C composite from rice husks as an anode material for lithium-ion batteries

Accepted Manuscript Title: SiOx /C composite from rice husks as an anode material for lithium-ion batteries Author: Yanming Ju Joel A. Tang Kai Zhu Yuan Meng Chunzhong Wang Gang Chen Yingjin Wei Yu Gao PII: DOI: Reference:

S0013-4686(16)30097-4 http://dx.doi.org/doi:10.1016/j.electacta.2016.01.095 EA 26477

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

9-10-2015 14-1-2016 14-1-2016

Please cite this article as: Yanming Ju, Joel A.Tang, Kai Zhu, Yuan Meng, Chunzhong Wang, Gang Chen, Yingjin Wei, Yu Gao, SiOx/C composite from rice husks as an anode material for lithium-ion batteries, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.01.095 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

SiOx/C composite from rice husks as an anode material for lithium-ion batteries Yanming Jua, Joel A. Tangb, Kai Zhua, Yuan Menga, Chunzhong Wanga,c, Gang Chena,c, Yingjin Weia, Yu Gaoa* [email protected] a

Key Laboratory of Physics and Technology for Advance Batteries (Ministry of

Education), College of Physics, Jilin University, Changchun 130012 (P. R. China) b

c

Department of Chemistry, Johns Hopkins University, Baltimore MD, 21218 (USA)

State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012 (P.

R. China) *

Corresponding author: Tel & Fax: +86-431-85155126

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Highlights Rice husks were utilized to prepare SiOx/C as an anode material for lithium ion battery. SiOx/C composite was prepared by a two-step fire process. SiOx/C contains low valence silicon owing to thermal treatment at argon/hydrogen atmosphere. SiOx/C exhibits a high specific capacity of nearly 600 mAh g-1 at 100 mA g-1 current density after 100 cycles.

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Abstract SiOx/C composite material derived directly from agricultural rice husk byproducts through an economically viable and environmentally benign approach has been explored to be used as an anode for rechargeable lithium batteries. Rice husks were converted into a SiOx/C composite directly by heat treatment under argon/hydrogen atmosphere, at a temperature of 900 °C. The composite contains SiOx surrounded by an amorphous carbon matrix. A steady state reversible capacity of nearly 600 mAh g-1 was delivered at 100 mA g-1 current density after 100 cycles. The improved performance of the SiOx/C composite anode over other agricultural byproduct derived carbon materials is believed to be due to the presence of low valence silicon. The filth-to-wealth conversion of rice husks to battery material is a highly energy efficient process with great economic and environmental benefits

Keywords: Rice Husks; Argon/hydrogen atmosphere; SiOx/C; Lithium-ion batteries

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1. Introduction Lithium-ion batteries (LIBs) have become the most important and widely used rechargeable battery as an energy source/storage with advantages of high voltage, low self-discharge, long cycling life, low toxicity, and high reliability [1]. The performance of LIBs is closely related to the electrode materials used. In current technology, graphite is widely used as a commercial anode material due to its low cost, high yield and long cycle life, however, the theoretical capacity of graphite is only 372 mAh g-1. Therefore, in recent years, considerable efforts have been devoted to search for alternative anode materials to improve the capacity of LIBs. Currently, Si and Si-related materials show great promise in improving the LIBs capacity. Advanced Si materials, such as silicon nanotube [2], silicon nanowire [3], silicon oxide [4,5] and silicon composite materials [6-8] have shown good performance in LIBs, however their production for commercial use is hindered due to their high costs and complicated preparation processes. Therefore, seeking sustainable precursors with scalable processes will be of great benefit to large scale applications of LIBs in energy storage and conservation. Compared to other Si materials, silicon-carbon composites draw more attention because of uncomplicated production methods, low cost and excellent electrochemical performance. Considering economical and efficient production of materials, biomass waste products become an appealing resource. To date, banana fibers, rice hulls, peanut shells, wheat straw and charcoal biomass precursors have been tested in the production of LIBs anode

