A team from Nankai University (Tianjin, China) has shown that “MXenes”—exfoliated 2D carbide and carbonitride nanosheets that are structurally similar to graphene, where M represents transition metals, and X is either C or/and N—are promising anode materials for Li-ion batteries. A paper on their work appears in the Journal of the American Chemical Society.
Graphene—a material consisting of single sheets of carbon atoms, has been extensively investigated as an anode material. However, because its chemical and electrical properties cannot be tuned, researchers are also investigating other 2-D materials composed of atomic species other than just carbon. Zhen Zhou and colleagues performed density function theory (DFT) computation to investigate Ti3C2 monolayers and their fluorinated and hydroxylated surfaces, Ti3C2F2 and Ti3C2(OH)2 as representative MXene materials.
...graphene has predominated as the most studied 2D material in the past several years. Nevertheless, its simple chemistry with only carbon networking might limit its practical applications. Complex layered materials composed of more than one element may offer new opportunities due to their large variety of structural compositions that can be tuned for specific properties and applications.
There are many types of inorganic layered materials that occur in nature or can be postsynthesized. If their 2D monolayer or few-layered structures can be isolated, the family of 2D inorganic materials should be expanded significantly.—Tang et al.
Among their findings was that Ti3C2 layers can store up to one Li ion per carbon atom—comparing favorably with the storage capacity of one Li ion per six carbon atoms for pure graphite.
The study found that the bare Ti3C2 sheet behaves as a magnetic metal, while Ti3C2F2 and Ti3C2(OH)2 can be narrow-band gap semiconductors or metals depending strongly on how the surface F and OH groups are geometrically terminated.
In the most stable forms, the F and OH groups prefer to be located above the hollow sites between the three neighboring C atoms, and the resulting I-Ti3C2F2 and I-Ti3C2(OH)2 are all semiconductors with tremendously small band gaps.
The metallic or narrow-band gap semiconducting characteristics favor the potential applications of Ti3C2-related materials to Li-ion batteries. For the bare Ti3C2 monolayer, its combined extraordinary properties, including good electrical conductivity, low diffusion barrier, low open circuit voltage, and high theoretical Li capacity, offer it great potential as an alternative anode material to TiO2 in Li-ion batteries. For its fluorinated and hydroxylated derivatives, however, the surface functionalization tends to degrade the Li diffusion and decrease the Li storage capacity and thus should be avoided in the practical synthetic experiments. Our results give insightful prospects for experimental peers in exploring the potentials of Ti3C2 as electronic and energy storage materials.
Noteworthy, the Ti3C2 monolayer only represents one example of the new family of MXenes, and the implications of this work can be helpful to design more MXenes with better performances. Further experimental and computational investigations on MXenes are highly desirable to shed light on their prospects as advanced materials.—Tang et al.
Qing Tang, Zhen Zhou, and Panwen Shen (2012) Are MXenes Promising Anode Materials for Li Ion Batteries? Computational Studies on Electronic Properties and Li Storage Capability of Ti3C2 and Ti3C2X2 (X = F, OH) Monolayer. Journal of the American Chemical Society 134 (40), 16909-16916 doi: 10.1021/ja308463r