Synthesis of Si-Sb-ZnO Composites as High-Performance Anodes for Lithium-ion Batteries
© Li et al. 2015
Received: 11 August 2015
Accepted: 16 October 2015
Published: 23 October 2015
The Si-Sb-ZnO composites were prepared by a chemical reduction-mechanical alloying method and were employed as anode materials for lithium-ion batteries. The electrochemical performance of the Si-Sb alloy was significantly improved by the addition of ZnO nanoparticles. Especially, the initial specific charge and discharge capacities for Si-Sb-(ZnO)0.3 composite were 845.1 and 1301.5 mAh/g, respectively, while the initial coulombic efficiency was 64.9 %. The capacity remained at 690 mAh/g after 200 cycles, and the capacity retention ratio was 81.6 %, which demonstrated excellent cycling stability and rate capability of the composite materials.
Lithium-ion batteries are promising energy storage systems for the rapid growing field of hybrid electric vehicles and electric vehicles due to their high energy density and long cycle life, etc [1–3]. Silicon (Si) is an ideal anode material for lithium-ion battery owing to its ultrahigh specific capacity of ~4200 mAh/g, low discharging potential, and safety. However, poor electrical conductivity and the huge volume change during lithium-ion insertion/extraction processes are two critical obstacles for Si-based anode materials [4, 5].
Recently, Si-based alloys have been developed to overcome the two disadvantages; the Si-M (M = Mg [6, 7], Sn [8, 9], Ag [10, 11], Zn [12, 13], Fe [14, 15], and Ni [16, 17]) alloys are able to alleviate the volume expansion and consequently increase the cyclability for the batteries. In our previous research, the immiscible Si-Sb alloy was synthesized and demonstrated moderate electrochemical and cycling performance with a reversible capacity of 596.4 mAh/g . Nevertheless, the Si-based alloy particles lose electrical contact with each other because of the huge volume expansion during charge/discharge processes. Therefore, improving the cycling stability as well as maintaining the high capacity has been an attractive research area in Si-based anode materials.
Zinc oxide (ZnO) was one of the candidates for anode materials due to its high theoretical capacity, low cost, and nature abundance . However, its poor cyclability, caused by volume expansion during the charge/discharge, affects the practical application [20–22]. In this work, ZnO was introduced to the binary Si-Sb materials by a chemical reduction-mechanical alloying method. ZnO has a high theoretical capacity of 978 mAh/g, which could improve the specific capacity of the composite materials. In addition, ZnO was able to effectively relieve the agglomeration of the Si-Sb alloy particles, preventing the loss of electric contact between the current collector and electrode materials induced by the volume expansion. The electrochemical performance of Si-Sb-(ZnO) x (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9) composite anodes was studied, and the dependence of the cyclic behavior of the electrodes was discussed. It was found that when the molar ratio of ZnO was 0.3, the initial specific charge capacity and specific discharge capacity of the synthesized Si-Sb-(ZnO)0.3 composite anode material were 870.3 and 1301.5 mAh/g, respectively. The addition of ZnO was able to effectively relieve the agglomeration of the Si-Sb alloy particles, resulting in greatly improved cycling stability for the Si-Sb-(ZnO) x composite anode materials.
Synthesis of Materials
All reagents were of analytical quality and used without any further purification. Si-Sb-(ZnO) x composite materials were synthesized by a chemical reduction-mechanical alloying method. Typically, 0.1 mol of SbCl3, C6H5Na3O7, and Zn(NO3)2 with the molar ratio of 1:25:x (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9) as well as a 0.2 mol aqueous solution of NaBH4 (pH > 12) were prepared, respectively. Excess amount of NaBH4 was added dropwise to the SbCl3/Zn(NO3)2 solution with strong magnetic stirring at room temperature. The solutions were aged in a water bath at 80 °C for 5 h. The precipitates were collected by centrifugation, washed with distilled water, and then mixed with 0.1 mol of Si nanopowders for the ball-milling process. The milling was performed under argon atmosphere for 15 h at 500 rpm, and the final products were dried at 120 °C for 10 h under vacuum and then stored in the Ar-filled glove box.
The phases of the composite materials were analyzed using a D8 Advance X-ray diffraction (XRD) instrument. The microstructure and morphology of the particles were observed using field-emission scanning electron microscopy (FE-SEM) by a Hitachi Limited S-3400 N instrument.
To evaluate their electrochemical properties, 70 wt.% of active materials, 15 wt.% of acetylene black and 15 wt.% of carboxyl methyl cellulose (CMC), and styrene butadiene rubber (SBR) were mixed to form slurry, which was then pressed onto a copper grid as electrodes and dried at 120 °C for 10 h under vacuum. The loading of active materials was about 1.2 mg cm−2 on the electrodes.
