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Review Article Chem Soc Rev However, large volume expansion/contraction upon the sodiation– desodiation process accelerated tremendous damage of electrodes, which led to the loss of electrical contact and subsequently rapid capacity fading. Moreover, sluggish mobility of Na+ ions due to the large ionic size (1.02 Å) is still a challenge to fully utilize their theoretical capacity. In recent years, to deal with such problems, strategies such as advances in the nano- technology and/or carbon-conducting technique have been introduced, facilitating the development of high performance conversion materials as anodes for SIBs. In this section, the recent research achievements for transition metal oxides, transition metal sulfides and transition metal phosphide as Na conversion hosts are summarized and discussed. 3.2.1 Transition metal oxide (TMO) based anode materials. Alcantara et al. first introduced the conversion material concept by using NiCo2O4 spinel oxide as an anode material for SIBs365 (Fig. 29a). Alcantara et al. described a reversible conversion reaction of sodium with a metal oxide in which Na2O and metals are formed: NiCo2O4 + 8Na - Ni + 2Co + 4Na2O. Subsequent to this work, many research groups have proposed various transi- tion metal oxides (TMOs) such as iron oxide (Fe3O4, Fe2O3),366–377 cobalt oxide (Co3O4),378–382 tin (di)oxide (SnO, SnO2),383–390 copper oxide (CuO),391–396 molybdenum oxide (MoO2),397 nickel oxide (NiO, NiO/Ni),398,399 and manganese oxide (Mn3O4).398 3.2.1.1. Iron oxides (Fe3O4, Fe2O3). Komaba et al. reported applicability of Fe3O4 in Na batteries through the insertion reaction in the voltage range of 1.2–4.0 V.366 Recently, Hariharan et al. proposed the possibility of Fe3O4 materials in the conver- sion reaction mechanism with Na+ ions at a discharge voltage of 0.04 V: Fe3O4 + 8e + 8Na+ 2 3Fe + 4Na2O.367 Through the conversion reaction, a discharge capacity of 643 mA h g1 was delivered during the initial cycles with a high Coulombic efficiency of 57%. Oh et al. suggested pitch carbon as a coating additive for nano-sized Fe3O4 (Fig. 29b). Oh et al. also observed the conversion reaction of the C/Fe3O4 electrode upon the sodiation–desodiation process51 (Fig. 29c). To further ensure electric conductivity, Park et al. introduced a composite of C/Fe3O4 embedded on carbon nanotubes, which delivered 440 mA h g1 for the first discharge and 321 mA h g1 for the first charge with a high Coulombic efficiency of 73%.368 More recently, Liu et al. synthesized extremely small Fe3O4 quantum dots on hybrid carbon nanosheets, which demonstrated a high capacity of 416 mA h g1 at 0.1 A g1 and a superior cycle retention of 70% for 1000 cycles at 1.0 A g1.369 Fe2O3 has also been considered to be a promising electrode material for SIBs due to its excellent chemical stability, high capacity, easy fabrication, low cost and nontoxicity.373–377 Similar to Fe3O4 materials, sodium storage in Fe2O3 is mainly achieved via a reversible conversion reaction, by forming Fe nanoparticles dispersed in the Na2O matrix. 3.2.1.2. Cobalt oxides (Co3O4). Rahman et al. proposed the reversible conversion reaction mechanism of Co3O4 with sodium ion via cyclic voltammogram and ex situ XRD analyses: Co3O4 + 8Na+ + 8e 2 4Na2O + 3Co.378 Based on the XRD Shen et al. predicted the structure and average voltage change of reduced phases at various compositions of Na2+xTi6O13 (x = 0–4) by using density functional theory (DFT) calculations (Fig. 28e). Shirpour et al. suggested layered sodium titanate, structurally identical to sodium nonatitanate, which was capable of reversibly intercalating Na+ ions at a low potential of about 0.3 V vs. Na/Na+.357 During the discharge process, typical voltage curves of a dehydrated nonatitanate electrode had a reversible capacity of 125 mA h g1 at 30 mA g1 accompanied by irreversible decomposition of the electrolyte which includes formation of a SEI layer in the voltage range of 0.3–0.9 V similar to the other titanium-based oxides. The sodium titanate form of Na4Ti5O12 has been investigated as the two crystal structures of trigonal Na4Ti5O12 (T-Na4Ti5O12) and monoclinic Na4Ti5O12 (M-Na4Ti5O12) by Woo et al. and Naeyaert et al., respectively.358,359 T-Na4Ti5O12 has a tunnel- structured three-dimensional framework, whereas M-Na4Ti5O12 has a quasi-2D layered structure. Both electrodes can incorporate intercalated Na+ ions into their structures, however, 2D channels with partially occupied Na sites, providing broader pathways, can deliver higher reversible capacity than T-Na4Ti5O12. The tunnel structured sodium titanate form of Na2Ti7O15 was also investi- gated as a possible Na+ insertion host material for SIBs anodes. Li et al. proposed Na2Ti7O15 nanotubes on the Ti net substrate, which exhibited a high reversible capacity of 258 mA h g1 at 50 mA g1 and an excellent capacity retention of 96% after 200 cycles at 1.0 A g1.364 Recently, as promising anode materials, layered struc- tured sodium titanate compounds of O3-NaTiO2 and P2- Na0.66[Li0.22Ti0.78]O2 were introduced.360–362 Through the in situ X-ray diffraction studies, Wu et al. demonstrated the reversible O3–O03 phase transition of O3-NaTiO2 and proposed the Na+ insertion/extraction mechanism (Fig. 28f). In an opti- mal voltage window of 0–1.6 V, approximately 0.5 mol of Na+ can be reversibly intercalated in NaTiO2, showing a reversible capacity of 152 mA h g1 and stable cycle retention after 60 cycles.361 For P2-Na0.66[Li0.22Ti0.78]O2, a reversible capacity of 116 mA h g1 was delivered at an average storage voltage of 0.75 V362 (Fig. 28g). In addition, it exhibited zero strain characteristics of only B0.77% volume change during sodium insertion–extraction, ensuring a potentially long cycle life for over 1200 cycles. 3.2 Conversion materials Some kinds of transition metal oxide (TMO),365–399 transition metal sulfide (TMS)400–454 and transition metal phosphide (TMP)455–466 compounds can adopt Na+ ions through conver- sion reactions. Unlike intercalation and alloying reactions, where metal atoms are reversibly shuttled in and out of a host lattice, conversion reactions involve the chemical transforma- tion of one or more of the atomic species into a host lattice to form a new compound.426 Depending on the transition metal, insertion–extraction or alloying–dealloying was combined with conversion reactions. Analogous to the reaction in LIBs, conversion materials have been considered as potential anode materials for SIBs due to their high theoretical specific capacities. View Article Online 3572 | 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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