ABSTRACT
Sodium-ion batteries (NIBs) are an emerging alternative to lithium-ion batteries because of the abundance of sodium resources and their potentially lower cost. Here we report the Na0.7MnO2 solid state synthesized at 1000 °C that shows two distinct phases; one adopts hexagonal P2-type P63/mmc space group symmetry, and the other adopts orthorhombic Pbma space group symmetry. The phase ratio of P2 to the orthorhombic phase is 55.0(5):45.0(4). A single-phase P2 structure is found to form at 1000 °C after modification with alkali metals Rb and Cs, while the K-modified form produces an additional minor impurity. The modification is the addition of the alkali elements during synthesis that do not appear to be doped into the crystal structure. As a cathode for NIBs, parent Na0.7MnO2 shows a second charge/discharge capacity of 143/134 mAh g-1, K-modified Na0.7MnO2 a capacity of 184/178 mAh g-1, Rb-modified Na0.9MnO2 a capacity of 159/150 mAh g-1, and Cs-modified Na0.7MnO2 a capacity of 171/163 mAh g-1 between 1.5 and 4.2 V at a current density of 15 mA g-1. The parent Na0.7MnO2 is compared with alkali metal (K, Rb, and Cs)-modified NaxMnO2 in terms of surface morphology using scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy, scanning electron microscopy, 23Na solid-state nuclear magnetic resonance, and X-ray photoelectron spectroscopy and in terms of electrochemical performance and structural electrochemical evolution using in situ or operando synchrotron X-ray diffraction.
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.
