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Review Article Chem Soc Rev 3.1.2.3. Sodium titanate. The Na2Ti3O7 structure has been most investigated as a promising anode with the lowest operat- ing potential for SIBs347–353 (Fig. 24f). Senguttuvan et al. firstly reported that sodium titanate, Na2Ti3O7, can reversibly uptake 2 mol of Na+ ions per formula unit with a low operating potential plateau at 0.3 V vs. Na/Na+ 347 (Fig. 28a). A plateau with this voltage can be quite advantageous in a full cell with a suitable cathode in terms of energy density compared to the other oxide type anodes. Typical voltage profiles of carbon black and a composite of Na2Ti3O7 with carbon black indicated an irreversible electrochemical process at ca. 0.7 V vs. Na/Na+, which corresponds to the reaction of carbon black and a reversible plateau around 0.3 V vs. Na/Na+ with concomitant intercalation of additional 2Na+ ions in the structure. Xu et al. reported the computational and experimental results for an in-depth understanding of the sodium storage mechanism of the Na2Ti3O7 structure.348 Based on the calculations of the electro- static interaction in the crystal structure, 2 mol of Na+ ions are intercalated into the Na2Ti3O7 structure to form Na4Ti3O7 producing strong electrostatic repulsion, leading to structural instability and low operating voltage. Also, this strong electro- static repulsion in the fully sodiated state induces the self- relaxation phenomena. In addition, they observed the Ti4+/Ti3+ redox couple upon cycling via the XAS technique (Fig. 28b). Furthermore, their carbon-coated Na2Ti3O7 electrodes delivered a theoretical capacity of 177 mA h g1 (corresponding to uptake of 2 Na+ ions per Na4Ti3O7). Rudola et al. investigated the physicochemical and electro- chemical properties of Na2Ti3O7 and calculated the diffusion coefficient of Na+ ions in the Na2Ti3O7/CB electrode.349 Later, Rudola et al. observed the intermediate phase of Na3xTi3O7 during the sodiation/desodiation process350 (Fig. 28c). During the sodiation process, based on the ex situ XRD results, two discharge plateaus revealed the following two phase reactions: Na2Ti3O7 - Na3–xTi3O7 (black curve in Fig. 28c) and Na3–xTi3O7 - Na4Ti3O7 (red curve in Fig. 28c). The lower discharge plateau causes an irreversible transformation that leads to the loss of the sodium storage pathway in subsequent cycles. By controlling the cut-off potential from 0.01–2.5 V (with low plateaus) to 0.155–2.5 V (without low plateaus), the Na2Ti3O7 # Na3–xTi3O7 pathway has the lowest redox voltage of 0.2 V vs. Na/Na+ with a moderate capacity of 89 mA h g1. In addition, it showed excellent rate performance up to 80C-rate and a good cycle life for over 1500 cycles. On the other hand, Pan et al. observed that nanosized Na2Ti3O7 intermediate phase is avoided and Na4Ti3O7 directly translates to Na2Ti3O7 during the Na+ extraction process.351 They also discussed two main reasons for the low Coulombic efficiency and continuous capacity fading of the Na2Ti3O7 electrode: (i) partial decomposi- tion on the Na2Ti3O7 particle surface (instability of the SEI layer) and (ii) structural distortion upon Na+ insertion/extraction. Later, Mun ̃oz-M ́arquez et al. observed the instability of SEI formation during the charge process and precisely investigated the com- position and evolution of the solid–electrolyte interphase in Na2Ti3O7 electrodes.352 Although Na2Ti3O7 is considered as a promising anode with the lowest operating potential for They also improved the electrochemical properties of Li4Ti5O12 by varying the electrolyte and binders. Usually, for LIBs, Li+ insertion into the Li4Ti5O12 anode occurs through a two-phase reaction between spinel Li4Ti5O12 and rock-salt Li7Ti5O12. In Na cells, Na+ insertion into Li4Ti5O12 results in a three-phase reaction. Due to the different ionic sizes of Li+ and Na+, the Na+ ions are favorably occupied in the 16c site of the Li4Ti5O12 lattice, and they simultaneously induce phase separation into two rock-salt phases of LiNa6Ti5O12 and Li7Ti5O12 as follows: 2Li4Ti5O12 + 6Na+ + 6e 2 Li7Ti5O12 + Na6LiTi5O12. Later, Kim et al. observed the structural evolution and the chemical state of Ti at the initial cycle based on ex situ XRD and XPS measurements339 (Fig. 27c–e). Yu et al. observed three phase transition behavior of Li4Ti5O12 during Na+ insertion through in situ X-ray diffraction. They also investigated a size-dependent sodium storage mechanism in Li4Ti5O12.340 For 440 nm size Li4Ti5O12, only a small amount of Na can be inserted (0.27Na+ per formula unit of Li4Ti5O12), which corresponds to a specific capacity of 16 mA h g1. While the degree of the reversible Na+ insertion into 44 nm size Li4Ti5O12 was greatly increased (3Na+ ion per formula unit of Li4Ti5O12) and nearly reached its theoretical capacity of 175 mA h g1 (Fig. 27f). They claimed that downsizing the Na+ host structure of Li4Ti5O12 is a crucial factor to improve the sluggish Na+ ion diffusion kinetics. Analogous to this scenario, Hasegawa et al. reported nano- sized Li4Ti5O12 materials with hierarchically porous structures and flower-like morphologies.341 According to their reports, Na+ insertion/extraction capability is strongly dependent on their nanoarchitectural design and calcination temperature. These nanostructured Li4Ti5O12 electrodes calcined at 700 1C displayed a flower-like porous structure and exhibited a remark- ably high rate performance of 146 mA h g1 and 105 mA h g1 at 10C and 30C-rates, respectively, even without carbon-coating. On the other hand, Yu et al. reported Na+ ion transport kinetics and coupled pseudocapacitive charge in thin film Li4Ti5O12 electrodes.342 The synergistic effect of typical Na+ insertion and the extra pseudocapacitive charge storage produced an unexpected high capacity of 225 mA h g1 in thin film Li4Ti5O12 electrodes. They claimed that the pseudocapacitance effect on a typical insertion electrode is a potential solution to overcome the capacity limit for Na+ insertion anodes. Another approach to improve the electrochemical performances is combining carbon additives with Li4Ti5O12 materials. Kim et al. proposed pitch carbon-coated Li4Ti5O12 nanowires, which signifi- cantly increased the electronic conductivity and delivered a high capacity of 168 mA h g1 at 0.2C-rate.339 Chen et al. fabricated porous Li4Ti5O12 nanofibers confined in a highly conductive 3D- interconnected graphene framework for SIB anodes343 (Fig. 27g). This unique structure of porous Li4Ti5O12 nanofibers wrapped with 3D graphene offers not only short pathways for Na+ diffusion and highly conductive networks for electron transport but also abundant Li4Ti5O12–electrolyte (solid–liquid) and Li4Ti5O12–graphene (solid– solid) interfacial sites for Na+ adsorption, giving rise to additional interfacial Na+ storage, a high reversible capacity of 195 mA h g1 at a 0.2C-rate (exceeding the theoretical capacity based on the Na+ insertion reaction) and a long cycle life of 12 000 cycles. View Article Online 3570 | 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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