Developing single-site catalysts featuring maximum atom utilization efficiency is urgently desired to improve oxidation-reduction efficiency and cycling capability of lithium-oxygen batteries. Here, we report a green method to synthesize isolated cobalt atoms embedded ultrathin nitrogen-rich carbon as a dual-catalyst for lithium-oxygen batteries. The achieved electrode with maximized exposed atomic active sites is beneficial for tailoring formation/decomposition mechanisms of uniformly distributed nano-sized lithium peroxide during oxygen reduction/evolution reactions due to abundant cobalt-nitrogen coordinate catalytic sites, thus demonstrating greatly enhanced redox kinetics and efficiently ameliorated over-potentials. Critically, theoretical simulations disclose that rich cobalt-nitrogen moieties as the driving force centers can drastically enhance the intrinsic affinity of intermediate species and thus fundamentally tune the evolution mechanism of the size and distribution of final lithium peroxide. In the lithium-oxygen battery, the electrode affords remarkably decreased charge/discharge polarization (0.40 V) and long-term cyclability (260 cycles at 400 mA g−1).
is restricted to their inferior energy densities (≈250 Wh kg −1) due to intercalation chemistry. Consequently, "beyond Li-ion batteries" such as lithium-sulfur batteries and metal-air batteries, which have fundamental discrepant energy storage mechanism, have been attracted worldwide attention recently. [4,5] Of all the candidates, rechargeable lithium-oxygen battery (LOB) is considered to be one of the most fascinating next-generation batteries with extremely high theoretical energy density (≈3500 Wh kg −1). [6,7] The low-cost and environmental positive active material oxygen originates from air, endowing LOBs more attractive to satisfy the soaring demand of large-scale energy storage system. Generally, there are four architecture systems of LOBs determined by different electrolyte: aqueous, nonaqueous, hybrid, and all-solid-state. [8-11] In 1996, Abraham and Jiang first reported the rechargeable lithium-oxygen battery consisted of Li metal anode, solid polymer electrolyte membrane, and composite carbon cathode. [12] After ten years, nonaqueous LOBs actually caught worldwide attention due to the outstanding capacity and reversibility. [6] A typical nonaqueous lithium-oxygen battery comprises a metallic Li anode, organic electrolyte containing Li salt, a separator, and a porous gas diffusion cathode loading with catalysts (Figure 1). [13] Generally, the electrochemical reactions of nonaqueous lithium-oxygen battery can be described as following equations [14-16] Overall reaction E 2Li O Li O 2.96 V vs Li/Li 2 2 2 () + ↔°= + (1) Cathodic process (oxygen reduction reaction (ORR))
The commercial application of lithium−oxygen (Li−O 2 ) batteries has been seriously hindered by their large overpotential and inferior cycling performance caused by the insoluble and insulated traits of the discharge product, Li 2 O 2 . Herein, hierarchical hollow NiCo-LDH/MnO 2 hybrid nanostructures derived from metal−organic frameworks (MOFs) are successfully constructed as cathodes for Li−O 2 batteries to fundamentally improve the decomposition kinetics of Li 2 O 2 . The hollow NiCo-LDH/MnO 2 nanostructures assembled by hollow NiCo-LDH and MnO 2 nanosheets possess a highly special surface area, abundant open active sites, and a fast diffusion path for Li + and oxygen species. As expected, accelerated sluggish oxygen reduction reaction/oxygen evolution reaction kinetics and reduced charge/discharge overpotentials can be obtained. The toroidal Li 2 O 2 assembled by nanoflakes formed on the surface of the cathode can be conducive to form a low-impedance Li 2 O 2 /cathode contact interface to achieve the reversible formation and decomposition of Li 2 O 2 . The Li−O 2 battery based on the NiCo-LDH/MnO 2 cathode shows a high charge/discharge specific capacity of 13,380 mA h g −1 at 100 mA g −1 and a continuous cycling stability for 162 cycles at a fixed capacity of 500 mAh g −1 as well as a low overpotential of 0.63 V. Moreover, the application of MOFderived porous hierarchical nanostructures expands the selection range of electrocatalysts and offers a new idea of structure design for Li−O 2 batteries.
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