Sodium Insertion/Extraction in Carbon-Coated Nanoscale Anatase TiO2
Authors: Ding Changsheng , Nohira Toshiyuki , Hagiwara Rika 
Published: February 2020

Rechargeable sodium batteries, as the replacement of rechargeable lithium batteries, are attracting much attention for large-scale applications such as electric vehicles and stationary energy storage due to the abundant resource of sodium compared to lithium. Up to now, there are only a few materials which have reasonable performances as negative electrodes of rechargeable sodium batteries, such as alloy materials [1], hard carbon [2], and sodium titanium oxides [3]. Recently, TiO2, an attractive alternative negative electrode material to graphite for rechargeable lithium batteries, was also investigated for rechargeable sodium batteries [4, 5]. However, the reported reversible capacities of TiO2 electrodes are less than 200 mAh g-1 and the Na insertion/extraction mechanism is still unclear. In the previous study, carbon-coated anatase TiO2 (TiO2/C) nanopowders were investigated as negative electrode material for rechargeable sodium battery with Na[FSA]-[C3C1pyrrr][FSA] (FSA = bis(fluorosulfonyl)amide; C3C1pyrr = N-methyl-N-propylpyrrolidinium) ionic liquid electrolyte [6]. In the present study, the electrochemical performance of TiO2/C electrode was further investigated in Na[FSA]-[C3C1pyrrr][FSA] ionic liquid electrolyte, and the Na insertion/extraction mechanism was explored.

  The TiO2/C electrode shows a high reversible discharge capacity of 275 mAh g-1 at 10 mA g-1 at 363 K. The observed capacity is higher than that reported for anatase TiO2 nanorods (193 mAh g-1 at 10 mA g-1) in organic electrolytes at room temperature [5]. Moreover, this capacity is comparable to that of a hard carbon negative electrode (260 mAh g-1) in Na[FSA]-[C3C1pyrrr][FSA] electrolyte [7]. Since the density of anatase TiO2 (ca. 3.9 g cm-3) is much higher than that of hard carbon (ca. 1.52 g cm-3), the TiO2/C nanopowder electrode is a promising negative electrode material for the rechargeable sodium batteries with high volumetric energy densities.

  The Na insertion/extraction mechanism was investigated by means of ex-situ X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray (EDX) spectroscopy and electron diffraction. Ex-situ XRD analysis of the TiO2/C electrodes in the different charged/discharged states show that during the charge process, the anatase TiO2 crystal phase disappears and a new crystal phase is formed. During the discharge process, the newly formed crystal phase disappears and no anatase TiO2 phase appears again. According to the TEM, EDX and electron diffraction studies, the fully discharged TiO2/C electrode is suggested to consist of amorphous TiO2, amorphous Na2TiO3 and cubic TiO phases, and the crystal phase in the fully charged TiO2/C electrode is possibly a solid solution of cubic Na2TiO3 and cubic TiO phases. Thus, amorphous TiO2, amorphous  Na2TiO3 and cubic TiO are regarded as active electrode materials, and reversibly react with Na ions to form Na2TiO3•TiO solid solution.

  The TiO2/C negative electrode also exhibits a high rate capability and good cycling performance. When the charge-discharge rate is increased, the shapes of the charge-discharge curves remain unchanged. The discharge capacities decrease gradually with an increase in the current rate. The TiO2/C electrode shows reversible discharge capacities of 174 and 93 mAh g-1 at current rates of 500 and 2000 mA g-1, respectively, corresponding to about 69 and 37% of the capacity at 20 mA g -1. The discharge capacity is 174 mAh g-1 after 400 cycles at a current rate of 200 mA g-1, exhibiting only 10% drop in capacity. After 1000 cycles, the capacity decreases by approximately 21%.

Acknowledgements

This study was partly supported by the Advanced Low Carbon Technology Research and Development Program (ALCA, No. 3428) of the Japan Science and Technology Agency (JST) and the “Elements Strategy Initiative to Form Core Research Center" program of the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT).

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