Fly ashes as a sustainable source for nanostructured Si anodes in lithium-ion batteries
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Recycling industrial wastes for high-value items is of strategic importance for commercial production. Fly ashes are solid waste by-products generated by coal-fired power plants. Although a large amount of annual output approach 150 million tons worldwide, fly ashes have been recycled merely into some low-value items. In order to realize the high additive value of fly ashes, we transform the solid waste fly ashes into nanostructured silicon powders and apply them as anodes active materials for lithium-ion batteries. The nanostructured silicon exhibits good electrochemical performance as lithium-ion batteries anodes with high rate capacity (1450.3 mAh g−1 at current density 4000 mA g−1) and reversible capacity (1017.5 mAh g−1 after 100 cycles), indicating that fly ashes can be a useful resource of anode materials to meet the need of high performance lithium-ion batteries.
KeywordsFly ashes Nanostructured Si Anodes Lithium-ion batteries
Fly ashes are solid waste by-products generated by coal-fired power plants, and the yield of fly ashes approach 150 million tons/year worldwide in 1990 . Moreover, the increasing energy demands of developing countries is possible to ensure that the coals are key components of energy consumption despite the climate-change policy [2, 3]. As a result, it can be expected that the amount of fly ashes will increase with the energy demand increases in the future. The super yield of waste by-products results in an environmental issue inevitably, hence, the comprehensive utilization of this solid waste resources is closely related to global sustainable development strategy. However, less than a half of the solid waste fly ashes are recycled for concrete, cement and as a solid additive, and they are mainly used in a narrow range of low-value items . The remainder still being directly discharged into fly ashes ponds and landfills leading to various environmental problems . Typically fly ashes are composed primarily of aluminosilicate glass, mullite (Al6Si2O13) and quartz (SiO2), and they can provide a ready source for Si and Al, which are necessary for the preparation of zeolites . In this context, some valuable applications are focus on the conversion of fly ashes to zeolites [7, 8], solid catalyst [9, 10], porous materials  and adsorption of textile dyes to data [12, 13]. However, other more valuable applications must be investigated to enhance the value of the fly ashes and to recycle the high fly ashes output.
It is well known that Si have important industrial and potential applications in the field of high technology, such as semiconductor , nanoelectronic , biotechnology [16, 17, 18], and energy storage [19, 20, 21, 22]. In these applications, Si has aroused considerable attention as the alternative lithium-ion batteries anode materials for portable electronic devices and electric vehicles on account of its large theoretical specific capacity (4200 mAh g−1 for Li4.4Si), which can be up to ten times greater than that of conventional graphite anode (370 mAh g−1) [19, 23, 24, 25]. However, the giant volume changes (> 300%) is caused by the process of lithium insertion and extraction, which lead to the fracture and pulverization of Si structure . As a result, Si typically suffers a severe capacity fading during cycling. Many research works have shown that nanostructured Si can effectively accommodate such volume changes and significantly improve the cyclic performance [19, 20, 21, 22, 27, 28, 29], nevertheless, the high cost of nanostructured Si production is a restricted facror for Si anodes compared to graphite anodes. Therefore, an increased interest has been noted lately in the use of low-cost waste products (rice husks and reed plants) as Si sources for the preparation of nanostructured Si anodes [30, 31, 32, 33]. However, the research on how to extract the nanostructured Si out from solid waste fly ashes generated by coal-fired power plants is paid less attention . Considering this, current work explores the expensive nanostructured Si materials can be easily produced from coal-fired power wastes present in abundance by a green and economical approach.
In a typical synthesis, the industrial fly ashes were used as the Si source. The raw fly ashes materials were obtained from a coal-fired electric power station in China. Mg powders (250 mesh) were obtained from Aladdin Reagent, China. For comparison with the derived Nano-Si from fly ashes, Si powders (6000 mesh) were obtained from a commercial vendor (CNPC Powder Material Co., China).
In order to realize the high additive value of fly ashes, in this work, we transform the solid waste fly ashes into nanostructured silicon (Nano-Si) powders and apply them as anodes active materials for lithium-ion batteries.