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and were demonstrated to have high capacities and good capacity retentions [9-13]. Rice husks are the main by-product in the rice milling industry, and contain abundant carbon and silicon (rice husks contain 20 wt% of silica [14]). With an annual production of 1.2×108 tons of rice husks worldwide [15], some countries have utilized the product as a cheap energy resource. However the excess that is not used is either burned in the field or discarded. There is great motivation to further utilize the rice husks in porous carbon materials [16] and the production of silicon [17]. This is an efficient way to utilize discarded rice husk resources. Recently, Wang et al. were able to isolate the SiO2/C composite in a one-step fire process. The as-obtained SiO2/C composite derived from rice husks delivered a high initial discharge capacity of 325 mAh g-1 and increased to 485 mAh g-1 after 84 cycles [18]. The purpose of this article is to explore the application of SiOx/C composite derived from rice husks in LIBs. The composite is prepared by a two-step fire process. The first stage is carbonization, which could also be designed to produce biofuel simultaneously. After that, the carbonized material is further heated in an argon/hydrogen atmosphere to produce the SiOx/C composite. The SiOx/C composite was characterized using different analytic approaches such as XRD, XPS and TEM. The suitability of the composite obtained for electrodes in a LIB was evaluated by using a series of electrochemical techniques including galvanostatic charge-discharge cycling and cycle voltammetry.

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2. Experimental 2.1. Precursor The original rice husks were washed completely by deionized water to remove any dirt and dust. The husks were dried at 110 °C overnight, then carbonized into the precursor under N2 gas flow at 450 °C for 0.3-1.0 h. The material was ground into a fine powder. 2.2. Preparation of SiOx/C The powdered precursor was placed in a combustion boat and calcined in a tube furnace at the desired temperature (900 °C) with a heating rate of 2 °C/min for several hours under argon/hydrogen (Ar/H2) mixed atmosphere. The sample was then cooled to room temperature to obtain SiOx/C composite. The preparation process of SiOx/C is shown in Scheme 1. 2.3. Characterization of SiOx/C X-ray diffraction (XRD) of the synthesized material was measured on a Bruker AXS D8 X-ray diffractometer with a Cu-Ka X-ray source operating at 40 kV and 100 mA. Microscopic analysis was conducted using a field emission scanning electron microscope (FE-SEM, HITACHI SU8020) and a transmission electron microscope (TEM, JEOL JEM-2200FS). An energy dispersive X-ray spectroscopy (EDX) attached to TEM apparatus was used for local elemental analysis. X-ray photoelectron spectroscopy (XPS) was carried out with a VG scientific ESCALAB 250 spectrometer to study the valence

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state of silicon. The Raman spectra were recorded on a Renishaw inVia Raman microscope using an argon-ion excitation laser with a wavelength of 514.5 nm. 2.4. Electrochemical characterization Electrochemical properties of SiOx/C were studied with 2032-type coin cells, which were assembled with the working electrode (composed of 60 wt% of the SiOx/C composite, 30 wt% of conductive additives and 10 wt% of PVDF dissolved in N-methyl pyrrolidone), metallic lithium as the counter electrode, copper foil current collectors and a Celgard 2320 membrane in an argon-filled glove box. 1 M LiPF6 was dissolved in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) (1:1:8 in weight) to act as the electrolyte. Galvanostatic charge-discharge was performed in the voltage window of 0.01-3.0V on a LAND automatic battery tester. Bio-Logic VSP multichannel potentiostat-galvanostat electrochemical workstation was used to analyze the electrochemical performance of the LIB via cyclic voltammetry (CV). CV was tested between 0.01 to 3.0 V using a scanning rate of 0.1 mV s-1. 3. Results and Discussion XRD was carried out to analyze the crystal structures of the precursor and SiOx/C composite. The characteristic XRD pattern of amorphous silica in the precursor and SiOx/C composite is shown in Fig. 1a, with a diffraction peak at approximately 2θ = 22.5°. The SiOx/C composite does not show sharp diffraction peaks, indicating the

