碳纳米管材料的储锂性能

碳纳米管较低的碳原子密度、管径和管间的空隙可以为锂离子提供大量的嵌入空间,从而拥有更高的储锂能力。本文结合实验与理论研究的最新成果,综述了这一领域的主要进展和前景。实验上,对单壁碳纳米管进行适当处理,可以将锂存储量提高到常规石墨材料的2~3倍。根据密度泛函理论计算,锂在不同碳纳米管束中的最高理论嵌入量可以达到Li0.5C。嵌入后锂和碳纳米管之间发生了完全的电荷转移,碳纳米管的Fermi能级上移到导带中,所有碳纳米管都转变为金属。纳米管自身的电子结构对锂的吸附是至关重要的,缺电子体系更有利于锂的吸附。锂在B掺杂的复合管如BC3纳米管中有很大的吸附能。锂穿透纳米管壁从管壁外进入纳米管内的能垒,随着纳米管壁拓扑缺陷结构的尺寸变大而显著降低,B在纳米管壁的存在会进一步降低锂穿越纳米管壁的能垒。同时B的掺杂会降低相同拓扑缺陷的生成能,导致在BC3纳米管中出现更多的拓扑缺陷,从而有利于锂离子的扩散。实验与理论计算的结合可望加深对锂离子在纳米管材料中嵌入过程的理解,指导设计具有更高储锂性能的新材料。

LITHIUM STORAGE IN CARBON NANOTUBE MATERIALS

Both the interior of the carbon nanotube and the interstitial space of the nanotube bundles are susceptible for Li intercalation,so carbon nanotubes may have high Li storage capacity.Combining the experimental results and computational investigations,we review the progress and prospect in this field.According to density functional calculations,after intercalation almost complete charge transfer between Li and SWNTs occurs,Fermi level shifts into conduction band,and all the nanotubes become metallic.The theoretical highest Li storage capacity can reach about Li0.5C.Li adsorption depends critically on the electronic structure of nanotubes.Due to the strong propensity of boron to accept electrons from lithium energetically,the electron deficient B-doped and BC3 nanotubes adsorb lithium very favorably.There are very high energy barriers to be overcome when Li penetration from exohedral to endohedral sites through the perfect sidewalls of pure C and BC3 nanotubes.However,these barriers are reduced substantially when topological defects(seven-,eight-,and especially nine-membered rings) are present,and the effect for composite BC3 nanotubes is more significant.Moreover,topological defects are easier to form on the BC3 tubes than on pure C tubes.The combination of these two effects will result in enhanced Li penetration rates into nanotubes interiors through the sidewalls.Combination of experiments and calculation can lead to further understanding of Li intercalation in nanotubes,and provide guidance to design novel materials with higher performances for Li ion batteries.