Background:

Energy storage devices, such as supercapacitors and lithium ion batteries comprise active materials, electrolytes and separators. For optimal energy storage, active materials should have large surface areas, chemical and mechanical stability, and good electrical properties, especially electrical conductivity. As such, carbon-based materials are widely used in practical and commercially-feasible energy storage devices. Modified carbon-based materials such as activated carbon, carbon nanotubes and graphene have been suggested for such applications, but are limited in their performance due to self-aggregation and the presence of micropores, both phenomena restricting ion diffusion and causing loss of active surface area.

Summary:

In the present invention, a graphene-carbon nanotube multi-stack three-dimensional architecture (“graphene-CNT stack”) comprises alternating layers of graphene and carbon nanotubes (CNT). The CNT function as both physical spacers and electrical conduits between the graphene layers, and may be substantially parallel to each other. Also, a method of fabrication of graphene-CNT stacks includes the steps of: (i) depositing a first graphene layer on a metal foil; (ii) transferring the first graphene layer to a current collector; (iii) depositing a first layer of a catalytic metal on the first graphene layer; (iv) alternately depositing graphene and catalytic metal layers one upon the other so as to form a stack of alternating graphene and catalytic metal layers on the first graphene and catalytic metal layers; (v) transforming the catalytic metal layers into arrays of metal nanoparticles by thermal breakdown of the catalytic metal layers; and (vi) precipitating CNT outward from the metal nanoparticles. In embodiments of the present invention, the CNT are precipitated in a single execution of step (vi), resulting in simultaneous growth of the CNT and expansion of the graphene-CNT stack. Such simultaneous growth and expansion, implemented in a single step, is referred to herein as “popcorn-like growth”. In some embodiments of the present invention, the catalytic metal is a transition metal. In some such embodiments, the transition metal is nickel. In some embodiments, the graphene layers are formed by a chemical vapor deposition process. In some embodiments, the CNT are formed by a chemical vapor deposition process. In some embodiments, the catalytic metal layers are formed by a physical vapor deposition process.

Full Patent: Popcorn-like growth of graphene-carbon nanotube multi-stack hybrid three-dimensional architecture for energy storage devices

Image Source: https://link.springer.com/article/10.1007/s41918-019-00042-6