Li NMR of Fast Lithium Ion Conductors

One of the most important practical applications of lithium compounds is as fast ion conductors with potential electronic applications such as solid electrolytes for lithium batteries. Li20 is a fast ion conductor in which the Li ions occupy a simple cubic sublattice with the antifluorite structure. Both MAS and static Li NMR spectra of Li20 have been reported, the former recorded as a function of temperature up to 1000 K (Xie et al. 1995). The effect of introducing vacancies on the Li sites by doping with LiF has been studied by high-temperature static Li NMR, which reveals the interaction of the Li defects 600 K and the appearance of 2 distinct quadrupolar interactions at about 900 K. Measurements of the relative intensities of the satellite peaks as a function of temperature have provided evidence of thermal dissociation of an impurity-vacancy complex (Xie et al. 1995). 

Lithium intercalation of compounds such as SnS2 are of technical interest for pho-tochromic display materials and lithium electrodes which reversibly take up and release Li . Li and Sn NMR has been used to investigate the location of the Li insertion sites in this material (Pietrass et al. 1997). The Li spectra show a central transition which can be decomposed into 2 components with different xq values corresponding to Li in octahedral and tetrahedral interlayer sites. As the Li concentration increases, the additional ions enter tetrahedral intralayer sites surrounded by 3 tin and 4 sulphur atoms and characterised by a broad Li NMR spectral component. Further insertion of Li results in the material becoming amorphous by rupture of the layers (Pietrass ef a/. 1997). 

LiCo02, an important electrode material for secondary lithium batteries, occurs in 2 polytypes, both of which have been investigated by Li and Co NMR at 3 magnetic fields (Siegel et al. 2001). Both polytypes show only 1 Li resonance corresponding to lithium in octahedral coordination with oxygen, with similar Li xq values (25-36 kHz for the 02 polytype and 31-39 kHz for the 03 polytype). 

The superionic compound Li3Sc2(P04)3, studied by Li NMR up to 575°C (Vash-man et al. 1992) has revealed the operation of three types of Li ion motion and allowed 

Lithium-doped BPO4, another candidate ceramic electrolyte material for lithium batteries has been studied by Li NMR relaxation and linewidth measurements of samples with Li doping levels up to 20 mol % (Dodd et al. 2000). Comparison of the NMR data with values of the second moment calculated for both random and homogeneous models of Li distribution indicate the existence of Li clusters with an intemuclear distance of 3A, possibly consisting of 1 Li ion fixed at a boron vacancy with additional 2 Li ions in the conduction channels surrounding the vacancy. The atomic jump time, determined from measurements of the Li motional narrowing behaviour, indicate a maximum in the Li ionic mobility at the 10 mol % doping level (Dodd et al. 2000). 

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