Abstract
To meet the growing energy demands, sodium-ion batteries can be a potential substitute for lithium-ion batteries. Here, we report the solid-state synthesized alkali Rb- and Cs-modified Na0.7Mn0.8Mg0.2O2, which adopts hexagonal P63/mmc symmetry. The second charge/discharge capacity for the as-synthesized Rb- and Cs-modified P2 Na0.7Mn0.8Mg0.2O2 are 118/114 and 130/125 mA h g-1, which reduces to 62/62 and 77/76 mA h g-1, respectively, after 100 cycles. In situ synchrotron X-ray diffraction data illustrate that a solid solution reaction occurs for most of the charge/discharge process in both cases. Rb-modified P2 Na0.7Mn0.8Mg0.2O2 shows multiple phases near the charged state, whereas Cs-modified P2 Na0.7Mn0.8Mg0.2O2 shows the formation of a new phase (P2new) at about 2.5 V and multiple phases below 1.7 V. The P2new phase is found to evolve in conjunction with the original P2 phase until about 1.7 V, where the P2 reflection appears to split into multiple reflections and a single P2 phase is recovered at 2.7 V on the second charge. The Cs-modified P2 Na0.7Mn0.8Mg0.2O2 shows better energy density in comparison with the K- and Rb-modified P2 Na0.7Mn0.8Mg0.2O2 and comparable to the parent P2 Na0.7Mn0.8Mg0.2O2. Ex situ scanning electron microscopy images show no noticeable change in surface morphology of the Cs-modified Na0.7Mn0.8Mg0.2O2, whereas in the case of Rb-modified P2 Na0.7Mn0.8Mg0.2O2, rods and irregular-shaped particles are observed after the 100th cycle. The solid-state 23Na NMR shows a distinct shift in the peak position in comparison with the parent P2 Na0.7Mn0.8Mg0.2O2, and two Na environments are observed with no local disordering in Rb- and Cs-modified P2 Na0.7Mn0.8Mg0.2O2 samples. Overall, this article illustrates the influence of using larger alkali ions to modify P2 Na0.7Mn0.8Mg0.2O2 and compares this scheme in terms of phase transitions and electrochemical performance.