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NATURE COMMUNICATIONS | DOI: 10.1038/ncomms2513 ARTICLE 0 mol l–1 2 mol l–1 4 mol l–1 7 mol l–1 Figure 4 | Lithium polysulphide dissolution experiments. (a) The colour changes of four samples with different salt concentrations containing the same amount of Li2S8 were recorded by digital camera along with time: 0 mol per l solvent, 2#: 2 mol per l solvent, 4#: 4 mol per l solvent, 7#: 7 mol per l solvent. (b) The ultraviolet-visible spectrophotometry. The 2#, 4# and 7# for UV-Vis measurement are the solutions from the standing samples for 18 days, which are then diluted with the corresponding electrolytes in a volume ratio of 1:7. The curves of 4#’ and 7#’ are the pure electrolytes of 4 mol per l solvent and 7 mol per l solvent for comparison, respectively. the dissolution of lithium polysulphide is not totally prohibited because in this case the highest concentration used was 5M LiTFSI in DME/DOL. Second, a metallic lithium anode is more stable in the SIS-7# electrolyte than in other electrolytes. Compared with other kinds of rechargeable metallic lithium batteries, it is more complicated to stabilize a metallic lithium anode in a Li–S battery owing to double damage from dendrite formation and the side reaction between lithium polysulphide and metallic lithium during cycling. From the scanning electron microscopy (SEM) images shown in Fig. 5a–d, it is obvious that SIS-7# shows the lowest roughness and damage level of metallic lithium anode compared with the other three samples (2#: Fig. 5b, 4#: Fig. 5c and SIS-7#: Fig. 5d), which demonstrates that the SIS electrolyte system can effectively reduce the corrosion and suppress the formation of lithium dendrites owing to the ultrahigh lithium salt concentra- tion and high viscosity. It is generally accepted that dendrites start to grow in the non-aqueous liquid electrolyte when the anion is depleted in the vicinity of the electrode where plating occurs according to Chazalviel model50,51. In the case of SIS electrolyte with ultrahigh salt concentration, there is a mass of anion to keep the balance of cation (Li þ ) and anion (TFSI ) near metallic lithium anode, and the space charge that is created by TFSI depletion is minimal, thus this is not a favourable condition for dendrite growth. Furthermore, owing to both ultrahigh lithium salt concentration and high lithium-ion transference number (0.73), SIS electrolyte provides a large amount of available lithium-ion flux and raises the lithium ionic mass transfer rate between electrolyte and metallic lithium electrode, thereby enhancing the uniformity of lithium deposition and dissolution in charge/discharge process. Besides, influenced by high viscosity, on the one hand, it possibly increases the pressure from the electrolyte to push back growing dendrites, resulting in a more uniform deposition on the surface of the anode. On the other hand, high viscosity limits anion convection near deposition area, which is also helpful to deposit uniformly52. Meanwhile, lithium cycling efficiencies in different electrolyte systems were investigated by means of a Li deposition-disso- lution experiment according to Aurbach et al.53 (2#: Fig. 5e, 4#: Fig. 5f and SIS-7#: Fig. 5g). In the commonly used low-salt concentration electrolyte system, the lithium cycling efficiency is estimated to be below 50%, which means a large excess of lithium as an anode needed in a real battery, thus decreasing the energy density and increasing the cost. In contrast, interestingly, a lithium cycling efficiency as high as 71.4% is obtained in the SIS-7# electrolyte system, which is much higher than that of other LiTFSI-based non-aqueous electrolytes54. Furthermore, from SEM images (Supplementary Fig. S9), a smoother and more uniform lithium deposition was also clearly observed in the SIS-7# electrolyte system. This important issue for real applications has seldom been discussed in previous work on Li–S batteries. The different lithium cycling efficiencies in different electrolyte systems could be ascribed to the nature of solid electrolyte interphase (SEI) formed on the surface of lithium electrodes. To confirm our conjecture, X-ray photoelectron spectroscopy analysis of Li electrode surfaces was performed. Based on the previous work from Aurbach group53–57 and our experimental data, it could be concluded that the solvents of DME and DOL in electrolytes could possibly be reduced to ROLi species and oligomers with OLi end group, and the salt of LiTFSI could possibly be reduced to Li3N, LiF, C2FxLiy and sulphur-containing compounds such as Li2S, Li2S2O4, Li2SO3 or SO2CFx during the cycling process to form an SEI layer on the metallic lithium surface (Supplementary Fig. S10). The composition of SEI for both 2# and SIS-7# electrolytes seems to be similar; however, the thickness is different. In the case of SIS-7# electrolyte, it can be seen that the lithium metal signal appears after 150 s sputtering with Arþ ions, which etches the surface layer-by-layer from a NATURE COMMUNICATIONS | 4:1481 | DOI: 10.1038/ncomms2513 | www.nature.com/naturecommunications 5 & 2013 Macmillan Publishers Limited. All rights reserved. –1 –2 7 6 5 4 3 2 1 0 4#’ 7#′ 150 300 450 600 750 900 1,050 Wavelength (nm) 18 Days 1 Day 1h Visible light region (400–760nm) 4# 2# 7# AbsorptionPDF Image | Solvent-in-Salt electrolyte for high-energy rechargeable metallic lithium batteries
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