PDF Publication Title:
Text from PDF Page: 007
Review Article Chem Soc Rev Na0.7FeO2 is not stable in P2 type layer compounds because of the intrinsic instability of tetravalent Fe in an oxide frame- work, as described in Section 2.1.1. Yabuuchi and Komaba et al. introduced a P2-Nax[Fe0.5Mn0.5]O2 layer compound that delivers 190 mA h g1 of reversible capacity (Fig. 4a).46 Partial substitution of Fe by Mn made it possible to utilize the electrochemical oxidization of Fe3+ to Fe4+ reversibly, in parti- cular in a voltage range of 3.8–4.2 V. Upon desodiation, the P2 phase was maintained at 3.8 V by the oxidation of Mn3+/4+, Na0.42[Fe0.5Mn0.5]O2, after which the oxidation of Fe3+/4+ led to phase transformation from P2 to OP4 at 4.2 V (Na0.13[Fe0.5Mn0.5]O2) as shown in Fig. 4b. An in situ XRD study also demonstrated that P2-Na2/3[Fe0.5Mn0.5]O2 undergoes a reversible P2–OP4 phase transition at the end of charge.54 This achievement is remarkable in terms of capacity in comparison to the O3 type Na[Fe0.5Mn0.5]O2 that delivers less than 120 mA h g1 in early cycles. Despite a high capacity approximating 190 mA h g1 on the first cycle, capacity fading was inevitable in the P2-Nax[Fe0.5Mn0.5]O2 compound. Morternard de Boisse et al. raised another issue regarding the structural change during the first discharge from OCV to 1.5 V.55 The P2 phase, Na0.62[Fe0.5Mn0.5]O2, was transformed into orthorhombic P02 (space group: Cmcm), Na0.97[Fe0.5Mn0.5]O2, by cooperative Jahn–Teller distortion at the end of discharge. In a P2 type layer structure, Nae and Naf sites are both simultaneously occupied at a ratio close to 2 : 1. This distribution results from the strong sodium–sodium repulsion interaction, which prevails over sodium–transition metal repulsion. Hence, Nae and Naf cannot be occupied simultaneously in the P2 phase. All Na+ ions are located at one site, and the stoichiometry of Na[Fe0.5Mn0.5]O2 can be stabilized in the orthorhombic structure. Note that the atomic displacement parameter of Na in the P02 phase (0.7(2) Å2) is much lower than that of the P2 phase (1.9(3) Å2), indicating that Na+ ions are less mobile in the orthorhombic P02 phase. One interesting finding that contrasts with the results of Yabuuchi and Komaba et al. is that when electrochemical sodiation forms the orthorhombic P02 phase, the resulting desodiation to 4.3 V leads to the formation of not OP4, but also a new and unindexable ‘‘Z’’ phase (Nax[Fe0.5Mn0.5]O2, x o 0.25) with poor crystallinity (Fig. 4c).55 Talaie et al. revealed phase ‘‘Z’’, which is a result of migration of Fe3+ into tetrahedral sites in the interlayer space, showing a short range order between two adjacent layers.56 This migration is highly reversible, although it induces polarization of the cell. Addition of Ni instead of Fe was very effective at mitigating migration of Fe3+ and thus improved the cycling performance. Thorne et al. determined the relationship between the Na and Fe content in Na1x[Fe1xMnx]O2 (0 r x r 0.5).57 The Na content is a decisive factor establishing the phase (O3, O 0 3, P2, etc.). As clearly seen in Fig. 4d, the length of the first desodia- tion plateau increased with x in Na1x[Fe1xMnx]O2. The length of the first sodiation plateau also increased as x decreased due to more sodium vacancies in Na1x[Fe1xMnx]O2. The oxidation states in Na1x[Fe1xMnx]O2 are Fe3+ and Mn4+, as suggested by Mo ̈ssbauer spectroscopy. Therefore, desodiation oxidized Fe3+ to Fe4+, while the sodiation of Na1x[Fe1xMnx]O2 selec- tively reduced Fe4+/3+ at high voltages above 3.5 V and