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Exploring an Interesting Si Source from Photovoltaic Industry Waste and Engineering It as a Li-Ion Battery High-Capacity Anode Tzu-Yang Huang, Baskar Selvaraj, Hung-Yu Lin, Hwo-Shuenn Sheu, Yen-Fang Song, Chun-Chieh Wang, Bing-Joe Hwang, and Nae-Lih Wu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01749 • Publication Date (Web): 29 Aug 2016 Downloaded from http://pubs.acs.org on September 3, 2016

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ACS Sustainable Chemistry & Engineering

Exploring an Interesting Si Source from Photovoltaic Industry Waste and Engineering It as a Li-Ion Battery High-Capacity Anode Tzu-Yang Huang1,#, Baskar Selvaraj1, 2,#, Hung-Yu Lin1, Hwo-Shuenn Sheu3, Yen-Fang Song3, Chun-Chieh Wang3, Bing Joe Hwang2, Nae-Lih Wu1,* 1

Department of Chemical Engineering, National Taiwan University, No. 1, Roosevelt Road,

Section 4, Daan district, Taipei 10617, Taiwan, ROC 2

Sustainable Energy Development Center, National Taiwan University of Science and

Technology, No. 43, Keelung Road, Section 4, Daan district, Taipei 10607, Taiwan, ROC 3

National Synchrotron Radiation Research Center, No. 101, Hsin-Ann road, Hsinchu Science

Park, Hsinchu 30076, Taiwan, ROC

# Authors contribute equally. *

Correspondence and requests for materials should be addressed to N. -L. Wu.

([email protected])

ACS Paragon Plus Environment

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ABSTRACT

Low cost electrode materials are essential for the expansion of the applications of large-format Li-ion batteries (LIBs). Kerf-loss (KL) Si waste from the photovoltaic industry represents a low cost, high-purity Si source for the production of high capacity anodes of LIBs. Producing an energy storage device from solar-panel industry waste is a potential environment-friendly energy development. This study addressed the challenges of employing KL Si as high-capacity LIB anode. The abrasive SiC particle impurities in KL waste powder were used not only as a milling agent to reduce silicon particle size but also as mechanically and electrochemically robust pillars that resist microstructural degradation of the electrode caused by the expansion of Si during lithiation. High energy ball milling of Si with rigid SiC produced fused nanosilicon particles that were supported on micron-sized SiC; this resulted in substantially mitigated capacity fading. In addition, an effective conducting network was formed by incorporating Ni into the Si agglomerates, enabling high rate density and maintaining high powder tap density. The resulting Si-SiC-Ni composite powder exhibits high capacity and long-term stability.

Keywords: kerf loss, high-energy ball milling, Si-SiC-Ni composite, practical Si anode.


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1. Introduction Developing low cost, environmentally friendly electrode materials is crucial for expanding the applications of Li-ion batteries (LIBs) from small electronic devices to electric vehicles and energy storage stations. Si is abundant and environment friendly and, because of its high theoretical lithiation capacity (> 3000 mAh/g), has substantial potential to replace carbonbased materials as a high capacity anode for LIBs. Resolving the severe volume expansion (> 300%) problem of Si upon lithiation has been a major focal point of research over the preceding decade.1–4 Nanosized Si, nanowires, patterned thin films, and porous structures have been developed to accommodate this volume expansion and have demonstrated sufficient cycle stability.5–7 However, for the development of a practical Si anode, not only the volume expansion problem but also the costs of manufacturing for mass production, electrode loading and density, safety, and eco-friendliness must be addressed. For example, Si nanoparticles (< 150 nm) can more effectively accommodate the volume expansion8 but their practical viability is questionable because of high production cost, low tap density, and toxicity of such a material. Being a cost effective and scalable method, industrially mature high energy ball milling (HEBM) is well suited for the low cost production of micron-sized agglomerates composed of nanostructured silicon; furthermore these micron-sized agglomerates have high tap density, high capacity, long term cycle stability, and are nontoxic.9–11 However, finding low cost and reliable Si sources for mass production remains a challenge despite previously proposed methods for deriving Si from rice husk and beach sands, which used high-temperature magnesium reduction, an expensive process.12, 13 The photovoltaic industry generates tons of high-purity kerf-loss (KL) Si waste in the fabrication of Si wafer-based solar panel modules. Nearly 40% of Si is lost during the slicing of


