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Chem Soc Rev Review Article As cycles went by, the length of the upper voltage plateau related to Ni2+/4+ and the lower voltage plateaus related to Mn3+/4+ 2 V decreased. This can be interpreted in two ways: (1) the progressive formation of the O2 phase at high voltage and the effect of the Jahn–Teller distortion in the crystal lattice. This further limited the cycling region to three parts: 1.5–4.3 V, 1.5–4.0 V, and 1.7–4.0 V. It is evident that, although the Jahn– Teller distortion is present in low voltage regions, severe capacity fade occurs in high voltage regions. This explains that the P2–O2 phase transition is not favored when maintaining the capacity because of the large difference in the c-axis, which causes a drop in the capacity (Fig. 9d). Jahn–Teller distortion seems to be less pronounced in capacity retention. In contrast, instead of Co, Yuan et al. suggested that Ni substitution with Fe (P2-Na0.7[Mn0.65Ni0.15Fe0.2]O2) led to an improvement in capa- city and retention: 208 mA h g1 with 71% retention over 50 cycles.115 The phase transition was P2 to OP4 in this case.117 Tetravalent Ti-substituted P2-Na2/3[Ni1/3Mn2/3xTix]O2 yielded a wide solid solution range of 0 r x r 2/3.118,119 Due to the similarity of both Mn4+ and Ti4+ in the valence and ionic radius, such a solid solution can be formed readily throughout the range. The reason remains unclear, but an increase in the Ti content resulted in a decrease in capacity. In particular, Na2/3[Ni1/3Mn2/3xTix]O2 (x = 1/6) delivered approximately 127 mA h g1 with an average operating voltage of approximately 3.7 V on discharge, which corresponds to 470 W h kg1. More importantly, the stepwise voltage profiles of Na2/3[Ni1/3Mn2/3]O2 were obviously diluted by Ti substitution. XRD suggested that the volume change of the fully charged state was reduced from 23.1% in Ti-free compounds to 12–13% for Ti-substituted compounds, confirming the suppression of Na/vacancy ordering and phase transitions during sodium intercalation. A new mechanism was introduced in P2-Na2/3[Mg0.28Mn0.72]O2 and P2-Na5/6[Li1/4Mn3/4]O2 by Yabuuchi and Komaba et al.120,121 In this compound, since Mg2+ and Li+ are electrochemically inactive, the only possible reaction should be associated with Mn redox upon de-/sodiation. While the average oxidation states of Mn are 3.84+ for Na2/3[Mg0.28Mn0.72]O2 and 3.88+ for Na5/6[Li1/4Mn3/4]O2. Nonetheless, both materials exhibited large capacities of approximately 220 mA h g1 for Na2/3[Mg0.28Mn0.72]O2 (Fig. 9e) and 180 mA h g1 for Na5/6[Li1/4Mn3/4]O2. Similarly, a recent report by Slater et al. also showed a large capacity of 200 mA h g1 for P2-Na0.85[Li0.17Ni0.21Mn0.64]O2.7 Taking the Mn3+/4+ redox species into account, it is not possible to explain the delivery of high capacity. Indeed, there is almost no delivery of capacity in the voltage cutoff up to 4 V on charge; however, those capacities are delivered above 4 V, which is related to oxidation of oxide ions, resulting in the release of oxygen from the oxide lattice. Oxygen removal causes formation of Mn3+, which simultaneously participates in oxidation on charge and is reduced on discharge. This causes rearrangement of the in-plane cation ordering of the crystal structure, which is similar to a Li-rich Li2MnO3 system.122 P2–O2 transition was also observed in these compounds (Fig. 9f), which is worth mentioning because of the theoretical capacity of 173 mA h g1 based on the Ni2+/4+ redox reaction. Ni2+ was successfully overcome by the oxygen compensation accompanied by the Mn3+/4+ redox reaction. P2–O2 transition up to 4.4 V on charge, showing a simple solid solution reaction across the entire range.105 As a result, there was no characteristic voltage plateau in a voltage range of 4.1–4.4 V. This feature ensured excellent capacity retention as high as 91% for 50 cycles (B120 mA h g1 at the first discharge). Substitution of divalent elements such as Mg2+ and Zn2+ in Ni is effective for diluting the effect of the above-mentioned P2–O2 phase transition.106–113 A decrease in the initial charge capacity is natural because the electro-active species Ni2+ was reduced by substitution, as shown in Fig. 9a, which reflects the origination of the capacity drop from the Ni2+/4+ redox reaction. Stepwise voltage plateaus were not dominant through- out the operation range of 2–4.5 V.106,107 Compared with Na2/3[Ni1/3Mn2/3]O2, the capacity drop in Na0.67[Ni0.2Mg0.1Mn0.7]O2 was only 6 mA h g1 for 50 cycles.107 Using in situ XRD, instead of the P2 phase, a new phase identified as an OP4 phase was found at voltages above 4.2 V, where a short voltage plateau was observed compared to Mg-free Na2/3[Mn1/3Mn2/3]O2 (Fig. 9b). The presence of electro-inactive Mg2+ in transition metal layers facilitates occupation of Na+ ions in prismatic sites, stabilizing the overall charge balance of the compounds. Since the Na+ ions are less extracted from the host structure assisted by Mg2+ in the transition metal layers, the original P2 phase is retained as the major phase, though transition to OP4 because deep desodiation is observed at high voltage as a minor phase. Hence, the P2–O2 transition and the Na+/vacancy ordering in Na2/3[Ni1/3xMgxMn0.7]O2 are suppressed during cycling via Mg substitution. This P2–OP4 phase transition is highly reversible; the cycling performance demonstrates its superiority compared to the P2–O2 phase transition.101 While Zn-substituted P2-Na2/3- [Ni1/3xZnxMn2/3]O2 induces reversible phase transition during electrochemical reactions.111,112 Although the capacity was still limited to approximately 140 mA h g1 by the Ni2+/4+ redox reaction due to reductions in electro-active Ni2+, the enhanced structural integrity achieved by Zn2+ in the transition metal layer enabled good cyclability due to readiness for transformation and a less variation in unit volume. Further studies explored partial replacement of Ni sites by Al.113 The substitution displayed sloping charge–discharge curves due to the diminution of the electro-active Ni2+ concentration, suggesting that Ni2+/4+ redox has superior capacity delivery. The substituent stabilized the host structure and the resulting cyclability was significantly improved, in particular for Al-doped P2-Na2/3[Ni1/3Mn2/3]O2, which had 94.8% (from B147 mA h g1) retention over 30 cycles. Replacement with a trivalent 3d transition metal element such as Co and Fe is also interesting because they are electro- chemically active in the same operating range.114–116 Yoshida et al. investigated the composition of P2-Na0.7[Mn0.6Ni0.3Co0.1]O2, in which the oxidation state of Mn is approximately 3.4+, in a voltage range of 1.5–4.3 V.114 Note that the oxidation state of Mn is 4+ in P2-Na2/3[Ni1/3Mn2/3]O2. As expected, the material exhibited multiple voltage plateaus. In comparison with P2-Na2/3- [Ni1/3Mn2/3]O2, the improvement in capacity during the first cycle (approximately 204 mA h g1) is due to the additional capacity contributed by Mn3+/4+ redox below 4 V (Fig. 9c). The long plateau above 4 V confirmed the validity of the Ni2+/4+ redox reaction. View Article Online Thisjournalis©TheRoyalSocietyofChemistry2017 Chem.Soc.Rev.,2017,46,3529--3614 | 3543 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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