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Chem Soc Rev Review Article Scheme 1 Illustration of a Na-ion battery system. Thus, it is important to use a true Na-ion system, where Na ions are exchanged between cathodes and anodes in a ‘rocking-chair’ format. A new type of electrolyte for SIBs is needed, as the use of organic liquid electrolytes raises practicality and safety issues. The most common electrolyte formulations for SIBs are NaClO4 or NaPF6 salts in carbonate ester solvents, particularly propylene carbonate (PC). Metallic sodium anodes corrode continuously in the presence of these organic electrolytes, rather than forming a stable solid electrolyte interface (SEI). According to XPS and TOF- SIMS analyses performed by Komaba et al.,27 when NaPF6 is used as the electrolyte salt, the SEI film on hard carbon is predomi- nantly an inorganic salt that contains precipitated species such as NaF on the surface.7 Palacin and colleagues28 found that NaClO4 and NaPF6 in an EC:PC solvent mixture represent the best electro- lyte for a hard carbon anode. Developing aqueous electrolytes instead of organic electrolytes could be essential to the success of SIBs. Recently, an aqueous rechargeable battery with Na2NiFe(CN)6 and NaTi2(PO4) as the cathode and anode, respectively, demon- strated a good rate and cycle life with a theoretical energy density of 42.5 W h kg1.29 Thus, it is possible to achieve higher energy density by selecting the appropriate electrode material. Nevertheless, an aqueous electrolyte system is more complicated than an organic system because of the (1) elimination of residual O2 from the electrolyte, (2) maintenance of electrode stability in the aqueous electrolyte, (3) inhibition of H3O+ co-intercalation into the electrode and (4) efficiency of the internal consumption of O2 and H2 produced from the cathode and anode sides when overcharged, overdischarged or improperly operated in a closed aqueous battery system. All of these issues are important for practical applications of aqueous battery systems.30,31 Scheme 1 and Fig. 1 illustrate a schematic SIB image that can adopt several representative candidate materials such as cathode and anode materials, electrolytes, separators, and binders that are discussed in the present paper. Most studies of SIBs explored the electrochemical performance of new electrodes and materials used with Na metal in half-cells, as this field is still in its early stages, and it is thus difficult to create full cells. In that regard, discussion and justification of SIBs are complicated compared to those of LIBs. In addition, experimental conditions such as the purity of Ar gas, electrolyte quality, and the glove box can influence the performance of SIB cells. Hence, comparison of battery performance between studies can be challenging.18 2. Cathode materials Similar to LIBs, highly reversible cathode materials based on the intercalation reaction, which involves interstitial introduc- tion of a guest species (Na+ in the present context), are needed for high capacity and good cyclability of SIBs. These electrode materials are mainly categorized into oxides, polyanions such as phosphates, pyrophosphates, fluorosulfates, oxychlorides, and NASICON (Na super ionic conductor) types, and organic compounds, which are mentioned in detail in Sections 2 and 3. These cathode materials exhibit a minimal structural change with intercalation, which ensures a reversible intercalation reaction that affects the cycle life. However, continuous struc- tural evolution is inevitable during Na+ ion intercalation into the host structure interaction because of the large Na+ ion size (coordination number 6: 1.02 Å) relative to that of Li+ View Article Online Thisjournalis©TheRoyalSocietyofChemistry2017 Chem.Soc.Rev.,2017,46,3529--3614 | 3531 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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