Solid-state NMR Relaxation Measurement For Nucleus Subject To Large Anisotropic Interactions And Its Application To The Phase Transition In Li₆CoO₄

With the rapid development of modern society,the demand of various energy sources is also increasing.Therefore,it is urgent to develop clean,environmentally friendly,and efficient energy storage materials.Lithium-ion secondary battery,as an energy storage technology with high efficiency and high performance,occupies an important position in the research field in recent years.It has become an important issue for scientists to study the microstructures,charge/discharge mechanisms,and capacity decay mechanisms of battery materials by using advanced characterization tools.High-resolution solid-state NMR,as an important spectroscopy method,has significant applications in the field of materials science.It is very suitable for the study of microstructures and dynamics in various crystalline and amorphous solid materials due to its capability to provide structural information at the atomic and molecular levels in a nondestructive and high-resolution manner.The common cathode materials of Li-ion batteries are mainly transition metal oxides.Their SS-NMR spectra often lack sufficient resolution,because the unpaired electrons in the transition metal center form strong hyperfine interaction with the local nucleus.The isotropic part of this interaction leads to a significant resonance shift,whereas the anisotropic part leads to a severe spectral broadening that relies on Magic Angle Spinning（MAS）to distinguish the isotropic peaks.The existing MAS speed is still limited,and the signals of different sites tend to overlap with various spinning sidebands under strong anisotropic interactions,preventing effective differentiation between different chemical environments.As a result,relaxation measurements of nuclear spins at different sites are very difficult.Therefore,it is necessary to develop a novel method for such systems to accurately predict the relaxation parameters with site-specific resolution,which will also permit quantitative probe of different atomic environments.We will also focus on the study of tetragonal-to-cubic phase transition in Li₆CoO₄,a typical super lithium-rich oxide,by using SS-NMR and Electron Paramagnetic Resonance（EPR）spectroscopy.The antifluorite oxide Li₆CoO₄ has a great potential to be applied as a high-capacity cathode material due to its rich lithium content.The electrochemical results show that the reversibility of Li₆CoO₄ cathode material is very poor.It is likely that only the transition metal redox can be utilized,and the reversibility of the two-electron reaction is not satisfactory.It has been reported that it is difficult for the antifluorite structure to adapt the large crystal structure change during the tetragonal-to-cubic phase transition upon delithiation.Therefore,reducing the lattice distortion by forming a cubic structure can be an effective way to improve the electrochemical reversibility of Li₆CoO₄.It is common to apply mechanical ball milling to synthesize cubic rock-salt structures.However,fundamental explanation needs to be given to understand the cation distribution,the electronic structure of transition metal ions in the cubic structures,and the mechanism of tetragonal-to-cubic phase transition.Therefore,the main work of this study is as follows:（1）We will develop a suitable relaxation measurement method,especially the T₁relaxation,for nucleus subject to strong anisotropic interaction.This new method not only permits the differentiation of different atomic environments but also accurately predicts T₁ relaxation for each site.The method development targets at nucleus with strong chemical shift anisotropy or strong electron-nucleus dipole interaction;and the relaxation measurement method was experimentally verified in LiMn₂O₄,Li₂MnO₃,ScF₃ and other typical samples.（2）For the Li₆CoO₄ model system,the tetragonal-to-cubic phase transition is quantitatively studied by using SS-NMR and EPR,together with Raman and XPS characterization techniques.The phase transition process can be quantitatively described with the help of relaxation measurement method developed in this work.Pure disordered phase structure was obtained by optimizing the milling conditions.On this basis,the electrochemical performance and stability of the two forms of structures are preliminarily explored,which provides a new idea for the subsequent development of high-performance lithium cathodes with super lithium-rich structures.