Articel of chemistry

        Making batteries from waste                              glass bottles

" Researchers are turning glass bottles into high performance lithium-ion batteries for electric vehicles and personal electronics " 

Abstract

    Every year many tons of waste glass end up in landfills without proper recycling, which aggravates the burden of waste disposal in landfill. The conversion from un-recycled glass to favorable materials is of great significance for sustainable strategies. Recently, silicon has been an exceptional anode material towards large-scale energy storage applications, due to its extraordinary lithiation capacity of 3579 mAh g−1 at ambient temperature. Compared with other quartz sources obtained from pre-leaching processes which apply toxic acids and high energy-consuming annealing, an interconnected silicon network is directly derived from glass bottles via magnesiothermic reduction. Carbon-coated glass derived-silicon (gSi@C) electrodes demonstrate excellent electrochemical performance with a capacity of ~1420 mAh g−1 at C/2 after 400 cycles. Full cells consisting of gSi@C anodes and LiCoO2 cathodes are assembled and achieve good initial cycling stability with high energy density.

Introduction

    Conventionally graphite-based anodes used in commercial lithium ion batteries (LIBs) have a limited theoretical capacity of 372 mAh g−1 due to the inadequate Li-ion intercalation in LiC6. Silicon is extensively considered the most encouraging material for the next generation anodes owing to the low discharge potential (~0.1 V vs. Li/Li+) and the high theoretical capacity of 3572 mAh g−1 corresponding to the formation of Li15Si4 phase at room temperature3, 4. If commercially used LiCoO2 (~145 mAh g−1) is assembled as the common cathodes, the full cells based on Si anodes lead to a 34% increase in the total capacity over that of graphite-anode based full cells5, 6. However, Si is able to alloy with a large amount of Li-ions during lithiation, resulting in a large volume expansion upwards of 300%7. The lithium-induced mechanical stresses during alloying with subsequent contraction during dealloying can cause Si to fracture, which promote the pulverization of active materials and the deterioration of the conductive network. 

    However, the high-cost electronic grade wafers coupled with the milligram-per-wafer yield of active material limit the large-scale production on industry level. The hydrolysis of tetraethyl orthosilicate (TEOS) to produce nano-SiO2 with subsequent reduction into Si has been investigated to generate the high performance anode materials30. However, the extensive procedure to achieve TEOS as SiO2 precursor is inefficient for industry-level manufacturing.

       

Discussion

     The high frequency semicircle represents the resistance of SEI layer (RSEI) coupled with resistance stemming from the imperfect contact between metal current collector and active materials (RINT)51, 52. The RSEI+INT decreases in diameter as the cycle increases, while the semicircle at mid frequency drops sharply for the first 7 cycles and then stabilizes, indicating the stabilization of charge transfer impedance as shown in Fig. S5c. Interfacial impedance remains constant as cycling, which signifies the contact impedance among active particles and current collect is independent with cycling53. This phenomenon can be attributed to the buffering effect of carbon coating on the Si volume expansion54

The reaction process generates a large amount of heat as shown in Equations 1–2 as follows33:

2Mg(g)+SiO2(s)→Si(s)+2MgO(s),ΔH=−546.42kJmol−1.$$2\mathrm{Mg}(\mathrm{g})+{\mathrm{SiO}}_2(\mathrm{s})\to \mathrm{Si}(\mathrm{s})+2\mathrm{MgO}(\mathrm{s}),\mathrm{\Delta }\mathrm{H}=-546.42\mathrm{kJ}{\mathrm{mol}}^{-1}.$$

(1)

Conclusion

     In summary, we have demonstrated the direct conversion from glass bottles to high purity silicon using a scalable, facile and low-cost Mg reduction process. The excellent electrochemical performance of gSi@C anodes can be mainly attributed to the mitigated volume expansion and improved system conductivity resulting from the interconnected gSi network and the conformal carbon coating. 

  

Methods

Materials synthesis

     Collected beverage glass bottles were first sealed in several thick bags and crushed into small pieces by hammer. Crushed glass was hand-milled in an alumina mortar for several minutes, transferred into tubes with ultrasonication for 2 hours in isopropanol (IPA), and then left for settling big quartz down for 2 hours. Light-weight suspended quartz particles in IPA were collected and dried at 90 °C under vacuum for 2 hours. 

Materials characterization

  

   The surface morphology is investigated using optical microscopy, scanning microscopy (SEM; Leo-Supra, 1550) with an X-ray energy-dispersive spectroscopy (EDS). Transmission electron microscopy (TEM, Titan Themis 300) operated at 300 KV is used to further characterize the purity and morphology of gSi and gSi@C. The TEM samples are prepared by dispersing the powder in water for 15 minutes, diluted and then dropped onto TEM grids. The phase identification is performed by X-ray diffraction (XRD, PANalytical Empyrean) from 10° to 80°. Raman spectroscopy (Renishaw DXR) with a 532 nm laser (8 mW excitation power, 100X objective lens) source is carried to check the purity of gSi. Electrochemical impedance spectroscopy (EIS) analysis is obtained with a Biologic VMPs.

Electrochemical measurements

      The anode electrodes were prepared by doctor-blading a slurry on pre-cleaned Cu foil with a pre-area mass loading for 0.5–0.6 mg cm−2. The slurry comprises 70% active material (gSi@C), 20% PAA binder and 10% conductive additive (carbon black). A button-type (CR 2032) half-cell configuration was used for the electrochemical measurements. Cells were assembled in an Argon-filled VAC Omni-lab glovebox with oxygen and H2O level below 0.5 ppm. Pure Li metal was used as the counter electrode for half-cell tests. Full cells were prepared and evaluated in TET USA Corporation facility with custom made LiCoO2(lithium cobalt oxide) cathodes with a LiCoO2 mass loading of 5.3–5.5 mg cm−2. A Celgard 3501 porous PP membrane was used as the separator. The electrolyte comprising 1 M LiPF6 in fluoroethylene carbonate and dimethyl carbonate (FEC:DMC = 1:1, v/v) was used as electrolyte for half and full cells. Cycling performance and galvanostatic charge-discharge behaviors were conducted on Arbin BT300 with a voltage window ranging from 0.01 to 1.5 V (vs. Li+/Li). Capacity and C-rates were determined using 1 C = 3.6 A g−1. Cyclic voltammetry scans were tested at a fixed voltage window between 0.01 V and 1.5 V (vs. Li+/Li). Electrochemical impedance spectroscopy measurements were performed to evaluate the impedance information of gSi@C anodes on a Biologic VMPs with a frequency range between 0.01 Hz and 1 MHz.

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