The electrochemical performances were measured using coin cells assembled in an argon-filled glove box; these cells contained metallic lithium foil as a counter electrode, 1 M LiPF6 in ethylene carbonate (EC)-dimethyl carbonate (DMC) (1:1 v/v) as the electrolyte, Celgard 2400 membrane as a separator, and the synthesized composite alloy as a working electrode. The galvanostatic charge/discharge performance was assessed using a CT2001A battery tester from 0.1 to 2.0 V (vs. Li+/Li) at a constant current density of 0.1 mA/cm2 at room temperature unless noted. Cyclic voltammetry (CV) from 0 to 2.0 V (vs. Li/Li+) at 0.01 mV/s and electrochemical impedance spectroscopy (EIS) in the frequency range 105 to 0.01 Hz and with an amplitude of 5 mV were performed using a Solartron 1470e electrochemical testing system.
Results and Discussion
The quantification results of the EDX measurements for Si-Sb-(ZnO)0.3 anode material
Cycling performances of Si-Sb-(ZnO) x composite anode materials were investigated by galvanostatic charge/discharge at 0.1 mA/cm2; the results are shown in Fig. 4b. The initial discharge capacities of Si-Sb-(ZnO) x (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9) composites are 1161.2, 1262.9, 1301.5, 1159, 1146.9, and 1112.2 mAh/g, respectively. Due to the formation of Sb and Si oxides during the synthesis process which would react with lithium in the first discharge cycle, the irreversible capacities are also increased. The discharge capacities for the second cycle are 818.5, 963, 870.3, 734.4, 740.3, and 777 mAh/g, and the capacities retained at 242.4, 436.8, 690.6, 548.1, 519, and 456.7 mAh/g after 200 cycles. The Si-Sb-(ZnO)0.3 exhibited 2.5 times of capacity retention compared to Si-Sb alloy and showed the best cycling performance among the composite anode materials. It was demonstrated that with the addition of ZnO, the electrochemical performance of Si-Sb-(ZnO) x was significantly improved (Additional file 1: Figure S1). The reason is that ZnO has a high theoretical capacity of 978 mAh/g, which could improve the specific capacity of the composite materials. In addition, ZnO was able to effectively relieve the agglomeration of the Si-Sb alloy particles, preventing the loss of electric contact between the current collector and electrode materials induced by volume expansion.
To confirm the superior electrochemical properties of the Si-Sb-(ZnO)0.3 composite material, the performance at different current densities was evaluated. As shown in Fig. 4c, the anodes discharged at increasing current densities during the preceding 60 cycles. The initial discharge capacities of Si-Sb-(ZnO) x (x = 0.2, 0.3, 0.4) are 1260, 1267.3 and 1004 mAh/g, respectively, and the average specific capacities decreased as the current density increased. The anode materials became fully lithiated at a low current density, therefore exhibiting high capacities; while at high current densities, the diffusion of lithium ions in the liquid phase and solid electrode are different, resulting in different equilibrium rates for the reactions, while the capacities decreased. The Si-Sb-(ZnO)0.3 composite anode material exhibited the highest discharge capacities among the three composites, even at high current density of 5 mA/cm2. When the current density varied from 5 to 0.1 mA/cm2, the discharge capacity of Si-Sb-(ZnO)0.3 recovered to 910.8 mAh/g, showing excellent rate capability.
Impedance parameters associated to Si-Sb-(ZnO) x anode materials at open circuit voltage
Si-Sb-(ZnO) x (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9) composite anode materials were prepared by a chemical reduction-mechanical alloying method. The materials exhibited superior capacity and good cycling stability for lithium ion-batteries. When the molar ratio of ZnO was 0.3, the initial specific charge capacity and specific discharge capacity of the synthesized Si-Sb-(ZnO)0.3 composite anode material were 870.3 and 1301.5 mAh/g, respectively. An appropriate amount of ZnO was able to effectively relieve the agglomeration of the Si-Sb alloy particles, not only greatly shortening the lithium-ion diffusion distance, but also increasing the space available for volume expansions, resulting in greatly improved cycling stability for the Si-Sb-(ZnO) x composite anode materials. This study gives a rational direction for tailoring the components and structures of composite anode materials to improve the performance of lithium-ion batteries.
We are grateful for the financial support provided by the National Natural Science Foundation of China (Grant #51374146, 50874074), Shenzhen Government’s Plan of Science and Technology (Grant #JCYJ20120613173950029, JCYJ20140418095735619).
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