A 7Li Nuclear Magnetic Resonance Study of Metal-Substituted Lithium Manganese Oxide Spinels
Metal-substituted spinels, LiMxMn2−xO4 (M=Li,Zn,Ni,Al,Co,Cr), are synthesized with low levels of substitution by solid-state techniques and studied with 7Li magic angle spinning nuclear magnetic resonance (MAS NMR). The NMR spectra are characterized by several distinct peaks, spanning a wide shift range. The shifts are interpreted in terms of the supertransferred hyperfine interaction. The as-prepared spinels show peaks in the vicinity of 510 ppm, assigned to “normal” lithium in a tetrahedral site surrounded by 12 manganese nearest neighbors, and 530-580 ppm, assigned to “near-defect” lithium in a tetrahedral site with one or more metal substituents as nearest neighbors. Upon substitution, the peak arising from normal lithium broadens and reduces in intensity, whereas the peaks arising from near-defect lithium increase in intensity. Li-, Co-, and Al-substituted spinels also give rise to a peak in the vicinity of 1700 ppm, assigned to electrochemically inactive lithium in the 16d octahedral site. NMR spectra of electrochemically cycled spinels reveal that the local environment of lithium changes upon cycling. After cycling, the normal lithium peak reduces in intensity and broadens, while the near-defect lithium peak increases in intensity. The extent of these changes is least for spinels that show robust capacity retention. It is found that damage from moisture contamination results in a shift and reduction in intensity of the peak arising from tetrahedral lithium in the spinel. In addition, a new peak at 0 ppm is observed and assigned to multiple diamagnetic, lithium-containing solid electrolyte interface species. No effects of moisture contamination are observed in the electrochemically prepared samples.