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amorphous state of the product. The weak peaks at ~ 26° and ~ 44° are caused by diffusion scattering of amorphous carbon. The intensity of these diffraction peaks increases with the calcination temperature, indicating improved graphitic degree of the amorphous carbon. Fig. 1b shows the Raman spectra of the precursor and the SiOx/C composite. Both samples show two characteristic Raman peaks of carbonaceous materials (disordered carbon: D-mode at 1342-1353 cm-1; graphitic carbon: G-mode at 1590-1601 cm-1). Only the D- and G-modes of disordered and graphitic carbon peaks are observed in the precursor. But, the SiOx/C composite prepared at 900 °C has a larger full-width at half maximum (FWHM) and a lower the D/G band intensity ratio with respect to the precursor, indicates a higher degree of graphitization. The main volume contribution of carbon shows the typical dependency of carbon ordering on the treatment temperature, since the content of graphitic carbon increases with temperature. To explore the valence state of Si in the precursor and the SiOx/C composite in detail, XPS analysis was conducted. Fig. 2a and Fig. 2b show the 2p-binding energy spectra of Si in the precursor and the SiOx/C composite, respectively. In the precursor only Si4+ (103.90eV) is found. After heating under Ar/H2 gas at 900 °C, Si4+ (103.70eV) is partially reduced to Si3+ (102.61eV) and Si2+ (101.7eV) [19]. Judging from the result, H2 plays the role of reducing the amount of SiO2, although the overall change in the valence state of Si seems to be small. According to the XPS analysis, the addition of H 2

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resulted in a decrease of SiO2 phase, which causes a reduction in the valence state of Si as a result. Fig. 3 shows the SEM images of the precursor and the SiOx/C composite. The surface of the precursor is typical of surface morphologies of rice husks [16]. The surface after low temperature calcination (Fig. 3a) is highly similar to the composite after heat treatment in Ar/H2 at 900°C (Fig. 3b), indicating that high temperatures do not affect the morphology of the samples. The TEM images in Fig. 4 (a, b) show the presence of nanoparticles (size range from 10-50 nm) in the composite. From the enlarged image, one can see that SiOx nanoparticles (highlighted by the red circle) are evenly distributed in the carbon matrix (indicated by the yellow circle). The elemental mappings (Fig. 4d-f) indicate homogeneous mix of the Si and C components. Meanwhile, energy dispersive X-ray spectroscopy (Fig. 4c) reveals that the area contains C, Si, O and some other minor elements (Ca, P) (possibly because the rice husk is not washed with an acid solution). From the TEM results, it can be concluded that SiOx/C is a natural composite because carbon and SiOx are homogeneously distributed in SiOx/C composite. The electrochemical testing of two samples at a current density of 100 mA g-1 is plotted in Fig. 5a and Fig. 5b. Fig. 5a shows the initial discharge-charge plot of the precursor and the prepared SiOx/C. SiOx/C electrode exhibits a discharge capacity of 998.5 mAh g-1 and a charge capacity of 466 mAh g-1, with an initial Coulombic efficiency of 46.67 %. The first irreversible capacity of SiOx/C electrode is attributed to the formation of solid

