Indium-Doped Rutile Titanium Oxides As Anode Materials for Na-Ion Batteries

Abstract

Na-ion batteries (NIBs) have been recently considered as an attractive alternative to Li-ion batteries (LIBs) for large-sized rechargeable batteries because Na’s resource has a lower cost and a wider availability compared to Li’s resource. Na+ has a 2.4 times larger ionic volume than Li+, resulting in slower kinetics of its host materials of negative electrode (anodes). We have recently developed various compounds as novel anode materials of NIB [1-6], and have particularly focused on rutile-type TiO2 with very favorable ion-diffusion path along its c-axis. However, rutile TiO2 has two problems as anode materials: the very low diffusion coefficient along ab in-plane direction and the poor electronic conductivity.To address the issues, the authors have conducted two kinds of approaches: the doping of impurity elements into the crystal structure [1] and the improvement in the crystallinity of TiO2 particles [2]. The approaches could improve the electronic conductivity and the trapping at grain boundaries, leading to a remarkable enhancement in the anode performances. In particular, the Nb-doped TiO2 electrode [6] and Ta-doped TiO2 electrode [5] showed remarkably-enhanced anode performances. Nevertheless, there still remains a serious problem that the capacity steeply increases or decreases in the initial several tens of cycles even by Nb-doping and Ta-doping. The dopings of Nb and Ta have another effect of expanding crystal lattice of rutile TiO2 because the sites of Ti4+ (ionic diameter: 121 pm) are substituted with Nb5+ (128 pm) and Ta5+ (128 pm). This expansion is accompanied by an increase in size of Na+ diffusion path, which is favorable for improving the Na+ insertion−extraction properties. Since the ionic diameter of Na+ (204 pm) is larger than that of Li+ (152 pm), the size expansion of ion diffusion path is more important for NIB compared with LIB.Indium (In) has a much larger ionic size of 160 pm than Nb and Ta. It is thus expected that the larger size will change the doping effects on Na+ insertion−extraction properties. In the previous study, the authors have not systematically investigated the dependence of In doping amount on the anode properties. Therefore, in this study, we evaluated the anode properties of In-doped rutile electrodes with various doping amounts to clarify the influence of In doping. In addition to this, we used an anionic surfactant for the hydrothermal synthesis of TiO2 to reduce its particle length [6].We prepared In-doped rutile TiO2 by a facile one-pot hydrothermal synthesis [6-8]. For the reduction of the particle length, we used a solution of the anionic surfactant (Sodium dodecyl sulfate: SDS) for the synthesis. We investigated the influences of In doping and the particle morphology on the Na+ storage properties. The X-ray diffraction analyses revealed that the solid solubility limit was found to be 0.8 at.% in In-doped TiO2. With increasing the In amount from to 0.8 at.%, the lattice parameters of rutile TiO2 were increased, indicating the expansion of diffusion path size for Na+. The degree of size expansion by In doping is comparable to the change by 6 at.% of Nb doping. The remarkable feature is that the similar size increasing can be achieved even with a much smaller doping amount in the case of In doping because of its larger ionic size. The improvement in the electronic conductivity was also confirmed by 0.8 at.% In doping into TiO2.In the case where no surfactant was used, the best anode performance was obtained for 0.8 at.%. In-doped TiO2 electrode by the benefits of three doping effects: (i) expanded diffusion-path size, (ii) improved electronic conductivity, and (iii) reduced electron charge density in the path. A further enhancement in the performance was achieved for the In-doped TiO2 with a reduced particle-length by the synthesis in the SDS surfactant solution. This electrode exhibited a better cycle stability, and maintained high discharge capacities of 240 mA h g−1 for 200 cycles. The reason is probably that Na+ can be inserted in the inner part of TiO2 particles because of its reduced particle-length.