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

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 NaMnO solid state synthesized at 1000 °C that shows two distinct phases; one adopts hexagonal P2-type 6/ space group symmetry, and the other adopts orthorhombic 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 NaMnO shows a second charge/discharge capacity of 143/134 mAh g, K-modified NaMnO a capacity of 184/178 mAh g, Rb-modified NaMnO a capacity of 159/150 mAh g, and Cs-modified NaMnO a capacity of 171/163 mAh g between 1.5 and 4.2 V at a current density of 15 mA g. The parent NaMnO is compared with alkali metal (K, Rb, and Cs)-modified NaMnO in terms of surface morphology using scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy, scanning electron microscopy, Na solid-state nuclear magnetic resonance, and X-ray photoelectron spectroscopy and in terms of electrochemical performance and structural electrochemical evolution using or synchrotron X-ray diffraction.