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electrolyte interface (SEI) passivation film (0.75V) and some irreversible component (1.5V) (that might be due to the remaining metal oxides in the materials not being washed away with an acid solution [18,20]), as supported by cycle voltammetry (Fig. 5c). Compared to the precursor, SiOx/C has a higher specific capacity because of the existence of a low valence state silicon [21-23] and slightly graphitized carbon. From the charge curve, no obvious plateau is observed. Even though some Si4+ is reduced through calcination in the atmosphere of argon/hydrogen gas, there is a lot of SiO2 existing in SiOx/C. SiO2 as anode materials in LIBs was reported in previous literature, and there is no obvious plateau in the charge plot [24,25]. Fig. 5b shows the specific capacity of SiOx/C at different cycle numbers. After first discharge-charge cycle, the capacity stays relatively stable, having a high discharge capacity of 582.1 mAh g-1 after 100 cycles. Fig. 6a shows the cycle performance of the precursor and prepared SiOx/C composite. Based on the two curves, both samples have a good capacity retention rate. This can be attributed to the natural composite from having a good electron transport path in the bulk carbon and the buffer effect from the volume expansion of silicon in the discharge and charge process (Scheme 1). In our case, it is obvious that the specific capacity of SiOx/C improves as the cycle number increases. It is believed that the size of SiO2 nanoparticle becomes smaller gradually with each cycle and more SiO2 in the core gets electrochemically activate; while silicon in a low valence state can be partially coated by

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carbon and silica raw material. With increasing cycle number, more silicon with the low valence state participates in the electrochemical reaction, however, SiOx/C has a higher capacity. Fig.6b shows the rate performance of the prepared SiOx/C at different current densities. The cell was first cycled at 0.1 A g-1 for 20 cycles, and the capacity remains at about 600 mAh g-1. The cell was then cycled at 0.2 A g-1, 0.5 A g-1, 1 A g-1, returning to 0.1 A g-1 at the end of the cycle. As shown in Fig.5b, even at a current density of 1 A g-1, the capacity still reaches around 300 mAh g-1, displaying an excellent performance rate. 4. Conclusions SiOx/C anode prepared from rice husks exhibited superior electrochemical performance (around 600 mAh g-1 at 0.1 mA g-1 and 300 mAh g-1 at 1 A g-1). The superior electrochemical performance of SiOx/C anode over materials derived from other biomass material is attributed to the presence of low valence silicon. The preparation process is well industrialized and without complicated procedures. Considering that an abundant amount of rice husks are accumulated per year and that the Si-related material can be produced at low costs under more environmentally friendly conditions, the SiOx/C LIBs is a valuable product for fast growing markets in electrical energy storage. If this route comes true in industry, it will generate a positive influence to energy sustainable use and be of great social value.

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Acknowledgments This work was supported by Ministry of Science and Technology of China (No. 2015CB251103), National Natural Science Foundation of China (No. 21473075), Defence Industrial Technology Development Program of China (No. B1420133045), Jilin Provincial Science and Technology Department (No. 20140101083JC) and Open Project of State Key Laboratory of Superhard Materials (Jilin University) (No. 201513).

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Figure Captions Fig. 1. (a) XRD patterns and (b) Raman scattering patterns of the precursor and the SiOx/C composite. Fig. 2. Si 2p XPS of the precursor and the SiOx/C composite. Fig. 3. SEM images of (a) the precursor and (b) the SiOx/C composite. Fig. 4 (a and b) TEM images of the SiOx/C composite, (c) Energy-dispersive X-ray spectrum of the SiOx/C composite and (d-f) EDX element mapping of C and Si of SiOx/C composite Fig. 5. (a) The first galvanostatic charge-discharge curves of the precursor and the SiOx/C composite. (b) The 1st, 20th, 50th, 80th and 100th galvanostatic charge-discharge curves of SiOx/C between 3 and 0.01V (vs. Li+/Li) at a current density of 100 mA g-1. (c) CV curves of SiOx/C at a scan rate of 0.1 mV s-1. Fig. 6. (a) Cycle performance of the precursor and SiOx/C at a current density of 100 mA g-1. (b) Rate-dependent cycling performance of SiOx/C between 0.1 A g-1 and 1 A g-1. Scheme 1. Schematic of the preparation process of the SiOx/C composite.

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Fig. 1

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Fig. 2

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Fig. 3

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Fig. 4

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Fig. 5

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Fig. 6

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Scheme 1

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