PDF Publication Title:
Text from PDF Page: 027
Review Article Chem Soc Rev in an inorganic ionic liquid NaFSA-KFSA (FSA: bis(fluoro- sulfonyl)amide) at 363 K.200 Ha et al. extended the composition to Na3.32Fe2.34(P2O7)2, which can be represented as Na1.66Fe1.17P2O7, of which the crystal structure is identical to Na2FeP2O7, but the new compo- sition is able to increase the theoretical capacity to approxi- mately 110 mA h g1.201 The first irreversible capacity was negligible and the lower voltage was shortened. The electrode performance was superior to that of Na2FeP2O7 in terms of capacity retention and rate capability up to a rate of 10C. However, Na3.32[Fe0.5Mn0.5]2.34(P2O7)2 and Na3.32Mn2.34(P2O7)2 had very low capacities, which was a controversial result of the computational study of Clark et al.199 The poor electrochemical activity seems to be similar to that of LiMnPO4, which requires strategies for nanosizing and doping with the other divalent elements to improve electric conductivity. Recent work of Barpanda et al. identified a new polymorph of b-Na2MnP2O7, which was also stabilized into a triclinic structure, P1%.202 The structure consists of distorted MnO6 octahedral and PO4 tetrahedral blocks creating tunnels accom- modating Na atoms along the [001] direction. The structure has corner-sharing MnO6–MnO6 (Mn2O11) dimers, which are in turn connected by PO4–PO4 (P2O7) diphosphate units in a mixed-edge and corner-sharing fashion (Fig. 16c). A galvanostatic test of the electrode provides a reversible discharge capacity of 80 mA h g1 associated with an average operating voltage of 3.6 V (Fig. 16d). This coincided with earlier DFT calculations suggesting that this compound can undergo a Mn2+/3+ redox reaction at around 3.65 V.199 The sloppy discharge curve is the signature of b-Na2MnP2O7, which is different from Na2FeP2O7 operating with two distinct voltage plateaus. Structural investigation is further required to elucidate the reaction mechanism. They also suggest a new pyrophosphate oxyanionic framework com- pound, tetragonal Na2(VO)P2O7 with space group P4bm.202 This material belongs to the fresnoite family consisting of a VO5 or (VO)O4 square pyramid and PO4 tetrahedral units (Fig. 16e). Each VO5 pyramid is connected to four independent PO tetra- hedra in a corner-sharing fashion, thus forming [VP2O11] units. Also, each PO4 tetrahedron is connected to two VO5 units and one PO4 unit that share corners. The delivered capacity was approximately 80 mA h g1, which is close to the theoretical capacity (93 mA h g1) (Fig. 16f). The discharge curve was sloppy with the V5+/4+ redox reaction showing an average operating voltage of 3.8 V. Apart from the above-mentioned compounds, the other pyrophosphates with Ni(II), Cu(II), Ti(II), Co(II) and higher transition metal oxidation states have not yet been explored as host materials for Na+ insertion. 2.3.3. Mixed phosphates. Mixed phosphates are composed of phosphate (PO4)3 and pyrophosphate (P2O7)4. This phase was recently discovered in a Na4M3(PO4)2P2O7 (M: Mn, Co, Ni) system by Sanz et al.204 These compounds crystallize in the orthorhombic structure with a Pn21a space group. The struc- ture is built up from MO6 octahedra and PO4 groups with shared corners. M3P2O13 blocks parallel to the bc plane are linked with the P2O7 unit along the a-axis (Fig. 17a). Four Na+ sites are located in the three-dimensional channel, which can A high-resolution diffraction study using synchrotron radiation revealed a suitable orthorhombic structure with space group Amam rather than a tetragonal structure with space group P42/ mnm (Fig. 15g). Interestingly, when F was absent, the structure was crystallized to NASICON-type rhombohedral Na3V2(PO4)3 with space group R3%c, which will be addressed in Section 2.3.4. Similar to Na1.5VOPO4F0.5186 and Na1.5VPO4.8F0.7,187 the presence of fluorine with phosphorus ions raises the operating voltage originating from the strong inductive effects of the anion group, even though the V3+/4+ redox couple is available for delivery of high capacity (approximately 120 mA h g1) in Na3V2(PO4)3F3 compounds, with two voltage plateaus at 3.7 and 4.2 V (Fig. 15h). Computational calculations revealed that Na+ ions located at the Na2 site are more readily extracted due to the lower diffusion barrier. Compared with two earlier reports by Barker et al.181 and Sauvage et al.,186 this work is notable because variation in the anion configuration and the content of oxygen fluorine dramati- cally improved electrochemical performance, even at high rates. 2.3.2. Pyrophosphates. Na+ insertion was introduced in (MoO2)2P2O7 in 2003.192 The crystal structure was stabilized in an orthorhombic system with a Pnma space group, which is the same as olivine LiFePO4. The presence of an empty channel enabled Na+ insertion into (MoO2)2P2O7, with a discharge capacity of 190 mA h g1 corresponding to 3.1 mol Na+ per formula unit in (MoO2)2P2O7, although the rate capability was limited due to the large ionic size of the Na+ ion. In 2008, Adam et al.193 introduced the crystal structure of Li2MnP2O7, and it was confirmed using Li+ intercalation cathode materials.194,195 Recent developments in SIBs have motivated the exploration of facile Na+ intercalation materials towards pyrophosphates. Na2FeP2O7 with triclinic P1% was first studied by Honma et al.196 and Barpanda et al.,197 and the crystal structure was stabilized into the triclinic P1% phase (Fig. 16a). The structure was comprised of corner-sharing FeO6 octahedra creating Fe2O11 dimers, which are interconnected via corner- sharing and edge-sharing with P2O7 pyrophosphate groups. FeO6 octahedra and PO4 tetrahedra are connected in a stag- gered fashion, thus creating large tunnels along the [110] direction, which accommodate Na atoms at four distinct sites. However, these materials are not competitive cathode materials because of the two phosphate groups per transition metal in terms of capacity (theoretical capacity approaching 97 mAh g1) even though the related material chemistries are interesting. The appearance of stepwise voltage plateaus (roughly at 2.5 and 3 V) may indicate the presence of Na+/vacancy ordering in the structure (Fig. 16b). An ex situ study suggested that a two-phase reaction prevails in the upper voltage plateau and a single phase reaction is dominant in the lower voltage region.198 According to calculation by Clark et al.,199 low activation energy was found for long-range diffusion in all crystallographic directions in Na2FeP2O7 (M: Fe and Mn), indicating three- dimensional Na+ ion diffusion. Therefore, electrochemical studies indicated moderate performance at high rates, which is related to the availability of three-dimensionality and low migration energy for Na+ ion diffusion in Na2FeP2O7. The good electrode performances of Na2FeP2O7 were further evidenced View Article Online 3554 | 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 |