PDF Publication Title:
Text from PDF Page: 014
Chem Soc Rev Review Article Phase transformation towards spinel was possible in b-LiMnO2, because Mn can migrate into tetrahedral Li sites to form the spinel phase. This phase transition is unlikely to occur because tetrahedral sites rarely form in spinel with Na due to the large ionic size of Na+ relative to Li+. Reducing the Na content to 0.7 induces formation of a different crystal structure: P2-, P3, and orthorhombic (P02) Na0.7MnO2 (Fig. 8c).95 As mentioned in Section 2.1, P3 is the low temperature type and P2 is the high temperature type (Fig. 2). Stoyanova found that orthorhombic Na0.7MnO2 (space group: Cmcm) is stable at 1000 1C (Na2/3[Mn3+3/2Mn4+1/3]O2).95,96 Among these polymorphs, P2-Na0.7MnO2 and its derivatives have been intensively studied.97–99 An early report by Caballero et al. showed reversible capacity delivery of more than 150 mA h g1 in a voltage range of 2–3.8 V in Na cells.98 They also suggested that intercalation of Na+ ions occurred in several steps, but the resulting capacity fade progressed upon successive cycling tests. Increasing the synthetic temperature to 900 1C, Yabuuchi et al. could improve the capacity up to 190 mA h g1 in a voltage range of 1.5–4.3 V during several early cycles (Fig. 8d).99,100 The electrode performance is similar to that of O30-NaMnO2, although the related phase transition is different. P2-Na2/3[Ni2+1/3Mn4+2/3]O2 was first reported in 2001 by Lu and Dahn et al.101 This compound is stable in moist air, and hydration via the insertion of water molecules does not occur.102 Although it has a relatively low theoretical capacity (173 mA h g1), P2-Na2/3[Ni2+1/3Mn4+2/3]O2 showed an average operating voltage of 3.5 V with a Ni2+/4+ redox reaction, deliver- ing approximately 160 mA h g1 in a voltage range of 2–4.5 V (Fig. 8e). Because of the similarity in the ionic size between Ni2+ and Mn3+, Ni2+ prefers to occupy the Mn3+ sites instead of Mn4+ in Na2/3MnO2. Their in situ XRD study revealed a reversible P2–O2 phase transition stemming from the oxygen shift, in which the O2 phase prevails at a voltage plateau above 4 V on charge and 3.8 V on discharge (Fig. 8f). This is the main difference from P2-Na0.67MnO2; the absence of the Jahn–Teller distortion Mn3+ is responsible for the occurrence of the P2–O2 phase transition. This transition is associated with a large volume change when the O2 phase appears. Recently, Meng et al. revis- ited P2-Na2/3[Ni1/3Mn2/3]O2 and observed phase transformation from P2 to O2 at 4.2 V.103 The long voltage plateau is the evidence of two forms of Na+ ion ordering: one row of Naf and two rows of Nae in Na1/2[Ni1/3Mn2/3]O2 and Na orders in rows on either Nae or Naf order in Na1/3[Ni1/3Mn2/3]O2, in which the latter corresponds to the region where the O2 phase is dominant. In the P2 phase, the path with the minimum energy passes through a shared face between two neighboring Na prismatic sites. Here, Na+ ions need around 170 meV for diffusion in the P2 phase. In the O2 phase, Na+ ions cross the tetrahedron between two octahedral sites by means of a vacancy mechanism.104 The required energy for Na+ diffusion in the O2 phase is 290 meV, indicating slow Na+ mobility in the O2 phase, as was confirmed experimentally. It is reasonable because the diffusion path of Na+ ions is more spacious in the P2 phase relative to the O2 phase; this leads to a lower activation barrier (Fig. 8e inset). Hence, a cycling test in the voltage range of 2.3–4.1 V, sodium content, a-NaMnO2 and b-NaMnO2 are stable, although the phase stability of both compounds is dependent on temperature; for example, a-NaMnO2 (space group: C2/m) is the low-temperature form and b-NaMnO2 (space group: Pmnm) is the high-temperature form. P2-Na0.7MnO2 is no more stable when the Na layer is fully sodiated