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Chem Soc Rev Review Article accommodate the Na+ ions. Since both a- and b-MnO2 have a different tunnel structure, both materials should exhibit differ- ent charge and discharge behavior.149 Nevertheless, the same voltage profiles showing sloppy discharge curves are more likely to be related to pseudo-capacitance behavior. l-Type MnO2, which is produced by electrochemical delithiation of LiMn2O4, showed discharge of approximately 200 mA h g1 during the first cycle with reasonable capacity retention.150 The l-type MnO2 phase was transformed into O03 NaMnO2 via electro- chemical cycling and showed stable cycling performance. 2.2.2. Vanadium oxides. Vanadium oxides have been inten- sively studied as cathode materials for lithium-ion batteries. In Na cells, a-, b-NaxV2O5, and Na1+xV3O8 were investigated for Na+ ion insertion by West et al. in the 1980s. a-V2O5 and Na1+xV3O8 have layered structures, and b-NaxV2O5 has a three-dimensional structure with wide channels.151 In experiments, upon Na+ ion insertion, a-V2O5 underwent a phase transition resulting in a new structure. Meanwhile, sloppy charge and discharge curves were obtained for b-NaxV2O5. Na+ ion insertion in Na1xV3O8 proceeded as a multi-phase reaction. Although structural evolu- tion did not occur during Na+ insertion, the absence of a plateau on Na+ extraction indicates the slow kinetics of Na extraction. Nevertheless, the capacity retention was above 98% for 100 cycles in a voltage range of 1–3.5 V. Recently, Tapavcevic et al. reintroduced layered a-V2O5 using a nanostructured bilayer concept, of which the interslab distance was approximately 13.5 Å, which is significantly larger than that of conventional V2O5 showing 4.4 Å.152 This was possible because they used electrochemical deposition from aqueous vanadyl sulfate on a Ni foil substrate followed by heat treatment at 120 1C for removal of water molecules present in the interslabs (Fig. 13a). The electrode was activated by a V5+/4+ redox reaction in a voltage range of 1.5–3.8 V, but showed sloping charge– discharge curves. The electrode delivered a reversible capacity of approximately 250 mA h g1, which is close to the theoretical capacity, with excellent cyclability of 300 cycles (Fig. 13b). Surface modification of VO2(B) using reduced graphene oxide (rGO) substantially improved repetitive Na+ insertion ability.153 Although it has an open bronze structure to accommodate ion species into the empty channels, structural collapse or amorphi- zation was observed in Na cells. The charge and discharge curves were sloppy, delivering approximately 150 mA h g1, in which the V4+/3+ reaction was related to the electrochemical reaction. Both Na+ insertion into VO2(B) and pseudo-capacitive behavior were confirmed via XRD and XAS studies. 2.2.3. Metal fluorides. Fluorine compounds have a high discharge voltage due to their ionic metal–ligand bonds. Perovskite-type metal trifluorides with a corner-sharing matrix (R3%c) have large bottlenecks in diffusion pathways for ion carriers such as Li+ and Na+. Although they have a high theoretical capacity (B200 mA h g1), their electrochemical performance is affected by the low electric conductivity of metal fluorides. Success has been achieved through mechanical milling and compositization with nanosized carbons. Okada’s group introduced several metal fluorides (metal: Fe, V, Ti, Co, and Mn) including sodiated compounds.154–157 A reversible that the Na1 and Na2 sites located in the S-shaped tunnels were very accessible in the range of 0.22 to 0.66 in NaxMnO2, while the Na+ ions located in the Na3 site were not generally extracted. An in situ XRD study revealed the structural evolution during the electrochemical reaction in Na cells. A biphasic reaction was found upon reduction in the range of x = 0.2–0.44 in NaxMnO2. In a range of x = 0.44–0.612 NaxMnO2, not a single solid solution, but several multiphase reactions were associated with the electrochemical oxidation. Unfortunately, the high rate test caused a drastic capacity drop (to B1016 S cm1) due to some kinetic limitations, as calculated by Cao et al.142 Kim et al. suggested that the composition of Na0.44MnO2 and Na0.55MnO2 in a voltage range of 2.6–2.8 V is unfavorable because of electro- static repulsion, which causes slow Na+ diffusion.143 They suggested that Jahn–Teller distortion was another parameter of electrode performance decay. Recent work of Cao et al. reported the synthesis of single crystalline Na0.44MnO2 nanowires with a reversible capacity that could be maintained for over 1000 cycles.142 High crystallinity provided the long-term durability for Na+ insertion and extraction and the reduced diffusion path also contributed to excellent capacity retention. The application of Na0.44MnO2 electrodes has also been highlighted in aqueous systems. Whitcare et al. documented a full cell with a Na0.44MnO2 cathode and an activated carbon anode in a 1 M NaSO4 aqueous electrolyte.144 Despite a smaller capacity in the aqueous solution relative to the aprotic ones, the full cell could be tested without apparent capacity loss over 1000 cycles. In the NaTi2(PO4)/Na0.44MnO2 system, the cell was capable of operation at over 100C-rates and stable cycling for over 1000 cycles (Fig. 12b).145 Na0.4MnO2 consists of a 2 3 tunnel structure called romanechite.89 Ba+ and water molecules have been introduced to stabilize the large tunnel structure. Although the Na+ inser- tion and extraction mechanism remains unclear, 0.3 mol of the Na+ ion could be inserted into the host structure. Few studies have explored Na-free MnO2 for Na+ insertion. Because of the relatively large tunnel size of a- and b-MnO2, Na+ insertion is also possible for both a- and b-MnO2. a-MnO2, which is called hollandite, is composed of double chains of edge-sharing MnO6 octahedra that are linked at the corners to form a 2 2 and 1 1 tunnel structure (Fig. 12c). Because of the large size of the 2 2 tunnel, Na+ insertion into the empty tunnel is possible, as suggested by Su et al.146 and Islam et al.147 (Fig. 12d). A relatively large capacity of approximately 280 mA h g1 was obtained at the first discharge in a-MnO2 nanorods, whereas the retained capacity was only 75 mA h g1 after 100 cycles. In contrast, rutile-type b-MnO2 shows a 1D channel 1 1 tunnels along the c-axis composed of individual chains of the MnO6 octahedral units. In general, insertion of ion species, i.e., Li+, is not easy due to the small size of the 1 1 tunnels.148 Su et al. also tested b-MnO2 in Na cells, which delivers approximately 300 mA h g1 during the first cycle.146 In comparison with a-MnO2, the b-MnO2 nanorods exhibited a higher discharge capacity of approximately 145 mA h g1 after 100 cycles. They attributed the better electrochemical perfor- mance of b-MnO2 to the large number of empty tunnels that View Article Online Thisjournalis©TheRoyalSocietyofChemistry2017 Chem.Soc.Rev.,2017,46,3529--3614 | 3549 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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