Structural Origins of Voltage Hysteresis in the Na-Ion Cathode P2Na0.67[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[Mg0.28Mn0.72]O-2. Using ex situ X-ray diffraction, Mn K-edge extended X-ray absorption fine structure, and Na-23 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 Mg2+ migration in the Z 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[Mg0.28Mn0.72]O-2 and identify qualitative changes in the Na+ ion mobility.

Automated summary

This study reports the structural changes in P2-Na-0.67[Mg0.28Mn0.72]O-2, including the formation of a high-voltage Z phase and intergrowth of OP4 and O2 phases. The research shows that reversible Mg2+ migration in the Z phase is favorable at high voltages and is a significant contributor to observed voltage hysteresis.