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NATURE COMMUNICATIONS | DOI: 10.1038/ncomms2513 ARTICLE a Solid-state-based electrolytes Liquid-based electrolytes Salt content b ∞ 1 1. Lithium ion conductor 2. LiI/AI2O3 “Polymer-in-Salt” ([salt] > [polymer]) Plastic crystal electrolyte Dry polymer ([salt] < [polymer]) 100% 50% Lithium molten salt ‘Solvent-in-Salt’ ([salt] > [solvent] fig.1b) Traditional non-aqueons electrolytes ([salt] < [solvent]) Liquid phase AB “Solvent-in-Salt” by volume 100% Solid-phase content (%) 0% 01∞ Weight ratio of salt-to-solvent Soggy sand electrolyte Gel polymer CD Figure 1 | A general concept for Solvent-in-Salt electrolyte. (a) The overview of the available electrolytes58–60. (b) The distribution map of non-aqueous liquid electrolytes with the weight and volume ratios of salt-to-solvent. A, B and D regions are Solvent-in-Salt electrolyte, in which the ratio of salt-to- solvent is over 1.0 by volume or weight. C region is [solvent]4[salt] by weight and volume. 2.2 2.0 C region (fig. 1b) D region (fig. 1b) B region (fig. 1b) SIS-7# Solvent-in-Salt –1.6 –2.0 –2.4 –2.8 –3.2 –3.6 –4.0 –4.4 “Solvent-in-Salt” by volume and weight “Solvent-in-Salt” by weight 1.8 1.6 Salt-in-Solvent Volume ratio 1.2 Weight ratio 1.4 1.0 0.8 0.6 0.4 0.2 0.0 16 14 12 10 1234567 Ratio of salt-to-solvent (mol per l solvent) 3.0 1# 2# 3# 4# SIS-5# SIS-6# SIS-7# 3.2 25 °C 3.4 3.6 3.8 4.0 1,000/T (K–1) Tg = –77.3 °C SIS-7# Tg = –83.6 °C SIS-6# 80 70 60 50 40 30 20 10 0.9 0.8 0.7 0.6 0.5 0.4 0.3 Tg = –91.9 °C Tg = –98.9 °C 4# Tg = –109 °C 3# 2# 1# 8 6 4 2 00 1234567 Ratio of salt-to-solvent (mol per l solvent) –150 –120 –90 –60 –30 0 30 Temperature (°C) Figure 2 | Physicochemical properties of Solvent-in-Salt electrolytes. (a) Weight and volume ratio of salt-to-solvent with different ratios of LiTFSI to DOL:DME (1:1 by volume). (b) Arrhenius plots of the ionic conductivity as a function of 1,000/T for electrolytes with different ratios of LiTFSI to solvent, (1#: 1 mol per l solvent, 2#: 2 mol per l solvent, 3#: 3 mol l solvent, 4#: 4 mol per l solvent, 5#: 5 mol per l solvent, SIS-6#: 6 mol l per solvent and SIS-7#: 7 mol l per solvent). (c) Viscosity, ionic conductivity and lithium-ion transference number at room temperature for the aforementioned different electrolytes. (d) DSC traces of the aforementioned different electrolytes. It shows an initial specific discharge capacity of 1,041mAhg1 at a current rate of 0.2C (that is, 335mAg1) and maintains a reversible capacity of 770 mA h 1 g 1 with capacity retention of 74% after 100 cycles (Fig. 3b). More importantly, the coulombic efficiency reaches nearly 100% after the first cycle (it is 93.7% for the first cycle.) for the SIS-7# electrolyte (also see Supplementary Fig. S5), which is higher than previous reports in a similar system12–22, owing to effectively avoiding the polysulphide shuttle effect in the charging process (Fig. 3c) (ref. 41). In contrast, other less concentrated electrolytes exhibit an apparent coulombic efficiency greater than 100% (Fig. 3c and Supplementary Fig. S6), a sign of the ‘polysulphide shuttle effect’. The only shortcoming of the use of SIS-7# is that the polarization becomes slightly larger due to relatively higher viscosity compared with traditional electrolytes with low-salt concentration (Fig. 3a). Yet Li–S batteries using SIS-7# electrolyte still show excellent rate capability as shown in Fig. 3d. They can achieve capacities of 1,229, 988, 864, 744 and 551 mA h g 1 of sulphur at current rates of 0.2, 0.5, 1, 2 and 3C, respectively. When the current rate returns to 0.2C, a reversible capacity of 789 mA h 1 g 1 remains. The capacity still decays after rate measurement, although the polysulphide dissolution is inhibited, which is probably related to the unstable C/S electrode. In the charge–discharge process, the electrode suffers from a large volume change by a conversion reaction between S8 (2.07 g cm 3) and Li2S (1.66 g cm 3), which could result in not only the sulphur redistribution but also the structural damage of the carbon–sulphur composite. Thus, we can expect that through optimization of cathode materials, the cyclic perfor- mance will be further improved. To further prove the power of this new SIS electrolyte, the Ketjenblack without mesoporous structure was used as a support NATURE COMMUNICATIONS | 4:1481 | DOI: 10.1038/ncomms2513 | www.nature.com/naturecommunications 3 & 2013 Macmillan Publishers Limited. All rights reserved. SIS-5# σ25 (mS cm–1) Salt/solvent ratio Exothermic Endothermic Volume ratio of salt-to-solvent “Solvent-in-Salt’’ Log σ (S cm–1) Lithium-ion transference Viscosity (cP)PDF Image | Solvent-in-Salt electrolyte for high-energy rechargeable metallic lithium batteries
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