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Review Article Chem Soc Rev but the other transition metal elements such as Ni, Co, V, and Mn, among others. Sodiated metal carbonophosphates, such as sidorenkite Na3MPO4CO3 (M: Mg, Mn, Fe, Co, Ni, and Cu), were recently introduced as a new intercalation cathode by Chen et al.220,221 Two different series of (PO4)3- and (CO2)2-ions have bonds with M in Na3MPO4CO3 compounds, which are stable with a sidorenkite structure and a P21/m space group (Fig. 19e). In Na3MnPO4CO3, the MnO6 octahedra are connected by a PO4 tetrahedra to form a double layer. The CO3 group shares an oxygen edge with MnO6. The two-dimensional interslab accom- modates two Na atoms at two different interstitial sites, where Na1 and Na2 coordinate with seven and six O atoms, respectively. As anticipated, their computations suggested that the sidorenkite structure may have good intercalation properties.222 Carbopho- sphate showed high discharge (125 mA h g1) (Fig. 19f), which represented 66% of the theoretical capacity (191 mA h g1). It is interesting that Na3MPO4CO3 underwent a topotactic solid solution reaction during cycling, activated by a two-electron electrochemical reaction of Mn2+/3+ and Mn3+/4+ redox couples. 2.3.6. Cyanides and organic compounds. Prussian blue and its analogues were investigated as hosts for alkali ions, but those works received less attention due to low energy density com- pared with the above-mentioned materials.223,224 Recently, Cui and coworkers reported potassium copper hexacyanoferrate to intercalate Na+ ions in aqueous solution at less than 2 V.225–227 In an attempt to increase the operation voltage, Wang et al. utilized KMFe[CN]6 (M: Fe and Mn, which are divalent with high spin), which exhibited high operation voltage plateaus at 3.8 V for charge and 3.5 V for discharge.228 Their extended work varied the concentration of Na in the MFe[CN]6 perovskite framework: Na1.4MFe[CN]6 (Fm3m) and Na1.72MFe[CN]6 (R3% m).229 The double-perovskite ordering of both the anion orientations and the cations introduces a rhombohedral site symmetry along each of the four [111] axes that would stabilize alkali-ion displacement along a cubic [111] axis toward the more octahedral-site complex. The four-fold degeneracy of these displacements keeps the cubic phase at higher temperature and lower Na concentration, but a cooperative Na displacement at higher Na concentration reduces the crystal symmetry to rhombohedral, stabilizing the rhombohedral phase (Fig. 20a). Note that the larger is the concen- tration of Na+ ions in the compound, the higher is the tem- perature of transition from rhombohedral to cubic symmetry. Electrochemical reactions occurring at low and high voltage are the redox reactions of low-spin Fe3+/2+ and high-spin Mn3+/2+, respectively. The post-cycled electrode did not show a notable structural change after 50 cycles. Na1.72MFe[CN]6 reacted at a rate of 40C (Fig. 20b). Replacement of Mn with Fe, FeFe[CN]6, lowered the operation voltage of the lower voltage plateau to 3.1 V on charge and 2.8 V on discharge, delivering a discharge capacity of approximately 120 mA h g1.230 The electrode was substantially stable over 600 cycles with excellent capacity retention. Na4Fe[CN]6 was electrochemically active in Na cells, showing approximately 90 mA h g1 on discharge with a flat voltage plateau at 3.4 V.231 The lower capacity is due to the one- electron reaction by the low-spin Fe3+/2+ redox. rate of 40C (4.68 A g1) with superior rate capability (Fig. 18c). Their symmetrical cell also evidenced the cycling stability for over 5000 cycles. Jian et al. further analyzed the crystal structure during the electrochemical reaction.215 As mentioned in Fig. 18a, Na atoms are localized to two different sites (6b, M1 and 18e, M2) in Na3V2(PO4)3. The flat curve during desodiation of Na3V2(PO4)3 indicates a biphasic reaction based on the V3+/4+ redox reaction as a result of Na+ extraction at the M2 site, which is related to the formation of NaV2(PO4), in which only one Na (6b, M1) resides in the crystal structure. NMR further revealed that, in Na3V2(PO4)3, the Na atoms were not randomly distributed at M2 sites, but appeared in an ordered arrangement locally; an Na+ ion at the M2 sites in Na3V2(PO4)3 is mobile, whereas an Na+ ion at the M1 site is immobile, corresponding to 2 mol Na+ per formula unit in Na3V2(PO4)3, while 1 mol Na+ per formula unit in Na3V2(PO4)3 can retain the framework during the electrochemical reaction. 2.3.5. Sulfates, fluorosulfates, and carbonophosphates. Substitution of (SO4)2 for the (PO4)3 anion results in a higher operating voltage because of the greater ionicity. When successful, this improves the operating voltage in a tavorite-type material, leading to an OCV increase of 0.6–0.8 V in LiFeSO4F compared to LiFePO4F.216 Similarly, this substitution is also applicable in Na systems, namely, NaMSO4F (M: Fe, Co, and Mn), as suggested by Barpanda et al.217 The compounds are crystallized in a monoclinic structure with a P21/c space group (Fig. 19a). Tripathi et al. calculated the activation energy for Na+ diffusion. Although NaFeSO4F has a three-dimensional structure in only one direc- tion, [010], it has relatively low activation energy (0.6 eV).218 The measured ionic conductivity of the compound was sufficiently high at approximately 107 S cm1. Notwithstanding, the elec- trode delivered only 6% of the theoretical capacity (137 mA h g1) (Fig. 19b). Barpanda et al. further developed a sulfate-based sodiated iron compound, which was crystallized to an alluaudite- type sulfate framework, Na2Fe2(SO4)3, with a P21/c space group.219 In this case, the crystal structure was similar to earlier work on NaMSO4F, while the resulting volume of the unit cell was larger by over two-fold. In contrast to AxM2(XO4)3-type compounds, which usually have NASICON-related structures, Na2Fe2(SO4)3 does not contain [M2(XO4)3] units, but has a unique structure with an alluaudite type framework (Fig. 19c). FeO6 octahedra share edges, forming Fe2O10 dimer units. These Fe ions could be assigned to two distinct groups, Fe(1) and Fe(2), since local structures of Fe(1) and Fe(2) were found to have two doublets in the M ̈ossbauer spectra. The Fe2O10 dimers were linked with SO4 units in a corner-sharing fashion, generating a three- dimensional network framework with large tunnels along the c-axis, with Na located in the tunnels. Electrochemical testing of the material revealed that a Fe2+/3+ redox reaction led to a high operating voltage of 3.8 V on average, the highest that appeared in the Fe2+/3+ reaction, which confirms strong ionicity when (SO4)2 anions occur in the crystal structure instead of (PO4)3. Based on one-electron transfer, the material could deliver approximately 85% of the theoretical capacity (120 mA h g1) with a moderate rate capability (Fig. 19d). The structure may be able to extend to several family compounds, adopting not only Fe, View Article Online 3558 | 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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