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crystalline Si ingots using wire sawing with SiC abrasive particles. KL-Si has potential as a low cost, reliable Si source for LIBs. Moreover, developing an energy storage device from photovoltaic industry waste represents a sustainable, environmentally friendly energy development. The presence of abrasive SiC particles and the separation of this impurity have long been regarded as a crucial problem in utilizing KL waste for LIBs. Several methods have been developed for extracting Si particles from KL powder based on centrifugation or filtration.14–16 Recently, aerosol-assisted extraction of Si has been reported as a method for the fabrication of LIB anodes from recovered Si.17 These additional processes designed to remove the SiC particles lead to substantial increases in the manufacturing cost; the direct conversion of KL waste into a usable anode is highly desirable. In this study, SiC was not separated from the KL powder but intentionally retained to serve multiple functions. Micron-sized SiC particles were used as part of the milling medium to assist in the reduction of the particle sizes of the Si particles during ball milling.9 Crucially, the SiC particles were designed to act as “giant” pillars (relative to the Si nanoparticles) within the final composite anode; they thus enhanced the microstructural integrity against the volume expansion caused by lithiation.18–20 This material design strategy for the production of LIB anodes of high capacity and cycle stability, combined with the application of Ni as a conductive additive, enables KL Si waste to be used without the abrasive SiC particles being removed. 2. Materials and methods 2.1 Powder preparation. Industrial KL powder was acid treated and washed with water to remove cutting fluid and metal particles. For HEBM, KL powders, either with or without NiO (−325 mesh, Aldrich), were placed in zirconium dioxide vials containing zirconium dioxide balls using a ball-to-powder weight ratio of 14. The vials were sealed under argon atmosphere and


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subjected to HEBM for 4 h at 400 rpm by using the Fritsch Pulverisette 7 mixer. After milling, the composite was calcined under 3% H2–N2 atmosphere at 500 °C or 700 °C with a heating rate of 200 °C/h for 12 h. Commercial Si nanoparticles (>100 nm) were obtained from Alfa Aesar for the purpose of comparison. 2.2 Materials characterization. Material morphology was examined using scanning electron microscope (SEM; JEOL JSM-7600F) with energy dispersive X-ray spectroscopy (EDS, XMaxN, Oxford Instruments) and transmission electron microscope (TEM; JEOL JEM-1200EX II). Synchrotron X-ray diffraction (XRD) analysis was performed at the beamline 01C2 and transmission X-ray microscopy (TXM) was performed at the beamline 01B1 at the National Synchrotron Radiation Research Center, Taiwan. The carbon content of the KL powder was evaluated through elemental analysis (Elementar vario EL cube). The particle size distribution (PSD) was measured using a laser particle analyzer (LS230, Coulter). The crystal structure and phase composition were identified using a powder X-ray diffractometer (Ultima IV, Rigaku) with Cu Kα X-ray radiation. Temperature program reduction (TPR) was carried out using a 10% H2/Ar mixture in an AutoChem II set (Micromeritics) from 50 °C to 800 °C at 4 °C/min. Tap densities were determined by tapping a graduated cylinder containing a known amount of powder; 2000 taps were applied with a frequency of 260/min and a 3 mm tapping height (Autotap, Quantachrome). The Brunauer-Emmett-Teller (BET) surface area was determined using ASAP 2020 (Micromeritics). Resistance measurement was carried out by pressing the powder into a pellet with two copper pistons connected to an ohmmeter (502BC, Chen Hwa). Pressure ranging from 0 to 5.7 kgf/cm2 was applied to the pellets during the measurements. 2.3 Electrochemical characterizations. Electrodes were prepared from water-based slurries comprising 70 wt. % (dry weight) active material, 3 wt. % carbon black (Super P, TIMCAL), 15


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wt. % flake graphite (KS6, TIMCAL), and 12 wt. % of binder (Aginic acid sodium salt, Acros) coated onto a thin copper foil and vacuum dried at 150 °C for 7 h. The active-material loading was 2 mg/cm2, and the material had a density of approximately 0.9–1.2 g/cm3. Coin cell batteries (CR2032) were assembled in a glove box. Li foil was used as a counter electrode. The electrolyte comprised 1 M LiPF6 in a 1:2 (v/v) mixture of ethylene carbonate and ethyl methyl carbonate with 2 wt. % vinylene carbonate (Dongguan Shanshan Battery Material Co., LTD, H2O ≤ 20ppm, HF ≤ 50ppm) and 10 wt. % fluoroethylene carbonate (TCI, 98%) additives. The electrochemical performance of the cells was measured by using an Arbin battery test station (BT-2000, model: MCN6410) at various current densities ranging from 0.03 to 3 A/g and applying voltages ranging from 0.005 to 1.2 V.

