Microspheric Na2Ti3O7 consisting of tiny nanotubes: an anode material for sodium-ion batteries with ultrafast charge-discharge rates

Conventionally, rechargeable batteries with a fast charge-discharge rate, while being able to be implemented in large-scale applications with low prices, are critical for new energy storage systems. In this work, first-principles simulations were employed to theoretically investigate the insertion of sodium into the Na2Ti3O7 structure. The result discovered that the theoretical capacity of Na2Ti3O7 was 311 mA h g(-1). Furthermore, a microspheric Na2Ti3O7 material consisting of tiny nanotubes of ca. 8 nm in outside diameter and a few hundred nanometers in length has been synthesized. The galvanostatic charge-discharge measurements, using the as-prepared Na2Ti3O7 nanotubes as a working electrode with a voltage range of 0.01-2.5 V vs. Na+/Na, disclosed that a high capacity was maintained even under an ultrafast charge-discharge rate. At a current density of 354 mA g(-1), the discharge capacity was maintained at 108 mA h g(-1) over 100 cycles. Even at a very large current density of 3540 mA g(-1), the discharge capacity was still 85 mA h g(-1). HRTEM analysis and electrochemical tests proved that sodium ions could not only intercalate into the Na2Ti3O7 crystal, but could also be stored in the intracavity of the nanotubes. All of the results disclose that the as-prepared Na2Ti3O7 nanotubes are able to be used as anode materials in large-scale applications for rechargeable sodium-ion batteries at low cost while maintaining excellent performance.