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Materials 2020, 13, 3453 32 of 58 cycles. This result demonstrates that this strategy is efficient and opens the route to the construction of binder-free anodes for SIBs with metals as the active element. (ii) Antimony. According to the reaction between Sb and Na3Sb, the theoretical capacity is 660 mA·h·g−1 [358,359] To approach this result and relieve the stress caused by the three Na uptake to obtain a good cycle ability, the synthesis of nanocomposite with different forms of carbon has been investigated. 1D carbon nanofibers, which trap Sb nanoparticles via a simple electrospinning process delivered a capacity of 631 mA·h·g−1 at C/15, 337 mA·h·g−1 at 5C, and demonstrated a good rate capability (90% capacity retention after 400 cycles at C/3) [360]. Other forms of carbon include acetylene black [361] and porous carbon [362]. In this last case, a capacity of 385 mA·h·g−1 (capacity retention of 88.5%) after 500 cycles at 100 mA·h·g−1 was demonstrated. Antimony/nitrogen-doping porous carbon (Sb/NPC) composite with polyaniline nanosheets as a carbon source delivered a capacity of 529.6 mA·h·g−1, with 97.2% capacity retention after 100 cycles at 100 mA·g−1 [363]. Binding Sb nanoparticles in ionic liquid-derived nitrogen-enriched carbon (Sb@NC) via pyrolysis of an SbCl3/1-ethyl-3-methylimidazolium dicyanamide mixture improved the sodium storage [364]. This anode delivered a capacity of 440, 285 and 237 mA·h·g−1 at a current density of 0.1, 2 and 5 A·g−1, respectively. At the current density of 100 mA·g−1, the capacity maintained at 328 mA·h·g−1 after 300 cycles. An electrode material composed of Sb nanoplates on Ni nanorod arrays exhibited a capacity of 580 mA·h·g−1 at a current density of 0.5 A·g−1 with 80% retention over 200 cycles [365]. The full cell with P2-Na2/3Ni1/3Mn2/3O2 as the cathode delivered a capacity of 580 mA·h·g−1 over 200 cycles and an energy density as high as 100 Wh·kg−1. These results show that antimony is one of the best-performing anode materials in terms of both capacity and cycling stability. This is surprising since the theoretical sodium-storage capacity of silicon is 954 mA·h·g−1, In practice, Si has never reached such a capacity. However, by combining silicon and antimony amorphous films with bilayer thickness down to 2 nm, and an amount of Si of 7 at.%, the mesoporous Si0.07Sb0.93 reached a capacity of 663 mA·h·g−1 after 140 cycles at a low rate of 20 mA·g−1. This is more than the theoretical capacity for Sb (660 mA·h·g−1) and more than the highest experimental capacity for pure Si reported so far (∼600 mA·h·g−1) [366]. Additional results for metallic Sn- and Sb-anodes can be found in [367]. (iii) Phosphorous. The theoretical capacity according to the reaction between P and Na3P is 2596 mA·h·g−1, and phosphorous has aroused growing interest as an anode element for sodium-ion batteries [368]. The allotropes of interest for SIBs are the red and the black phosphorous. The black-phosphorous is a good conductor (∼300 S·m−1 ), and the interlayer channel size is large (3.08 Å), so that Na+ ions of radius 2.04 Å can be stored between the phosphorene layers. Few phosphorene layers sandwiched between graphene layers shows a specific capacity of 2440 mA·h·g−1 (calculated using the mass of phosphorus only) at a current density of 0.05 A·g−1 and 83% capacity retention after 100 cycles while operating between 0 and 1.5 V [369]. This very high capacity was attributed to a dual mechanism of intercalation of sodium ions along the x axis of the phosphorene layers followed by the formation of a Na3P alloy that accompany the P-P bond breaking, in agreement with theoretical calculations [370]. Poly(3,4-ethylenedioxythiophene) (PEDOT) functionalized on surface-modified black-phosphorous nanosheets delivered a capacity of 1078 mA·h·g−1 at a current density of 0.1 A·g−1 after 100 cycles. The capacity delivered at higher rate were 750 (1 A·g−1) and 370 mA·h·g−1 (10 A·g−1) [371]. A black phosphorus/Ketjenblack–multiwalled carbon nanotubes (BPC) composite with 70 wt.% phosphorus content was fabricated by high energy ball milling [372]. This composite delivered a capacity of 1700 mA·h·g−1 after 100 cycles at 1.3 A·g−1 based on the mass of P. More recently, 4-nitrobenzene-diazonium-modified P was used to bond chemically with RGO to enhance the electrical connection between two species [373]. The additional functional groups enlarged the channels of the modified black phosphorous with RGO layers, thus improving the rate capability. A capacity of 650 mA·h·g−1 at 1 A·g−1 over 200 cycles was obtained with this composite. The drawback, however, was a capacity at low rate (1400 mA·h·g−1 at 0.1 A·g−1) smaller than the best results that can exceed 2000 mA·h·g−1: 2060 mA·h·g−1 at 0.2 C, with capacity retention of 75.3% after 200 cycles for a composite of black phosphorus and multiwall carbon nanotubes (BP–CNT)PDF Image | Electrode Materials for Sodium-Ion Batteries
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