3. Results and discussion 3.1 KL powder generation and characterization.

Figure 1. Schematic illustration of KL powder generation. Multiwire sawing of Si wafer ingots is schematically illustrated in Figure 1; fast moving metal wires in a slurry consisting of SiC abrasives and cutting fluid grind and slice the ingots


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into thin wafers. This generates KL powder that contains Si debris and metal fragments from the sawing wire, mixed with the slurry. KL powder was purified through hydrochloric acid treatment and washed to remove cutting fluid and metal particles. The XRD pattern of the resulting KL powder (Figure 2a) showed crystalline phases of Si and SiC and the removal of metal contaminants was confirmed. Elemental analysis indicated a carbon content of 11.9 wt. %, corresponding to a SiC content of 39.7 wt. %. The TEM and SEM analyses (Figure 2b-d) showed particles ranging from a few hundred nanometers to nearly 20 micrometers in size. The selected area electron diffraction ring-pattern and further calculation of the d-spacing of the rings identified the ragged edge nanoparticles as polycrystalline Si and the sharp-edged micron-sized particles as SiC (Figure 2c and 2d). The BET surface area was 55 m2/g. Notably, the high level of SiC produced a high tap density value of 0.73 g/cm3 for the KL powder, in contrast to the 0.16 g/cm3 tap density of a commercial Si nanoparticles.

Figure 2. Characterization of purified KL powder: (a) XRD patterns (b) SEM micrograph and (c, d) TEM micrographs (insets show SAED patterns).


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3.2 Conversion of KL-Si into an anode composite. The PSD of the KL powder (Figure 3a) had a range of approximately 50 nm to 20 µm and populated the nanometer and micrometer regions, in agreement with the SEM and TEM observations. The HEBM of the powder slightly reduced the population of the largest particles and increased the population of the submicron particles (Figure 3a). SEM analysis of the milled powder (Figure 3b) revealed that the particles transformed during milling (albeit with a minor change in PSD) and confirmed the existence of smaller particles (< 130 nm) fused together to form larger particles of a few hundred nanometers to several micrometers in size. In contrast to the clear-edged SiC particles observed in the original KL powder (Figure 2d), the milled micron-sized particles were deformed and had smaller particles on their surfaces. Mapping of these micron particles and TEM images depicting dark centers contrasting with lighter outer surfaces showed that these particles were not just SiC, but a combination of Si and SiC (Figure S1). These results demonstrate that the milling of Si with rigid SiC reduced the Si particle size and that the constant fracturing and welding of the particles during milling resulted in larger fused particles of Si and Si/SiC. This fracturing and fusion of nano-Si into micron-sized particles reduced the specific surface area from 55 m2/g to 17 m2/g, as indicated by the BET analysis; however, it is notable that the powder maintained the tap density (0.67 g /cm3) close to the original value (0.73 g /cm3). Milling of the KL powder together with NiO (Figure 3a) produced the Si-SiC-NiO (SSN) composite having even larger particles that had a major particle size population of approximately 30 µm and a minor peak population of approximately 2–3 µm; the presence of particles in the nanometer region of the distribution curve was negligible. SEM observation (Figure 3c and Figure S2) of the mainly micron-sized particles of this milled composite led to the conclusion that NiO accelerated the fusion of the particles and produced larger agglomerates by binding


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Figure 3. (a) Change in particle size distribution of the KL powder (black line) after 4 h of milling without (red line) and with NiO (blue line); SEM images of the 4 h-milled powders without (b) and with NiO (c); (d) XRD analysis of the KL powder with NiO before and after four hours of milling. with the other components. Ball milling did not change the powder diffraction pattern (Figure 3d) except for the obvious broadening and reduction in the intensity of the NiO peak (2θ–43.3° for (200)) (Figure S3); this indicated size reduction and amorphization of the NiO. A suitable temperature for the reduction of the NiO in the SSN composite into metal Ni was evaluated using TPR measurements under hydrogen (H2) atmosphere (Figure 4a). The reduction profiles of the milled and unmilled composite powders were considerably different. Maximum H2 consumption occurred at 240 °C and above 700 °C for milled powder, whereas the


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maximum for the unmilled composite mixture was observed at approximately 350 °C and resembled that of pure NiO (Figure S3). The decreased NiO reduction temperature may be caused by the formation of fresh surfaces during milling and the higher density of surface defects

