Electrochemical characteristics of silicon films with phosphorus-doped C60 coating layer as anodes of lithium secondary batteries

Abstract

Fullerenes C60 were first synthesized by Kroto and Smalley in 1985 using the carbon arc method. The pristine C60 thin films are known as high resistive semiconductor materials and this may limit their application. In order to improve their electrical properties, C60 films can be changed to extrinsic semiconductor (n-type or p-type) materials by effective doping using various elements such as Na, P or B. In this work, phosphorus-doped C60 (P:C60) films were synthesized by a radio frequency (rf) plasma assisted thermal evaporation technique using C60 powder as a carbon precursor and phosphine (PH3) gas as a P source. P:C60 thin films were then used as coating materials for silicon film anodes of lithium secondary batteries. The presence of phosphorus doping was confirmed by Raman and XPS analysis. In comparison with the undoped C60 coating layer, electrochemical measurements show that the P:C60 film contributes to the more improved capacity retention after the 50th cycles when discharged at a high C rate. It is attributed to the better Li-ion transfer and smaller charge transfer resistance at the interface between anode and the electrolyte.