High-Rate and High-Areal-Capacity Air Cathodes with Enhanced Cycle Life Based on RuO₂/MnO₂ Bifunctional Electrocatalysts Supported on CNT for Pragmatic Li-O₂ Batteries

Despite their potential to provide high energy densities, lithium-oxygen (Li-O₂) batteries are not yet widely used in ultrahigh energy density devices like electric vehicles, owing to various challenges, including poor cyclability, low efficiency, and poor rate capability, especially at high areal mass loading. Even the most promising Li-O₂ cells are unsuitable for practical applications, owing to a limited areal mass loading below 1 mg cm^-2, resulting in low areal capacity. Here, we demonstrate air cathodes of unprecedentedly high areal capacity at a high rate with sufficient cycle life for pragmatic operation of Li-O₂ batteries. A separator-carbon nanotube (CNT) monolith-type cathode of massive loading is prepared to achieve high areal capacity, but the cycle life and round-trip efficiency of CNT-only separator monolith cathodes are limited. The reversible and energy-efficient operation at high areal capacity and a high rate is enabled by adopting RuO₂/MnO₂ solid catalysts on the CNT (RMCNT). RMCNTs show a bifunctional catalytic effect in both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) and also completely decompose LiOH and Li₂CO₃ byproducts that may exist in discharged electrodes. This separator-RMCNT monolith offers beneficial features such as high mass loading, binder-free, intimate contact with the separator, and most importantly, catalysts for reversibility. Together, these features provide a remarkably long cycle life at unprecedentedly high capacity and high rate: 315, 45, and 40 cycles, with areal capacity limits of 1.5, 3.0, and 4.5 mAh cm^-2, respectively, at a rate of 1.5 mA cm^-2. Cycling is possible even at the curtailing capacity of 10 mAh cm^-2.