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Chem Soc Rev Review Article high discharge capacity of 185 mA h g1 in a voltage range of 2.5–4.5 V (Fig. 7b), whereas optimization of the electrode performance limited the upper voltage cutoff to 3.8 V, resulting in a capacity above 100 mA h g1 over 20 cycles. In contrast to NaNiO2, the O3 2 O03 2 P3 2 P03 phase transition was highly reversible in a voltage range of 2.5–3.8 V (Fig. 7c). Fe-substituted Na[Ni0.5Mn0.5]O2, however, underwent different structural transformation during desodiation compared with Na[Ni0.5Mn0.5]O2.84 Phase evolution from P3 to P300 was pre- dominant in Na[Ni0.5Mn0.5]O2, whereas Na+ extraction from Fe-substituted Na[Ni0.5Mn0.5]O2 facilitated a gradual evolution from P3 to OP2 due to the migration of Fe3+ ions from the octahedron of the transition metal layers to the interstitial tetrahedron or the octahedron of the Na layers (Fig. 7d). None- theless, capacity retention was improved in a voltage range of 2–4.3 V. In particular, a long irreversible reaction due to the formation of P300 phase transitioned to the OP2 phase in a voltage range of 4–4.3 V on charge, although the length of the plateau did not appear on discharge. This peculiar behavior seems to be related to the effect of Fe. Recently, Hwang et al. investigated the collation of Ni and Fe contents in Na[Ni0.75xFexMn0.25]O2 (x = 0.4, 0.45, 0.5, and 0.55).85 The structural and thermal stabilities, which affect cycle retention and rate capability, were dependent on the Fe content. Although the discharge capacity of higher Fe content materials decreased slightly, the improved cycling performance and rate capability compensate for the slightly reduced capacity. The increase in the Fe content seems to improve conductivity, which is associated with low band gap energy (B2.5 eV for Fe2O3).86 Therefore, enhanced rate performance with increasing Fe con- tent is attributed to the improvement in electric conductivity derived from Fe in the compound. Also, their DSC study revealed that reactive Ni4+ ions in the desodiated host material cause oxygen removal from the crystal structure, and that oxygen evolution can effectively suppress the Fe increase in the crystal structure (Fig. 7e). They also developed a radially-aligned hierarchical columnar structure in spherical particles with a varied chemical composi- tion from the inner end (Na[Ni0.75Co0.02Mn0.23]O2) to the outer end (Na[Ni0.58Co0.06Mn0.36]O2) of the structure.87 An electrochemical reaction based on Ni2+/3+/4+ had a discharge capacity of 157 mA h (g-oxide)1 with a capacity retention of 80% (125 mA h g1) over 300 cycles in combination with a hard carbon anode. The cathode also exhibited good temperature performance, even at 20 1C, which enables the Ni redox reaction. Ti-Substituted O3-Na[Ni0.5Ti0.5]O2 is also interesting due to its superior cyclability under moderate conditions, with an average operating potential of 3.1 V (vs. Na+/Na) and delivery of a reversible capacity of 121 mA h g1 at 20 mA g1. Since the average oxidation states of Ni and Ti are 2+ and 4+, respectively, the Ni2+/4+ redox reaction is responsible for electrochemical activity.88 Increasing the Ni content resulted in higher capacity, while a dramatic degradation in capacity and thermal properties was observed. In contrast, increasing the Fe content improved capacity retention and thermal stability in a highly desodiated state. Hence, it is worth noting that Ni redox such as Ni2+/4+ or high, delivering approximately 125 mA h g1 with retention of 91% for 100 cycles. Even at high rates, half capacity was delivered in the voltage range of 1.5–2.1 V associated with the Co3+/2+ and Mn4+/3+ reactions. The structural stabilization achieved in the biphasic compound may explain the excellent electrochemical performance. However, the capacity obtained in the low voltage region may decrease the energy density as use these electrode materials for cathodes. Recently, Matsui et al. explored the possibility of Ca-doping at Na sites to form Na2/3xCaxCoO2.79 The similarity in the ionic radius of Ca2+ (1.00 Å) versus Na+ (1.02 Å) allows the incorpora- tion of Ca2+ into the Na sites. Although the delivered capacity decreased to some extent due to Ca2+ in the Na layers, Na2/3xCaxCoO2 could suppress the multiple phase transition during charge and discharge. For example, Na5/8Ca1/24CoO2 could be cycled even at 5 mA cm1 with very little capacity decay, while capacity fade was inevitable in Ca-free Na2/3xCoO2. Post-cycled electrodes showed formation of a sodium poor phase of Na2/3CoO2, while Na2/3xCaxCoO2 suppressed this phase separation. This work emphasizes that stability in the Na environment is another topic of investigation that will contribute to development of a long-term cyclable cathode for Na cells. Xia et al. tested the reactivity of desodiated Na0.35CoO2 derived from P2-Na0.65CoO2 in the NaPF6-based electrolyte.80 Na0.35CoO2 decomposed to Na0.7CoO2 and Co3O4 with oxygen release from the crystal structure. The NaPF6 salt rapidly reacted with Na0.35CoO2, and NaCoF3 was produced via the exothermic reaction. This reaction has not been reported in a LixCoO2 system thus far. Selection of the electrolytic salt is another important issue to utilize the electrode in Na cells. 2.1.5. Na1xNiO2 and derivatives. NaNiO2 is stable as two polymorphs, a low temperature type with an O3 layer structure and a high temperature rhombohedral phase. Similar to a-NaFeO2, the Ni–O layer shares edges between NiO6 octahedra in which the Jahn–Teller Ni3+ ions are elongated.81 Since the starting material, Ni, is divalent, an oxidative environment is needed to synthesize O 0 3 type NaNiO2 with the space group C2/m. Early work reported by Braconnier et al. suggested that 0.2 mol of Na+, which has approximately 50 mA h g1 of capacity, was deintercalated from NaNiO2 via O03 2 P03 2 P03 2 O03 multiple phase transition based on the Ni3+/4+ redox reaction in the voltage range of 1.7–3.5 V.33 Later work by Vassilaras reported delivery of a high capacity of approximately 145 mA h g1 in a 2.2–4.5 V voltage range.81,82 By charging to 4.5 V, however, the capacity retention stabilized when the upper voltage cutoff was lowered to 3.75 V, with 94% of the initial capacity (115 mA h g1) after 20 cycles. The considerable Coulombic efficiency was due to oxidation of the electrolyte when charged to high voltage (Fig. 7a). Komaba et al. applied the findings of research on Li[Ni0.5Mn0.5]O2 to Na[Ni0.5Mn0.5]O2, in which the average oxidation states of Ni and Mn are 2+ and 4+, respectively.83 The material was also crystallized into an O3-type layer structure and could be solely activated by the redox reaction of Ni2+/4+. In comparison with NaNiO2, the presence of tetravalent Mn induced the formation of Ni2+ in Na[Ni0.5Mn0.5]O2. The two-electron reaction led to a View Article Online Thisjournalis©TheRoyalSocietyofChemistry2017 Chem.Soc.Rev.,2017,46,3529--3614 | 3539 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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