Improved rate capability of lithium-ion batteries with Ag nanoparticles deposited onto silicon/carbon composite microspheres as an anode material
Highlights
► Si/C composite microspheres were prepared by solution polymerization. ► Ag nanoparticles are evenly decorated onto the surface of Si/C microspheres. ► Ag/Si/C composite electrode shows higher capacity and improved rate capability. ► Ag nanoparticles decrease contact resistance between particles and electrolyte.
Introduction
Graphite has been used as an anode material for lithium-ion batteries (LIBs) for decades due to its excellent behavior during continuous charge/discharge cycles [1], [2], [3]. However, since its low theoretical capacity (372 mAh g− 1) is insufficient for high energy applications, alternative anode materials such as silicon (Si), tin (Sn), aluminum (Al), and antimony (Sb) have all been investigated for their potential to improve the energy density of LIBs [4], [5], [6]. Among these materials, silicon has attracted much attention as a promising anode material through the formation Li4.4Si alloys due to its much higher theoretical specific capacity (4200 mAh g− 1) than graphite [7], [8]. However, lithium/silicon alloys experience huge changes in volume, which results in poor cyclability because of the strong physical stresses on the crystallites [9]. To resolve this problem, many groups have investigated silicon-based composite anodes consisting of active silicon and amorphous carbon phases, such as amorphous carbon-coated Si composites [10], [11], carbon coated Si-graphite composite anodes [12], Si/disordered carbon composite anodes [13], [14], [15], and core-shell Si/carbon nanocomposites [16]. Our research group synthesized multi-core/shell structured Si/C microspheres for use with high performance anodes, in which nanosized silicon particles are embedded in amorphous carbon microspheres. The amorphous carbon phase prevents changes in the volume of the silicon nanoparticles during the lithiation and de-lithiation processes, thus improving cyclability [17]. Although the Si/carbon composite anodes showed better cycling performance than pure Si anodes, these composite anodes have the drawback of severe capacity fading at high current densities.
In this study, Ag-deposited Si/poly(AN-co-TMSPM) multi-core/shell microspheres were synthesized by suspension polymerization and a wet chemical method using aqueous silver nitrate (AgNO3) and hydrazine as a reducing agent. Decorated Ag nanoparticles remained on the surface of the Si/C composite microspheres even after heat treatment. The composite microspheres were used as anodes in a lithium-ion battery, which exhibited better rate capability compared to the Si/C composite microspheres. The deposited Ag nanoparticles on their surface reduce the charge-transfer resistance during the charging and discharging processes.
Section snippets
Materials
Hydrazine, monohydrate (NH2NH2·H2O, JUNSEI, Chuo-ku, Tokyo, Japan), silver nitrate (99.999%, Aldrich Chemical Co., Milwaukee, WI, United States), acrylonitrile (AN, JUNSEI, Chuo-ku, Tokyo, Japan), 3-(trimethoxysilyl)propyl methacrylate (TMSPM, 98%, Aldrich Chemical Co., Milwaukee, WI, United States), 2,2′-azobis(2,4-dimethylvaleronitrile) (ADVN, Wako), divinylbenzene (DVB, isomer mixture; 55%, Aldrich Chemical Co., Milwaukee, WI, United States, Wako Chemical, Dalton, GA, United States),
Characterization of the Si/C and Ag/Si/C composite microspheres
Fig. 1 shows OM images of samples at each step in the synthesis process. Spherical-shaped droplets were formed from as-prepared mixture consisting Si nanoparticles, Si/poly (AN-co-TMSPM), and MC, as shown in Fig. 1a. After stirring for 12 h at room temperature, black-colored droplets could be observed after evaporation of MC, as shown in Fig. 1b. Fig. 1c shows Ag-deposited Si/poly(AN-co-TMSPM) composite microspheres after decoration with Ag nanoparticles by chemical method. These Ag-deposited
Conclusion
Surface decoration of Si/C composite microspheres with Ag nanoparticles was performed by a wet chemical method using as-synthesized Si/poly(AN-co-TMSPM) composite microspheres. After heat treatment, the electrochemical performances of the microspheres as anode materials in lithium-ion batteries were evaluated by galvanostatic charge–discharge measurements and electrochemical impedance spectroscopy. The Ag-deposited Si/C composite electrode decreased the charge-transfer resistance after SEI
Acknowledgments
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2012-0002074).
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