Figure 4. (a) TPR profile of NiO-containing KL powder before and after milling; XRD pattern of milled composite calcined at 500˚ C and 700˚ C after milling. and grain boundaries (the formation of smaller crystallites was revealed by XRD, as shown in (Figure S4). However, the small hydrogen consumption peak above 700 °C might be a consequence of NiO becoming embedded inside the fused particles; the covering of NiO by a silica layer has been shown to hinder the reduction of the oxide phase.21, 22 The TPR study indicated that calcination of the milled powder performed at 700 °C for 12 h completely reduced the NiO and successfully produced a final SSN composite. The resulting powder had a specific surface area of 21 m2/g-1 and the addition of Ni increased the tap density to 0.80 g/cm3. XRD analysis (Figure 4b) showed that the NiO peak remained after calcination at 500 °C but became invisible after calcination at 700 °C. The data confirmed that a calcination temperature of 700 °C was necessary for the complete reduction of NiO. 3.3 Characterization of the SSN composite. High resolution synchrotron XRD was performed


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for KL and SSN powders (Figure S5) since silicon and its alloy peaks exist close to each other. The calcined powder determined the presence of Si, SiC, Ni, and a small amount of a nickel silicide (NiSi2) (Figure 5a). NiSi2 is believed to originate from interfacial reaction between the Si and Ni particles. Presence of this phase is an added advantage to the material since this is known to improve conductivity of the material due to its metallic nature and help to mitigate volume expansion problem of silicon during the charge and discharge process. 23, 24 The morphology of the final calcined SSN composite was similar to that before calcination and mainly consisted of micron-sized particles (approximately 1–10 µm) with ranging from a few tens to several hundreds of nanometers on their surfaces. EDS analysis of the composite (Figure 5b) produced an intense signal for Si and moderate signals for Ni, indicating uniform distribution of the components after milling and provided a rough estimation of the surface composition of the particles.

Figure 5. Characterization of SSN; (a) synchrotron XRD of the KL and SSN powders. (b) SEM, (b-1–3) EDS elemental mapping, (c) TXM, and (d, e) TEM images of SSN.


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Detailed images of the SSN micron particles were obtained using TXM analysis (Figure 5c) and showed contours of differently sized particles and dark areas at various focal depths and confirmed that the micron particles were composed of many smaller particles. TEM images of the SSN composite (Figure 5d and 5e) showed dark areas ranging from a few hundred nanometers to several microns in size at the center of the particles, as observed in the milled KL powder (Figure S1); this proved that the larger, rigid SiC particles act as a core for the Si nanoparticles. Tiny nanometer-sized dark spots were observed throughout the lighter areas and were identified as being reduced Ni. Particles of a few hundred nanometers in size having many smaller dark spots were also observed. These results demonstrate that the SSN composite is a mixture of micron-sized particles consisting of SiC cores fused with Si nanoparticles, tiny Ni conducting networks supported on these micron-sized particles, and smaller (< 2 µm) fused silicon nanoparticles having Ni conducting networks. 3.4 Electrochemical performance of the SSN composite. The conductivity of the SSN powder was tested with binder and conductive additives before preparing the slurry and compared with that of the KL powder (Figure S6). The SSN conductivity was demonstrated to have been substantially improved by the addition of Ni. Coin cells were fabricated using KL powder, KL powder subjected to 4 h milling (KL-4h), and SSN, and their electrochemical performance was then evaluated using a current density of 30 mA/g for the formation cycles and 120 mA/g for the cycling performance (Figure 6). The KL electrode produced first and second discharge capacities of 1445 and 971 mAh/g and coulombic efficiencies of 72% and 87%, respectively (Figure S7). By contrast, the KL-4h electrode exhibited discharge capacities of 1579 and 1288 mAh/g for the first and second cycles (Figure 6a), which are considerably higher than those of the KL electrode. The KL-4h capacity being


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higher than that of the KL powder indicates that milling, which caused particle reduction, has increased the accessibility of Si to Li ions during the first-cycle lithiation. However, both the KL and KL-4h electrodes exhibited fast capacity fading as like Si nanoparticles, losing more than 50% of their original capacity in 50 cycles (Figure 6c).

Figure 6. (a and b) Voltage profiles of KL-4h and SSN electrodes; (c) plots of discharge capacity and coulombic efficiency versus cycle number for KL, Si nanoparticles, KL-4h, and SSN-3-2 composite and (d) cycling performance of SSN-3-2 composite at various current densities (C-rate, 1C=600 mAh/g; lithiation and de-lithiation were at the same rate). The SSN electrodes were prepared using two Si-to-Ni weight ratios, namely 3:1 (SSN-31) and 3:2 (SSN-3-2). The SSN-3-1 electrode exhibited first- and second-cycle discharge capacities of 1314 and 907 mAh/g. Relative to KL-4h, the improvement in cycle performance was minimal. However, the SSN-3-2 electrode had substantially enhanced cycle stability (Figure 6b and 6c). Notably, the discharge capacities of the initial two cycles of the two SSN electrodes


