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Review Article Chem Soc Rev Fig. 17 (a) Schematic representation of Na4Fe3(PO4)2(P2O7) and (b) galvanostatic charge/discharge profiles of Na4Fe3(PO4)2(P2O7) under a C/40 rate and the calculated average voltage at each region, where the inset shows the dQ/dV curve of initial charge/discharge profiles. (Reproduced with permission from ref. 205, Copyright 2012 American Chemical Society.) (c) Galvanostatic charge/discharge curves at 1st, 10th and 50th cycles of Na4Co3(PO4)2P2O7. (Reprinted from ref. 206, Copyright 2012, with permission from Elsevier.) (d) The projected crystal structure along the c-axis: gray tetrahedrons, PO4; red octahedrons, VO6; yellow spheres, Na atoms. The (VP2O7)4PO4 unit is denoted as a dashed circle and (e) galvanostatic profiles of Na7V4(P2O7)4PO4 measured at C/20 and C/40 in the first cycles. (Reproduced with permission from ref. 208, Copyright 2014 PNAS.) View Article Online Although stepwise charge and discharge curves were observed, the average operating voltage was approximately 4.5 V, which is the highest value that has ever been observed in the Na system (Fig. 17c). In contrast to Na4Fe3(PO4)2P2O7, the appear- ance of a stepwise curve would be ascribed to the presence of Na+/vacancy ordering in the crystal structure. The material could deliver a capacity of approximately 80 mA h g1 even at 25C-rates (4.25 A g1). This superior electrochemical performance was attributed to the help of the three-dimensional Na+ diffusion pathways in the crystal structure. In an attempt to mitigate the multistep voltage plateaus, they synthesized Na4[Co2.4Mn0.3Ni0.3]- (PO4)2P2O7. The material also exhibited an average operating voltage of 4.5 V, and a 103 mA h g1 discharge capacity was obtained at a rate of 5C. As expected, partial replacement of Co by Ni and Mn led to sloppy charge and discharge curves. In addition, not only Co, but two substituents, Ni and Mn, participated in the electrochemical reaction in a voltage range of 2–5 V according to an XAS study. 3556 | 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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