Unraveling the Control Mechanism of Carbon Nanotubes on the Oxygen Reduction Reaction and Product Growth Behavior in Lithium–Air Batteries
Xiaoping Yi, Xunliang Liu, Ruifeng Dou, Zhi Wen, Wenning Zhou
Abstract
Despite the excellent specific energy of lithium-air batteries, their unclear discharge mechanism and product morphology remain great challenges for successfully replacing commercial lithium-ion batteries. We report, with periodic density functional theory calculations, the LixOy (x and y = 0, 1, and 2) reaction pathway and (Li2O2)n growth mechanism of lithium–air batteries on carbon nanotube (CNT) electrodes. The stable tube spacing (internal or external) of the optimized CNT cathode is theoretically confirmed to be 3.4 Å via noninterference between electrons. We demonstrate that upon interaction with CNTs, Li2O2 and Li2O molecules that are preferentially generated through lithiation reaction rather than disproportionation reaction are physically adsorbed on the surface of CNTs. Interestingly, the (Li2O2)n cluster with n > 1 formed by aggregation is unstable on the CNT surface, which proved to be beneficial to the formation of toroidal Li2O2. Compared with single-walled CNTs, double-walled CNTs exhibit stronger LixOy adsorption, larger (Li2O2)2 generation threshold potential, smaller (Li2O)2 generation threshold potential, clearer product morphology distribution, and higher (Li2O2)n@CNT electronic conductivity. The abovementioned innovative conclusions inspired us to pay attention to the influence of the CNT structure on the electrochemical properties. Therefore, the adjustment of the spacing and layer of CNTs inevitably plays a vital role in improving the cycle and rate performance of lithium–air batteries. Our results provide theoretical guidance for the design of advanced cathode catalysts in lithium–air batteries and other potential metal-O2 batteries.