Structural Origins of Voltage Hysteresis in the Na-Ion Cathode P2–Na0.67[Mg0.28Mn0.72]O2: A Combined Spectroscopic and Density Functional Theory Study

P2-layered sodium-ion battery (NIB) cathodes are a promising class of Na-ion electrode materials with high Na + mobility and relatively high capacities. In this work, we report the structural changes that take place in P2-Na 0.67 [Mg 0.28 Mn 0.72 ]O 2 . Using ex situ X-ray diffraction, Mn K -edge extended X-ray absorption fine structure, and 23 Na NMR spectroscopy, we identify the bulk phase changes along the first electrochemical charge-discharge cycle-including the formation of a high-voltage " Z phase", an intergrowth of the OP4 and O2 phases. Our ab initio transition state searches reveal that reversible Mg 2+ migration in the Z phase is both kinetically and thermodynamically favorable at high voltages. We propose that Mg 2+ migration is a significant contributor to the observed voltage hysteresis in Na 0.67 [Mg 0.28 Mn 0.72 ]O 2 and identify qualitative changes in the Na + ion mobility.