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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To lower the overpotential of a lithium-oxygen battery, electron transport at the solid-to-solid interface between the discharge product Li2O2 and the cathode catalyst is of great significance. Here we propose a strategy to enhance electron transport property of the cathode catalyst by the replace of oxygen atoms in the generally used metal oxide-based catalysts with nitrogen atoms to improve electron density at Fermi energy after nitridation. Hierarchically porous CoN nanorods were obtained by thermal treatment of Co3O4 nanorods under ammonia atmosphere at 350 °C. Compared with that of the pristine Co3O4 precursor before nitridation, the overpotential of the obtained CoN cathode was significantly decreased. Moreover, specific capacity and cycling stability of the CoN nanorods were enhanced. It is assumed that the discharged products with different morphologies for Co3O4 and CoN cathodes might be closely associated with the variation in the electronic density induced by occupancy of nitrogen atoms into interstitial sites of metal lattice after nitridation. The nitridation strategy for improved electron density proposed in this work is proved to be a simple but efficient way to improve the electrochemical performance of metal oxide based cathodes for lithium-oxygen batteries.

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Developing efficient sodium-ion-storage mechanisms to increase the energy capacity in organic electrodes is a critical issue even after this period of prolonged effort. Uric acid (UA), a simple organic compound with three carbonyl groups is demonstrated to be electrochemically active in the insertion/extraction of Na ions. Theoretical calculations and experimental characterizations reveal that the sodium-ion storage by UA is a result of the stepwise mechanisms of p-π conjugation and the carbon anion. Aside from C═O, the functional group C═C(NH-) also provides an efficient Na-storage activated site in which the lone-pair electrons is stabilized through the planar-to-tetrahedral structural transition and low-energy orbital hybridization of N atoms. For further improvement of the electrochemical performance, a uric acid and carbon nanotube (UA@CNT) composite is prepared via a vacuum solution impregnation method. When employed as an anode material for sodium-ion batteries, the UA@CNT composite exhibits high specific capacity, excellent rate capability, and long cycling life even at high current densities. A reversible capacity of over 163 mA h g is maintained even after 150 cycles at a current density of 200 mA g. The present study paves a way to develop reversible high-capacity organic electrode materials for sodium-ion batteries by a carbon-anion stabilization mechanism.