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Chem Soc Rev Review Article particles via carbonization of pitch, which improved the elec- trical conductivity to approximately 101 S cm1.131 The carbon-coated NaCrO2 electrode exhibited excellent cyclability and an ultrafast rate capability of up to a 150C-rate (Fig. 10b). High electrical conductivity successfully promotes reversible insertion and extraction of sodium ions accompanied by a facile complementary redox reaction of the Cr3+/Cr4+ couple as confirmed by XANES (Fig. 10c–e). The NaCrO2 electrode was also sufficiently stable in an intermediate temperature ionic liquid, NaFSA-KFSA, at 363 K.132 Excellent electrochemical per- formance of these cathodes was demonstrated in both half and full sodium ion cells. Apart from the electrode performance, the carbon coating layer also delays exothermic decomposition by preventing oxygen evaporation from the crystal lattice; specifically, the lower heat generation from the desodiated Na0.5CrO2 was ascribed to the suppression of oxygen evolution during the exothermic reaction, which results from the presence of carbon coating layers. A major challenge associated with sodiated cathode materials is immediate water absorption after exposure to air. This leads to the formation of NaOH and Na2CO3 on the surface of active materials. The sodium diffuses to the surface, and electro- chemically inactive parts are formed within the particles. Even worse, the NaOH and Na2CO3 formed are electrical insulators. The carbon coating can prevent moisture uptake due to its hydrophobic characteristics. 2.1.9. Na1xVO2 and derivatives. Among several Na1xVO2 compounds, only two compositions, x = 0 and x = 0.3, have been identified;133,134 the former represents O3 and the latter, P2. However, O3-NaVO2 reacts in air for a few seconds, leading to desodiated Na1xVO2 phases.134 NaVO2 shows highly reversible Na+ insertion and extraction in a voltage range of 1.2–2.4 V (Fig. 11a). The first charge plateau is a typical characteristic of the biphasic domain until the composition reaches Na2/3VO2. Complicated behavior is observed upon further sodiation, showing three voltage plateaus. Approximately 120 mA h g1 was delivered. Upon desodiation, the O3 structure transforms into the O03 structure via monoclinic distortion. Na+ extraction above x = 0.5 in Na1xVO2 deteriorates electrode performance, resulting from the migration of vanadium ions into interslab vacancies. This phenomenon is similar to Na1xCrO2.128,130 Despite considerable voltage variation and oxygen sensitivity, capacity fading was negligible for 15 cycles, as reported by Hamani et al.135 In P2-Na0.7VO2, the resulting charge and dis- charge behavior is very reversible in a voltage range of 1.2–2.6 V, delivering approximately 105 mA h g1 (Fig. 11b). In contrast to the Fe- and Mn-based compounds presented in Sections 2.1.2 and 2.1.6, the related phase transition is more or less compli- cated (Fig. 11c and d). An abrupt voltage drop occurs due to the single phase domain and the plateaus are associated with the solid solution reaction. Hence, four single phase domains and solid solutions are present between the single phase domains, which have been identified in XRD studies.136 Similarly, these multi-domains are also observed in P2-NaxCoO2.62 Since NaVO2 is stable in a reducing atmosphere, carbon coating of NaVO2 could further minimize oxygen uptake in air. Successful carbon facilitate the formation of Al2O3-coated P2-Na2/3[Ni1/3Mn2/3]O2, although there is some possibility of Al doping in the crystal structure because additional heating after the removal of the aqueous medium was performed at 650 1C for 10 h. Al2O3- coated P2-Na2/3[Ni1/3Mn2/3]O2 had good cycling performance for 300 cycles with approximately 72% retention in a voltage range of 2.5–4.3 V. Although the detailed mechanism related to this high capacity retention remains unclear, structural stabili- zation of the host material by the coating was suspected. 2.1.7. Na1xTiO2 and derivatives. NaTiO2 was first synthe- sized by Hagenmuller et al. and Maazaz et al. evaluated its potential as a Na+ insertion/extraction material.124,125 This material underwent phase transformation from O3 2 O03, in which the chemical composition reaches Na0.7TiO2 (approxi- mately 75 mA h g1) on desodiation, showing an average operating voltage of 1 V, which is suitable for an anode. In contrast to the other materials, the electrode exhibited a low operating voltage of B1 V, followed by the Ti3+/4+ redox reaction. When a solid solution is formed with NaNiO2 (Na[Ni0.5Ti0.5]O2) as suggested above by Yu et al., the material is sufficiently stable for long-term cycling as a cathode because Ti enables significant structural stability and the Ni2+/4+ redox couple contributes to capacity delivery.88 2.1.8. Na1xCrO2 and derivatives. The first study of O3-NaCrO2 showed limited capacity, which was desodiated to Na0.85CrO2.126 Miyazaki et al. also investigated Na+ desodiation via chemical and electrochemical methods, in which extraction facilitated the formation of Na0.4CrO2.127 They also confirmed the formation of Cr4+ for desodiated Na0.5CrO2, as analyzed based on magnetic susceptibility. Recently, Komaba et al. revisited O3 type layer-structured NaCrO2, in which Na ions could be inserted into/extracted from the host structure.128 In contrast to LiCrO2 in Li cells, NaCrO2 could deliver a capacity of approximately 110 mA h g1 in the voltage range of 2–3.6 V due to the greater inter-slab distance provided by the presence of large Na+ ions in the Na layers. Despite the high theoretical capacity of about 250 mA h g1, the practical reversible capacity was approximately 110 mA h g1 (Na1xCrO2, 0 r x r 0.5), with a flattened voltage plateau at 3 V (Fig. 10a). Dahn’s group also reported that desodiated Na0.5CrO2 has excellent thermal stability in Na-based non-aqueous electrolytes.129 However, NaCrO2 electrodes suffer from capacity fading during cycling.128,129 In consideration of synthetic conditions, NaCrO2 is usually produced in a reducing atmosphere to retain the oxidation state of Cr at 3+. This condition enables carbon coating, which can dramatically improve electrode performance. Ding et al. found that citric acid-assisted carbon coating slightly improved the cycling stability of NaCrO2 electrodes.130 Although the cycling performance of carbon-coated NaCrO2 was improved compared to the bare material, operation at high rates was not possible, presumably due to inhomogeneous or excessively thick carbon layers. The XANES study revealed that Cr3+ is oxidized to a higher chemical state on charge. The above results are not satisfactory for use in practical applications because of rapid capacity fading (ca. 80% in the 50th cycle) and disappointing rate capability. Yu et al. modified the surfaces of the NaCrO2 View Article Online Thisjournalis©TheRoyalSocietyofChemistry2017 Chem.Soc.Rev.,2017,46,3529--3614 | 3545 Open Access Article. Published on 28 March 2017. Downloaded on 7/1/2019 3:41:21 AM. 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