Abstract: High-field solid-state nuclear magnetic resonance (SSNMR) technology, due to its high sensitivity and multi-nuclear detection capabilities, is an important characterization method for the analysis of the microstructure of solid materials and the study of molecular dynamics behavior. However, for some paramagnetic materials such as lithium-ion batteries, the transition metal ions (such as Mn3+, Fe3+, etc.) commonly present in the materials endow the system with strong paramagnetic properties. Under high-field conditions, it will face a series of challenges such as magnetic field inhomogeneity, spectral line broadening, signal attenuation, and the inability to perform magic angle spinning. As a result, it is difficult to obtain useful information. Under low-field conditions, the magnetic field distortion caused by the paramagnetic effect is significantly weakened, which is expected to solve the problems of reduced resolution or the inability to perform magic angle spinning of paramagnetic samples in high-field nuclear magnetic resonance. This paper analyzes the advantages of a low-field environment for the study of paramagnetic substances through theoretical analysis, and develops a low-field solid-state MAS probe for a 0.5T Halbach magnet. It mainly includes research work such as the miniaturization design of the MAS unit, the finite element simulation optimization of the pneumatic drive gas circuit, and the simulation analysis of the parameters of the radio frequency coil. It is integrated with the spectrometer control unit, magnet unit, gas circuit control unit, etc. to form a complete low-field solid-state MAS spectrometer, realizing the MAS NMR testing of lithium-ion batteries under low-field conditions. Moreover, the NMR signals of 7Li of various paramagnetic samples are collected at a rotation speed of 12kHz, verifying the feasibility of the self-developed low-field solid-state MAS probe for solid high-resolution NMR acquisition of paramagnetic samples under low-field conditions. It solves the problems of the superposition of spinning sidebands of paramagnetic samples or the inability to perform magic angle spinning under high-field conditions, providing a new solution for the NMR study of paramagnetic samples.

Key words: Solid-state NMR, Low-field NMR, Magic angle spinning technology (MAS), Low-field MAS, Radio frequency coil