reduced (102 mA h g1). Wang et al.48 stabilized the crystal structure of NaFeO2 by forming a solid solution with NaNiO2, NaFe1xNixO2 (0 r x r 1). The dilution of the Fe concentration, NaFe0.3Ni0.7O2, effectively increased capacity and retention (135 mA h g1 and 74% retention after 30 cycles) via Fe3+/4+ and Ni3+/4+ redox couples (Fig. 3b). In addition, they also recorded the 57Fe Mo ̈ssbauer spectra to determine the valence state and investigate the mecha- nism of the electrochemical reaction of Fe in O3-NaFe1yNiyO2 (y = 0, 0.5, and 0.7). Recent report by Nanba et al. also well supported the electrochemical reaction mechanism of NaFe1xNixO2 (0 r x r 1) electrodes.49 Extension of the layer structure toward a three component system, Na[Ni1/3Fe1/3Mn1/3]O2, was suggested by Kim et al.50 For synthesis, they used oxalate coprecipitation, [Ni1/3Fe1/3Mn1/3]C2O4, due to the difficulty of forming hydroxides when Fe is involved. Na[Ni1/3Fe1/3Mn1/3]O2 could deliver a discharge capacity of 120 mA h g1 in the voltage range of 2–4 V for a half cell. They also tested the cyclability after adopting a hard carbon anode (100 mA h g1 after 150 cycles). Oh et al. also synthesized spherical Na[Ni0.25Fe0.5Mn0.25]O2 using [Ni0.25Fe0.5Mn0.25](OH)2.51 They found that the electronic states of Ni, Fe, and Mn were 2+, 3+, and 4+, respectively, of which Ni2+/4+ and Fe3+/4+ were respon- sible for electrochemical activity in the range of 2.1–3.9 V via the X-ray absorption nearest edge spectral (XANES) analysis technique. Mn remained electrochemically inactive, but pre- served the crystal structure during the electrochemical reaction. A Fe3O4/Na[Ni0.25Fe0.5Co0.25]O2 full cell with a conversion anode and an insertion cathode delivered a capacity of approximately 130 mA h (g-Na[Ni0.25Fe0.5Mn0.25]O2)1 with approximately 76.1% retention at the 150th cycle (Fig. 3c). Despite the good reversibility of Na[Ni0.25Fe0.5Mn0.25]O2, the discharge capacity limit of 140 mA h g1 in O3 type layer materials needs to be overcome. The voltage limitation to 3.9 V prevents iron migration during charge. Oh et al. suggested another approach to raise the upper voltage cutoff to 4.4 V, in order to offer more capacity in O3 type compounds using the Mn3+/4+ redox, but reducing the concentration of Fe3+ in Na[Li0.05(Ni0.25Fe0.25Mn0.5)0.95]O2.52 As anticipated the compound was active based on the Ni2+/4+, Fe3+/4+, and Mn3+/4+ redox reactions, resulting in a large capacity of approximately 180 mA h g1 at a 0.1C-rate in the voltage range of 1.7–4.4 V. Additionally, the presence of a stronger Li–O bond relative to that of Ni–O and Mn–O in the transition metal layer was responsible for stabilization of the crystal structure, enabling better capacity retention during cycling. 2.1.2. Na1x[Fe1yMny]O2 (x r 0.3) and derivatives. As men- tioned in Section 2.1.1, O3 type compounds deliver limited reversible capacities of less than 160 mA h g1, of which the O3 phase follows a reversible structural transformation (O3 2 O0 3 2 P3 2 P0 3). Further desodiation results in hexagonal P300 from P03 that shows much greater interslab distances of approximately 7 Å compared to the P03 phase (5.6 Å).53 This greater distance is not preferred because it induces intercalation of electrolytic molecules formed from oxidative decomposition of electrolytes at high voltage. Hence, delivery of a high capacity of 180 mA h g1 or above is not possible in O3 systems. View Article Online 3534 | 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
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 |