Rb/Cs-Modified P2 Na_0.7Mn_0.8Mg_0.2O₂: 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_0.7Mn_0.8Mg_0.2O₂, which adopts hexagonal P6₃/mmc symmetry. The second charge/discharge capacity for the as-synthesized Rb- and Cs-modified P2 Na_0.7Mn_0.8Mg_0.2O₂ 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 Na_0.7Mn_0.8Mg_0.2O₂ shows multiple phases near the charged state, whereas Cs-modified P2 Na_0.7Mn_0.8Mg_0.2O₂ 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_0.7Mn_0.8Mg_0.2O₂ shows better energy density in comparison with the K- and Rb-modified P2 Na_0.7Mn_0.8Mg_0.2O₂ and comparable to the parent P2 Na_0.7Mn_0.8Mg_0.2O₂. Ex situ scanning electron microscopy images show no noticeable change in surface morphology of the Cs-modified Na_0.7Mn_0.8Mg_0.2O₂, whereas in the case of Rb-modified P2 Na_0.7Mn_0.8Mg_0.2O₂, 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 Na_0.7Mn_0.8Mg_0.2O₂, and two Na environments are observed with no local disordering in Rb- and Cs-modified P2 Na_0.7Mn_0.8Mg_0.2O₂ samples. Overall, this article illustrates the influence of using larger alkali ions to modify P2 Na_0.7Mn_0.8Mg_0.2O₂ and compares this scheme in terms of phase transitions and electrochemical performance.