Rutile-type titanium dioxide (TiO₂) has the most unique characteristics of Li⁺ diffusion in some polymorphs of TiO₂; its diffusion coefficient along the c-direction is approximately 10^–6 cm² s^–1, while it is only 10^–15 cm² s^–1 in the ab-plane. Due to its highly anisotropic diffusion, three-dimensional Li⁺ diffusion is kinetically restricted in micrometer-sized rutile TiO₂, resulting in its poor anode property. Concerning this subject, Maier and Tarascon reported that rutile TiO₂ with several tens of nanometers in size exhibited enhanced reversible capacities and improved cycle stabilities as a Li-ion battery anode. We have found that the anode performances of the TiO₂/Si composite electrodes were considerably improved by uniform coating of Si particles by the rutile TiO₂ particles with 10−50 nm size. The authors have proposed that a smooth Li⁺ diffusion in rutile can be successfully exerted based on its nanostructure, thereby providing improved anode performances. We are expecting that this effect will be useful for Na-ion battery anode also. In this study, we applied the rutile TiO₂ particles to the Na-ion battery anode, and investigated its Na-insertion/extraction property for the first time. In addition, we tried to enhance its anode performance by Nb doping into the rutile TiO₂.

   Active materials of Nb-doped rutile TiO₂ powders were synthesized a typical sol–gel method. By using a gas-deposition method, we prepared Nb-doped TiO₂ thick-film electrodes which were consisted only of the active materials and titanium current collectors to clarify an original anode performance of its active materials. We assembled 2032-type coin cells consisted of the thick-film electrodes as working electrode, Na foil as counter electrode, electrolyte, and propylene-based separator. The electrolytes used in this study were 1 M NaClO₄-dissolved in propylene carbonate. Galvanostatic charge–discharge tests were carried out using an electrochemical measurement system at 303 K.

   As the results of charge–discharge tests, all the electrodes showed potential shoulders in the charge (sodiation) reaction at 0.1–0.7 V vs. Na/Na⁺ and the discharge (desodiation) reaction at 0.5–1.0 V. However, a Na-insertion/extraction mechanism has never been explored to the best of the authors’ knowledge. To elucidate the mechanism, we performed ex-situ XRD measurements for the electrodes during the charge–discharge reactions. During the charge process, the diffraction peaks of the rutile TiO₂ shifted toward lower angles. Although the diffractions significantly weakened at 0.005 V, the (101) and (211) reflections still remained. During the discharge process, the peak positions shifted toward higher angles associated with the increasing diffraction intensities. The change in the patterns revealed that the crystal phase of rutile was basically maintained during the charge–discharge process, and that there are reversible reactions of Na-insertion and Na-extraction into/from crystal lattice of the rutile TiO₂. Reversible lattice expansion and contraction were clearly observed. In the tetragonal rutile structure, the lattice parameter c was not changed during the Na-insertion/extraction. In contrast, the lattice parameter a showed about a 1.1% increasing by the Na-insertion. From this result, we firstly revealed the reversible Na-insertion/extraction of rutile TiO₂(Fig.1).

   Figure 2 shows the cycling performances of the Ti_1−xNb_xO₂ electrodes with various Nb amounts between x = 0 and x = 0.18. All the electrodes showed a gradual increase in the discharge capacities until the 5th–15th cycles. It is suggested that the side reactions were gradually suppressed by the 5th–15th cycles. The discharge capacity increased with x. At x= 0.11 and 0.18, the electrode temporarily showed higher capacities in the initial several cycles and a greater capacity decay after the cycles compared to other electrodes. The capacity decay is presumably caused by the reduced crystalline sizes and the formation of an impurity crystal phase, as already mentioned. Among all the Ti_1−xNb_xO₂s, the electrode with x = 0.06 exhibited the best performance with the reversible capacity of 160 mA h g^–1 at the 50th cycle. It is a noteworthy result that our rutile Ti_1−xNb_xO₂ electrode delivered a comparable performance even though its active material layer is a thick film of approximately 14 µm and does not contain any conductive material and binder. These improved performances are mainly attributed to the three orders of magnitude higher electronic conductivity of Ti_0.94Nb_0.06O₂ compared to TiO₂. These results suggest that Nb-doped TiO₂ is an attractive candidate as a conductive-additive-free active material for the Na-ion battery anode as well as for the Li-ion battery anode.