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Chem Soc Rev Review Article oxygen environments (Fig. 18a).209 Early work by Gopalakrishnan et al. demonstrated the possibility of Na+ extraction to form V2(PO4)3 using chlorine in CHCl2 at room temperature.210 Electro- chemical Na+ extraction was investigated by Masquelier211 and Plashnitsa et al.212 and two voltage plateaus were found at 3.4 and 1.6 V. The latter work also tested a full symmetric cell, adopting Na3V2(PO4)3 for both cathodes, but the performance was unsatisfactory. Jian et al. modified the surface of Na3V2(PO4)3 using electro-conducting carbons derived from sugar.209 The Na3V2(PO4)3/C electrode exhibited improved capacity in both the upper and lower voltage regions compared with earlier works (Fig. 18b). They further reduced the irreversible capacity, changing the conventional electrolyte to NaFSI/PC from the PC-based compound. The new combination electrolyte increased the initial Coulombic efficiency above 98%.213 Saravanan et al. employed a solution-based template method using CTAB as a cationic surfactant that left approximately 6 wt% of carbon after synthesis in a reducing environment.214 They demon- strated stable long-term cyclability for over 30 000 cycles at a Another interesting compound, Na7V4(P2O7)PO4, was suggested by Lim et al.208 This material has a crystallized tetragonal structure with a P4%21c space group. The crystal structure has a basic unit, (VP2O7)4PO4, which is composed of a central tetrahedron [PO4] that shares corners with four [VO6] octahedra, and each diphosphate group [P2O7] bridges the two adjacent [VO6] octa- hedra with shared corners. The interconnected (VP2O7)4PO4 units comprise a three-dimensional channel for Na+ ion diffusion (Fig. 17d). Based on the V3+/4+ redox reaction, Na7V4(P2O7)PO4 exhibited a flat plateau at 3.88 V associated with a biphasic reaction governed by the first-order transition, delivering approximately 90 mA h g1 (Fig. 17e). Addition of rGO further extended its cycle life to 1000 cycles with 78% capacity retention. 2.3.4. NASICON type phosphates. Na3V2(PO4)3, which crystal- lizes to a rhombohedral structure with a R3%c space group, has a NASICON-type framework built up from isolated VO6 octahedra and PO4 tetrahedral units interlinked via corners to form a frame- work anion, [V2(PO4)3]3. Two different Na atoms are located in the empty channels of the framework with two different View Article Online Fig. 18 (a) The crystal structure of Na3V2(PO4)3. (Reprinted from ref. 209, Copyright 2012, with permission from Elsevier.) (b) TEM bright-field image of carbon-coated Na3V2(PO4)3. (Reproduced from ref. 213 with permission, Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA.) (c) Charge and discharge curves in a voltage range of 1.0 to 3.8 V for carbon-coated Na3V2(PO4)3. (Reprinted from ref. 209, Copyright 2012, with permission from Elsevier.) (d) Long term cycle life and Coulombic efficiency for 30 000 cycles at a rate of 40C for carbon-coated Na3V2(PO4)3. (Reproduced from ref. 214 with permission, Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA.) Thisjournalis©TheRoyalSocietyofChemistry2017 Chem.Soc.Rev.,2017,46,3529--3614 | 3557 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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