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Review Article Chem Soc Rev electrochemical stabilities.99 However, some problems remain in terms of making electrode slurries, such as the relatively high production cost and the necessary use of a volatile and toxic organic solvent (N-methyl pyrrolidone).99,598,599,610 Recently, in both LIBs and SIBs, alternative water-soluble binders have been introduced such as sodium carboxymethyl cellulose (Na-CMC), poly(acrylic acid) (PAA) and sodium alginate (Na-Alg)600–612 (Fig. 42g). Generally, such binders were introduced for alloying reaction materials undergoing a large volume change during lithiation–delithiation or sodiation–desodiation to enhance cycle performance due to the thermally cross-linked three- dimensional interconnection of those binders. A Na-CMC binder is an environmentally friendly and inexpensive material that is derived from cellulose as natural polymers. During the electro- chemical reaction process, the Na-CMC binder could play an important role in improving the solid electrolyte interface (SEI) passive layer on the electrode surface, which reduces the irreversible capacity and somehow leads to a better cycle life.601–603 Na-Alg is a high-modulus natural polysaccharide extracted from brown algae, which yields a remarkably stable battery anode.604–606 In particular, Na-Alg is much more polar than the Na-CMC polymer chains, which can ensure a better interfacial interaction between the polymer binder and parti- cles, as well as stronger adhesion between the electrode layer and the Cu substrate. Comparing the chemical character between PVDF and PAA, it is a general assumption that the carboxylic groups of the PAA binder play an important role in achieving better uniformity and stronger binding ability in the composite electrode; this is due to their amorphous and cross-linking nature via the hydrogen bonds between carboxylic groups.608–611,613 A PAA binder allows for the formation of a stable deformable SEI layer on the elastic binder-coated elec- trode surface, in which the elasticity of the polymer matrix may prevent cracking in the SEI when the volume changes.609,610 The polymeric cross-linkage network can regulate the mechanical/ chemical stress that results from large volume expansion in the composite electrode.517,610 In addition, the decomposition reac- tion of an electrolyte was reduced compared to the PVDF binder due to the stable SEI layer formation via a PAA binder.458,613,614 In 2014, Dahbi et al. compared and examined the binder and electrolyte additive effects in a hard carbon electrode using Na-CMC and PVDF binders.601 Decomposition of the electrolyte on hard-carbon during the initial cycle was sufficiently sup- pressed by the CMC binder. Moreover, Dahbi et al. noted that uniform coverage with CMC was proposed to be advantageous in stabilizing the electrochemical performance, leading to excellent capacity retention with a higher efficiency compared to the PVDF binder.601,615 To overcome the capacity fading resulting from volume expansion of insertion materials, Sun et al. used the Na-CMC binder in their Li4Ti5O12 electrode.337 Recently, Zhao et al. introduced the Na-doped Li4Ti5O12 electrode using the Na-CMC and Na-alginate binder instead of the PVDF binder. These binders exhibited a slightly higher Coulombic efficiency than that of the PVDF binder. After 100 cycles, the Na-CMC and Na-alginate binders exhibited an excellent cyclability compared to that of the PVDF binder616 (Fig. 42h). When applied to alloy 4.3. Ionic liquids, polymer electrolytes, all-solid electrolytes The study of ionic liquids (ILs) and polymer electrolytes contain- ing a fraction of sodium salts is still at a very early stage.99,568 Ionic liquids generally have a negligibly low volatility, non- flammability, and high thermal and electrochemical stability. The use of an ionic liquid electrolyte solution has been intro- duced by several research groups.589–592 However, unsolved drawbacks such as high viscosities at room temperature and expensive production cost still remain for practical utilization as an electrolyte in SIBs. Polymer electrolytes are mainly considered as solvents because of their thermal, and chemical stabilities, as well as their flexibility for cell manufacturing.568 In addition, all-solid-state batteries with inorganic solid electrolytes and electrodes are promising power sources for a wide range of applications because of their safety, long-cycle lives and versatile geometries.18,99,568,593–597 By West et al. a Na/NaClO4-PEO/V2O5 cell was proposed in 1985 to avoid the use of a Na metal anode.593 Later, Ma et al. reported on a SIB cell built using Na0.7CoO2 as the cathode, Na15Pb4 as the anode and NaTf(PEO)8 as the solid polymer electrolyte.72 A phosphate-based all-solid- state symmetrical cell, Na3V2(PO4)/Na3Zr2Si2PO12/Na3V2(PO4), has been successfully fabricated, but the reported electrode performances are for under 80 1C conditions.596 Recently, Hayashi et al. realized an all-solid-state rechargeable sodium cell based on a Na-Sn/Na3PS4 glass-ceramic/TiS2 cell configu- ration at room temperature. Although the capacity was limited to approximately 40% of the theoretical capacity of TiS2, the all- solid-state cell has the potential to realize good charge–discharge reversibility at ambient and moderate temperatures.597 In summary, the main electrolyte for practical use in SIBs is as follows. First, NaPF6 has been largely used as electrolyte salts in carbonate-ester binary or ternary mixtures, which have become the main electrolyte composition used for SIBs. Second, propylene carbonate (PC) is a key component in both binary and ternary solvent mixtures, which has intrinsic advantages derived from its high dielectric constant and wide operating electrochemical and temperature windows.568 Third, the use of FEC as an additive plays the important role of stable passivation film formation on the electrode surface, which effectively prevents electrode destruc- tion as a result of the sodiation–desodiation process.577 Finally, the choice of high purity solvents and Na-salt is necessary for identifying a suitable electrolyte solution.18,28,568,574 4.4. Binders Another important key component to improving the electrode performance is the choice of a binder used for powdery active materials. Especially, in anode materials, Na storage perfor- mances are dramatically affected by selecting the appropriate binders. Thus, investigation of desirable binders is also necessary to stabilize the electrode surface and suppress electrode distortion during the sodiation–desodiation process. Commonly, most electrodes of commercial and/or scientific studies are made using poly(vinylidene fluoride) (PVDF) as a binder to form an electrode layer from a powdery electrode material on a metal current collector due to its good chemical and View Article Online 3594 | 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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