidation of high-order LiPS into S8 on the ECF and the Page 9 of 24PDF Image | A lithium-sulfur battery with a solution-mediated pathway
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