Structural Origins of Voltage Hysteresis in the Na-Ion Cathode P2–Na(0.67)[Mg(0.28)Mn(0.72)]O(2): A Combined Spectroscopic and Density Functional Theory Study

[Image: see text] 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.