Joong Kee Lee

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.

Effects of Doping and C60 Coating on the Electrochemical Characteristics of Silicon Film Anodes for Lithium Secondary Batteries

Effects of B or P doping and Fullerene C60 film coating on the anodic characteristics of Si film were studied by some electrochemical measurements such as charge-discharge tests, cyclic voltammetry (CV) and electrochemical impedance spectroscopy. The structural and morphology of deposited films were also investigated by scanning electron microscope (SEM), energy dispersive x ray spectroscopy (EDS) and x ray photo electron spectroscopy (XPS). Experimental results showed that the P doped Si (n-type) anodes coated by fullerene film coating exhibit a more enhanced cycle performance and smaller charge transfer resistance compared with those of intrinsic and B doped Si coated by fullerene film. The C60 coated n-type Si electrodes demonstrated a high discharge capacity of 2500 mAhg after 70 cycle under a constant current density of 100 Acm. The improved electrochemical characteristics were attributed to the improvements of electronic conductivity of Si film by P doping and the suppression of side reactions with the electrolyte by the fullerene film coating. In addition, the diffusion coefficients of Li-ions in the Si film anodes can be improved after doping and surface modification with C60 film coating layer. Abstract #1359, 220th ECS Meeting,

Interfacial Properties of Fullerene Coated Silicon Film as an Anode Material for Lithium Secondary Batteries: Effect of Coating Layer Thickness

Silicon has been known as alternative anode materials for lithium ion batteries since it possesses large theoretical capacity of 4200 mAhg. One main drawback preventing the silicon from the commercial application is the rapid capacity fading due to the huge volume changes during the charge-discharge reaction and the unstable interfacial properties [1-2]. In the previous work, the surface modification of silicon thin film anodes with approximated thickness of 300 nm was proposed by using fullerene films as a coating material [3-4]. It was found that in the case of silicon thin film, its electrochemical properties were significantly improved by fullerene coating. In the present work, fullerene film was deposited onto the surface of silicon thin film anodes using plasma assisted thermal evaporation technique by varying the thickness of fullerene coating layer (50-200 nm). Electrochemical properties were done using half cells with the fullerene coated silicon thick film as a working electrode and lithium metal as a counter electrode and 1 M LIPF6 dissolved in EC-PC-DMC (1:1:1 v/v) as the electrolyte. The half cell properties were investigated by charge-discharge tests, cyclic voltammetry (CV) measurements with scan rate of 5 mVs and electrochemical impedance spectroscopy (EIS) using Zahner IM 6 with amplitude of 5 mV and frequency range between 0.01 Hz and 10 Hz. Recent results regarding the interfacial properties of the electrodes coated with fullerene film of different thicknesses will be studied specifically using cyclic voltammetry (CV) and impedance spectroscopy. The results will be compared with those of uncoated electrodes.