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Materials 2020, 13, 3453 24 of 58 b. Cobalt Oxides Cobalt-based compounds are among the most investigated materials for sodium storage [263]. For Co3O4, the complete reaction is based on the conversion mechanism [264]: Co3O4 + 8Na → 3Co + 4Na2O. (1) It possesses a high theory capacity of 890 mA·h·g−1, which justifies a lot of investigations. In the charge process, Co nanoparticles are partially oxidized to Co3O4, the other part of the CoO remain leading capacity loss [265]. Meso-porous cobalt oxide-based anode (Co3O4 mass loading of 0.6–0.8 mg·cm−2) for SIB was tested in 1 mol·L−1 NaPF6 in 1:1 (vol.%) fluoroethylene carbonate (FEC): Anhydrous diethyl carbonate (DEC) optimized electrolyte. It retained 80% of its maximum 204 mA·h·g−1 capacity at current density of 445 mA·g−1 through 200 cycles (and retained 75% capacity through 250 cycles) with near 100% coulombic efficiency [266]. As any material that operates via conversion reaction, nano-structuring and porosity are mandatory to alleviate the change of volume during cycling. Yang et al. used mesoporous silica as the template for the generation of dual porosity Co3O4 with spherical mesopores and porous nanochannels. The dual porosity mesopores allow better transport pathways and increase the effective surface area in contact with the electrolyte. Consequently, this Co3O4 electrode delivered an initial capacity of 707 mA·h·g−1 at a current density of 90 mA·g−1, retaining a capacity of 416 mA·h·g−1 after 100 cycles [267]. The synergetic effect of Co3O4 nanoparticles and conductive carbon has been explored with different kinds of carbon including carbon nanotubes [268,269], graphene [270] and N-doped graphite (NC) [271]. This Co3O4/NC hybrid with flower-like structure as an anode for SIB delivered a capacity of 214 mA·h·g−1 after 100 cycles at 0.1 A·g−1, excellent rate capability (145 mA·h·g−1 at 2 A·g−1 and 130 mA·h·g−1 at 4 A·g−1) and long-term cycling stability (120 mA·h·g−1 after 2000 cycles at 0.5 A·g−1). Porous hollow Co3O4 with N-doped carbon coating (Co3O4/N-C) polyhedrons delivered a capacity of 229 mA·h·g−1 within 150 cycles at 1 A·g−1 [272]. Various shapes have also been investigated: nanosheets [273–275], nanocubes [276], shale-like [277], yolk–shell dodecahedrons [278], flower-like [271] with similar results. We can then conclude that the experimental capacity reported in the literature is far below the theoretical one. One of the largest reversible capacity has been obtained 523.5 mA·h·g−1 after 50 cycles at rate of 25 mA·g−1 in the voltage range of 0.01–3 V vs. Na+/Na on mesoporous Co3O4 sheets/3D graphene networks nanohybrids [279]. Unfortunately, the investigation of the cycle ability has been tested on 50 cycles only. Rambutan-like hybrid hollow spheres of carbon confined Co3O4 nanoparticles synthesized by a facile one-pot hydrothermal treatment delivered a capacity of 712 mA·h·g−1 at a current density of 0.1 A·g−1, and 223 mA·h·g−1 at 5 A·g−1. It also demonstrated 74.5% capacity retention after 500 cycles [280]. In an attempt to increase the cycle life Co3O4/metal oxide heterostructures were synthesized. An example is the graphene/SnO2/Co3O4 (GSC) heterojunction [281]. Consequently, this graphene oxide/SnO2/Co3O4 anode achieved a reversible capacity of 461 mA·h·g−1 after 80 cycles at a current density of 0.1 A·g−1. At a high current density of 1 A·g−1, a high reversible capacity of 241 mA·h·g−1 after 500 cycles was demonstrated. Co3O4 is a p-type semiconductor while SnO2 is an n-type semiconductor. In the discharge process, the internal electric field then points from the SnO2 side to the Co3O4 side as an effective p-n junction. As a result, a depletion region is formed, reducing the accumulation of charge at the interfaces, which is favorable to the diffusion and insertion of Na+ ions. Other heterostructures have been synthesized. For example, carbon-encapsulated wire-in-tube Co3O4/MnO2 heterostructure nanofibers (Co3O4/MnO2@C) synthesized via electrospinning followed by calcination delivered 306 mA·h·g−1 at 100 mA·g−1 over 200 cycles, but also showed a cycling stability of 126 mA·h·g−1 after 1000 cycles at a high current density of 800 mA·g−1 [282]. We can also cite ZnO/Co3O4 [283].PDF Image | Electrode Materials for Sodium-Ion Batteries
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