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Chem Soc Rev Review Article Fig. 5 (a) First charge and discharge curves of P2-Na0.7CoO2. (Reprinted from ref. 33, Copyright 1981, with permission from Elsevier.) (b) Comparison of open circuit EMF composition curves obtained from P2-Na0.7CoO2y. (Reprinted from ref. 69, Copyright 1989, with permission from Elsevier.) (c) Comparison of dQ/dV curves of O03-Na0.83CoO2 (left), P3-Na0.67CoO2 (center), and P2-Na0.67CoO2 (right). (Reproduced with permission from ref. 71, Copyright 1988 The Electrochemical Society.) View Article Online whereas P3-Na0.7CoO2 exhibited retention of approximately 60% for over 250 cycles. The higher capacity for the P2 phase may be due to its higher crystallinity compared with the P3 material produced at low temperatures. Ma et al. confirmed that P2-Na0.7CoO2 can be used for long-term cycling in a P(EO)8NaCF3SO3 electrolyte at 90 1C.72 Berthelot et al. revisited P2-NaxCoO2 to investigate the phase diagram in a range of 0.45 r x r 0.9, in which nine potential drops were found during the second discharge (Fig. 6a).62 Weak polarization, which is an intrinsic property of P2-Na0.7CoO2 due to its high conductivity, enabled characterization of each voltage drop via in situ XRD. The (008) diffraction peak some- times splits into two peaks, which is indicative of a two-phase domain when the discharge curve exhibits voltage plateaus, while the voltage drop indicates a strictly fixed peak due to the single phase reaction, demonstrating the presence of ordered phases (Na0.5CoO2, Na0.67CoO2 (No. 3), Na0.72CoO2 (No. 7), Na0.76CoO2 (No. 8), and Na0.79CoO2 (No. 9), although they are present only in very narrow ranges. Between the voltage plateau and the voltage drop, the diffraction peak shifts slightly with solid solution behavior. Na ordering arises from two different repulsive interactions, sodium–sodium and sodium–cobalt. Occupation of Na at the shared edges (Nae) for P2-Na2/3CoO2 was predicted by first-principles calculations63,67 and was experimentally verified.62,68 At the same time, some of the Na ions are located at the shared faces (Naf). For P2-Na0.5CoO2, an ordered phase, half of Na occupies Nae sites while the rest appears at Naf sites, which probably minimizes the repulsive in-plane interaction between Naf and Nae. The distribution of Na+ ions in these two positions depends on the Na content in the compound, which affects the Na+/vacancy-ordered structure. Croguennec et al. obtained T#2-Li2/3[Co2/3Mn1/3]O2 via ion- exchange from P2-Na2/3[Co2/3Mn1/3]O2, in which the oxidation states of Co and Mn were 3+ and 4+, respectively.73,74 Cheng et al. also confirmed that the oxidation states of Co and Mn for P2-Na2/3[Co2/3Mn1/3]O2 are stabilized at 3+ and 4+, respectively.75 Carlier et al.76 evaluated P2-Na2/3[Co2/3Mn1/3]O2 in the Na cell and Yang et al.77 extended the stability of P2-Na2/3[Co1xMnx]O2 to x = 0.5. The resulting P2-Na2/3[Co2/3Mn1/3]O2 shows suppression of Na+/vacancy ordering in the structure because the stepwise voltage plateau disappeared in a range of 0.5 r x r 0.83 in Nax[Co2/3Mn2/3]O2 (Fig. 6b). Note that P2-Na0.74CoO2 formed nine distinct phases in a range of 0.5 r x r 0.9 in NaxCoO2 (Fig. 6a). However, the single phase domain for x = 0.5 in NaxCoO2 is still found in Na2/3[Co2/3Mn1/3]O2, although the voltage drop at that composition is lower than in NaxCoO2. The main differ- ence after Mn substitution for Co is abrupt voltage decay in a range of 0.65 r x r 0.83 in Nax[Co2/3Mn1/3]O2. They further Thisjournalis©TheRoyalSocietyofChemistry2017 Chem.Soc.Rev.,2017,46,3529--3614 | 3537 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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