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Review Article Chem Soc Rev Fig. 13 (a) Synchrotron XRD and SEM images of electrodeposited bilayered V2O5 (top) and orthorhombic V2O5 (bottom) and (b-1) first four charge– discharge cycles of bilayered V2O5 and orthorhombic V2O5 and (b-2) the cycle life of bilayered V2O5. (Reproduced with permission from ref. 152, Copyright 2012 American Chemical Society.) (c) Initial charge and discharge curves of NaFeF3 (reprinted from ref. 155, Copyright 2009, with permission from Elsevier.) View Article Online Fe3+/2+ redox reaction achieved Na+ insertion and extraction, which could deliver 150 mA h g1 during the first cycle. All MF3 compounds except FeF3 (M: V, Ti, Co, and Mn) exhibited disappointing Na+ storage. They further developed sodiated metal fluoride crystallized into the perovskite structure, NaMF3 (M: Fe, Mn, Ni). Among those materials, NaFeF3 had a relatively a large discharge capacity (128 mA h g1) with an average cell voltage of 2.7 V (Fig. 13c), while NaNiF3 and NaMnF3 suffered from capacities below 40 mA h g1 with sloppy voltage decay. Even when the synthesis method was switched from mechano- milling to a solution-based method, the resulting capacity did not exceed the prior report at the same current density (0.2 mA cm1), but instead reached 180 mA h g1 at a rate of 0.01C.157 The strong ionic character of the M–F bond must be overcome to achieve a high capacity even at high rates. Homogeneous dispersion of nanosized metal fluoride materials onto a conducting carbon matrix might facilitate high rate performance. 2.3. Three-dimensional polyanion compounds In comparison with oxide and fluoride systems, transition metal polyanion materials have shown significant thermal stability, which is supported by the presence of covalent bonds such as P and O, in particular cathodes are in a deeply-charged (oxidized) state. Oxygen evolution is common in layered compounds at temperatures above 200 1C;51,85,158–162 however, such behavior is dramatically suppressed by the presence of P–O covalent bonds in the crystal structure. These phenomena prevail in Li cells and also apply to Na systems, since the related material chemistry during electrochemical reactions does not vary significantly from Li systems with the exception of the charge carrier, Na+, in Na cells. Polyanion-based materials usually exhibit lower electric conductivity relative to oxides, such that surface modifications using electro-conducting carbons, which contribute to a dramatic increase in electrical conductivity, improve electrochemical performance. The basic form begins from NaFePO4; interestingly, some factors such as (i) variation in the charge carrier number of Na in Na sites, (ii) partial or full replacement of Fe by the other transition metals, (iii) a mixed anion system with F, OH, CO2, and (iv) extension towards mixed phosphate (PO4)3 and pyrophos- phate (P2O7)4 ions also stabilize the crystal structure in the Na system. Unfortunately, because moisture absorption occurs easily in these polyanion systems, avoiding hydration and formation of NaOH on the surfaces of particles can be a challenge. Heterogeneous surfaces may lead to misinterpreta- tion of the electrode performance in Na-containing aprotic electrolytes. 3550 | 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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