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Review Article Chem Soc Rev high average discharge voltages (3.3 V), high energy efficiencies and an extraordinary long-term cycling stability of 80% capacity retention after 700 cycles. However, such full cells based on hard carbon anodes suffer from an inferior rate capability, and the voltage plateau related to most capacities is too close to the sodium plating voltage, causing a safety concern. Li et al. designed a full sodium-ion battery based on nanostructured Na2Ti3O7 and VOPO4 materials as the anodes and cathodes625 (Fig. 43f). In order to reduce the polarization and irreversibility effect of the electrode materials in the first discharge process (both for the anode and cathode sides), pre-desodiated Na2Ti3O7 and pre-sodiated VOPO4 were prepared in advance. This full cell shows outstanding rate capability and excellent cycling stability in the wide temperature range of 20 to 55 1C. Recently, sodium-ion full cells were proposed using the same materials as both cathodes and anodes, namely symmetric full cells.626–629 The greatest advan- tage of using the same materials for both cathodes and anodes is that they significantly decrease the material’s processing cost. Wang et al. proposed the P2-Na0.6[Cr0.6Ti0.4]O2 cation-disordered electrode for high-rate symmetric rechargeable SIBs626 (Fig. 43g). This symmetric full cell exhibited an average operating voltage plateau at B2.53 V, as well as an extraordinary rate capability of 75% retention at 12C-rate and superior cycling performance. The Na3V2(PO4)3 materials also exhibit both cathode and anode potentials (3.4 V with a V4+/V3+ redox reaction and 1.6 V with a V3+/V2+ redox reaction), so a novel symmetric full cell system could be designed by employing Na3V2(PO4)3 as a bipolar electrode material.628 Such present results regarding sodium-ion full cells based on several platforms of cathodes and nonmetallic anodes have brought forth great advances in practical issues. Nevertheless, to obtain practical SIBs with high safety, outstanding rate capability, and cycling stability, further investigation of rationally designed full cells, including capacity balancing between the cathode and the anode, the voltage range, and stable electrolyte solution, is still necessary. Further studies of functional additives and binders to effectively control the solid electrolyte interface for- mation of cathodes and anodes are essential parameters for high safety SIBs. In addition, the total production cost of active materials of the electrode and the cost of battery components should be considered. In summary, a strategy for battery design in realizing practical SIBs is needed to find a good balance between an increase in battery performance with high safety and a decrease in the total cost of the batteries. 6. Summary Even though SIBs were studied around the same time as LIBs, they were also abandoned at one point, particularly after the commercialization of LIBs by Sony in the early 1990s. However, the growth of technology and the exigency for large-scale applications such as ESSs have opened the door for SIBs to be utilized again. Lithium is not uniformly scattered on the Earth’s crust, and given the demand that it is continually Applying the Na3V2(PO4)2F3 cathode and hard carbon anode for fabricating sodium-ion full cells, this full cell exhibited a high capacity of 110 mA h g1 with an operation voltage of 3.65 V, as well as excellent capacity retention upon cycling with a satis- factory Coulombic efficiency (498.5%) and very good power performance in the EC : PC : DMC (45 : 45 : 10, v/v) electrolyte solution574 (Fig. 43c). Later, Nose et al. assembled a new type of Na-ion full cell by combining Na4Co3(PO4)2P2O7 as a cathode and hard carbon as an anode; this battery demonstrated a 4.0 V-class high operating voltage with long-term operation.206 Layered structured cathodes such as P2-type and O3-type materials are also considered for sodium ion full cells with hard carbon anodes. Previous work has shown that P2-type cathode materials (NaxMO2; x r 0.7, M = transition metal) result in high recharge- able capacities.46 However, a lower initial sodium content in the crystal structure of P2-type layered cathode led to an abnormal Coulombic efficiency above B100% in the 1st cycle.100 There- fore, the intrinsic properties hinder practical full cell fabrication. Meanwhile, the practical benefits of O3-type (NaxMO2; x E 1.0, M = transition metal) cathodes are that they are able to fabricate sodium-ion full cells similar to commercial LIBs. Komaba et al. fabricated the full cell combined with a hard carbon anode and O3-tpye Na[Ni1/2Mn1/2]O2 cathode, which demonstrated acceptable battery performance with approximately 3 V for the operating voltage.27 Later, Kim et al. proposed O3-type layered Na[Ni1/3Fe1/3Mn1/3]O2 cathode materials and fabricated a high capacity full cell with hard carbon, which exhibited a high discharge capacity of 100 mA h g1 and stable cycle retention after 150 cycles with a high operating potential above 3 V50 (Fig. 43d). On the other hand, the sodium ion-full cells used in some works utilize a hard carbon anode that was presodiated in order to reduce the irreversible capacity during the first cycle. Moreover, such a presodiated system needs a different electrode balancing to prevent sodium plating.624 This technique is not intended for commercial applications, but rather for scientific queries, to test the long-term electro- chemical performance of such materials using full cells; this is because metallic sodium deposition on the Na metal anode does not warrant a long-term cycling test.87 Hwang et al. fabricated sodium ion full cells with a compositional graded O3-type Na[Ni0.60Co0.05Mn0.35]O2 cathode and presodiated hard carbon anodes, which demonstrated a reversible discharge capacity of 257 mA h g1 (Na0.68C6) with non-irreversible capacity (Fig. 43e). This full cell exhibited an excellent rate capability of 132.6 mA h g1 at 1.5 A g1 and stable cycle retentions at various temperatures (20, 0, 30 and 55 1C). Especially, it demonstrated a superior cycle retention of B80% under extensive cycling conditions for over 300 cycles with an average operation voltage of 2.84 V on discharge at 30 1C. More recently, Keller et al. proposed sodium-ion full cells based on a presodiated hard carbon anode and a mixed layered oxide cathode of P2/P3/O2-Na0.76[Mn0.5Ni0.3Fe0.1Mg0.1]O2.624 Presodiation of hard carbon was carried out until 135 mA h g1 to prevent excess sodium in the full cell. The developed sodium-ion full cells demonstrated high specific energies (200–240 W h per kg of cathode and anode active materials), View Article Online 3596 | 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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