Core–Shell‐Structured CNT@RuO₂ Composite as a High‐Performance Cathode Catalyst for Rechargeable Li–O₂ Batteries

Abstract

A RuO 2 shell was uniformly coated on the surface of core CNTs by a simple sol-gel method, and the resulting composite was used as a catalyst in a rechargeable Li-O 2 battery. This core-shell structure can effectively prevent direct contact between the CNT and the discharge product Li 2 O 2 , thus avoiding or reducing the formation of Li 2 CO 3 , which can induce large polarization and lead to charge failure. The battery showed a high round-trip efficiency (ca. 79 %), with discharge and charge overpotentials of 0.21 and 0.51 V, respectively, at a current of 100 mA g total À1 . The battery also exhibited excellent rate and cycling performance.Rechargeable lithium-oxygen (Li-O 2 ) batteries were first introduced by Abraham and Jiang in 1996.[1] These batteries have a very large theoretical gravimetric energy (3505 Wh kg À1 based on the reversible reaction of 2 Li + O 2 !Li 2 O 2 ) and have been attracting increasing attention in recent years.[2] Li-O 2 batteries can deliver five times the energy density observed for Li-ion batteries and are thus promising for electric-vehicle applications.[3] However, the development of rechargeable Li-O 2 batteries faces a lot of challenges, such as low round-trip efficiency, low rate capability, a poor cycle life, and electrolyte instability. [4] A typical rechargeable Li-O 2 battery is composed of a lithium-metal anode, an organic electrolyte, and a porous cathode exposed to O 2 during cell operation. On discharge, O 2 is reduced when it enters the porous cathode and combines with Li + to form solid Li 2 O 2 , which is decomposed during the charging process. Until now, carbon has been widely used as a cathode catalyst in rechargeable Li-O 2 batteries.[5] Its overpotential is as large as 1.5 V, which induces a low round-trip efficiency (53-64 % [6] ). Carbon has a discharge overpotential of approximately 0.3 V and a charge overpotential higher than 1 V; these values indicate that carbon exhibits sufficient catalytic activity for the oxygen-reduction reaction (ORR) but low catalytic activity for the oxygenevolution reaction (OER). [5,6] The OER catalytic activity for carbon-based catalysts is more crucial. Metal or metal-oxide nanoparticles have been added to reduce the charge overpotential. [7] In this way, the corresponding round-trip efficiency can be enhanced.McCloskey et al. [3a] recently found that a carbon electrode can react with Li 2 O 2 to form Li 2 CO 3 . Gallant et al. [8] confirmed the reaction between the carbon electrode and Li 2 O 2 . To avoid this reaction, carbon-free electrodes were suggested.[9] Peng et al. [10] introduced porous Au as a catalyst in rechargeable Li-O 2 batteries. They reported reversible formation/decomposition of the main discharge product Li 2 O 2 and excellent cycling performance. However, the use of carbon-free electrodes, such as metals and metal oxides, has some disadvantages. Noble metals are expensive, and other metals can be oxidized easily or have little catalytic activity. Most metal oxides suffer from low electronic c...

Long life rechargeable Li-O2 batteries enabled by enhanced charge transfer in nanocable-like Fe@N-doped carbon nanotube catalyst

“…Wang et al [36] developed a MnCo2O4-graphitic carbon hybrid material as the non-precious metal catalyst, which delivers a lower overpotential (0.8 V) and a cut-off capacity of 1000 mA h g −1 for 40 cycles. On this base, Jian et al [37] further reduced the overpotential for Li-O2 reactions to 0.72 V by using carbon nanotubes (CNTs) supported RuO2 shells as the cathode catalyst. Xu et al [38] also demonstrated that the rate capability and lifespan of Li-O2 batteries can be significantly improved by tailoring the deposition behavior and morphology of the discharge products on a Pd-loaded hollow carbon air cathode.…”

Enhanced electrocatalytic performance of Co3O4/Ketjen-black cathodes for Li–O2 batteries

“…However, the proposed replacement by Li-O 2 batteries still faces several technical challenges such as large voltage hysteresis, low round-trip efficiency and short cycle life [4][5][6]. Intensive research efforts towards better understanding of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) mechanisms, and the design principles of highly efficient ORR & OER catalysts are critical in improving battery performance [7][8][9][10]. Different from the intercalation mechanism in LIBs, Li-O 2 batteries are based on an electrocatalytic mechanism for both ORR and OER processes, where "electro" emphasizes the essentially smooth transportation for electrons, and "catalytic" indicates the necessity of catalysts with intrinsically high activity.…”

A Review of Carbon-Composited Materials as Air-Electrode Bifunctional Electrocatalysts for Metal–Air Batteries

“…For the scale-up production of next-generation bifunctional catalysts in commercial MABs, low costs, high bifunctional activities, and good stability are strongly desired. In terms of material cost and availability, noble metals are being replaced by low-cost materials such as transitional metals, oxide, nitrides, carbides, sulfides, β-FeOOH, Fe phthalocyanine, and conducting polymers to Table 5 Electrochemical performance for typical metal-free carbon-composited bifunctional catalysts as air-electrode materials in the tested MABs rLOB [183] composite with carbon to enhance bifunctional activities and stability. With the rapid development of nanotechnologies and related sciences, processing techniques are becoming more facile in the fabrication of non-noble metal-based and metalfree bifunctional catalysts in scale-up productions.…”