Superstrong Ultralong Carbon Nanotubes for Mechanical Energy Storage

Energy storage in a proper form is essential for a good grid strategy. The systems developed so far mostly use batteries or capacitors in which energy is stored electrochemically or electrostatically. Mechanical energy storage is also one of the most important ways for energy conversion. In fact, water reservoirs on high mountains store mechanical energy using the gravitational potential on the earth, and the surplus energy can be mechanically stored in water pumped to a higher elevation using pumped storage methods. Other systems for mechanical energy storage were realized, such as flstoring mechanical energies by the use of a rapidly rotating mass and steel springs storing mechanical energies by their elasticity. However, such mechanical energy storage usually is operated on a macroscopic scale, and the energy density is not very high. With the fast development of nano- and micro-electromechanical systems (N/MEMS) and actuators, nanoscale mechanical energy storage is highly required. Developing a robust nonmaterial with good mechanical performance and stable supply is the fi rst step. Ultralong carbon nanotubes (CNTs) with the properties of 1‐2 TPa modulus and 100‐200 GPa strength, [ 1‐4 ] the strongest material ever known, have shown promising potential for the storage of mechanical energy, either by their deformation in the composite materials, [ 5‐7 ] or by their elastic deformation produced by stretching or compressing the pristine tubes or tube arrays. [ 8 ] Theoretical calculation suggested that the energy storage capacity, in the latter case, can be at least three orders higher than that of steel spring and several times that of the fl ywheels and lithium ion batteries. [ 9 , 10 ] The mechanical energy storage capacity of CNTs depends on their mechanical properties, while which directly depend on their molecular structures. Besides, CNTs that simultaneously have theoretically high strength (100‐200 GPa), high tensile modulus (1‐2 TPa) and high breaking strain ( > 15%) are not yet experimentally available on the macroscale. [ 2 , 11‐20 ] This is mainly due to the existence of defects in the fabricated CNTs. Even for CNTs with little defects, the highest reported breaking strain is 13.7% ± 0.3%, [ 21 ] which is still lower than the theoretical value. [ 22 , 23 ] Here we experimentally demonstrate that the as-grown defect-free CNTs with length over 10 cm, have breaking strain up to 17.5%, tensile strength up to 200 GPa and Young’s modulus up to 1.34 TPa. They could endure a continuously repeated mechanical strain-release test for over 1.8 × 10 8 times. The extraordinary mechanical performance qualifi es them with high capacity for the storage of mechanical energy. The CNTs can store mechanical energy with a density as high as 1125 Wh kg − 1 and a power density as high as 144 MW kg − 1 , indicating the CNTs can be a promising medium for the storage of mechanical energy.