to NaMnO2 at a low tempera- ture. In a P2 layer structure, Nae and Naf sites are simultaneously occupied, due to the strong sodium–sodium repulsion interaction in the Na layers (Fig. 2). Hence, simultaneous distribution of Nae and Naf is not possible in the P2 phase, but all Na+ ions are located in one site to form a-NaMnO2 (space group: C2/m, O03 structure, Fig. 8a). The prime symbol is an indication of a monoclinic structure with respect to the hexagonal lattice. Since the average oxidation state of Mn is 3+, the Jahn–Teller distortion prevails in the crystal structure. Ma and Ceder et al. reported that monoclinic O 0 3-NaMnO2 could deliver charge and discharge capacities of approximately 210 mA h g1 and 197 mA h g1, respectively, in a voltage range of 2–3.8 V and at a rate of C/30 (Fig. 8b).90 This behavior differs from an earlier report by Mendiboure and Hagenmuller.91 The charge and discharge curves show many plateaus and voltage drops, with eight and five charge and discharge plateaus, respectively. The hysteresis was reversible even after cycling. A long plateau observed at 2.63 V from Na0.93MnO2 to Na0.7MnO2 is associated with a two-phase reaction. Their ex situ XRD investigation revealed that the second phase was Na0.7MnO2, although this was not consistent with orthorhombic Na0.7MnO2 (space group: Cmcm) or P2-Na0.7MnO2 (space group: P63/mmc). Capacity retention of O03-NaMnO2 was approximately 74% for 10 cycles without a significant structural change compared to the fresh electrode. In Mn3+-containing cathode materials, capacity fade is usually mentioned with Mn dissolution, which causes disproportionation to Mn2+ and Mn4+. Only a small percentage of Mn was dissolved from the active material (less than 32 mg from 2–3 mg of the active material), such that dissolution was not likely to affect electrode performance. Their successive work demonstrated the readiness of the 1801 Na–O– Mn3+–O–Na strip formation in contrast to VNa–O–Mn3+–O–Na, albeit insufficient Na+ ions in Na5/8MnO2, because Na+ ions relax to the highly distorted octahedral sites, where they share the symmetric attraction of two neighboring Jahn–Teller distorted –O–Mn3+–O–Na configurations along the [100] axis.91 This struc- tural imperfection may be the reason for the gradual capacity fade observed during cycling in this system. High temperature-type b-NaMnO2 has a zigzag layer structure composed of two edge-sharing stacks of the MnO6 octahedra. Between two neighboring sheets, the sodium ions occupy octahedral sites.92 In the Li system, orthorhombic LiMnO2 is directly synthesized, indicating that the orthorhombic structure is energetically favored relative to monoclinic LiMnO2, which is usually produced via ion exchange from a-NaMnO2. In the Na system, however, first principles calculations indicate that the monoclinic a-NaMnO2 is energetically more stable than orthorhombic b-NaMnO2.93,94 b-NaMnO2 is active in a narrow range (0.85 r x r 0.96) in NaxMnO2.91 In this range, a two- phase domain is attributed to b-NaMnO2 and NaxMnO2. View Article Online Thisjournalis©TheRoyalSocietyofChemistry2017 Chem.Soc.Rev.,2017,46,3529--3614 | 3541 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
PDF Search Title:
Sodium-ion batteries present and futureOriginal File Name Searched:
Sodium-ion batteries present and future.pdfDIY PDF Search: Google It | Yahoo | Bing
Salgenx Redox Flow Battery Technology: Salt water flow battery technology with low cost and great energy density that can be used for power storage and thermal storage. Let us de-risk your production using our license. Our aqueous flow battery is less cost than Tesla Megapack and available faster. Redox flow battery. No membrane needed like with Vanadium, or Bromine. Salgenx flow battery
| CONTACT TEL: 608-238-6001 Email: greg@salgenx.com | RSS | AMP |