Alkali Metal-Modified P2 NaxMnO2: Crystal Structure and Application in Sodium-Ion Batteries

Sodium-ion batteries (NIBs) are an emerging alternative to lithium-ion batteries because of abundant sodium re-sources and the potentially lower cost. Here we report the Na0.7MnO2 solid-state synthesized at 1000 oC which shows two distinct phases, one adopts hexagonal P2-type P63/mmc and the other adopts orthorhombic Pbma space group symmetry. The phase ratio of the P2 to orthorhombic phase is 55.0(5):45.0(4). A single phase P2 structure is found to form at 1000 oC after modification with alkali metals Rb and Cs while the K-modified produces an additional minor impurity. Modification is the addition of the alkali elements during synthesis which do not appear to dope 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 184/178 mAh g-1, Rb-modified Na0.9MnO2 159/150 mAh g-1 and Cs-modified Na0.7MnO2 171/163 mAh g-1 between 1.5-4.2 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 micros-copy coupled with energy-dispersive X-ray spectroscopy (STEM-EDS), scanning electron microscopy (SEM), 23Na sol-id-state nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), electrochemical performance and structural electrochemical evolution using in situ/operando synchrotron X-ray diffraction (SXRD).