Alkali Metal-Modified P2 NaMnO₂: 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 Na_0.7MnO₂ 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 Na_0.7MnO₂ shows a second charge/discharge capacity of 143/134 mAh g^-1, K-modified Na_0.7MnO₂ a capacity of 184/178 mAh g^-1, Rb-modified Na_0.9MnO₂ a capacity of 159/150 mAh g^-1, and Cs-modified Na_0.7MnO₂ 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 Na_0.7MnO₂ is compared with alkali metal (K, Rb, and Cs)-modified NaMnO₂ 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.