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Review Article Chem Soc Rev time304 (Fig. 25a). On the other hand, Mattsson et al. claimed that high crystalline and/or micronized TiO2 cannot easily support Na+ insertion because of the ionic size of Na+ and a much higher sodium diffusion barrier compared with Li.305 Recently, high electrochemical activity of TiO2 with Na+ ions was achieved by reducing the particles to the nanometer size for shortening of the migration length for Na+ insertion. Xu et al. used anatase nanocrystalline TiO2 for Na+ storage.306 Wu et al. interpreted the reaction process during Na+ insertion and extrac- tion, highlighting the substantial influence of the electrolyte composition (salt and solvent) and cut-off potential307 (Fig. 25b). Through NMR and electrochemical studies, Gonzalez et al. explained the irreversible process (electrolyte decomposition) up to 0.3 V observed at the first Na+ insertion process in the TiO2 electrode.308 Kim et al. observed that anatase TiO2 nanorods can be stored in the host structure through the Na+ insertion and extraction reaction coupled with the Ti4+/Ti3+ redox reaction via ex situ XRD and XAS studies309 (Fig. 25c and d). Their carbon coating on the anatase TiO2 nanorod surface played an important role in improving the capacity and rate capability. Furthermore, they also suggested that the presodiation technique was an effective way to minimize the initial irreversible reaction of the TiO2 anode. Recently, Passerini’s group found that only the (de-)insertion of Na+ ions in the newly formed amorphous sodium titanate phase appears to be reversible (uptake/release 0.41 Na per TiO2), while all the other processes (metallic titanium, sodium superoxide, and oxygen evolution) appear to be irreversible (Fig. 25e and f).310 Usui et al. investigated rutile-TiO2 that showed the reversible reaction of Na+ insertion and extraction into and from the crystal lattice of rutile TiO2326 (Fig. 26a). In addition, the appropriate amount of Nb doping (0.06 mol%) on rutile TiO2 materials could substantially improve the electronic conductivity. for decreasing the initial irreversible capacity. And morphology and size control is a substantial strategy to facilitate mass transport and storage, which can significantly improve the rate capability. Most of all, for developing practical SIBs using high mass loading electrodes, a comprehensive study of the impact of the electrode thickness on the rate capability, energy and power density and long-term cycling behavior is required.298 3.1.2 Titanium based oxides. In general, the low operational potential can cause safety issues for practical applications; such as, metallic sodium plating and sodium dendrite formation on the surfaces of anodes.241,247,249,250,347 Analogous to LIBs, metal oxide compounds have been studied as Na+ ion insertion host materials. Titanium-based oxides are particularly interesting as anodes due to their reasonable operation voltage, cost, and nontoxicity;299,300 representatively, titanium dioxides,301–333 spinel-lithium titanate,334–346 and sodium-titanate compounds.347–364 Note that these compounds are driven by a Ti4+/3+ redox couple in Na cells. Recently, most related works have focused on finding the sodiation/desodiation mechanism and improving the electro- chemical performance of such materials. 3.1.2.1. Titanium dioxides. The several TiO2 polymorphs, including anatase-TiO2,301–314,316–323 rutile-TiO2,324–328 brookite- TiO2,329,330,332 and bronze-TiO2333 have been investigated as anode materials for SIBs294–296 (Fig. 24a–d). Among them, most research results were reported using anatase TiO2 because the activation barrier for Na+ insertion into the anatase lattice is comparable to that of lithium, which is rather remarkable considering its significantly larger ionic radius.301–303 Xiong et al. reported the feasibility of electrochemically-grown amorphous titanium dioxide nanotube electrodes in Na cells for the first View Article Online Fig. 24 Crystal structures of TiO2. (a) Rutile, (b) anatase, (c) bronze, (d) brookite. (Reprinted from ref. 299, Copyright 2015, with permission from Elsevier.) Crystal structures of (e) spinel-type Li4Ti5O12 and (f) Na2Ti3O7. Li and Na atoms are represented by green and yellow spheres, respectively. (Reproduced with permission from ref. 300, Copyright 2016 The Royal Society of Chemistry.) 3566 | 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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