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 Na₂Ti₃O ₇ was 311 mA h g^-1. Furthermore, a microspheric Na ₂Ti₃O₇ 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 Na₂Ti₃O ₇ crystal, but could also be stored in the intracavity of the nanotubes. All of the results disclose that the as-prepared Na ₂Ti₃O₇ 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.