A Composite of Carbon‐Wrapped Mo₂C Nanoparticle and Carbon Nanotube Formed Directly on Ni Foam as a High‐Performance Binder‐Free Cathode for Li‐O₂ Batteries

Abstract

8514 wileyonlinelibrary.com electrolyte, and an O 2 -penetrating cathode. Though the operation of the Li-O 2 battery is based on a simple reaction (2Li + + O 2 + 2e − → Li 2 O 2 , E 0 = 2.96 V vs Li/Li + ) consisting of oxygen reduction reactions (ORR) and oxygen evolution reactions (OER), the practical application of Li-O 2 batteries is constrained by their low coulombic efficiency, poor rate capability, and especially short cycle life. [2,3] The rational design of cathode catalysts with high ORR and OER activities is of particular importance for promoting the practical application of Li-O 2 batteries. Porous carbon materials are a desirable choice due to their advantageous properties such as a wide source of raw materials, good electronic conductivity, tuneable porous structures, and high specific capacity; as a result, they are a popular choice for ORR catalysts for Li-O 2 batteries. [2,4,5] However, their poor OER activity leads to high charging potentials of over 4.5 V. These charging potentials are high enough to cause decomposing of the electrolyte and the carbon electrode, thus deteriorating the performance of the Li-O 2 battery. [6] Noble metals such as Pt and Au, and metal oxides (particularly RuO 2 ) give new insight into the design of high performance Li-O 2 battery cathodes. [7][8][9][10][11][12] These cathodes usually exhibit much higher efficiency in the Li 2 O 2 decomposition than that of carbon materials. Consequently, low charging potentials are observed (less than 4.0 V) and high cycling stability (more than 100 stable cycles) is achieved. However, the expensive raw materials, poor specific capacity, and complicated preparation procedures limit the further application of these cathode catalysts. [13] Porous transition metal oxides, on the other hand, are characterized by their low cost and accessible porous structures. However, issues such as poor electrical conductivity, low OER activity, and limited specific capacity impact their general application in Li-O 2 batteries. [14][15][16][17][18][19][20] Recently, metal carbides, particularly Mo 2 C, have received considerable attention due to their multiple valence states, high electrochemical activity, low electrical resistivity, and affordable cost. [21][22][23][24][25] Inspiring results have been reported for Mo 2 C as a cathode in Li-O 2 batteries. [26,27] Mo 2 C nanoparticle catalysts are believed to promote the formation of well-dispersed Li 2 O 2 Cathode design is indispensable for building Li-O 2 batteries with long cycle life. A composite of carbon-wrapped Mo 2 C nanoparticles and carbon nanotubes is prepared on Ni foam by direct hydrolysis and carbonization of a gel composed of ammonium heptamolybdate tetrahydrate and hydroquinone resin. The Mo 2 C nanoparticles with well-controlled particle size act as a highly active oxygen reduction reactions/oxygen evolution reactions (ORR/ OER) catalyst. The carbon coating can prevent the aggregation of the Mo 2 C nanoparticles. The even distribution of Mo 2 C nanoparticles results in the homogenous...

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