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Review Article Chem Soc Rev redox center for sodium storage.551 The reduction reaction proceeds in two steps in the voltage range from 0.005 to 1.6 V versus Na+/Na corresponding to two different processes. Later, the oligomeric Schiff bases are also investigated. Lo ́pez-Herraiz et al. reported for the first time the electrochemical activity of the 10-p-electron end group (–OOC–f–CQN–) (f: refers to the phenyl group) and the central (–NQC–f–CQN–) Hu ̈ckel units.565 They claimed that the maximum capacities are achieved for oligomers in which H+ ions are replaced by Na+ ions due to the fact that the hydrogen bond of the carboxylic end groups acting as crosslinks is removed, thus helping to accommodate more easily for the inserted Na+ ions. 4. Electrolytes, additives, and binders SIB technology is a very important and promising follow-up to LIB technology. The significance of SIBs is cost effectiveness owing to the geographical distribution of sodium in comparison with lithium. Therefore, in the long term, sodium-ion batteries will replace and/or substitute lithium-ion batteries in mid-large scale battery market.566 However, SIBs have still faced several challenges in terms of developing optimized electrode materials and electrolytes with a suitable capability for stable sodium storage. To date, while a number of efforts have been directed toward the searching for new electrode materials for SIBs, studies dealing with the electrolyte itself are much scarcer.28,567 However, looking back through the history of LIBs, it is clear that a suitable choice of electrolyte and binders is equally as important as the choice of electrode material for making operational SIBs; this is because the electrolyte and binders form a protective layer at both the cathode and anode, the surface layer (SL) and the solid electrolyte interfaces (SEI), respectively.568 Therefore, identifying suitable formulation electrolytes is indispensable to developing high performance SIBs. To accomplish this, we borrow ideas and techniques from those typically used in LIB electrolyte development. In the case of LIBs, various electrolytes, including organic electrolyte solutions, solid- and gel-polymer electrolytes, inorganic solid electrolytes, and ionic liquids have been investigated, and their development is still in progress.99 Organic electrolyte solution based on carbonate-ester polar solvents, where sodium salts are dissolved with complex containing functional additives, are mainly used in the practical development of SIBs due to their large potential window, high ionic conductivity and good temperature performance. On the other hand, a water-based electrolyte has also been proposed as a cost-effective energy storage system, which was successfully commercialized.18,569 In this section, we discuss SIB electrolytes, including salts, solvents, and additives. In addition, we briefly summarized the currently employed binders as well as their effects on the electrochemical performances based on various electrodes and/or sodium ion full cells. 4.1. Electrolytes A general list of properties needed for SIB electrolytes complies with those usually compiled for LIB oriented electrolytes: (1) chemically stable, (2) electrochemically stable, (3) thermally conjugated structure between the carbonyl group and the phenyl rings. Br–Na2TP and No2–Na2TP electrodes delivered a high capacity of 300 mA h g1; however, NH2–Na2TP delivered a relatively low capacity of 200 mA h g1. Abouimrane et al. first incorporated a disodium terephthalate-based organic anode material into sodium-ion full cells with transition-metal cathode materials547 (Fig. 41b). The Na4C8H2O6 electrode revealed that two reversible electrochemical reactions occurred with two redox couples of Na2C8H2O6/Na4C8H2O6 as the cathodes at 2.3 V and Na4C8H2O6/Na6C8H2O6 as the anode at 0.3 V.236 By using symmetric reactions, Wang et al. fabricated an all-organic SIB using Na4C8H2O6, which delivered a reversible capacity of 180 mA h g1 with an average operation voltage of 1.8 V. Later, to enhance the fast insertion/extraction of Na-ions at high current densities, Wang et al. suggested an extension of the p-conjugated system by using sodium 4,40-stilbene-dicarboxylate (SSDC).552 Remarkably, the designed electrodes exhibited a much enhanced high rate performance with reversible capacities of 105 mA h g1 at a current density of 2 A g1 and 72 mA h g1 at a current density as high as 10 A g1. According to their report, excellent sodium storage performances under high rate charge–discharge condi- tions can be ascribed to the following two main reasons: (1) improvement of the charge transport and stabilization of the charged and discharged states, and (2) enhancement of the intermolecular interactions and the resulting terrace packing structure; both of these can facilitate the insertion/extraction of Na+ ions. On the other hand, one main reason for the capacity fading of organic compounds is the dissolution of the active compound in polar liquid electrolytes.556 To circumvent unwanted active mass dissolution during cycling, various strategies are introduced such as the application of polymers as active materials or the immobilization of active molecules onto the conductive additives.543,556–558 Recently, Chen et al. proposed the use of PNTCDA, a kind of polyimide.557 The intrinsic stability and insolubility of the polyimide ensure that it is not dissolved in the electrolyte, and thus allows for an excellent cycling stability and a high initial coulomb efficiency of 97.6%. Another promising candidate for organic compounds, biomolecule- based electrodes have also been widely studied. A biomolecule- based organic compounds contained the quinone and carbonyl group.559–563 Recently, Wang et al. proposed renewable-juglone biomolecules with well-defined redox-active quinone carbonyl groups, which exhibit promising electrochemical performance in reversibly transferring sodium ions561 (Fig. 41c). Also, juglone can be immobilized onto reduced graphene oxide (rGO) nanosheets owing to the strong p–p interaction between the aromatic struc- ture and the carbon scaffold. Optical and photoelectron spectra results demonstrated non-covalent immobilization of the redox molecules via p–p interactions on the rGO carbon scaffold, which suppresses the dissolution puzzle of organic materials and enhances both the conductivity and sodium-ion accessibility of the electrode. As a result, juglone/RGO electrode demonstrated a high capacity of 305 mA h g1 and stable cycle retention after 100 cycles. Armand’s group reported polymeric and oligomeric Schiff-based electrodes564,565 (Fig. 41d). Polymeric Schiff bases have the (NQCH–Ar–HCQN) repeat unit, which can work as a View Article Online 3590 | Chem. Soc. Rev., 2017, 46, 3529--3614 This journal is © The Royal Society of Chemistry 2017 Open Access Article. Published on 28 March 2017. Downloaded on 7/1/2019 3:41:21 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.PDF Image | Sodium-ion batteries present and future
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