Twisted carbon nanotubes store more energy than lithium-ion batteries

> By making single-walled carbon nanotubes (SWCNTs) into ropes and twisting them like the string on an overworked yo-yo, Katsumi Kaneko, Sanjeev Kumar Ujjain and colleagues showed that they can store twice as much energy per unit mass as the best commercial lithium-ion batteries.

> SWCNTs are made from sheets of pure carbon just one atom thick that have been rolled into a straw-like tube. They are impressively tough – five times stiffer and 100 times stronger than steel – and earlier theoretical studies by team member David Tománek and others suggested that twisting them could be a viable means of storing large amounts of energy in a compact, lightweight system.

> [...] The maximum gravimetric energy density (that is, the energy available per unit mass) they measured was 2.1 MJ/kg (583 Wh/kg). While this is lower than the most advanced lithium-ion batteries, which last year hit a record of 700 Wh/kg, it is much higher than commercial versions, which top out at around 280 Wh/kg. The SWCNT ropes also maintained their performance over at least 450 twist-release cycles

"Giant nanomechanical energy storage capacity in twisted single-walled carbon nanotube ropes" (2024) https://www.nature.com/articles/s41565-024-01645-x :

> Abstract: A sustainable society requires high-energy storage devices characterized by lightness, compactness, a long life and superior safety, surpassing current battery and supercapacitor technologies. Single-walled carbon nanotubes (SWCNTs), which typically exhibit great toughness, have emerged as promising candidates for innovative energy storage solutions. Here we produced SWCNT ropes wrapped in thermoplastic polyurethane elastomers, and demonstrated experimentally that a twisted rope composed of these SWCNTs possesses the remarkable ability to reversibly store nanomechanical energy. Notably, the gravimetric energy density of these twisted ropes reaches up to 2.1 MJ kg−1, exceeding the energy storage capacity of mechanical steel springs by over four orders of magnitude and surpassing advanced lithium-ion batteries by a factor of three. In contrast to chemical and electrochemical energy carriers, the nanomechanical energy stored in a twisted SWCNT rope is safe even in hostile environments. This energy does not deplete over time and is accessible at temperatures ranging from −60 to +100 °C.