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Chem Soc Rev Review Article associated with their three-dimensional open framework. How- ever, these compounds can be used as electrode materials with the incorporation of transition metals into their structure. Na3V2(PO4)3, for example, showed improved capacity at both higher and lower voltage regions after the modification of its surface with carbons derived from sugar. Their earlier work showed unsatisfactory results when tested as cathodes in a full symmetrical cell. Prussian blue and its analogues have also attracted attention due to their high energy, power density and electrochemical properties. However, unlike polyanions, their poor thermal stabilities remain a hindrance. Types of cathode materials, their redox couple, capacity, and operation voltage are summarized in Table 1. Given that the electrochemical insertion of Na+ into graphite, which is a commonly used anode material in LIBs, has not been considered suitable for Na-ion batteries, an alternative system for large-scale energy storage is needed because of the thermodynamic instability of binary Na-intercalated GICs.258,259 Therefore, the search for appropriate anodes with proper Na voltage storage, large reversible capacity and high structural stability is necessary in order for SIBs to be successfully devel- oped. Sodium metal is not recommended for use in an anodes due to its high reactivity and formation of dendrite. Many non-graphitic materials such as carbon black, pitch-based carbon-fibers have been demonstrated to insert Na+. Hard carbons, i.e., non-graphitic but graphene containing, carbonaceous materials, are considered as the ‘‘the first-generation’’ anode of choice for SIBs.22,23,241 The low BET surface area of hard carbon appears to be a pivotal factor in achieving good rever- sible cycling of SIBs. However, the low operational potential of carbonaceous anodes raises severe safety issues for practical applications. Hence, transition metal oxide compounds, espe- cially titanium-based oxides, have been widely studied.300 The most common form of titanium oxide compounds, titanium dioxide (TiO2), has been tested in Na systems by downsizing the size to the nanometer-scale. It was found that the pseudo- capacitance mechanism was responsible for the charge storage. The pseudocapacitive effect on typical insertion electrodes is believed to be a potential solution to overcoming the capacity limitation for Na insertion anodes. Conversion reactions of metal oxides can also be useful for anodes in SIBs.243 These conversion reactions, which depend on the transition metals, occasionally combined with the insertion–extraction or alloy- dealloying reactions. The reaction concept was first introduced with spinel NiCo2O4, which delivered a reversible capacity of B200 mA h g1 after the initial discharge of B600 mA h g1. Previous research has shown that the conversion reaction that forms sodium oxide is the primary reaction because spinel NiCo2O4 cannot accommodate for the large size of Na+ into its vacant sites.365 Various metal sulfides such as cobalt sulfides (CoS, CoS2), molybdenum sulfides (Mo2S, MoS2), iron sulfides (FeS, FeS2), and tin sulfides (SnS, SnS2) have been extensively investigated as high capacity anode materials for NIBs. The weaker M–S bond in metal sulfides compared to the corresponding M–O bond in metal oxides is kinetically favorable for conversion reactions with Na ions.410 Hence, the smaller volume changes and better increasing, its reserves (specifically in lithium-constrained areas) will be exhausted. This will lead to an escalating price for lithium, and thereby the use of LIBs for ESS applications will not be cost-effective. With the recognition that SIBs have recently received, the hurdles for making them as applicable as LIBs cannot be taken lightly. Electrodes with open frameworks are more desirable in sodium systems, as the larger-sized Na+ (in comparison to Li+) can be introduced into the framework. However, given how the immense structural changes that occur when introducing Na+ cannot be avoided, disruption of structure’s durability while trying to maintain its original state is bound to happen.630 Moreover, sodiated transition metal materials are particularly hygroscopic, even upon brief exposure to air. Thus, the pre- paration of sodiated cathode materials and batteries requires meticulous handling and moisture-free conditions. The hydra- tion of materials will deteriorate the electrochemical perfor- mance of the electrode due to the formation of intrinsically insulating NaOH. One recent study on cathode materials for SIBs has focused on this hygroscopic problem. Specifically, the Sumitomo Chemical Co., Ltd research group has successfully synthesized O3-type Ca-doped NaFe0.4Ni0.3Mn0.3O2, and they managed to suppress its tendency to absorb moisture through Ca doping. Although it is reasonable to suspect that the kinetics of Na+ ion diffusion might be a problem due to the larger ionic size relative to that of Li+, Ceder et al. reported that Na+ extraction/ re-insertion from/into Na-containing materials is experimentally and computationally faster compared to Li systems due to the low Lewis acidity of Na+. By contrast, the insertion of Na+ into non Na-containing compounds with a smaller bottleneck size may be slower compared to the insertion of Li+. Furthermore, b-Al2O3; the fast Na+ ionic conductor, was discovered almost 50 years ago, which is even earlier than Li+. Studies on cathode materials are considerably broad, includ- ing oxides, polyanions, NASICON (Na Super Ionic Conductor) types and organic compounds. Through studies on the O3-type sodium-containing layered transition metal oxides Na1xFeO2, it has been found that this material is electrochemically active based on Fe3+/Fe4+ redox couples, which is in contrast to O3-type lithium containing the same layered transition metal oxides in a Li system. Moreover, the irreversible capacity that results from the migration of iron ions in Na1xFeO2 can be suppressed with the help of Co or Mn occupation in transition metal sites as a substitute to Fe.39,47,50 However, the problem of insufficient cycle stability has yet to be solved. In the case of polyanion compounds, they exhibit better thermal stabilities compared to those of oxides, owing to the presence of the P–O covalent bonds in the crystal structure. They have an operating voltage comparable to that of the Li system. In addition, as opposed to the earlier mentioned information regarding the kinetic diffusion of Na+ ions, a few studies on NaFePO4 have reported that the migration energy of Na+ ions is 0.05 eV higher than that of Li+, which is actually a slower kinetic diffusion than reported here.173 Hong et al.631 and Goodenough et al.632 initially proposed NASICON type compounds for use in Na+ ion solid electrolytes due to the high Na+ ionic conductivity View Article Online Thisjournalis©TheRoyalSocietyofChemistry2017 Chem.Soc.Rev.,2017,46,3529--3614 | 3597 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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