Rb/Cs-Modified P2 Na0.7Mn0.8Mg0.2O2: Application in Sodium-Ion Batteries

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 Na 0.7 Mn 0.8 Mg 0.2 O 2 , which adopts hexagonal P 6 3 / mmc symmetry. The second charge/discharge capacity for the as-synthesized Rb- and Cs-modified P2 Na 0.7 Mn 0.8 Mg 0.2 O 2 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 Na 0.7 Mn 0.8 Mg 0.2 O 2 shows multiple phases near the charged state, whereas Cs-modified P2 Na 0.7 Mn 0.8 Mg 0.2 O 2 shows the formation of a new phase (P2 new ) at about 2.5 V and multiple phases below 1.7 V. The P2 new 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 Na 0.7 Mn 0.8 Mg 0.2 O 2 shows better energy density in comparison with the K- and Rb-modified P2 Na 0.7 Mn 0.8 Mg 0.2 O 2 and comparable to the parent P2 Na 0.7 Mn 0.8 Mg 0.2 O 2 . Ex situ scanning electron microscopy images show no noticeable change in surface morphology of the Cs-modified Na 0.7 Mn 0.8 Mg 0.2 O 2 , whereas in the case of Rb-modified P2 Na 0.7 Mn 0.8 Mg 0.2 O 2 , rods and irregular-shaped particles are observed after the 100th cycle. The solid-state 23 Na NMR shows a distinct shift in the peak position in comparison with the parent P2 Na 0.7 Mn 0.8 Mg 0.2 O 2 , and two Na environments are observed with no local disordering in Rb- and Cs-modified P2 Na 0.7 Mn 0.8 Mg 0.2 O 2 samples. Overall, this article illustrates the influence of using larger alkali ions to modify P2 Na 0.7 Mn 0.8 Mg 0.2 O 2 and compares this scheme in terms of phase transitions and electrochemical performance.