Nankai University team shows “MXenes” promising anode materials for Li-ion batteries

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-Ti₃C₂F₂and 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.