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P-Doped Fullerene Films as Coating Materials for Silicon Film Anodes of Lithium Secondary Batteries

Silicon has attracted much attention for replacing the graphite as use as anode materials in lithium secondary batteries [1,2]. However, silicon suffers a 300% volume expansion during the insertion of Li-ions that will lead into the mechanical disintegration of the electrodes. As a result, the cycle performances of silicon film anodes are very poor due to the loss of electrical contact with the current collector, side reactions with the electrolyte and high cell resistance [3-4]. Our previous works concern with the un-doped fullerene C60 coatings on the Si film anodes and some improved electrochemical properties have been reported [5-6]. In order to further improve the electrochemical characteristics of silicon film anode in lithium secondary batteries, the effect of a phosphorus doped fullerene (P:C60) film as coating materials were studied in detail. The phosphorus doping of fullerene C60 films were expected to contribute the better properties of lithium ion transfer at the electrode/electrolyte interface. In this work, the P:C60 films were synthesized by plasma assisted evaporation technique using fullerene powder and a gas mixture of argon and phosphine (PH3). The surface morphology, structural, and electrochemical properties of P:C60 coated silicon film anodes were studied using scanning electron microscope (SEM), Fourier transform infra red (FTIR) spectroscopy, Raman spectroscopy and charge-disharge tests. FTIR and Raman analyses showed that the phosphorus atoms were successfully inserted into the fullerene film lattices. Electrochemical measurements revealed that the higher coulombic efficiency and lower irreversible capacity on the first cycle were observed for the P:C60 coated silicon electrodes in comparison with the bare silicon case. In addition, the stable cyclic property and high capacity retention after the 50 cycle were shown. All above mentioned results are attributed to the presence of P:C60 film coating as passive layer against the side reaction with electrolyte during cycle tests such that the more stable solid electrolyte interphase (SEI) layer has been established on the silicon film surface.