High‐Energy SWCNT Cathode for Aqueous Al‐Ion Battery Boosted by Multi‐Ion Intercalation Chemistry

Abstract The aqueous Al‐ion battery has achieved great progress in recent years. It now shows comparable performance to that of even non‐aqueous Al‐ion batteries. However, it also shows relatively low energy output and there is limited general understanding of the mechanism behind this restriction to its practical application. Thus, the development of a high‐performance cathode material is in great demand. Herein, a high‐capacity single‐walled carbon nanotube (SWCNT) is developed as a cathode for the water‐in‐salt electrolyte‐based aqueous Al‐ion battery, which provides an ultra‐high specific capacity of 790 mAh g^–1 (based on the mass of SWCNT) at a high current density of 5 A g^–1 even after 1000 cycles. Moreover, the SWCNT/Al battery shows a complicated multi‐ion intercalation mechanism, where AlCl₄^–, Cl^–, Al³⁺, and H⁺ can function at the same time, improving the battery output. Beyond recently revealed H⁺ and metal ion co‐intercalation, the Cl‐assisted intercalation of Al³⁺ ions mechanism is also studied by experimental characterization and modeling for the first time, which significantly boosts the Al³⁺ storage capacity. This multi‐ion intercalation mechanism combines the high‐voltage anion deintercalation and the high‐capacity cation intercalation, and thus, benefits the development and application of high‐energy Al‐ion batteries in the future.