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ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms2513 Lithium-based batteries as highly efficient energy storage devices have long been considered as promising power supply for various electric vehicles and smart grid storage systems1–4. However, presently available lithium-ion technology cannot satisfy the increasing demand for energy density. Metallic lithium batteries exhibit the highest theoretical energy densities among the secondary batteries5. Among many possible systems, lithium–sulphur (Li–S) and lithium–oxygen (Li–O2) batteries are quite attractive as candidates for next-generation high-energy density batteries5–22. However, the application of metallic lithium anode suffers from inevitable formation of lithium dendrites, which are caused by uneven current distributions at the metal–electrolyte interface during cycling. The formation of lithium dendrites will lead to poor cyclic performance and increase the probability of internal short circuit, resulting in safety issues. Thus, great effort has been paid to suppress the formation of lithium dendrites and reduce metallic lithium corrosion in liquid electrolytes23. For instance, the use of solid polymer or inorganic electrolytes could potentially suppress the formation of the lithium dendrites. However, for solid-state electrolytes, the kinetic properties are limited, due to both low conductivity at room temperature and high interfacial resistance24–28. Recently, a breakthrough has been achieved in inorganic sulphide-based electrolytes with quite high room temperature conductivity of 10 2 S cm 1 (ref. 29). The application of Li2S–P2S5 glass–ceramic electrolyte in Li–S batteries has been demonstrated30–32. To reduce the interfacial resistance, 30wt.% solid electrolyte and 35wt.% acetylene black were required to add into sulphur electrode32. This leads to a decrease in energy density of Li–S batteries. It seems that the strategy of physical mixing of active phase and solid electrolyte still suffers from point- to-point contact30–32, not like a complete surface-to-surface wetting effect as liquid electrolyte. Here we report a new class of non-aqueous liquid ‘Solvent-in- Salt’ electrolytes and apply them in Li–S batteries. It is demon- strated that the use of ‘Solvent-in-Salt’ electrolyte inhibits the dissolution of lithium polysulphide, effectively protects metallic lithium anodes against the formation of lithium dendrites and results in high lithium cycling efficiency, thus enhancing electro- chemical performance. Results The concept of ‘Solvent-in-Salt’ electrolyte. In 1993, Angell et al.33 proposed the innovative concept of ‘Polymer-in-Salt’ by reversing the ratio of solid polymer solvent to salt, in which glass transition (Tg) was low enough to remain rubbery at room tempe- rature to preserve good conductivity and high electrochemical stability. However, in practice the Tg remained above ambient temperature and/or the system would crystallize. For conventional non-aqueous organic electrolytes, the salt concentration is usually limited in a range of 1–2 mol l 1, which is a trade-off among ionic conductivity, viscosity and salt solubility. Thus, most of the studies focus on the region C of Fig. 1, in which there is much less salt than solvent. There are few reports of research in the A (yellow) or D (green) regions of Fig. 1, in which either the weight or volume ratio of salt-to- solvent exceeds 1.0 (refs 34–37). As a matter of fact, by choosing proper salt and solvent, we can move an electrolyte into those regions (A, B and D) and also obtain some unexpected properties. To distinguish from traditional electrolytes, this new class of electrolyte is denoted by ‘Solvent-in-Salt’ (SIS). A similar system of hydrated molten salt composed of KNO3 and Ca(NO3)2 4H2O, in which the water content is insufficient to satisfy more than a first coordination sheath for the cation that was reported in 1965 by Angell38,39. This kind of hydrated molten salts was usually used in heat storage but not called SIS. The physicochemical properties of ‘Solvent-in-Salt’. For the following discussion, we select an