Effect of the field strength on the MAS NMR spectra: Comparative study between diamagnetic and paramagnetic systems

Abstract

Solid-state NMR spectroscopy has gained increasing attention as a probe to investigate structures and dynamics of various solid materials, in particular, materials for rechargeable batteries. Cathodes and anodes for rechargeable batteries contain unpaired electrons (paramagnetic), which generate localized magnetic fields at molecules. Owing to this paramagnetic interaction, it is often complicated to measure and interpret the NMR spectra of the paramagnetic systems. Thus, understanding the effect of the interaction between unpaired electrons and nuclei is important. NMR spectroscopy at higher magnetic fields has been perceived as beneficial since this can provide higher sensitivity and resolution. However, the response to the magnetic field strength varies depending on the nuclei of interest and material properties because various factors affect NMR characteristics. In this work, we performed a systematic study of the effect of the field strength on the magic angle spinning (MAS) NMR characteristics by comparing diamagnetic and paramagnetic systems at two different magnetic fields. As diamagnetic materials, LiCoO₂ (LCO) and Li₂O are examined. As paramagnetic materials, LiFePO₄(LFP) and lithium nickel manganese cobalt oxides (LiNi_xMn_yCo_1–x–yO₂) with different compositions are investigated. We have demonstrated that higher signal intensity and narrower linewidths can be obtained at higher magnetic fields for diamagnetic systems, whereas higher signal intensity and better resolution are obtained at lower magnetic fields for paramagnetic systems. Our research will provide systematic and experimental evidence about the field strength dependence of paramagnetic systems and rationalized grounds for choosing proper NMR spectrometers for each material.