Abstract:

In order to know whether there is space for the improvement of the electrochemical performance of Na_1.25V₃O₈ as cathode for zinc-ion batteries, this study successfully synthesized pure Na_1.25V₃O₈ and Rb⁺ ion doped Na_1.25V₃O₈ nanorods through hydrothermal method and solid phase method. With the help of XRD patterns, the diffraction peak shifts to a low angle and no new diffraction peak is generated with the increase of rubidium content, indicating that the Na_1.25Rb _x V₃O₈ sample is pure phase and maintain the structural symmetry of P21/m space group. This result indicate that rubidium ions have successfully entered the interlayer of Na_1.25Rb _x V₃O₈ layered compound. The results of SEM images and TEM images analysis show that, with the insertion of Rb⁺, scale of Na_1.25V₃O₈ nanorods become thicker and larger, which would reduce the specific surface and lead to the increase of impedance and reducing the ion diffusion coefficients, resulting in poor electrochemical performance. EIS and GITT results further confirm that the insertion of Rb⁺ will lead to the decrease in capacity. Electrochemical testing results show that bare Na_1.25V₃O₈ nanorods exhibit about 370 mA·h/g initial capacity, but it decreases fast with the increasing times of charge and discharge circle. After 100 cycles of discharge and charge processes at current density of 100 mA/g, discharge capacity decays about 50% compared to initial capacity. Although the specific capacity of Rb+-doped nanorods is reduced, their cycle stability and rate performance are significantly improved. When the amount of doped Rb is 1%, the initial capacity is 336 mA·h/g, but after 100 cycles of discharge and charge processes under the same conditions, the discharge capacity only decays about 6% compared to initial discharge capacity. When the amount of Rb increases to 5%, the initial capacity is merely 217 mA·h/g, but no capacity lost compared to initial capacity. The mechanism of zinc ion storage was analyzed through in-situ XRD and ex-situ XPS. The redox reaction mechanism of Na_1.25Rb _x V₃O₈ cathode material in the ZIBs is mainly based on chemical conversion reaction, in which only the charging reaction process is reversible.

Key words: zinc-ion batteries, cathode material, Na_1.25V₃O₈, Rb-doped, nanorods