Overhauser dynamic nuclear polarization in alkali metals

Abstract

Dynamic nuclear polarization (DNP) of the 7Li NMR lithium metal signal via an Overhauser mechanism was predicted and demonstrated over 70 years ago, and applied more recently at high fields and room temperature to study electrodeposited microstructural lithium metal. Here we further explore the requirements and limitations of the Overhauser effect for alkali metals. Following a theoretical treatment of the Overhauser mechanism, we explain why only relatively modest enhancements are obtained (∼50 compared to a theoretical maximum of 1700 for 7Li) at a magnetic field strength of 9.4 T. Through a systematic investigation of the 7Li enhancement as a function of experimental parameters (magnetic field strength, B0, temperature, microwave power, and MAS frequency), we show that the experimental enhancements are limited by the short spin-lattice relaxation times (T1e and T2e) of the electrons in lithium metal, and thus the degree of saturation of the electron spin resonance (ESR) transition. The enhancements of 6Li and 23Na are similarly limited by the degree of electron spin saturation, the effect being more pronounced for 23Na; enhancements are improved by using higher microwave power and for 23Na, with its temperature-dependent T1e, by performing experiments at lower temperatures. Sample considerations, such as sample volume, metal particle sizes, and dilution of metal particles, are also important for obtaining high enhancements. Studies of the hyperpolarization mechanism of the spins in diamagnetic environment at the interfaces between lithium metal and its passivating film suggest that both spin diffusion and direct chemical exchange play a role, although the role of direct cross-relaxation with the conduction electrons requires further investigation. The hyperpolarization is then transferred to spins farther from the metal surface via spin diffusion, allowing only the spins within less than ∼10 nm from the metal surface to be hyperpolarized. Based on the insights gained through this work, lithium metal possesses a possibly unique combination of beneficial qualities, making it a highly suitable system for Overhauser DNP. The high enhancements achieved for 6Li (∼150) using a 1.3 mm rotor are particularly promising for studying lithium metal samples extracted from batteries, making use of the generally higher resolution of 6Li over 7Li.