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Review Article Chem Soc Rev further increase as more of the present cathode materials are consumed in future. Depletion of these resources, particularly cobalt, should be considered at the same time. Academic and industrial societies are recently raising the alarm about this situation and speaking up about the necessity of alternative rechargeable batteries that can substantially sub- stitute present LIBs. Indeed, sodium-ion batteries (SIBs) were developed together with LIBs in 1980s; however, the inferior battery performance of SIBs relative to LIBs was the main reason for the fade of SIBs, which led to the birth of commercialized LIBs by Sony in 1990s. However, the recent upsurge of EVs and energy storage using LIBs as power sources could cause a short- age of LIBs so a great deal of attention has been paid again to development of SIBs, in which sodium is inexpensive and evenly distributed. Since 2010 with the slogan of beyond lithium, research trends in SIBs have been intensively focused on elec- trode materials rather than battery systems in order to distin- guish the feasibility of commercialization. There are some merits of SIBs versus LIBs as follows; (i) possibility of high performance electrode materials because of the abundance of sodium transi- tion metal compounds relative to lithium compounds, (ii) fast diffusion of sodium ions in the solid phase, which indicates high rate performance of Na cells, (iii) lowering of the manufacturing cost due to the use of Al foil instead of Cu current collectors for anodes and the transfer of the production line of LIBs to SIBs, which does not need new production facility, (iv) superior safety properties of the cathode even at a highly charged state, and (v) less risk for sodium supply. By contrast, there are some demerits of SIBs; (i) low operation approximately 0.3 V versus Li, (ii) a large ionic radius of Na+ (1.02 Å) relative to Li+ (0.76 Å), which induces a simultaneous structure change during Na+ insertion and extraction that may cause gradual capacity fade, and (iii) high reactivity of Na metal which necessitates basic solution for formation of metallic sodium deposition onto anodes that can seriously threaten safety of Na cells. As mentioned in Fig. 1 and Table 1 development of cathode materials has been progressing towards layer and polyanion structure materials. In particular, know-hows accumulated by the development of LIBs are leading to significant progress in layered structure materials. O3 type layer compounds are very fascinating because they have the same crystal structure as commercial LiCoO2 and Ni-rich derivatives of LIBs, but their capacities are usually limited below 130 mA h g1 due to simultaneous structural changes induced from the presence of large Na+ ions in the crystal structure.83,634 Different from the O3 type materials, P2 type cathode materials are of interest due to their high capacity approaching approximately 200 mA h g1.46 Despite the high capacities, the serious flaw of the P2 materials is the lack of extractable sodium content in the compounds, namely, Na0.7MO2 (M: transition metals), of which the first discharge capacity is similar to that of O3 type compounds. Nonetheless, many works have been directed toward stabili- zation of cycling performances. Appropriate sacrificing agents can show oxidative decomposition at the first charge, which can be used to solve the irreversible capacity at the first cycle. This enables the resulting Coulombic efficiency approaching that NaPF6 in EC:PC exhibited a low generation of heat and a high thermal stability, which can be attributed to the more thermally stable SEI layer that is formed on the HC after cycling.28 As organic liquid electrolytes are less safe than the other electrolytes, using an aqueous electrolyte is a good approach to prevent safety hazards, and additionally they are of a relatively low cost. Developing aqueous Na-ion batteries is meaningful and practical because of the huge abundance of Na resources (NaCl, Na2SO4, NaNO3, etc.). Using poly(vinylidene fluoride) (PVDF) as a binder is very common due to its good chemical and electrochemical stabilities. However, with the use of a volatile and toxic organic solvent (N-methyl pyrrolidone) and relatively high production cost when using PVDF to make the slurry, water-soluble binders such as sodium carboxymethyl cellulose (Na-CMC), poly(acrylic acid) (PAA) and sodium alginate (NaAlg) have been introduced. Na-CMC, an environmentally friendly and inexpensive material, could play an important role in improving the solid electrolyte interface (SEI) passive layer, which reduces the irreversible capacity and somehow leads to a better cycle life. Na-Alg, which is more polarized than CMC, can ensure better interfacial interaction between the polymer binder and the particles, as well as create stronger adhesion between the electrode layer and the Cu substrate. Use of the PAA binder allows for the formation of a stable SEI layer on the elastic binder-coated electrode surface, in which the elasticity of a polymer matrix may prevent cracking at the SEI upon volume changes. Na-ion based electrochemical systems are alternatives to Li-ion systems. The study of electrode materials, carbon addi- tives, binders, electrolyte salts, solvents, and current collectors is important and must be established in order to realize the practicality of SIBs in large-scale applications such as ESS. We believe that improving the understanding and more findings of materials can accelerate the development of SIBs which are compatible to commercial level LIBs. 7. Perspectives LIBs have realized their adoption from portable devices to vehicle applications because of their high operation voltage and energy density, so that battery performances including price and safety are very important concern in technological evolution of electronic devices in future. In addition, demands on batteries for zero-emission EVs and energy storage are expected to increase exponentially from the year of 2020, and the secondary battery market will expand almost twice of the current level, 112 billion US dollars.633 Albeit the expandability of LIBs, the geological issue, in which lithium resources are localized in mainly South America, is thought of as a serious situation to be resolved. Use of cobalt or other rare metals as electrode materials is another concern on the price of batteries for future mass production for the purpose of mounting LIBs to EVs and energy storage applications. Indeed, the battery pack price should reach 20–30% of the current value to spread LIBs for those applications from 2020. This is controversial for the use of present high capacity lithiated Ni-rich cathode materials because each metal price except manganese is expected to View Article Online 3600 | 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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