Graphene Oxide Gel‐Derived,Free‐Standing,Hierarchically Porous Carbon for High‐Capacity and High‐Rate Rechargeable Li‐O2 Batteries

Lithium‐oxygen (Li‐O₂) batteries are one of the most promising candidates for high‐energy‐density storage systems. However, the low utilization of porous carbon and the inefficient transport of reactants in the cathode limit terribly the practical capacity and, in particular, the rate capability of state‐of‐the‐art Li‐O₂ batteries. Here, free‐standing, hierarchically porous carbon (FHPC) derived from graphene oxide (GO) gel in nickel foam without any additional binder is synthesized by a facile and effective in situ sol‐gel method, wherein the GO not only acts as a special carbon source, but also provides the framework of a 3D gel; more importantly, the proper acidity via its intrinsic COOH groups guarantees the formation of the whole structure. Interestingly, when employed as a cathode for Li‐O₂ batteries, the capacity reaches 11 060 mA h g^?1 at a current density of 0.2 mA cm^?2 (280 mA g^?1); and, unexpectedly, a high capacity of 2020 mA h g^?1 can be obtained even the current density increases ten times, up to 2 mA cm^?2 (2.8 A g^?1), which is the best rate performance for Li‐O₂ batteries reported to date. This excellent performance is attributed to the synergistic effect of the loose packing of the carbon, the hierarchical porous structure, and the high electronic conductivity of the Ni foam.     

Controllable Synthesis of a Monophase Nickel Phosphide/Carbon (Ni5P4/C) Composite Electrode via Wet‐Chemistry and a Solid‐State Reaction for the Anode in Lithium Secondary Batteries

A monophase nickel phosphide/carbon (Ni₅P₄/C) composite with a thin carbon shell is controllably synthesized via the two‐step strategy of a wet‐chemistry reaction and a solid‐state reaction. In this fabrication, the further diffusion of phosphorus atoms in the carbon shell during the solid‐state reaction can be responsible for a chemical transformation from a binary phase of Ni₅P₄‐Ni₂P to monophase Ni₅P₄. Galvanostatic charge‐discharge measurements indicate that the Ni₅P₄/C composite exhibits a superior, high rate capacibility and good cycling stability. About 76.6% of the second capacity (644.1 mA h g^?1) can be retained after 50 cycles at a 0.1 C rate. At a high rate of 3 C, the specific capacity of Ni₅P₄/C is still as high as 357.1 mA h g^?1. Importantly, the amorphous carbon shell can enhance the conductivity of the composite and suppress the aggregation of the active particles, leading to their structure stability and reversibility during cycling. As is confirmed from X‐ray‐diffraction analysis, no evident microstructural changes occur upon cycling. These results reveal that highly crystalline Ni₅P₄/C is one of the most promising anode materials for lithium‐ion batteries.