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The fibrous network of CNTs is also beneficial for retaining the conductive network in the electrode during the deposition/decomposition of the discharge solid. In contrast, CNTs provide paper-like flexible sheets as a result of their tubular constitution that are ready for use as air electrodes in LABs 17, 18, 19, 20, 21, 22. Powdery carbon materials require rigid solidification to be handled as air electrodes, but such firm solid electrodes are often not tolerant of the mechanical stress triggered by the discharge solid deposition, especially in deeply discharged conditions. In fact, enhancing the cell capacity per electrode area (mAh cm −2) is essential to developing a practical cell with an extremely large energy density.Īmong the proposed nanocarbons, CNTs have a unique character stemming from their nanofibrous structure.
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Although this areal capacity is 5 times larger than the capacity of the current LiB cells of ~2 mAh cm −2, an even larger areal capacity is needed to assemble practical LAB cells with the expected energy density. However, their lean carbon loadings mostly below 1 mg cm −2 limit their resulting cell capacities per electrode area to no more than 10 mAh cm −2.
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Some of these nanocarbons exhibit very high specific capacities of ~10,000 mAh g carbon −1 owing to their huge surface areas of ~1,000 m 2 g −1. Several nanocarbons, such as carbon black (such as Super-P ® or Ketjenblack ®) 6, 7, 8, graphene 9, 10, 11, 12, carbon aerogels 13, carbon fibers 14, 15, 16, or carbon nanotubes (CNTs) 17, 18, 19, 20, 21, 22, have been proposed as air electrode materials. The cell capacities of actual LAB cells are dominated by the oxygen-breathing cathode (air electrode), in which the discharge product Li 2O 2 is deposited/decomposed along with the discharge/charge through the fundamental electrochemical reaction of 2Li + + O 2 + 2e − ↔ Li 2O 2. on a rechargeable LAB cell in 1996 5, researchers have focused on assembling practically available LAB cells while concurrently working to fundamentally understand the battery reaction. Since the pioneering work by Abraham et al. By leveraging this high energy density nature, LABs are expected to be used to develop a cell with a larger capacity than ever before. Lithium-air batteries (LABs), which deliver electric energy from the aerial oxidation of lithium metal, have large theoretical energy densities of up to 3,500 Wh kg −1 (based on the weight of lithium peroxide (Li 2O 2) as a discharge product), which is more than 10 times that of current LiBs and is the largest among those developing batteries 1, 2, 3, 4. This behavior results from the CNT sheet characteristics of the flexible and fibrous conductive network and suggests that the CNT sheet is an effective air electrode material for developing a commercially available LAB cell with an ultra-high cell capacity.ĭue to the increasing demand for vast energy storage media, battery systems far exceeding the current lithium-ion battery (LiB) technologies, such as lithium-sulfur, multivalent ions, or metal-air batteries, have been under development. During discharge, the CNT sheet electrode experienced enormous swelling to a thickness of a few millimeters because of the discharge product deposition of lithium peroxide (Li 2O 2), but the sheet was fully recovered after being fully charged. Here, we demonstrate the use of flexible carbon nanotube (CNT) sheets as a promising air electrode for developing ultra-high capacity in LAB cells, achieving areal cell capacities of up to 30 mAh cm −2, which is approximately 15 times higher than the capacity of cells with lithium-ion battery (LiB) technology (~2 mAh cm −2). Lithium-air batteries (LABs) are expected to provide a cell with a much higher capacity than ever attained before, but their prototype cells present a limited areal cell capacity of no more than 10 mAh cm −2, mainly due to the limitation of their air electrodes.