Characterisation of Local Environments in Strongly Paramagnetic Battery Cathodes: A Combined Experimental and Theoretical Approach

Non-invasive techniques which probe the local environment around the paramagnetic redox-active transition metal (<i>TM</i>) centres in lithium- and sodium-ion battery (LIB and NIB, respectively) cathode materials are critical in developing a clear understanding of the phase transformations, redox processes and degradation mechanisms which take place at the cathode during operation of the battery.¹ Where NMR is increasingly used as a characterization technique for studying, for example, Li, Na and O local environments in paramagnetic battery materials, direct observation of the redox-active paramagnetic (<i>TM</i>) centres using NMR is not possible owing to rapid nuclear relaxation times.^2–4 EPR, however, enables investigation of these centres and is far less reported. Further, an understanding and assignment of these EPR spectra—made all the more challenging by strong electron-electron dipolar and magnetic exchange interactions—does not yet exist. In this work, we present a combined high-frequency EPR and density functional theory (DFT) study of the local paramagnetic environments in Li₂MnO₃, a model compound for the family of commercially successful, layered lithium-ion battery cathodes. By collecting EPR spectra at a range of frequencies (9 – 383 GHz) and temperatures (5 – 300 K) and comparing to DFT-calculated <i>g-</i>tensors, we clearly observe several unique local environments and assign these observed resonances to Mn⁴⁺ centres with different local environments [Figure 1]. We also examine the effect of magnetic exchange interactions on the spectra to explain the temperature evolution of these complex EPR spectra. The methodology presented in this work will be invaluable for studying paramagnetic centres in paramagnetic solids not only for LIBs and NIBs, but also in a range of <i>TM</i>-based materials for devices.<br/><br/><br/><b>References:</b><br/><br/>(1) Nguyen, H.; Clément, R. J. Rechargeable Batteries from the Perspective of the Electron Spin. <i>ACS Energy Lett.</i> <b>2020</b>, <i>5</i> (12), 3848–3859. https://doi.org/10.1021/acsenergylett.0c02074.<br/>(2) Grey, C. P.; Dupré, N. NMR Studies of Cathode Materials for Lithium-Ion Rechargeable Batteries. <i>Chem. Rev.</i> <b>2004</b>, <i>104</i> (10), 4493–4512. https://doi.org/10.1021/cr020734p.<br/>(3) Carlier, D.; Ménétrier, M.; Grey, C. P.; Delmas, C.; Ceder, G. Understanding the NMR Shifts in Paramagnetic Transition Metal Oxides Using Density Functional Theory Calculations. <i>Phys. Rev. B</i> <b>2003</b>, <i>67</i> (17), 174103. https://doi.org/10.1103/PhysRevB.67.174103.<br/>(4) Bassey, E. N.; Reeves, P. J.; Seymour, I. D.; Grey, C. P. 17O NMR Spectroscopy in Lithium-Ion Battery Cathode Materials: Challenges and Interpretation. <i>J. Am. Chem. Soc.</i> <b>2022</b>, <i>144</i> (41), 18714–18729. https://doi.org/10.1021/jacs.2c02927.