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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 Materials 2020, 13, 3453 20 of 58 after 250 cycles at a current density of 1000 mA g−1. Figure 8. (a) Charge-discharge behavior of coir pith derived carbon prepared at 850 ◦C (CPC-850) as an Figure 8. (a) Charge-discharge behavior of coir pith derived carbon prepared at 850 °C (CPC-850) as anode at 1000 mA·g−1. (b) Voltage vs. capacity behavior CPC-850 anode upon progressive cycles after an anode at 1000 mA g−1. (b) Voltage vs. capacity behavior CPC-850 anode upon progressive cycles formation cycle. Reproduced with permission from [217]. Copyright 2017 Royal Society of Chemistry. after formation cycle. Reproduced with permission from [217]. Copyright 2017 Royal Society of Chemistry. 3.2. Metal Chalcogenide-Based Anodes Metal oxides have also been extensively studied as anodes for sodium-ion batteries because of 3.2. Metal Chalcogenide-Based Anodes their low operating voltage vs. Na+/Na, and some of them have a high capacity [221]. They are Metal oxides have also been extensively studied as anodes for sodium-ion batteries because of divided in two families, depending on the intercalation or conversion reaction at the origin of their their low operating voltage vs. Na+/Na, and some of them have a high capacity [221]. They are divided electrochemical properties. in two families, depending on the intercalation or conversion reaction at the origin of their electrochemical properties. 3.2.1. Intercalation-Based Materials A huge effort of research has been devoted to the intercalation compounds [222]. Among them, 3.2.1. Intercalation-Based Materials TiO2 is abundant on earth and not toxic. Among the different polymorphs, the anatase phase looks A huge effort of research has been devoted to the intercalation compounds [222]. Among them, more promising [223]. TiO2 has a high capacity, provided that the discharge cut-off voltage is decreased TiO2 is abundan+t on earth and not toxic. Among the different p−o1lymorphs, the anatase phase looks to 0.01 V vs. Na /Na, in which case the capacity ≈ 193 mA·h·g is achieved [224]. There has been more promising [223]. TiO2 has a high capacity, provided that the discharge cut-off voltage is a debate on the origin of the electrochemical activity of anatase TiO2. Kim et al. attributed the decreased to 0.01 V vs. Na+/Na, in which case 4t+he c3a+pacity ≈ 193 mA h g−1 is achieved [224]. There has de/intercalation in to the redox activity of Ti /Ti during charge/discharge, while Gonzalez et al. been a debate on the origin of the electrochemical activity of anatase TiO2. Kim et al. attributed the suggested pseudo-capacitive reactions [225]. Wu et al. clarified the origin of the storage mechanism de/intercalation in to the redox activity of Ti4+/Ti3+ during charge/discharge, while Gonzalez et al. owing to in situ XRD and ex situ XPS experiments on TiO2 nanoparticles [226]. They determined suggested 3p+seu4d+o-capacitive reactions [225]. Wu et al. clarified the origin of the storage mechanism that the Ti :Ti ratio is approximately 2.23, corresponding to 0.69 Na per TiO2 after discharge, owing to in situ XRD and ex situ XPS experiments on TiO2 nanoparticles [226]. They determined that while it decreases to 0.35 after charge, corresponding to 0.28 Na per TiO2 remaining in the structure the Ti3+:Ti4+ ratio is approximately 2.23, corresponding to 0.69 Na per TiO2 after discharge, while it (intercalation reaction). In addition, the reduction of TiO2 to metallic Ti along with the structural decreases to 0.35 after charge, corresponding to 0.28 Na per TiO2 remaining in the structure rearrangement (conversion reaction) is observed. In conclusion, the chronological electrochemical (intercalation reaction). In addition, the reduction of TiO2 to metallic Ti along with the structural process is the following: (i) pseudo-capacitive reaction during the initial discharge process; (ii) structural rearrangement (conversion reaction) is observed. In conclusion, t0he chronological electrochemical rearrangement;(iii)disproportionationreactionandformationofTi andO2duringfurtherdischarge; (iv) reversible Na de-insertion occurring in Nax(TiO2) (0.28 ≤ x ≤ 0.69). In any case, TiO2 is not a good electrical conductor, so that is must be nano-structured. The 3D array architecture is particularly suited to obtain large accessible surface and yet maintains short ion-transport distance [227]. However, the electrochemical activity of TiO2 arrays might be compromised by the low surface reactivity, in which case surface functionalization is a key approach in the realization of high electrochemical activity [228–230]. Ni et al. combined the 3D nanotube architecture with phosphate functionalization [231]. The surface phosphorylated TiO2 nanotube arrays (noted P-TiO2) were obtained by electrochemical anodization of Ti metal in NH4F solution and subsequent phosphorylation using sodium hypophosphite. Another advantage is that the self-supported configuration eliminates the need for a binder and conducting additive so that the P-TiO2 nano-arrays can be directly adapted as an electrode. As a result, this electrode afforded a reversible capacity of 334 mA·h·g−1 at 67 mA·g−1 (0.2C) and a superior rate capability. At 3350 mA·g−1 (10C) the electrode delivered a capacity of retains a capacity of 143 mA·h·g−1 over 500 cycles and 141 mA·h·g−1 (≈ 94% of that in the 2nd cycle) over 1000 cycles. This result illustrates that the construction of binder-free and self-supporting electrodes improves the reaction kinetics and electrode stability. The reason is that it avoids the polymer binder/conductive additives that may cause virtual swelling in common electrolytes and result in a poor electrochemical performance [232].PDF Image | Electrode Materials for Sodium-Ion Batteries
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