Ultrahigh-Resolution 7Li Magic-Angle Spinning Nuclear Magnetic Resonance Spectroscopy by Isotopic Dilution

The functional importance of lithium in many types of materials requires reliable methods for studying its local structure and dynamics. Nuclear magnetic resonance (NMR) spectroscopy should be ideal for this purpose but suffers from poor resolution due to intrinsic dipolar broadening. We overcome this problem by diluting the Li-7 spins with Li-6, producing ultrahigh-resolution Li-7 magic-angle spinning (MAS) NMR spectra of lithium borates without appreciably sacrificing sensitivity. Resolution of up to six distinct lithium environments permits assignments, aided by density functional theory, which indicate separate chemical shift regions for different coordination numbers. With high sensitivity and site-specific Li resolution, two-dimensional NMR methods for probing Li hopping become more informative, revealing preferential site exchange in a complex lithium borate phase. The success of this approach will make Li-7 MAS NMR an attractive method for probing lithium coordination environments and mobility in diamagnetic lithium-bearing materials.