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Materials 2020, 13, 3453 19 of 58 material for use as an anode in Na-ion cells that delivered capacity of 85 mA·h·g−1 in the 1000th cycle at 5C (1 A·g−1) [198]. The best results can be obtained by combining synergetic effects of porosity, N-doping and nano-structuration, under the form of nanosheets [199] or nanofibers [200,201], the corresponding anodes delivering a typical capacity of 220 mA·h·g−1 at 50 mA·g−1. Nitrogen-rich carbon with interconnected mesoporous structure could deliver a reversible capacity of 338 mA·h·g−1 at a current density of 30 mA·g−1, and remarkable rate capability with a capacity of 111 mA·h·g−1 at a current density of 500 mA·g−1 over 800 cycles [202]. Yang et al. reported that, at 0.15C (1C = 375 mA·g−1), N-doped carbon sheets delivered a stable reversible capacity of 292 mA·h·g−1. Even at high mass loading of 7.12 mg·cm−2 the reversible capacity was maintained at 121.7 mA·h·g−1. At 4.5C, the capacity was stable at circa 50 mA·h·g−1 over 2000 cycles [203]. N-doped carbon sheets were also investigated by Yang et al. to produce an anode delivering a capacity of 165 mA·h·g−1 after 600 cycles at current density of 200 mA·g−1 [204]. N/S co-doped ordered mesoporous carbon delivered a capacity of 419 mA·h·g−1 at 0.1 A·g−1 after 150 cycles, retaining 220 mA·h·g−1 at 5 A·g−1 even after 3000 cycles [205]. Although N-doping is most popular, doping with other elements such as B, O, S, and P are also of interest, and a review on the design, synthesis, and electrochemical properties of heteroatom-doped carbon anodes can be found in [206]. Recently, 3D scaffolding S-doped carbon nanosheets produced from biomass delivered a reversible capacity of 605 mA·h·g−1 at 50 mA·g−1, 133 mA·h·g−1 at 10 A·g−1. The capacity was maintained ats ~211 mA·h·g−1 upon 2000 cycles at current density of 5 A·g−1 [207]. Yun et al. synthesized pyroprotein-based carbon nanoplates (CNPs) with varying degrees of carbon ordering [208]. They showed that the sodium-ion storage mechanism varies from chemi-physisorption insertion to nanoclustering of metallic states, depending on the carbon structure of CNPs, which display various potentials and capacities. Therefore, tailoring carbon orderings is a critical factor for tuning the electrochemical performance of carbonaceous materials for SIBs. A perspective for sodiation of hard carbon consists of Na-ion storage at defect sites, by intercalation and last via pore-filling [209,210]. In addition, ab initio calculations for disordered carbon show that large initial interlayer distances and defects, in particular vacancies can greatly enhance the Na+ ion intercalation [211]. In addition, hard carbon can be used with ionic liquid electrolytes to obtain less-flammable sodium-ion cells [212]. Moreover, Li et al. demonstrated that the rate capability of hard carbon is underestimated in prior studies that used carbon/Na two-electrode half-cells, because it is the overpotential of the sodium counter electrode that drives the half-cells to the lower cutoff potential prematurely during hard carbon sodiation [213]. There many ways to synthesize carbon anodes. However, since the practical application of the sodium-ion technology relies on the fact that it is cost effective with respect to the Li-ion technology as recalled in the introduction, attention has been focused on of cheap, scalable and facile synthesis of the carbon anodes. In this respect, attention has been focused on carbonaceous materials derived from biomass waste. Most of the carbon materials derived from biomass exhibit specific capacity in the range of 200–300 mA·h·g−1 at a current density of 50 mA·g−1 in sodium-ion batteries [214–216]. In particular, a coir pith waste derived carbon (CPC) electrode demonstrated a capacity of 220 mA·h·g−1 up to 300 cycles with negligible capacity fading have been observed at 50 mA·g−1. Furthermore, CPC prepared at 850 ◦C delivers ∼110 mA·h·g−1 for 1000 cycles at 1 A·g−1 (Figure 8) [217]. Carbon materials derived from biomass also demonstrate a good performance as sodium-ion supercapacitor [218]. A simple productive synthesis of carbon quantum dots with diameters in the range of 1.5–3.0 nm was discovered by Hou et al. by mixing acetone and NaOH, without any other treatment [219]. An outstanding cycle life was demonstrated with a capacity of 150.1 mA·h·g−1 after 3000 cycles at current density 2.5 A·g−1. At 5 A·g−1, a capacity of 99.8 mA·h·g−1 was maintained after 10,000 cycles. Soft carbons have been less investigated, but can be competitive to hard carbon, provided that the precursor and heat treatment are optimized so that the interlayer distance d is large [220]. The best results were obtained for a soft carbon with d = 3.65 Å, which delivered a capacity of 120 mA·h·g−1 after 250 cycles at a current density of 1000 mA·g−1.PDF Image | Electrode Materials for Sodium-Ion Batteries
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