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Chem Soc Rev Review Article inactive, as the structure blocks Na+ extraction and insertion.166 Olivine NaFePO4 shows a relatively high operating voltage (2.8 V) with two distinct voltage plateaus, delivering over 120 mA h g1 capacity (Fig. 14c).163–173 During the electrochemical reaction, the oxidation state of Fe is systematically altered followed by a redox reaction of Fe3+/2+, as confirmed by the XAS studies performed by Ali et al.173 Carbon coating led to excellent long- term cyclability. An interesting feature of triphylite NaFePO4 is the appearance of intermediate Na0.7FePO4, which has the same crystal structure as the triphylite NaFePO4, during electrochemical testing, whereas this phase is not found in LiFePO4. Moreau et al. suggested that the advent of Na0.7FePO4 could be related to cationic ordering in the crystal structure.164 Similar behavior is also observed in NaxCoO262 in comparison with LixCoO2.174 Casas-Cabanas et al. demonstrated that Na+ ion insertion into FePO4 occurs via the intermediate phases due to large volume mismatches between FePO4 and NaFePO4.170 Therefore, three phases, FePO4, Na0.7FePO4, and NaFePO4, appear simultaneously on discharge, while they are separated into two first-order phase transitions on charge, which may be related to Na+/vacancy ordering in the crystal structure. In comparison with LiFePO4, the charge transfer resistance of NaFePO4 is high, and the diffusion coefficient of Na+ ions is 2.3.1. Phosphates and fluorophosphates. Maricite NaFePO4 is a thermodynamically favored phase because it can be synthe- sized at high temperatures. Avdeev et al. demonstrated that chemically sodiated triphylite (olivine) NaFePO4 exhibited an irreversible phase transition from olivine to maricite NaFePO4 at around 480 1C, resulting in a significant volume shrinkage.163 Crystal structures of both polymorphs consist of slightly distorted FeO6 octahedra and PO4 tetrahedra. The maricite NaFePO4 has edge-sharing FeO6 units that share corners with neighboring PO4. There are no cationic channels for Na+ move- ment (Fig. 14a).164 In contrast, triphylite NaFePO4 has corner- sharing FeO6, which is linked with the PO4 edge (Fig. 14b). A one-dimensional Na+ diffusion channel is clearly seen along the b-axis in the crystal structure. The difference between these two polymorphs is the corner sharing and edge sharing FeO6 chains for triphylite and maricite, respectively. However, direct synthesis of triphylite NaFePO4 is not possible. Thus, chemical and electrochemical sodiation are effective ways to insert Na+ into heterosite FePO4 (space group, Pnma). Electrode performance is dependent on the crystal structure. Amorphous NaFePO4 exhibited a high discharge capacity of B150 mA h g1, but operated at a low voltage (2.4 V) with a sloping discharge profile.165 Maricite NaFePO4 is electrochemically View Article Online Fig. 14 (a) The crystal structure of maricite NaFePO4 and (b) olivine NaFePO4. (Reproduced with permission from ref. 164, Copyright 2010 American Chemical Society.) (c) Charge–discharge profiles of NaFePO4 for 50 cycles. (Reprinted from ref. 169, Copyright 2012, with permission from Elsevier.) (d) Structure of Na2FePO4F along the [100] direction, iron octahedra: blue and the phosphate tetrahedral: yellow, and Na (1) in green and Na (2) in pink. (Reproduced by permission from ref. 176, Nature Publishing Group, Copyright 2007.) (e) Charge/discharge curves of the Na2FePO4F cell cycled at a rate of 6.2 mA g1. (Reprinted from ref. 178, Copyright 2012, with permission from Elsevier.) Thisjournalis©TheRoyalSocietyofChemistry2017 Chem.Soc.Rev.,2017,46,3529--3614 | 3551 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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