2.2 Synthesis of Nano-SiO2 from fly ashes
To increase its activity in Nano-SiO2 formation, firstly, fusion of fly ashes powders with Na2CO3 was carried out at 800 °C for 2 h (1:0.8 fly ashes:Na2CO3 weight ratio), and the resultant fused mixture was then cooled down to the room temperature. After that, the product of fusion was leached with a 3 M HCl solution for 4 h in order to eliminate some metal ions. The leached product of fusion was washed with deionized water to neutral conditions, then heated in water bath at 100 °C for 6 h, then the Nano-SiO2 was collected by filtration to remove aluminium oxide, and dried at 105 °C.
2.3 Transformation of Nano-SiO2 to Nano-Si
300 mg of the synthesized Nano-SiO2 was fully mixed up with 300 mg of Mg powders. The mixture was spread evenly in a steel boat, then heated to 650 °C for 5 h inside a tube furnace under 95 vol% Ar/5 vol% H2 gas flow. The ramp rate was controlled at 5 °C min−1. After this reduction, the obtained dark brown powders were soaked in 2 M hydrochloric acid (HCl) aqueous solution for 12 h to selectively remove MgO and Mg2Si, and then treated in a 5 wt% HF solution for 5 min to insure that newly formed or any unreacted SiO2 was eleminated. The resultant Nano-Si powders were gathered by filtration, cleaned with de-ion water and absolute ethanol for 4 times, respectively, and finally dried under vacuum at 80 °C for 6 h.
XRD spectra were acquired at room temperature with a Rigaku SmartLab 9 diffractometer using Cu Kα radiation (40 mA, 40 kV) to analyze the structure of crystals. The morphologies of all materials were characterized by FESEM (JEOL JSM-7800F) and HRTEM (JEOL JEM-2100F). The EDS attached to TEM and SEM apparatus was served as elemental analysis. The nitrogen adsorption–desorption isotherm was acquired through the Brunauer–Emmett–Teller (BET) (Micrometrics ASAP 2020 analyzer) after vacuum degas of the samples at 200 °C for 3 h.
To obtain the Nano-Si working electrodes, the slurry was prepared by dissolving 10 wt% sodium alginate, 10 wt% Super P, and 80 wt% Nano-Si in water. The slurry was cast onto thin copper foils. The coated electrodes were heated in a vacuum oven at 75 °C for 12 h, and then they were punched into circular discs to fabricate the coin-cell. The load of active materials was about 1.2 mg/cm2. The electrolyte used in the current study is 1.0 M LiPF6 in the mixture of carbonates (Novolyte Technologies Inc. Suzhou, China). For electrochemical evaluation, coin-type cell (CR2016) was assembled by sandwiching separator (Celgard 2400) with the Nano-Si working electrode and the metal Li metal counter electrode inside the glove box. The cell was charged and discharged galvanostatically in a fixed voltage window from 5 mV to 1.0 V on a LAND battery test system (Wuhan Kingnuo Electronics Co., Ltd., China) at 25 °C. Cyclic voltammogram (CV) was conducted on an Autolab PGSTAT302 N workstation by 0.02 mV s−1 scan.
3 Results and discussion
Meanwhile, the peaks between 5 nm and 20 nm according to the pore-size distribution of the Barrett-Joyner-Halenda (BJH) further proves the main presence of mesopores (Fig. 2d inset). Next, the derived Nano-SiO2 was reduced to acquire Nano-Si through a magnesiothermic reduction.
The present study demonstrates that fly ashes, major solid waste by-products generated in coal-fired power plants, can be applied to synthesis Nano-Si with the porous structure as LIBs anodes. The Nano-Si derived from fly ashes can solve the key issues existing in Si anodes, showing excellent rate and cycling performance. In consideration of annual enormous supply of fly ashes on a global scale, this work show how the solid wastes, fly ashes, can be available resources that contribute to meeting the growing demand for Nano-Si materials in LIBs batteries.
We acknowledge the financial supports from the Project Foundation of Chongqing Municipal Education Committee under Grant No. KJ1601338, the National Natural Science Foundation of China (51472037) and Shenzhen Science and Technology Innovation Committee (JCYJ20170818160815002). We thank Material Analysis and Testing Center of Chongqing University of Science and Technology for the help with all analysis and characterization.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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