N-doped-carbon coated Ni₂P-Ni sheets anchored on graphene with superior energy storage behavior

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.