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Materials 2020, 13, 3453 28 of 58 of 620 mA·h·g−1 after 900 cycles at current density of 1 A·g−1, and 508 mA·h·g−1 can still be kept at 5 A·g−1 [311]. Similar to Co9S8, heterostructures consisting of CoS2 and other metal sulfides have also been reported: SnS2@CoS2-rGO [312], NiS2@CoS2@C@C [313]. In this last case, 600 mA·h·g−1 capacity was demonstrated after 250 cycles, at a current density of 1 A·g−1. CoS has also been considered as an anode for SIBs. The electrochemical reaction can be divided into the insertion step: and the conversion step: CoS+xNa+ +xe− →NaxCoS,x<2, (5) NaxCoS+(2−x)Na+ +(2−x)e− →Co+Na2S. (6) The conversion step takes place below 0.8 V [314–316]. As usual, the problem is to avoid the pulverization of the particles due to the big change of volume in the conversion reaction. One can always increase the lower voltage to avoid the conversion reaction to obtain an anode with good capacity retention, but in that case the capacity associated to the conversion reaction is lost and the energy density in a full cell with such an anode will be too small. For this purpose, Zhou et al. synthesized CoS nanoparticles embedded into porous carbon rods [317]. In case the CoS particles were 7 nm in diameter, this anode delivered a capacity of 542 mA·h·g−1 after 2000 cycles in the voltage range of 0.6–3 V vs. Na+/Na, with a capacity retention of 91.4% at 1 A·g−1) and demonstrated an excellent rate performance (discharge capacities of 510 mA·h·g−1 at 5 A·g−1 and 356 mA·h·g−1 even at 40 A·g−1). A full Na-ion cell with this anode and Na3V2(PO4)3 cathode exhibited a capacity of 352 mA·h·g−1 at 0.5 A·g−1. It should be noted that this small size of the particles was crucial, since the same anode synthesized with CoS particles with a diameter of 18.5 nm gave degraded electrochemical properties. Another strategy is the fabrication of a CoS@C yolk-shell microsphere composite, but the corresponding anode has been tested over 50 cycles only [318]. Carbon-coated Co–Sn–S hollow nanocubes synthesized through a solvothermal sulfuration show excellent rate performance (478 mA·h·g−1 at 10 A·g−1) owing to a pseudocapacitance-dominated sodium storage mechanism, but a moderate cycle ability (83% capacity retention after 100 cycles at 0.1 A·g−1) [319]. A recent regain of interest in antimony sulfide is due to the electrochemical performance of Multi-shell hollow structured Sb2S3 obtained from the ZIF-8 framework. In the first step, multi-shell ZnS particles were obtained after three quenching and sulfidation processes. Then, the multi-shell structured Sb2S3 microparticles were obtained via a simple ion-exchange method [320]. Used as an anode, they delivered a capacity of 909 and 604 mA·h·g−1 at the current densities of 100 and 2000 mA·g−1, respectively. After 50 cycles, the multi-shell Sb2S3 could still maintain a reversible capacity of over 500 mA·h·g−1 (against 200 mA·h·g−1 for the single shell Sb2S3). The high capacity is due to the high efficiency of the conversion reaction: Sb2S3 + 6Na+ + 6e− 2Sb + 3Na2S, (7) which enhances the alloying/dealloying reaction: 2Sb + 6Na+ + 6e− 2Na3Sb. (8) In addition, the pseudocapacitive contribution raises from 44% to 84% under a sweep rate raising from 0.2 to 10 mV·s−1 due to the multi-shell structure that offers both the exterior and interior surfaces for the electrochemical reaction. Iron sulfides FeF2 (pyrite), Fe1-xS (pyrrhotite), and FeS have also been considered as promising anode materials for SIBs [58]. FeS@C, FeS2@C, and FeS2/graphene anodes have been constructed, demonstrating that the carbon shell of the introduction of graphene was able to buffer the volume change during cycling and also improve the electrical conductivity and the C-rate [321–325]. Nanostructured FeS2 (50–80 nm) embedded in an N-doped carbon nanosheet composite (FeS2/CNS) via a combinedPDF Image | Electrode Materials for Sodium-Ion Batteries
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