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Materials 2020, 13, 3453 39 of 58 On the anode side, non-graphitic (hard) carbon can give good results, but only if it is porous and nano-structured. Cobalt compounds have been extensively studied. Among them, cobalt sulfides with ether-based electrolytes are more promising than cobalt oxides for anodes in SIBs, because of their higher electrical conductivity allowing for higher rate capability. However, cobalt is not only toxic, but also very expensive, which constitutes a severe drawback in terms of commercialization. Red or black phosphorous are most attractive anodes. They benefit from the highest capacity, with good cycle ability over a thousand cycles, and they have a good rate capability when they are combined with porous N-doped carbon. Anode materials that are electrochemically active by conversion reaction suffered for a long time from the huge variation of volume during cycling. However, the improvement of their synthesis under the form of nano-structured and porous materials has overcome this problem, since their capacity can now extend to hundreds of cycles. Among them sulfides are of particular interest, since the weak metal-sulfur bonds could kinetically promote the conversion reactions. Moreover, their capacity is enhanced by an important supercapacitance contribution. Besides electrode shape design and surface modification, the construction of binder-free and self-supporting electrodes has been demonstrated to boost the reaction kinetics and electrode stability. We have given many examples in this review according to which such electrodes are more performing. First of all, they avoid the weight penalty of the binder and additives, so that they allow for a larger loading and energy density. In addition, the polymer binder/conductive additives may cause virtual swelling in common electrolytes, limiting the electrochemical performance, and finally, the absence of binder allows the electrodes to better accommodate the change of volume during cycling. The current challenges of binder-free electrodes and an outlook for their future in energy conversion and storage with focus on advanced SIBs have been detailed in a recent work [391]. Until recently, the construction of binder-free electrodes was not possible with metal active elements because the adhesion of the nano-arrays on the substrate was not maintained due to the huge volume change during cycling associate with the conversion reaction, but a new strategy experienced in the case of Sn proves opens the route to the construction of such anodes. While alloying and doping solve are efficient to improve the structural stability of the electrochemically active materials, the biggest challenge for SIBs is the variation of volume during cycling. A promising strategy to solve this issue is 3D structuring, even in the case of metal-based electrodes, according to the first results published recently. The constant progress experienced these last five years evidenced in this review gives evidence that the sodium-ion batteries will find an increasing market. They will not compete with the lithium-ion chemistry in terms of energy density and rate capability, and the lithium chemistry will keep the market of the electric cars for instance. However, for static utilization, to buffer the intermittence problem and integration of the production of wind and solar plants to the smart grids, where the volume and weight of the batteries have much less importance than the price and security issues of the batteries, the SIBs should conquest a market. Author Contributions: Writing—original draft preparation, A.M.; writing—review and editing, C.M.J. All authors have read and agree to the published version of the manuscript. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest. References 1. Zhang, H.; Li, C.; Eshetu, G.G.; Laruelle, S.; Grugeon, S.; Zaghib, K.; Julien, C.; Mauger, A.; Guyomard, D.; Rojo, T.; et al. From solid-solution electrodes and the rocking-chair concept to today batteries. Angew. Chem. Int. Ed. 2020, 59, 534–538. [CrossRef] [PubMed] 2. Kikkawa, S.; Miyazaki, S.; Koizumi, M. De-intercalated NaCoO2 and LiCoO2. J. Solid State Chem. 1986, 62, 35–39. [CrossRef] 3. Miyazaki, S.; Kikkawa, S.; Koizumi, M. Chemical and electrochemical deintercalations of layered compounds, LiCrO2, LiCoO2 and NaCrO2, NaFeO2, NaCoO2 and NaNiO2. Synth. Met. 1983, 6, 211–217. [CrossRef]PDF Image | Electrode Materials for Sodium-Ion Batteries
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