Magnetic Resonance Studies of a Sodium-Ion Battery Cathode: Experiment and Theory
Sodium-ion batteries (NIBs) are increasingly of interest in modern society as more affordable energy storage alternatives to lithium-ion batteries. At present, the electrochemical performance—the charge/discharge rate, lifetime and capacity—of NIBs is not optimised for practical applications and generally limited by the cathode material. To address these performance problems, a deeper understanding of the evolution of the chemical and electronic structure of NIB cathodes is required. In this thesis, Na₀.₆₇[Mg₀.₂₈Mn₀.₇₂]O₂, a layered cathode material exhibiting fast charge/discharge rates, a large voltage hysteresis and a high reversible capacity, is studied. Experimental techniques which probe both the local and bulk structure are employed, in conjunction with first-principles calculations. The superstructure of Na₀.₆₇[Mg₀.₂₈Mn₀.₇₂]O₂ is elucidated for the first time using synchrotron X-ray diffraction (XRD), total neutron scattering and high-frequency electron paramagnetic resonance spectroscopy (EPR). The effect of this superstructure on Na⁺-ion mobility (charge/discharge rates) is explored using variable-temperature solid-state ²³Na nuclear magnetic resonance (NMR) spectroscopy. Simulation of these spectra enables rationalisation of the relative mobilities of Na⁺ in different local environments. Using operando and ex situ XRD, as well as ex situ ²³Na NMR spectroscopy and first principles transition-state searching calculations, electrochemically-induced phase transformations are then identified, revealing that Mg²⁺ migration takes place during charge and contributes significantly to the observed hysteresis. Finally, the charge compensation mechanism is presented. Previous reports have attributed the large reversible capacity of Na₀.₆₇[Mg₀.₂₈Mn₀.₇₂]O₂ to redox reactions involving oxide anions, O²⁻, but the mechanism remains unclear. Through a combination of spectroscopic techniques (¹⁷O and ²⁵Mg NMR, EPR and X-ray absorption spectroscopy), bulk magnetic susceptibility measurements and first-principles calculations, the origin of the high capacity is identified with the formation of delocalised electronic states between Mn and O. These states are generated by Mg²⁺ migration and stabilised by strong antiferromagnetic interactions. The properties of Na₀.₆₇[Mg₀.₂₈Mn₀.₇₂]O₂—the superstructure, phase transformations, Na⁺ ion mobility and charge compensation mechanism—are summarised and a set of design rules for long lifetime, fast charging and high capacity NIB cathode materials is presented.
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