Structure and Electronic Structure Evolution of P2-Na<sub><i>x</i></sub>CoO<sub>2</sub> Phases from X-ray Diffraction and <sup>23</sup>Na Magic Angle Spinning Nuclear Magnetic Resonance
Yohan Biecher, Anthony Baux, François Fauth, Claude Delmas, Gillian R. Goward, Dany Carlier
Abstract
P2-Na0.70CoO2 is considered as a model material for positive electrode applications in Na-ion batteries. In this study, we report an in-depth study and characterization of the P2-NaxCoO2 system in order to understand the material evolution from the point of view of the structure at different scales and electronic properties upon charge up to a high voltage (4.6 V). Using a combination of ex situ and operando X-ray diffraction (XRD) and ex situ 23Na magic-angle spinning nuclear magnetic resonance (MAS NMR), we discuss the structural changes occurring due to the deintercalation of Na+ ions from the interlayer slabs and the change in the electronic structure and magnetic properties. The XRD study allows discussing the general evolution in relation with previous studies. The novelty lies here in the observation for the first time of an ordered phase for x = 1/3 appearing above 4.3 V followed by a disordering in the slab stacking for higher voltages. The combination of the data obtained using different techniques allowed the interpretation of the NMR shift and shape evolution versus the Na content. This study reveals a complex behavior due to the presence of localized and delocalized electrons, whose relative proportions change versus Na content.