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were similar despite substantial quantities of the Si in the active material composition having been replaced with Ni. The data indicate that the presence of the Ni conductive additives within the particle matrix increased the capacity of the Si component by enhancing the accessibility of Si to Li ions through improved material conductivity. In addition, formation of the conductive network gave excellent rate performance, has little change in capacity while maintaining 99 % coulombic efficiency; significant decrease observed only when the current density reach to 3.75C (Figure 6d). The KL electrode exhibited the highest capacity fading (Figure 6c) because of its severe volume expansion (278%, Figure S8) and the loss of electronic connectivity between particles. Capacity fading was reduced through particle milling, as demonstrated by the KL-4h electrode, which exhibited the effect of Si particle size reduction and the unique morphology of fused SiCSi agglomerates. The addition of Ni nullified the electronic connectivity loss in the SSN-3-2 electrode; rapid decrease in capacity occurred only in the first two SEI formation cycles and stabilized starting from the third cycle. After the third cycle, the loss in total capacity over 100 cycles of the SSN-3-2 electrode was very small (9%) relative to those of the KL (43%) and KL4h (54%) electrodes. The volume expansion of the SSN-3-2 electrode after 50 cycles was found to be considerably less (39%) than that of the KL electrode, proving that the engineered SSN microstructure enhanced cycle stability. For both the KL and KL-4h electrodes, as the number of cycles increased, the potential of the discharge plateau decreased, whereas that of the charge plateau increased (Figure S7 and Figure 6a). The considerable variations in potential indicate the increasing resistance of the electrode during cycling, which is believed to be caused mainly by the severe volume expansion of the Si particles and the attendant deterioration of the conductive network within the electrode. By


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contrast, for the SSN-3-2 electrode, the potential of the discharge plateau remained essentially unchanged after the second cycle, suggesting that an effective conductive network is maintained throughout cycling. Notably, after prolonged cycling, the flat charge plateau became a sloped plateau (Figure 6b). The sloped profile is a characteristic of the lithiation of amorphous Si via a solid-solution reaction pathway; a crystalline lithiated Si phase, which would produce a twophase reaction pathway with a flat potential plateau of the type observed for the KL and KL-4h electrodes, is not formed. The evolution of the solid-solution reaction pathway facilitates the lithiation/delithiation process by obviating the requirement for a moving phase boundary. Although the breakup and amorphization of the crystalline Si nanoparticles in the SSN electrode are inevitable after charging and discharging, the presence of a well-connected conducting network is believed to keep the amorphized Si particles active and provide a smooth pathway for lithiation and delithiation.

4. Conclusions KL-Si waste containing SiC impurities was successfully transformed into a practical anode for LIBs. The milling of the KL mixture reduced the Si particle size and produced microparticles composed of fused nanosilicon particles supported on rigid SiC. Addition of NiO to the milling mixture and reductive calcination of the composite produced an effective conducting network that was distributed uniformly within the fused matrix of the Si nanoparticles. The fast capacity fading observed for the KL powder and Si nanoparticles was mitigated in the presence of SiC support fused with Si nanoparticles. However, the absence of a conductive network in the milled powder caused electronic connectivity loss and increased polarization during prolonged cycles. These problems were addressed by the introduction of Ni


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to produce a well-connected conducting network in the SSN, leading to smooth lithiation, reduced volume expansion, gradual reduction of polarization, and the retention of electronic connectivity in the longer cycles. The low cost Si source, scalable method, high tap density, and an adequate capacity with good stability produce an anode suitable for practical applications. ASSOCIATED CONTENT Supporting Information. Additional information of kerf-loss and SSN composite involving various characterization methods supplied as Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgment This study was supported by Ministry of Economic Affairs (105-EC-17-A-08-S1-183), Taiwan. The authors also thank S.J. Ji, Ministry of Science and Technology (National Taiwan University) for her help in the SEM analysis.

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Table of Contents Graphic and Synopsis Title: Exploring an Interesting Si Source from Photovoltaic Industry Waste and Engineering It as a Li-Ion Battery High-Capacity Anode Authors: Tzu-Yang Huang, Baskar Selvaraj, Hung-Yu Lin, Hwo-Shuenn Sheu, Yen-Fang Song, Chun-Chieh Wang, Bing Joe Hwang, Nae-Lih Wu

A novel low-cost process is demonstrated to turn kerf-loss Si waste from photovoltaic industry into a high-capacity anode material for Li-ion batteries.


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Exploring an Interesting Si Source from ... - ACS Publications

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