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Materials 2020, 13, 3453 33 of 58 prepared via a surface oxidation-assisted chemical bonding procedure [374], 2119 and 1700 mA·h·g−1 at 0.2 and 1.3 A·g−1, respectively, over 100 cycles for a black phosphorous/Ketjenblack-MWCNTs composite [372]. Red phosphorous has also a high sodium storage theoretical capacity (2595 mA·h·g−1), but it shows lower electronic conductivity (≈ 10−14 S·cm−1), so that it must be associated to a conductive material to fabricate performing anodes for SIBs [375]. In addition, the volume expansion upon sodiation is large, so that hollow and porous structure has been fabricated to increase the cycle life. For instance, wet-chemical synthesis of hollow red-phosphorus nanospheres with porous shells used as an anode delivered 1364 mA·h·gem−1 and 1100 mA·h·gem−1 (gem = gram of electrode materials) at 0.2C. The corresponding areal capacity was 2.3 and 1.8 mA·h·cm−2 at 0.52 and 1.3 mA·cm−2, respectively. At 1C, a stable capacity of 969.8 mA·h·g−1 was demonstrated over 600 cycles [376] Combining electroless deposition with chemical dealloying to control the shell thickness and composition of a red phosphorus (RP)@Ni–P core@shell nanostructure, Liu et al. obtained an anode with remarkable properties: 1256 mA·h·g−1 after 200 cycles at 260 mA·g−1, while at the high current density of 5.2 A·g−1, the capacity was 491 mA·h·g−1 retained at 409 mA·h·g−1 after 2000 cycles (the data are per gram of the composite) [377]. Hybridization of red phosphorous with functional conductive polymer-sulfurized polyacrylonitrile (P–SPAN) was obtained via a facile mechanical ball-milling process. The hybridization enabled an intimate contact of SPAN and the red phosphorous, which greatly improved the conductivity and helped forming a robust electrode that can endure large volume change upon cycling [378]. The corresponding anode delivered a capacity of 1300 mA·h·g−1 at 520 mA·g−1, with 91% capacity retention after 100 cycles. By confining nanosized amorphous red P into ZIF-8-derived nitrogen-doped microporous carbon matrix (denoted as P@N-MPC), Li et al. obtained an anode with a capacity of ≈ 600 mA·h·g−1 at 0.15 A·g−1 and improved rate capacity (≈ 450 mA·h·g−1 at 1 A·g−1 after 1000 cycles with a capacity fading rate reduced to 0.02% per cycle) [379]. Red phosphorous encapsulated into the cube shaped sandwich-like interconnected porous carbon building via the vaporization–condensation method demonstrated a capacity retention of about 93% at 2 A·g−1 after 100 cycles, and a capacity of 502 mA·h·g−1 at 10 A·g−1 [380]. (iv) Silicon. The large interstitial sites of amorphous Si (a-Si) facilitates Na intercalation and migration with respect to crystalline Si. Amorphous Si (a-Si) can theoretically absorb 0.76 Na atom per Si, corresponding to a specific capacity of 725 mA·h·g−1 [381]. In addition, the full sodiation process only induces a volume expansion of 114%, much less than that of other alloying materials like Sn, P, Sb. Nevertheless, the first attempts showed poor rate capability. More recently, however, an anode based on the rolled-up amorphous Si nano-membranes capacity of 152 mA·h·g−1 after 2000 cycles, corresponding to a capacity retention of ≈ 85% owing to a large supercapacitance contribution [382]. Crystalline Si is electrochemically active toward sodium storage through an amorphization mechanism of NaSi alloys [383]. In this work, Zhang et al. fabricated a flexible binder-free bamboo-rattle type Si/carbon nanofiber film via an electrospinning technology. As an anode, this composite delivered 454 mA·h·g−1 after 200 cycles at a current rate of 50 mA·g−1. The contribution of the carbon nanofibers was 157 mA·h·g−1, the remaining part coming from the 20 wt.% silicon. At a higher current density of 5 A·g−1, the capacity maintained at ≈ 200 mA·h·g−1 after 2000 cycles. Table 2 lists the electrochemical properties of selected anode materials reviewed in the text.PDF Image | Electrode Materials for Sodium-Ion Batteries
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