Strong Magnetic Exchange Interactions and Delocalized Mn–O States Enable High-Voltage Capacity in the Na-Ion Cathode P2–Na<sub>0.67</sub>[Mg<sub>0.28</sub>Mn<sub>0.72</sub>]O<sub>2</sub>
- Authors
- Publication Date
- Sep 09, 2024
- Source
- Apollo - University of Cambridge Repository
- Keywords
- Language
- English
- License
- Green
- External links
Abstract
The increased capacity offered by oxygen-redox active cathode materials for rechargeable lithium- and sodium-ion batteries (LIBs and NIBs, respectively) offers a pathway to the next generation of high-gravimetric-capacity cathodes for use in devices, transportation and on the grid. Many of these materials, however, are plagued with voltage fade, voltage hysteresis and O2 loss, the origins of which can be traced back to changes in their electronic and chemical structures on cycling. Developing a detailed understanding of these changes is critical to mitigating these cathodes’ poor performance. In this work, we present an analysis of the redox mechanism of P2-Na0.67[Mg0.28Mn0.72]O2, a layered NIB cathode whose high capacity has previously been attributed to trapped O2 molecules. We examine a variety of charge compensation scenarios, calculate their corresponding densities of states and spectroscopic properties, and systematically compare the results to experimental data: 25Mg and 17O nuclear magnetic resonance (NMR) spectroscopy, operando X-band and ex situ high-frequency electron paramagnetic resonance (EPR), ex situ magnetometry, and O and Mn K-edge X-Ray Absorption Spectroscopy (XAS) and X-Ray Absorption Near Edge Spectroscopy (XANES). Via a process of elimination, we suggest that the mechanism for O redox in this material is dominated by a process that involves the formation of strongly antiferromagnetic, delocalised Mn–O states which form after Mg2+ migration at high voltages. Our results primarily rely on non invasive techniques that are vital to understanding the electronic structure of metastable cycled cathode samples. / E.N.B. acknowledges funding from the Engineering Physical Sciences Research Council (EPSRC) via the National Productivity Interest Fund (NPIF) 2018 (EP/S515334/1). T.I. acknowledges funding from the European Research Council under the Advanced Investigator Grant awarded to C.P.G. (EC H2020 835073). Additional thanks are given to the staff scientists at beamline I11 of the Diamond Light Source for synchrotron data using block allocation group time under proposal CY34243. This work also utilised the ARCHER UK National Supercomputing Service via our membership in the UK’s HEC Materials Chemistry Consortium, funded by the EPSRC (EP/L000202). Research was also carried out at the Center for Functional Nanomaterials, Brookhaven National Laboratory, through the U.S. Department of Energy, Office of Basic Energy Sciences, Contract DE-AC02-98CH10866. This work made use of the shared facilities of the UC Santa Barbara MRSEC (DMR 720256), a member of the Materials Research Facilities Network (http://www.mrfn.org). H.N. acknowledges support from the Graduate Division at UCSB, through a Graduate Research Mentorship Fellowship. E.N.B. would also like to thank P.J. Reeves, M.A. Jones, J.D. Bocarsly, A Genreith-Schriever and H. Banerjee for illuminating discussions.
