Transition metal phosphides (TMPs) have been widely studied as electrode materials for supercapacitors and lithium-ion batteries due to their high electrochemical reaction activities. The practical application of TMPs was generally hampered by their low conductivity and large volume changes during electrochemical reactions. In this work, nitrogen-doped-carbon (NC) coated Ni₂P-Ni hybrid sheets were fabricated and loaded into highly conductive graphene network, forming a Ni₂P-Ni@NC@G composite. The highly conductive graphene, the NC coating layer, and the decorated Ni nanoparticles in combination offer continuous electron transport channels in the composite, resulting with facilitated electrode reaction kinetics and superior rate performance. Besides, the flexible graphene sheets and well-decorated Ni particles among Ni₂P can effectively buffer the harmful stress during electrochemical reactions to maintain an integrated electrode structure. With these favorable features, the composite demonstrated superior capacitive and lithium storage behavior. As an electrode material for supercapacitors, the composite shows a remarkable capacitance of 2, 335.5 F·g^-1 at 1 A·g^-1 and high capacitance retention of 86.4% after 2, 000 cycles. Asymmetrical supercapacitors (ASCs) were also prepared with remarkable energy density of 53.125 Whk·g^-1 and power density of 3, 750 Whk·g^-1. As an anode for lithium ion batteries, a high reversible capacity of 1, 410 mAh·g^-1 can be delivered at 0.2 A·g^-1 after 200 cycles. Promising high rate capability was also demonstrated with a high discharge capacity of 750 mAh·g^-1 at 8 A·g^-1. This work shall pave the way for the production of other TMP materials for energy storage systems.

Red phosphorus-carbon nanotube (P@CNT) composites were synthesized as anodes for advanced lithium ion batteries via a facile solution-based method at room temperature. In these composites, the entangled P@CNT nanostructure reduced the aggregation of both components and allowed their complete utilization in a synergetic manner. The highly conductive and porous CNT framework, along with the nanoscale red P particles intimately anchored on the CNT surface, conferred the composite with excellent ion/electron transport properties. Volume expansion within the red P particles was mitigated by their amorphous and nanoscale features, which can be well buffered by the soft CNTs, therefore maintaining an integrated electrode structure during cycling. When used as an anode in lithium ion batteries, the composite exhibited a reversible capacity of 960 mAh·g^-1 (based on the overall weight of the composite) after 120 cycles at 200 mA·g^-1. The composite also delivered excellent high-rate capability with capacities of 886, 847, and 784 mAh·g^-1 at current densities of 2, 000, 4, 000, and 10, 000 mA·g^-1, respectively, which reveals its potential as a high performance anode for lithium ion batteries.

An asymmetrical supercapacitor (ASC), comprising reduced graphene oxide (rGO)-encapsulated nickel phosphite hollow microspheres (NPOH-0.5@rGO) as positive electrode, and porous nitrogen/sulfur co-doped rGO aerogel (NS-3D rGO) as negative electrode has been prepared. The NPOH-0.5@rGO electrode combines the advantages of the NPOH hollow microspheres and the conductive rGO layers giving rise to a large specific capacitance, high cycling reversibility, and excellent rate performance. The NS-3D rGO electrode with abundant porosity and active sites promotes electrolyte infiltration and broadens the working voltage range. The ASC (NPOH-0.5@rGO//NS-3D rGO) shows a maximum voltage of up to 1.4 V, outstanding cycling ability (capacitance retention of 95.5% after 10, 000 cycles), and excellent rate capability (capacitance retention of 77% as the current density is increased ten times). The ASC can light up an light-emitting diodes (LED) for more than 20 min after charging for 20 s. The fabrication technique and device architecture can be extended to other active oxide and carbon-based materials for next-generation high-performance electrochemical storage devices.