Structural Origins of Voltage Hysteresis in the Na-Ion Cathode P2–Na_0.67[Mg_0.28Mn_0.72]O₂: A Combined Spectroscopic and Density Functional Theory Study

Abstract

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_0.28Mn_0.72]O₂. Using <i>ex situ</i> X-ray diffraction, Mn <i>K</i>-edge extended X-ray absorption fine structure, and ²³Na NMR spectroscopy, we identify the bulk phase changes along the first electrochemical charge-discharge cycle-including the formation of a high-voltage "<i>Z</i> phase", an intergrowth of the OP4 and O2 phases. Our <i>ab initio</i> transition state searches reveal that reversible Mg²⁺ migration in the <i>Z</i> phase is both kinetically and thermodynamically favorable at high voltages. We propose that Mg²⁺ migration is a significant contributor to the observed voltage hysteresis in Na_0.67[Mg_0.28Mn_0.72]O₂ and identify qualitative changes in the Na⁺ ion mobility.