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Materials 2020, 13, 3453 27 of 58 the carbon that helps to maintain the structural integrity and improves the rate capability by increasing the electrical conductivity, and the nano-structuration, the corresponding anode demonstrated a capacity of 340 mA·h·g−1 after 2000 cycles with the Coulombic efficiency of over 99%, at a current density of 1 A·g−1. At high current density of 10 A·g−1, the capacity was still maintained at 253 mA·h·g−1. Utilizing a Co-based metal-organic framework (MOF) as the precursor, in particular the zeolitic imidazolate framework [300] is another effective way to synthesize a hierarchical Co9S8-based material. In particular, a Co9S8 quantum dot/hollow carbon matrix/graphene aerogel synthesized with the ZIF-67 precursor delivered a capacity of 628 mA·h·g−1 after 500 cycles at a current density of 300 mA·g−1 and demonstrated an exceptional rate capability with a capacity of 330 mA·h·g−1 at 6400 mA·g−1 [301]. A yolk–shell structured Co9S8/MoS2 polyhedron with N-doped carbon composite, synthesized through a step by step process using again ZIF-67 as the precursor, delivered a capacity of 438 mA·h·g−1 within 150 cycles at a current density of 1.0 A·g−1, and 421 mA·h·g−1 within 250 cycles at a high current density of 2.0 A·g−1 [302]. These remarkable electrochemical properties were attributed to the MoS2 shell that brings a lower activation energy for Na+ ion diffusion and larger exposed surface to the electrolyte, and also improves the electrical conductivity. MoS2 is one of the most promising anode materials for SIBs, provided that the huge variation of volume during cycling is alleviated. In addition, its electronic conductivity is small. To overcome these problems, the strategy is always the same: fabricate nano-structured composites with conductive carbon. MoS2-graphene composites, MoS2-CNT hybrids, and MoS2-carbon spheres have been tested with significant improvement in the electrochemical properties [303,304] The best performance was achieved recently with a MoS2@CNFIG composite, where CNFIG stands for a carbon nanofiber interpenetrated graphene architecture. This anode delivered a capacity of 598 mA·h·g−1 at 0.1 A·g−1 based on the total mass of MoS2 and CNFIG matrix. At 1 A·g−1, the capacity was still 412 mA·h·g−1 in the 1000th cycle, which corresponds to a capacity retention of 86.2% based on its initial specific capacity (478 mA·h·g−1) in the 2nd cycle. The rate capability was also remarkable, with a capacity of 366 mA·h·g−1 achieved at 5 A·g−1 after 1000 cycles, achieving capacity retention of 86.9% [305]. CoS2 can react with four Na+ ions and this electrochemical reaction can be divided into 2 steps. Firstly, when the voltage is above 1 V vs. Na+/Na, no more than two Na+ ions inserted into CoS2 and an intermediate product is formed. Then, when the voltage is decreased below 1 V, Co4+ is reduced to metallic Co and Na2S emerges. To buffer the volume change of CoS2 during cycling, composites were synthesized with different forms of carbon: multi-wall carbon nanotubes (MWCN) [306], reduced graphene oxide [307], graphene and carbon nanotubes [308]. The best results were obtained with the CoS2-MWCN anodes with a capacity maintained at 568 mA·h·g−1 after 100 cycles (69% of first discharge capacity) at current density of 0.1 A·g−1 in NaCF3SO3-DGM electrolyte. Note the results are very sensitive to this choice of electrolyte, since poor results were observed for the same composite in NaClO4-EC/PC. Using 1 mol·L−1 NaCF3SO3 in diethylene glycol dimethyl ether (DEGDME) as the electrolyte, CoS2 nanoparticles wrapping on flexible freestanding multichannel carbon nanofibers delivered a similar capacity at low current rate (537 mA·h·g−1 at 0.1 A·g−1), but in addition, a remarkable rate capability and cycle ability was demonstrated with a capacity of 315 mA·h·g−1 at 1 A·g−1 after 1000 cycles. Even at 10 A·g−1, the capacity was maintained at ~202 mA·h·g−1 [309]. Pan et al. synthesized flower-like N-doped carbon/CoS2 spheres (N-C/CoS2) by a solvothermal method followed by sulfurization [310]. Owing to conductive interconnected wrinkled nanosheets that create mesoporous structures, and many extra defect vacancies and Na+ storage sites introduced by the nitrogen doping process, the results were outstanding. This anode delivered a capacity of 698 at 1 A·g−1 after 500 cycles, and the capacity was still 458 mA·h·g−1 at 10 A·g−1. The same group fabricated an anode with Double-Morphology (nanoparticle and nanosheet) CoS2 Anchored on N-doped multichannel carbon nanofibers (CoS2@MCNFs). CoS2 nanosheets were in situ formed on the surface of the carbon fiber, while CoS2 nanoparticles also grew in the channels of these carbon fibers. With this unique structure, the carbon nanofiber plays the dual roles of matrix and coating for CoS2 nanosheets and nanoparticles, respectively. This anode delivered a capacityPDF Image | Electrode Materials for Sodium-Ion Batteries
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