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Review Article Chem Soc Rev Fig. 12 (a) The crystal structure of Na0.44MnO2 perpendicular to the ab–plane. (Reproduced with permission from ref. 141, Copyright 2007 American Chemical Society.) (b) Discharge curves of NTP-C/NMO full cells in two-electrode configuration at different C-rates. (Reproduced from ref. 145 with permission, Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA.) (c) The crystal structure of a-MnO2 along the [001] direction (top) and b-MnO2 along the [001] direction (bottom) and (d) the resulting initial charge and discharge curves of a-MnO2 (top) and b-MnO2 (bottom). (Reproduced with permission from ref. 146, Copyright 2013 The Royal Society of Chemistry.) View Article Online to unexpectedly high rate performance. Since the ionic size of Na+ is larger than that of Li+, the host structure should be sufficiently rigid or have a large tunnel size to facilitate the entry of large ionic species. Otherwise, the crystal structure could collapse upon repetitive Na+ insertion and extraction. These electrodes are active compared to Na metal; however, the main difficulty is that a sodiated anode is needed to construct a full cell. Recent research introduced low voltage operating P2-Na0.62[Ti0.37Cr0.63]O2 anode materials.138 Plenty of possibili- ties remain regarding the development of full cells utilizing these transition metal oxides and sodiated anode materials. 2.2.1. Manganese oxides. Several types of Na–Mn–O com- pounds were introduced by Parant et al.89,90 A lower Na/Mn ratio results in adoption of a three-dimensional structure such as Na0.2MnO2,90 Na0.4MnO2,89 or Na0.44MnO2 (space group Pbam).89,90,139–145 Among these, Na0.44MnO2, which is isostructural with Na4Mn4Ti5O18, is particularly interesting because of its cycling stability with a reasonable capacity, B120 mA h g1. Doeff et al. first examined the Na+ insertion properties of Na0.44MnO2 with a solid-state polymer electrolyte at 85 1C.139,140 The crystal structure consists of four MnO6 octahedral sites with Mn4+ and one MnO5 square-pyramidal site with a half of Mn3+ (Fig. 12a). These are connected by corner sharing to form two types of tunnels. Each unit cell contains a large S-shaped tunnel with four sodium sites, as well as two identical penta- gonal tunnels. The Na sites in the small tunnels are nearly fully occupied, while the large tunnel sites are partially occupied: Na1 and 2 sites in the S-shaped tunnels, half-filled; Na3 sites with an inner position in the small tunnels, fully-filled. Both Na+ ions are highly mobile along the c-axis, contributing to capacity. Sauvage et al. performed carbon coating of Na0.44MnO2, which led to a capacity of 140 mA h g1.141 They speculated 3548 | Chem. Soc. Rev., 2017, 46, 3529--3614 This journal is © The Royal Society of Chemistry 2017 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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