锂嵌碳的磁共振研究

Magnetic Resonance Studies on Lithium Intercalation into Carbons

本文综述了十数年来电子自旋共振(ESR)和7Li核磁共振(7Li-NMR)技术用于锂嵌碳研究的进展.ESR研究发现锂嵌碳材料中存在两种电子自旋.一种来自碳材料中的载流子电子,称为Pauli自旋.从Pauli自旋的ESR强度可推算给定锂嵌碳样品的电子态密度曲线,并进而计算能带模型机理对该样品嵌锂容量的贡献.另一种来自局域化自旋,即Curie自旋,其与嵌锂位置的关系尚不清楚.7Li-NMR测试已发现几个不同的谱峰,其峰位和强度随碳样品性质和嵌锂深度而异.一般认为,45±5×10-6(即ppm,下同)处的NMR谱线源于深度嵌锂(在LixC6中x=0.5～1)石墨化结构中的Li+,属于Knight位移;而明显小于45×10-6的谱峰则可能是来自碳材料中石墨化微结构中低浓度Li+的Knight位移,也可能是于无序微结构中共价结合的Li的化学位移.ESR与7Li-NMR在研究锂嵌碳方面有很强的互补性,联合应用此两技术可望对深入认识锂嵌碳材料的构效关系作出新贡献.

The electron spin resonance(ESR) and 7Li_NMR magnetic resonance (7Li_NMR) studies on lithium intercalation into carbons were reviewed based on recently published papers, including those by the authors of this review. Two kinds of electronic spins were found in ESR studies. One corresponds to the charge carrier electrons and is called the Pauli spin. From the ESR intensity of Pauli spins, the curve of the density of electronic states can be deduced for the carbon material studied and, in turn, the contribution of the band model mechanism to the lithium intercalation can be calculated for the given carbon sample. The other is the Curie spin associated with localized spins but its relations with intercalation siteds are not clear at present. 7Li_NMR measurements revealed a number of different signals, their position and intensity depenging on the nature of carbon and the degree of intercalation. It is generally accepted that the NMR peak at 45±5×10-6 corresponds to the Li+ of LixC6(x=0.5～1) in the graphitised structure and this peak shift is the Knight Shift. The NMR peaks with a shift below 45 ×10-6 may correspond to the low concentration Li+ in the graphitised microstructures or the covalently bonded lithium in the disordered microstructures.It is pointed out that ESR and 7Li_NMR are well complementary to one another in the study of lithium intercalation into carbons and combined use of these two techniques will hopefully contribute much to the understanding of the structure_property relationship of lithiated carbons.