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Review Article Chem Soc Rev symmetry with a I4/mmm space group, which is an isostructure of a-Na3Fe2(PO4)2F3.182 This structure demonstrates facile Na+ ion diffusion through an extended three-dimensional frame- work constructed from VO4F2 octahedra and PO4 tetrahedra (Fig. 15a). Na+ ions are present in the empty channel and diffuse along the c-axis. Coupling with a hard carbon anode, NaVPO4F, led to two-step voltage responses (Fig. 15b), while the delivered discharge declined to less than 50% of the initial capacity (approximately 82 mA h g1). Zhao et al. found another crystal system in this compound: monoclinic (C2/c) as the low temperature form and tetragonal (I4/mmm) as the high temperature form.183 Cr-doping into the V site of NaVPO4F crystallized to the monoclinic phase showed a discharge capa- city of 80 mA h g1 with 91% retention for 20 cycles.184,185 Sauvage et al. reinvestigated Na–V–P–O–F materials because the structural parallelism between NaVPO4F and Na3Al2(PO4)2F phases was not straightforward owing to the difficulties encountered in determining the stoichiometry.186 They found that a 10 wt% excess of NaF could yield a single phase. Reliable refinement results were obtained assuming a crystal structure of Na3V2(PO4)2F, which can be presented as Na1.5VOPO4F0.5 instead of NaVPO4F. In this structure, Na+ ions are disordered across two sites (8h and 8j) and the oxidation state of V is 4+ (Fig. 15c). The crystal structure is composed of a VO5F octahedral and PO4 tetrahedra sharing O vertices parallel to the ab-plane. Along the c-axis direction, VO5F octahedra are connected via F vertices located in the same ab-plane as the disordered Na atoms. This structure could accom- modate 0.56 Na+ (approximately 87 mA h g1) per formula unit in Na1.5VOPO4F0.5 with two different voltage plateaus, 3.6 and 4 V (Fig. 15d). In consideration of the theoretical value (156 mA h g1) assuming a V4+/5+ reaction, the delivered capacity can be improved further with electro-conducting coating layers. Due to structural and electrolytic instability, the capacity retention in the upper voltage region is not promising compared to the capacity delivered at the lower voltage plateau. These results, including capacity and retention, agree with an earlier report by Barker et al.181 The oxidation state of V was lowered to 3.8+ in Na1.5VPO4.8F0.7.187 In comparison with Na1.5VOPO4F0.5,188 the oxygen content was reduced slightly, but the equivalent content of F increased. Although structural refinement was performed based on space group P42/mnm, the XRD pattern, including the Bragg peak position, is the same for both compositions. Na1.5VPO4.8F0.7 is comprised of tetrahedral PO4 and VO5F/VO4F2 octahedral units that share corners, which yields a three- dimensional open framework with Na+ ions located at interstitial sites (Fig. 15e). Na1.5VPO4.8F0.7 provided a discharge capacity of approximately 137 mA h g1 based on the V3.8+/5+ redox reaction (1.2 mol Na+ per formula unit in Na1.5VPO4.8F0.7) with excellent cyclability for 500 cycles (Fig. 15f). To further lower the average oxidation state of V efforts were made employing Na3V2(PO4)3F3 with space group P42/mnm.188–190 Very recently, Bianchini et al. revisited the Na3V2(PO4)3F3 com- pound to clarify its crystal structure, although the structure was successfully refined by Le Mine et al. in 1988.191 They claimed that structural discrepancies in the literature originated from the possible existence of F and O mixing in the structure.182,188–190 approximately 2 orders of magnitude lower.175 Recent density functional theory calculation revealed that the migration energy of Na+ ions in NaFePO4 is 0.05 eV higher than that of Li+ ions in LiFePO4, which reflects the slow kinetics in NaFePO4.172 The large ion size of Na+ relative to Li+ is asso- ciated with slow diffusion of Na+ ions into the crystal structure. An extension of NaFePO4, Na[Fe0.5Mn0.5]PO4, could be synthe- sized;168 however, the electrode did not show voltage plateaus, but rather a sloping curve due to the high interface strain between the Na-rich and Na-poor phases that was ascribed to the large size of Na+ ions, allowing 0.6 mol Na+ into Nax[Fe0.5Mn0.5]PO4. Since large Na+ ions are associated with migration, minimization of the interface energy is essential to the utilization of the other transition metals such as Co, Mn, and Ni, among others. The ionicity of fluorides over oxides and sulfides is ascribed to their higher electronegativity, which thus increases the operating voltage of the electrodes. This concept was success- fully applied to the discovery of Na2FePO4F, which is applicable to both Li and Na systems.176 This material crystallizes in orthorhombic structures with a Pbcn space group (Fig. 14d). Bioctahedral Fe2O7F2 units comprising face-sharing FeO4F2 octahedra are connected via bridging F atoms to form a chain, and are joined by PO4 tetrahedra to form FePO4F layers. The two Na cations located in the interlayer enable facile two- dimensional migration pathways. However, structural evolution from the layer structure to a tunnel one occurred (space group P21/n) when Fe was substituted for Mn greater than 25%. In the case of the tunnel structure, the Na cations are located within channels.177 Notwithstanding the advantages of high operating voltage relative to NaFePO4, the strong ionicity lowers the intrinsic electric conductivity. Hence, the bare Na2FePO4F delivered almost no capacity. Surprisingly, the carbon-coated Na2FePO4F could deliver approximately 110 mA h g1, which corresponds to a 90% theoretical capacity.178 Two voltage plateaus are evident with small polarization. As anticipated from the crystal structure, Tripathi et al. predicted Na+ ion conduction in Na2FePO4F, which occurs along the two-dimensional inter- layer with low activation energy.179 However, further investiga- tion is needed to improve cycling performance by adding more carbon to provide better electron conduction. Carbon-coated Na2[Fe0.5Mn0.5]PO4F (6 wt% of carbon) also showed a reversible capacity of 110 mA h g1, confirming the feasibility of Na+ ion migration into/out of the tunnel structure.180 Polarization of the Mn redox is clearly greater than that of the Fe2+/3+ redox, which is similar to those observed in LiFePO4 and LiMnPO4. In place of divalent transition metal elements, a trivalent element is also available in the form of NaVPO4F. Barker et al. first introduced the material as an electrode material for hybrid-ion cells in 2003.181 During the initial charging, Na+ ions are extracted from the host material while Na+ ions are plated on the surface of the Li metal anode in a Li cell. This process releases an atomically equivalent amount of lithium back into the electrolyte. Therefore, Li+ ions are reinserted into the host material during the first discharge because Na is immobilized on the Li metal surface. NaVPO4F has tetragonal View Article Online 3552 | 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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