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Materials 2020, 13, 3453 11 of 58 decrease of capacity [117]. Fluffy Na0.67FePO4/CNT nanocactus used as a cathode delivered the same capacity, also stable over 50 cycles [118]. Fluorophosphates as cathode materials exhibit similar capacities ≈ 100 mA·h·g−1 as NASICON, but they have a higher operational voltage so that the loss of energy density is smaller. For instance, orthorhombic Na2CoPO4F/C delivers a capacity of 107 mA·h·g−1 with a voltage plateau at 4.3 V [119]. Vanadium-based fluorophosphates also benefit from a high voltage (average potential of 3.7 V), owing to the enhanced inductive effect of the (PO4)3− polyanion and the larger ionicity of the F-V bond [120]. The recent progress on sodium vanadium fluorophosphates has been reviewed in [121]. In particular, Qiu et al. [122] synthesized a core/double shell structured Na3V2(PO4)2F3@C nanocomposite through in situ coating of the carbon and the prepared particles were uniformly distributed in the mesoporous carbon framework. As a cathode for SIB, this composite delivered 125, 123, 121, 116, 100, 92, 84 and 63 mA·h·g−1 at 0.5, 1, 5, 10, 20, 30, 50 and 100C, respectively. After 5000 cycles at 50C rate, the capacity was 62 mA·h·g−1, which corresponds to a retention of 65%. Following this result, many works synthesized Na3V2(PO4)2F3@C compounds using other forms of carbon, including graphene, carbon nanofibers, and carbon nanotubes [123–128]. All of them demonstrated very good rate capability. Among them, in situ carbon nanofibers coating on Na3V2(PO4)2F3@C (NVPF@C) particles were obtained through chemical vapor deposition (CVD) by using Fe as the catalyst. The optimum ratio of NVPF@C to Fe was 5:100 [127]. The corresponding cathode tested at 20C over 5000 cycles delivered a capacity of 93.3 mA·h·g−1 with a capacity retention of 86.3% at more than 99.5% coulombic efficiency. Nanosized Na3(VOPO4)2F electrode also showed promising electrochemical properties, with a delivered capacity of 112 mA·h·g−1 with capacity retention of 93.8% after 200 cycles at a current rate of C/5, for an average discharge potential of 3.75 V leading to an energy density of 384 Wh·kg−1 At 2C, the capacity was still ≈ 100 mA·h·g−1 with retention of 90% over 1200 cycles [129]. A major progress was obtained recently by Wang et al. who fabricated an advanced low-T sodium-ion full battery assembled with this high-voltage cathode, and an anode of 3D Se/graphene composite [130]. This cell exhibited ultra-long lifespan (over 15,000 cycles, the capacity retention is still up to 86.3% at 1 A·g−1), outstanding low-T energy storage performance (e.g., all values of capacity retention are > 75% after 1000 cycles at temperatures from 25 to −25 ◦C at 0.4 A·g−1). At high current density of 4 A·g−1, the capacity is still 72.7 mA·h·g−1 at room temperature. Therefore, this cell well satisfies the requirements of grid energy storage for batteries. A flexible and binder-free Na3(VOPO4)2F cathode with nanocubes tightly assembled on carbon cloth was recently fabricated by a facile solvothermal method [131]. About 90% (112 mA·h·g−1) and 86% (106 mA·h·g−1) of the 1C capacity were retained at 10C and 20C, respectively, a rate performance superior to prior works, taking into account the high mass loading > 2.0 mg·cm−2. In total, 88% of the discharge capacity was retained after 1000 cycles while cycled at 5C. 2.5. Sulfates Sulfates have raised interest, since alluaudite Na2 Fe3 (SO4 )3 as a cathode material [132]. This cathode delivered a capacity of 100 mA·h·g−1, but with poor capacity retention; but most of all, the Fe2+/Fe3+ redox potential is raised at 3.8 V vs. Na+/Na, one of the highest among all the Fe-based intercalation compounds, owing to the electron-drawing (SO4)2−. This feature was the motivation for further investigation of sulfates of the same family: Na2+2xFe2-x(SO4)3 [133–135], Na2+2xMn2-x(SO4)3 or Na2.5(Fe1-yMny)1.75(SO4)3 [136–138] without improving the electrochemical properties. The hydrated sulfate compounds Na2Fe(SO4)2·4H2O, Na2Fe(SO4)2·2H2O [139], or eldfellite-type NaFe(SO4)2 [140] also gave poor results. NaFe3(SO4)2(OH)6 is amorphous material that delivered a capacity of 120 mA·h·g−1 at low C-rate (C/20) with an average voltage of 2.72 V vs. Na+/Na in Na-ion cells [141]. However, the cycle life has been studied over 20 cycles only, and the rate capability has not been explored, yet. In an attempt to increase the capacity, the orthosilicate Na2FeSiO4 has been synthesized by electrochemical Li–Na ion-exchange. As it can theoretically exchange two Na+ ions, its use as an anode delivered a capacityPDF Image | Electrode Materials for Sodium-Ion Batteries
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