electrolyte system containing Li[CF SO ) N] (LiTFSI), one of the lowest lattice energy salts and 322 1,3-dioxolane (DOL): dimethoxyethane (DME) (1:1 by volume) as solvent, resulting in what we show as one of the most pro- mising electrolytes for Li–S batteries. The physicochemical properties of this electrolyte with different ratios of salt-to-solvent are illustrated in Fig. 2. When the mole amount of salt reaches 4 mol in 1-l solvent, the electrolyte enters the D region of SIS by weight, and beyond 5 mol salt in 1-l solvent, the salt begins to have a dominant role in either weight or volume ratio (Fig. 2a). Arrhenius plots of the ionic conductivity of the electrolytes with different salt concentrations in a temperature range of 20 to 60 °C are shown in Fig. 2b and exhibit the typical curvature of the Vogel–Tammann–Fulcher (VTF) equation (Supplementary Fig. S1). It can be seen that the ionic conductivity decreases with increasing salt concentration. The conductivity drops slowly in the high temperature region (20–60°C) but rapidly at a low-temperature region ( 20 to 20 °C), resulting from a rise in Tg. For a given electrolyte with fixed salt and solvent, ionic conductivity depends on both viscosity and lithium-ion mobility. When increasing salt concentration, more and more Li–ether complex pairs form due to incomplete solvation shell and the viscosity at room temperature increases markedly in the SIS region (Fig. 2c and Supplementary Fig. S2). At the same time, the lithium-ion transference number of SIS-7# electrolyte increases to an unexpected high value (tLi þ 1⁄4 0.73, tLi þ 1⁄4 sLi þ / (sLi þ þ sTFSI )) (see Fig. 2c and Supplementary Fig. S3), which is much higher than that of traditional salt-in-solvent electrolytes (0.2–0.4). The conductivity of specific ion i is proportional to the concentration of mobile ion (ci) and its mobility (mi) (si 1⁄4 ncimi). The mobility of an ion is determined by the viscosity (Z) of the medium and radius of mobile ion (mi 1⁄4 1/6pZri) (ref. 23). In low-salt concentration electrolytes, lithium ions are coordinated with ether oxygen and form a large solvation shell compared with anions, leading to relatively lower mobility of solvated Liþ cations. In the SIS system, it is plausible that the number of solvated Liþ cations is decreased and large anion (TFSI ) could be more seriously dragged than the small unsolvated cation (Liþ) in this high viscosity system. Nevertheless, SIS-7# even with a high viscosity of 72 cP retains a conductivity of 0.814 mS cm 1 at room temperature, which remains superior to that of all-solid-state dry polymer or most of the inorganic electrolytes, and especially, could form better interfacial contacts. Differential scanning calorimeter (DSC) traces reveal distinct glass transition temperatures (Fig. 2d), which shows that all the electrolytes are glass-forming liquids, and their glass transition temperatures shift from low to high, with increasing ratio of salt- to-solvent. For the pure solvent mixture without salt (Supplementary Fig. S4), the glass transition temperature is 138.6 °C, justifying the excellent low temperature performance for the DOL–DME-based electrolyte. In the SIS-7# electrolyte, the value of Tg is –77.3°C, which is much lower than typical ‘Polymer-in-Salt’ system (Tg4–10 °C) (ref. 33) and close to that traditional commercial electrolyte systems (1 mol l 1 LiPF6 in EC–DMC, Tg 1⁄4 –67 °C)) (ref. 40). The flexible hinge in the S–N– S bond of TFSI , specific to these imide anions, explains this ‘plasticizing effect’, which is also reflected in the low viscosities of ionic liquids based on this anion. Application in Li–S batteries. The power of the SIS electrolytes is demonstrated by their use in rechargeable metallic Li–S batteries. The Li–S battery using SIS-7# as electrolyte exhibits the best electrochemical performance (Fig. 3). 2 NATURE COMMUNICATIONS | 4:1481 | DOI: 10.1038/ncomms2513 | www.nature.com/naturecommunications & 2013 Macmillan Publishers Limited. All rights reserved.PDF Image | Solvent-in-Salt electrolyte for high-energy rechargeable metallic lithium batteries
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