Effects of the Thickness of Silicon Thin Films on the Charge and Discharge Performance As Negative Electrodes for Lithium-Ion Batteries
Silicon is an attractive high-capacity negative-electrode material for lithium-ion batteries because it can accommodate 3.75 lithium atoms per 1 silicon atom at room temperature, which corresponds to a theoretical capacity of about 4200 mAh/g . Silicon is also a light, abundant and cheap material. It represents the second most abundant element in the earth’s crust after oxygen. However, silicon have a problem; large volume expansion occurs when it reacts with lithium to form alloys. This leads to the degradation of the electrode with a loss of electrical contacts among silicon, conductive additives and current collector of copper foil, and as a result electrically isolated silicon particles should arise. This facts explain the low Coulombic efficiency and the decrease in reversible capacities for silicon electrodes. In addition, electrolyte solution is reductively decomposed at negative potentials where silicon is alloyed with lithium, which is followed by the formation of surface films on the surface of the silicon. Thus, the charge/discharge performance is influenced profoundly by the volume changes of silicon and the formation of surface films. The impacts of the former issue can be alleviated by use of silicon with short diffusion paths for lithium. Hence, in this study, we investigated the influence of the thickness of silicon on the charge/discharge performance using silicon thin-film electrodes with three different thicknesses (50, 100, 200 nm).
We used silicon thin-film electrodes deposited on Cu substrates by an electron beam evaporation method. The surface morphology was investigated by atomic force microscopy (AFM). AFM images were obtained in conventional contact-mode with pyramidal silicon nitride tips (spring constant; 0.02 N m-1, Olympus). Silicon thin-film electrodes were used as working electrodes. Electrochemical cells were assembled by stacking the silicon thin film electrode, a polymer separator soaked with electrolyte solution, and a lithium metal (Honjo Metal) for counter electrodes. The electrolytes used in this study were 1 mol dm-3 LiPF6 dissolved in a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) with an EC/EMC volume ratio of 1:1, and a mixture of fluoroethylene carbonate (FEC) and EMC with an FEC/EMC ratio of 1:1. The electrochemical cells were cycled between 0.02 V and 1.5 V at C/2 rate.
Results and Discussion
XRD patterns (not shown) of three kinds of silicon thin-films indicate no peak identified as silicon. These results suggest that the silicon thin film should be amorphous.
Figure 1 shows an AFM image (10 mm×10 mm) of the surface of as-deposited silicon thin-film. The roughness of the film was evaluated to be 2.5 nm, whereas an obvious bump is seen as indicated by white color.
Figure 2 shows charge and discharge curves of a silicon thin-film electrode. The thickness of silicon thin film is 100 nm. In the 1st cycle, a large irreversible capacity due to the formation of surface films was observed. In subsequent cycle, charge (alloying)/discharge (dealloying) reactions proceeded more reversibly.
Figure 3 shows cycleability of silicon thin-film electrodes. The discharge capacities for silicon thin-film electrodes with a thickness of 100 nm decreased only by 15 % after 50 cycles. This value is much smaller than those obtained for 50-(34 %) and 200-(32 %) nm-thick. These results clearly indicate that the high cycleability is achieved with use of silicon with a thickness of about 100 nm.
We will report the changes in the surface morphology of silicon thin-film electrodes studied by in situ AFM in our poster presentation.