High-Rate and High-Areal-Capacity of Pragmatic Li-O2 Batteries Based on High-Loaded Air Cathodes with Bi-Functional Electrocatalysts

Abstract

Despite their potential to provide the highest theoretical energy densities, lithium-oxygen (Li-O2) batteries are not yet widely used in ultrahigh energy density devices like electric vehicles, owing to specific challenges, including low cycle life, low efficiency, and poor rate capability, especially at high areal mass loading. Even the most promising Li-O2 cells are unsuitable for practical applications, due to a common areal mass loading below 1 mg cm−2, resulting in low areal capacity. Here, we demonstrate air cathodes of unprecedentedly high areal capacity even 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; however, 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 at a high rate is enabled by adopting RuO2/MnO2 solid catalysts on the CNT (RMCNT). RMCNTs show bifunctional catalytic activities in both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) and also completely decompose LiOH and Li2CO3 by-products that may exist in discharged electrodes. This separator-RMCNT monolith offers benefits such as high mass loading, binder-free, intimate contact with the separator, and most importantly, catalysts for reversibility. Together, these features provide 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.