Activated carbon aerogel-based electrodes for Li–O<sub>2</sub> batteries: The impact of MnO<sub>2</sub> crystallinity on cycling performance

This study examines the effect of MnO2 crystallinity on the performance of lithium–oxygen batteries (LOBs). Low- and high-crystallinity MnO2 (LC-MnO2 and HC-MnO2) were synthesized and used to fabricate positive electrodes composed of activated carbon aerogels (ACA). In this configuration, the ACA provides a high-surface-area, electronically conductive support that serves as a substrate for Li2O2 deposition and ensures efficient electron transport to the MnO2 catalyst. Structural analyses showed that LC-MnO2 exhibited broader X-ray diffraction peaks and a larger surface area than HC-MnO2, indicating lower crystallinity and a higher density of oxygen vacancies. Electrochemical measurements revealed that the LC-MnO2/ACA electrode delivered lower charging voltages and markedly improved cycling stability, sustaining approximately 70 more cycles than the HC-MnO2/ACA electrode at a fixed capacity of 300 mAh g−1. These enhancements are attributed to the increased surface area and oxygen vacancies, which facilitate Li+ insertion and promote Li2O2 formation on the catalyst surface. Together with the high-surface-area, electronically conductive ACA support, the findings highlight the crucial role of crystallinity in both discharge and charge reactions in LOBs, demonstrating that low-crystallinity MnO2 is a promising catalyst for improving overall battery performance.