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

Euan N Bassey; Philip J Reeves; Michael A Jones; Jeongjae Lee; Ieuan D Seymour; Giannantonio Cibin; Clare P Grey

doi:10.1021/acs.chemmater.1c00248

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

Chem Mater. 2021 Jul 13;33(13):4890-4906. doi: 10.1021/acs.chemmater.1c00248. Epub 2021 Jun 21.

Authors

Euan N Bassey¹, Philip J Reeves¹, Michael A Jones¹, Jeongjae Lee^{1

2}, Ieuan D Seymour³, Giannantonio Cibin⁴, Clare P Grey¹

Affiliations

¹ Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom.
² School of Earth and Environmental Sciences, Seoul National University, Seoul 08826, Korea.
³ Department of Materials, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom.
⁴ Diamond Light Source, Harwell Science and Innovation Campus, Didcot OX11 0DE, United Kingdom.

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 ex situ X-ray diffraction, Mn K-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 "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[Mg_0.28Mn_0.72]O₂ and identify qualitative changes in the Na⁺ ion mobility.