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Materials 2020, 13, 3453 21 of 58 Avoiding the binder may also be beneficial to relax the volume expansion. Another example is provided by TiO2 nanorods grown on carbon fiber cloth as binder-free electrode grown by a facile hydrothermal method [233]. As an anode for SIBs, it exhibited an exceptional electrochemical performance, including excellent rate capability and cyclic stability, maintaining a high capacity of 148.7 mA·h·g−1 after 2000 cycles at 1 A·g−1. Composites with conductive carbon helps to improve the electronic conductivity and thus the electrochemical properties, in particular the rate capability. In particular, graphene is the most conductive form of carbon and has thus been considered for this purpose [234–236]. A graphene-TiO2 composite delivered a capacity of 115 mA·h·g−1 at a current of 1 A·g−1 and a stable specific capacity of 102 mA·h·g−1 at 0.1 A·g−1 after 300 cycles. [237]. Chen et al. synthesized a graphene-coupled TiO2 sandwich-like hybrid (10 wt.% graphene) in which intercalation pseudocapacitance dominated the charge storage process [238]. At a current density of 500 mA·g−1 (∼2C), after the initial dozens of cycles, this composite delivered a reversible capacity of 120 mA·h·g−1 kept unchanged during the subsequent 4300 cycles. The rate capability was also excellent, with a reversible capacity of 90 mA·h·g−1 at an extremely high current density of 12,000 mA·h·g−1. Other forms of carbon have been used successfully to coat the TiO2 particles. Combining the synergetic effects of a small size (11 nm) of TiO2 particles and uniform carbon coating to improve the conductivity of the powder, carbon coated anatase TiO2 particles delivered a capacity of 134 mA·h·g−1 at 10C (3.35 A·g−1) and 1227 mA·h·g−1 at 0.1C, with high cycling stability (full capacity retention between 2nd and 300th cycle at 1C) and high coulombic efficiency (≈ 99.8%) [239]. Carbon coated anatase TiO2 hollow spheres prepared through the carbon wrapping of etched amorphous TiO2 solid spheres demonstrated a capacity of capacity of 140.4 mA·h·g−1 after 500 cycles at 5C rate, and 84.9 mA·h·g−1 after 80 cycles at 25C [240]. We know from Hou et al. that carbon quantum dots can be used as anodes for sodium-ion batteries [219]. In a subsequent work, this group designed a hierarchical anatase TiO2 homogeneously tuned by using carbon through Ti–C bonds, exploiting carbon quantum dots as uniform carbon additives with surface area (202 m2·g−1) and abundant mesopores. The corresponding anode delivered a high reversible specific capacity of 264 mA·h·g−1 at a rate of 0.1C (33.6 mA·g−1) and still maintains 108.2 mA·h·g−1 even after 2000 cycles at 10C with a retention of 94.7% [241]. Carbon dots were also used to decorate N-doped TiO2 nanorods [242]. Utilized as an anode, this composite delivered a capacity of 185 mA·h·g−1 with 91.6% retention even at a high rate of 10C over 1000 cycles. Inverse opal TiO with N-doped carbon layer and oxygen vacancies surface as an anode material for sodium-ion battery delivered a capacity of 140 mA·h·g−1 after 400 cycles under 1 A·g−1, owing to a pseudo-capacitive contribution of 73.38% at 1 mV·s−1 [243]. The performance of TiO2 depends, like any electrochemically active material, on its porosity and structure. This can be evidenced by the performance of TiO2 mesocages with high surface area (204 m2·g−1) and uniform mesoporous structure. A capacity of 93 mA·h·g−1 (per gram of TiO2) was retained after 500 cycles at 10C in the range of 0.01–2.5 V [244]. In that case, the active particles were not a composite, only TiO2 particles, but they were admixed with polyvinylidene fluoride (PVDF) binder and acetylene black carbon additive in a weight ratio of 70:20:10 to form the anode. The performance of titanate also depends very much on the type of carbon additive. The incorporation of graphene into the titanate films produced efficient binder-free anodes delivering a reversible capacity of 72 mA·h·g−1 at 5 A·g−1 after 10,000 cycles (Figure 9) [245].PDF Image | Electrode Materials for Sodium-Ion Batteries
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