Flexible supercapacitors (SCs) have attracted increasing attention as the power supply unit for portable/wearable electronics. Carbon nanotubes (CNTs) are promising candidate materials for flexible SC electrodes because of their outstanding mechanical property, high electrical conductivity, large surface area, and functionability. CNTs can assemble into various macroscopic materials with different dimensions. In this review, flexible CNT assemblies including 1D fibers, 2D films, and 3D aerogels and sponges are introduced with a focus on the design strategies and fabrication techniques. The recent developments and state-of-the-art applications of such structures as electrodes in flexible SCs are summarized based on device configurations including sandwiched, interdigital in-plane, and cable-type configurations. The flexible CNT-based electrodes have shown great advantages in bendability, stretchability and/or compressibility, as well as a long cycle lifetime. The current challenges and future research opportunities in this field are also discussed.

Bismuth oxides are important battery materials owing to their ability to electrochemically react and alloy with Li, which results in a high capacity level, which substantially exceeds that of graphite anodes. However, this high Li-storage capability is often compromised by the poor electrochemical cyclability and rate capability of bismuth oxides. To address these challenges, in this study, we design a hybrid architecture composed of reduced graphene oxide (rGO) nanosheets decorated with ultrafine Bi₂O_2.33 nanodots (denoted as Bi₂O_2.33/rGO), based on the selective and controlled hydrolysis of a Bi precursor on graphene oxide and subsequent crystallization via solvothermal treatment. Because of its high conductivity, large accessible area, and inherent flexibility, the Bi₂O_2.33/rGO hybrid exhibits stable and robust Li storage (346 mA·h·g^-1 over 600 cycles at 10 C), significantly outperforming previously reported Bi-based materials. This superb performance indicates that decorating rGO nanosheets with ultrafine nanodots may introduce new possibilities for the development of stable and robust metal-oxide electrodes.

Lithium iron silicate (Li₂FeSiO₄) is capable of affording a much higher capacity than conventional cathodes, and thus, it shows great promise for high-energy battery applications. However, its capacity has often been adversely affected by poor reaction activity due to the extremely low electronic and ionic conductivity of silicates. Here, we for the first time report on a rational engineering strategy towards a highly active Li₂FeSiO₄ by designing a carbon nanotube (CNT) directed three-dimensional (3D) porous Li₂FeSiO₄ composite. As the CNT framework enables rapid electron transport, and the rich pores allow efficient electrolyte penetration, this unique 3D Li₂FeSiO₄-CNT composite exhibits a high capacity of 214 mAh·g⁻¹ and retains 96% of this value over 40 cycles, thus, outstripping many previously reported Li₂FeSiO₄-based materials. Kinetic analysis reveals a high Li⁺ diffusivity due to coupling of the migration of electrons and ions. This research highlights the potential for engineering 3D porous structure to construct highly efficient electrodes for battery applications.

Layered bismuth sulfide (Bi₂S₃) has emerged as an important type of Li-storage material due to its high theoretical capacity and intriguing reaction mechanism. The engineering and fabrication of Bi₂S₃ materials with large capacity and stable cyclability via a facile approach is essential, but still remains a great challenge. Herein, we employ a one-pot hydrothermal route to fabricate carbon-coated Bi₂S₃ nanomeshes (Bi₂S₃/C) as an efficient Li-storage material. The nanomeshes serve as a highly conducting and porous scaffold facilitating electron and ion transport, while the carbon coating layer provides flexible space for efficient reduction of mechanical strain upon electrochemical cycling. Consequently, the fabricated Bi₂S₃/C exhibits a high and stable capacity delivery in the 0.01-2.5 V region, notably outperforming previously reported Bi₂S₃ materials. It is able to discharge 472 mA·h·g^-1 at 120 mA·g^-1 over 50 full cycles, and to retain 301 mA·h·g^-1 in the 40th cycle at 600 mA·g^-1, demonstrating the potential of Bi₂S₃ as electrode materials for rechargeable batteries.