A perspective: carbon nanotube macro-films for energy storage

The ever-increasing demand of electricity storage is a growing challenge among a broad range of renewable energy sources. The development of high-energy storage devices has been one of the most important research areas in modern days. In particular, rechargeable batteries and electrochemical capacitors are recognized as the primary power sources for applications from portable electronic devices to electric vehicles. In order to power the emerging flexible/stretchable electronics, power sources themselves must be able to accommodate high levels of deformation and stretchability in addition to high energy and power density, light weight, miniaturization in size, safety qualification, and other significant characteristics. Utilizing carbon nanotubes (CNTs) for various energy storage applications such as electrodes in lithium ion batteries and supercapacitors, are under close scrutiny because of the promising electrochemical performance in addition to their extraordinary tensile strength and flexibility, ultrahigh surface area, and excellent thermal and electrical conductivity. Recently, there has been growing interest in investigating CNT macro-films with large-scale organized nanostructures of desired shape and form and unique and enhanced properties: integrity and stability to realize the scaled-up energy storage devices. In this perspective, research efforts in assembling 2-D CNT macro-films using a chemical vapor deposition method and their applications for different energy storage devices including stretchable supercapacitors, supercapacitors working under extreme conditions such as high temperature and high pressure, and lithium-ion batteries are discussed. In details, this paper provides an original overview involving the effect of compressive stress on the electrochemical behavior of flexible supercapacitors assembled with CNT macro-film electrodes and electrolytes with different anions and cations; the demonstration of the dynamic and galvanic stability of stretchable supercapacitor using buckled CNT macro-films by an in situ dynamic electrochemical testing method; the understandings on the self-discharge mechanisms of CNT macrofilm-based supercapacitors from both electrode and electrolyte aspects; and the investigation of the electrochemical properties of the tandem structure of active materials (e.g. thin porous silicon film and CuO) with CNT macro-films acting as a flexible and adhesive layer between the active layers and current collectors for lithium-ion batteries. Future research on CNT macro-films-based lithium-sulfur batteries and lithium-air batteries is also discussed.