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Chem Soc Rev Review Article Kim et al. reported a reversible phase transition during inter- calation and deintercalation processes of solvated-Na-ions in natural graphite via an operando X-ray diffraction analysis261 (Fig. 21c). They investigated the unusual Na storage behavior in natural graphite through Na+-solvent co-intercalation using different solvents (EC/DEC, DME, DEGDME and TEGDME) and Na salts (NaPF6, NaClO4, NaCF3SO3). Ether-based electro- lytes could suppress electrolyte decomposition, resulting in the formation of a negligible SEI film on the graphite surface, enabling Na+-solvent transport to the graphite lattice. In con- trast, carbonate-based electrolytes (EC/DEC) form relatively thick insulating SEI layers on the graphite surface, which block Na+-solvent transport262 (Fig. 21d). As a result, under the limited conditions using ether-based electrolytes, the natural graphite delivered a capacity of 150 mA h g1 with reasonable retention for 2500 cycles, and produced over 75 mA h g1 at 10 A g1 in the DEGDME electrolyte containing NaPF6 salt. On the other hand, Wen et al. proposed expanded graphite with an enlarged inter- layer lattice distance of 4.3 Å. Note that graphite has a typical interlayer space of 3.4 Å.263 Their in situ TEM study revealed reversible insertion and extraction of Na+ ions during the electrochemical reaction (Fig. 21e). As a result, the expanded graphite could exhibit a moderate capacity of approximately 284 mA h g1 at a current of 20 mA g1 with good capacity retention over 2000 cycles (Fig. 21f). Recently, Kang et al. reported the sodium ion intercalation behavior of expanded graphite oxide (GO) as an anode material. According to their report, the electro- chemical properties of GO strongly depend on the amounts and ratios of different functional groups.264 3.1.1.2. Non graphitic carbon (hard carbon). In 1993, Doeff and co-workers first reported the Na storage performance of disordered soft carbon prepared by pyrolysis of petroleum coke251 (Fig. 22a). They demonstrated the extent of the Na+ insertion/extraction reaction into the soft carbon and discussed the possibility of its application for SIBs. Stevens and Dahn reported the insertion mechanism of Na+ ions into disordered hard carbon (Fig. 22b).22,23,247 The suggested mechanism was the ‘‘house of cards’’ for Na+ ion storage, which is composed of the two domains in a disordered hard carbon structure without staging transition. First, Na+ ions are inserted between parallel graphene sheets (in the sloping voltage region) upon increasing the interlayer space. Second, Na+ ions fill the nanopores (in the plateau region) of the disordered carbon structure.22,23,249 The hard carbon prepared by carbonization of glucose showed a reversible electrochemical reaction in Na cells and it delivered a high specific capacity of approximately 300 mA h g1 with a low operating potential of about B0 V. Later, Komaba and co-workers performed a systematic study on electrochemical sodium insertion into hard carbon to understand the related structural change.27 Upon reduction to 0.2 V (in the sloping region), the resulting XRD peaks shifted to lower angles, indicating that the interlayer spacing between the graphene sheets was expanded due to the sodium insertion (Fig. 22c). They also confirmed the reversible Na+ insertion process into the nanopores via small angle X-ray scattering measurements the continuous self-pulverization of the electrode materials.240,243 More seriously, to compete with lithium ion batteries, a great challenge to overcome is the sluggish reaction kinetics from the large ionic size of Na+ ions (1.02 Å), which impedes fast Na+ storage.99,240,246 Therefore, scientists should make more efforts to improve the electrode performances. In this section, we discuss present research progress in anode materials for SIBs. 3.1. Insertion materials Based on an insertion reaction, carbonaceous and titanium-based oxides have been extensively studied as anodes for SIBs. Several carbon-based materials such as graphitic and non-graphitic carbons have been investigated for Na+ storage.241 These carbon materials are widely accepted because of their ability to accom- modate Na+ ions into their structure. In particular, hard carbon is interesting because of its reasonable capacity of B300 mA h g1 and low operating potential (almost zero, B0 V vs. Na+/Na).22,23,247 However, the Na+ storage mechanism in a disordered carbon structure is still controversial.247–251 On the other hand, titanium- based oxide compounds have been widely studied because of their low operation voltage and cost as well.242 Analogous to LIBs, titanium-based oxide anodes, including various polymorphs titanium dioxide (TiO2), spinel-lithium titanate (Li4Ti5O12) and sodium titanate (NaxTiyOz), were reported as promising anode materials. 3.1.1. Carbon-based anode materials 3.1.1.1. Graphitic carbon (graphite). During the electro- chemical reduction, Li+ ions are inserted between graphene layers, and Li-graphite intercalation compounds (Li-GIC) are formed through stage transformations in Li cells.252,253 Since the 1980s, the electrochemical behavior of graphite with Na+ ions has been studied based on the graphite/polyethylene oxide NaCF3SO3/Na cell254 (Fig. 21a). However, Na+ insertion into graphite is significantly impeded and degradation of electrolyte and/or electrode materials was observed255 (Fig. 21b). First- principles calculation results of the formation energy for Na-GIC exhibited that Na hardly intercalates into graphite because of the energetic instability of the Na-GICs. Graphite is stressed when some Na+ intercalates into graphite because of the thermodynamic instability of binary Na-intercalated GICs (b-GIC), which is assumed to be the result of an unfavorable mismatch between the graphite structure and the size of the Na ion.256 Namely, due to the unfavorable formation of NaC6 and NaC8 that are thermodynamically unstable at the first stage of Na-graphite intercalation compound (Na-GIC) formation in SIBs, the capacity of natural graphite is limited.256–258 As a recent breakthrough, Jache et al. facilitated Na-ion storage in graphite using solvated-Na-ion intercalation, forming ternary GICs. That is the intercalation of solvated alkali ions ‘‘co-intercalation’’ by reduction of graphite according to the following equation: Cn + e + A+ + y sol v 2 A+(sol v)yCn.258 Zhu et al. also confirmed that solvated Na+ ions intercalate into graphite via a stage- evolution process, forming a set of ternary graphite intercalation compounds.259 Ether-based electrolytes with a high donor number can form stable Na+ solvated species with non-polar characteristics for co-intercalation into natural graphite.260–262 View Article Online Thisjournalis©TheRoyalSocietyofChemistry2017 Chem.Soc.Rev.,2017,46,3529--3614 | 3561 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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