Porous Silicon Nanoparticles Prepared By an Alkaline Process As an Anode for Use in Lithium Ion Batteries

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INTRODUCTION In future, lithium ion batteries (LIBs) for use in electric cars and other applications will be required to provide greater energy densities. At present, silicon (Si) is a candidate anode material for improved, commercially viable batteries, due to its high theoretical capacity (4200 mAh/g) compared to graphite (372 mAh/g). However, the use of Si-based anodes is still restricted because of two problems; the low electrical conductivity of Si, and the significant volume change during lithiation and delithiation. There has been much research devoted to mitigating the deterioration of Si during charging and discharging and to increasing the conductivity of various anode active materials. Recently, several approaches to the synthesis of porous Si-based anode materials that exhibit good cycling properties have been developed. Some of these techniques are based on wet-chemical etching using hydrofluoric acid (HF) and silver nitrate (AgNO3). Ag-deposited Si nanoparticles are initially fabricated using HF and AgNO3. Subsequently, these particles are immersed in a solution containing HF and hydrogen peroxide (H2O2), producing porous Si nanoparticles. However, since HF is quite hazardous, the fabrication of Ag-deposited Si particles without HF would be an improvement.1,2,3) Herein, we describe the preparation of porous Si nanoparticles intended for use as LIB anode materials via both an alkaline method and an alkaline and HF hybrid method. EXPERIMENTAL A planetary ball mill was used to produce Si nanoparticles. These were ultrasonically dispersed for 30 min in an immersion plating bath containing 2 g/L Si, 0.005 M AgNO3 and 0.010 M EDTA-2Na, adjusted to a pH of 9 to 12 with KOH. The reaction was carried out in a water bath at 30 °C for 1 min. In addition, Si nanoparticles were treated using an alkaline and HF hybrid technique.The porous silicon nanoparticles were collected by suction filtration and dried under vacuum at 80 °C for 2 h. The microstructures of the samples were observed using field emission scanning electron microscopy (FE-SEM). Electrochemical characterizations of samples were performed. The working electrodes were composed of the active material, carbon black and polyvinylidene fluoride (PVdF) as a binder, at a 70:15:15 mass ratio. The mixture was subsequently cast onto copper foil and dried under vacuum at 80 °C for 3 h. Coin-type CR 2032 cells were assembled in an argon-filled glove box using a separator and lithium foil as the counter electrode. The electrolyte was 1 M LiPF6 in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 v/v). The cut off potentials for the charge-discharge processes were 0.02-1.5 V (vs. Li/Li+) and cyclic voltammetry (CV) curves were acquired at room temperature. RESULTS AND DISCUSSION Porous Si nanoparticles were clearly not obtained when using the alkaline method but were fabricated when employing the hybrid approach. Figure 1 shows an SEM image of the nanoparticles resulting from the hybrid method. It is evident that these particles contain numerous nanosized pores. The discharge capacity of these porous Si nanoparticles was much better than that of untreated Si nanoparticles. It is believed that the pores work to mitigate the volume change of the silicon upon insertion. The charge/discharge characteristics of these materials will be detailed in our presentation. Figure 1