Convertibility of Anode Electrode with Microsized ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Convertibility of Anode Electrode with Microsized Wafer Scraps via Carbon Veil with Plasma Technique Bing-Hong Chen, Shang-I Chuang, and Jenq-Gong Duh* Department of Materials Science and Engineering, National Tsing Hua University, No. 101, Section 2, Kuang-Fu Road, Hsinchu 30013, Taiwan

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ABSTRACT: Low-cost and micrometer-sized silicon cutting scraps were extracted as an anode material within recycled silicon/silicon carbide (Si/SiC) composites (RSCs) for lithium ion batteries (LIBs). A particular approach, carbon veil (Cveil), was unprecedentedly casted onto the surface of electrode and a further combination with plasma treatment was involved to convert the surface status. Both C-veil and plasma-enhanced C-veil (PEC-veil) improved the RSCs-based electrode with more stable capacity retention and higher Coulombic efficiency. Measurements of electrochemical impedance spectra show that the surface modifications reduced the resistance of solid electrolyte interphase (SEI) and resistance of charge transfer, indicating that both C-veil and PEC-veil own the effects of stabilizing formation of solid electrolyte interphases and lower barriers for lithium ion transportation. Furthermore, the expansion thickness of electrode after first cycle gradually reduced, and the surface morphology of SEI changed from cracked pieces to a smooth surface with the integrated treatment. Overall, a simple but distinct C-veil demonstrated a new breakthrough for Si-based anode by C-veil-based surface modification, which also mutates the useless scraps as a potential candidate for LIBs. KEYWORDS: Recycled waste, Atmospheric pressure plasma jet, Evaporation, Lithium-ion battery, Silicon anode

1. INTRODUCTION Motivated by the need for renewable resources and the pursuit of the processing within environmental friendly, the tons of recycled waste materials from solar cell industries have been paid attention seeking the possibility for renewable material. Such wastes are made during the step of cutting silicon ingots into wafers, and such wastes include nanosized silicon (Si), microsized silicon carbide (SiC), and slicing fluid contaminants. Moreover, the recycled waste can be purified into Si-based composites after chemical cleaning and physical separation. In this study, such recycled Si/SiC composites (RSCs) are used as the main ingredient in anodic electrodes to meet the trend of renewability and are involved in the increasingly popular market for the energy storage, especially in lithium-ion batteries (LIBs).1−5 Recently, the graphitized or graphenized approaches were adopted to excite the surface of nanosized SiC and to transfer the inactive SiC into a potential material as anode. Otherwise, the role of SiC in composites is recognized as an inactive material without capacity contribution.7−10 Si can be recognized as a high-capacity anode in LIBs because it has 3590 mAh/g theoretical capacity that forms Li15Si4 alloy-phase during cycling processes.6,11−14 Nevertheless, the remarkable capacity fading by alloy formation to reach high energy density is the intrinsic disadvantage to prevent its application. During lithiation, Si changes its density, expands its volume almost three times, and pulverizes the © 2016 American Chemical Society

structural stability of electrode, which disconnects Si electrodes and causes capacity degradation.15−17 In addition, the reaction between surface cracking or fractured silicon particles and the decomposition of electrolyte forms solid electrolyte interphases (SEI) as an insulator that obstructs the conducting routes of lithium ions (Li+) and electrons.18−20 The growth of SEI in anodes strongly depends on the surface functional groups of particles in the electrode and on the ingredients of electrolyte. The formed morphologies also affect the further decomposition of electrolyte, resulting in the capacity retention.15,17−20 To prevent the formation of SEI, many researchers suggest various tactics, such as (1) wrapping conductive additives, (2) adding electrolyte additives, and (3) synthesizing special-architecture composites to isolate Si particles from electrolytes and to preserve the conductivity of Li+.19−32 Except for the mentioned strategies, the treatment of nitrogen plasma can be another breakthrough path to enhance the electrochemical properties by rearranging the chemical bonds and doping the nitrogen ions for electrodes and particles.8,28−30,32−39 A unique matrix of Li−N compounds could be induced during the first cycling process to confine SEI formation to a more stable state and affect the further growth of SEI in the followed cycles.40−42 Received: October 19, 2016 Revised: December 16, 2016 Published: December 30, 2016 1784

DOI: 10.1021/acssuschemeng.6b02522 ACS Sustainable Chem. Eng. 2017, 5, 1784−1793

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ACS Sustainable Chemistry & Engineering

Figure 1. Schematic diagrams of electrodes: pristine, C-veil, and PEC-veil.

Figure 2. (a) X-ray diffraction patterns of pristine (black), C-veil (blue), and PEC-veil electrodes (red), and the indexes of Si (light blue), SiC (green), copper (orange) and 4H-SiC(asterisk) as referred by ICDD database; various deposition of C-veil with (b) grazing diffraction, (c) Raman spectra, and (d) cycling tests of time from 0 to 8 s.

To emphasize, the microsized material in this work was extracted from renewable waste without designed nanostructure and complicated chemical synthesis, which differs from the highly processed materials of main-stream publications.15,18,42−45 In addition, an evolved technique, carbon coating barely on the electrode (C-veil), was first tried and subsequently combined with atmospheric pressure plasma to enhance the performance and the electrochemical behaviors of RSCs-based LIBs. Such carbon coating was composed of

randomly distributed nanoparticles as a decoration veil on the surface of electrode. Surprisingly, the attempted C-veil exhibited an extraordinary improvement in increasing the efficiency of lithiation, cross-linking the conductive paths of broken routes during cycling and suppressing the SEI formation. Furthermore, the subsequent plasma treatment was cast to modify the electrode with/without the deposited Cveil by converting chemical bonds and implanting nitrogen ions.40,46,47 These multiple surface modifications combine 1785


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Figure 3. FESEM images of (a) pristine, (b) homogeneous region of C-veil, (c) randomly nucleated C-veil, and (d) PEC-veil electrodes. subsequent casted onto the regions with/without C-veil to form up a PEC-veil with the effects of rearranging chemical bonds and doping nitrogen for the predeposited C-veil. 2.3. Analysis Tools and LIB Information. The purity levels of pristine, C-veil-treated, and PEC-veil-treated electrodes were identified using X-ray diffraction device (XRD, Bruker D8 advance) with Cu Kα1 radiation. The surface morphology was observed by a fieldemission scanning electron microscope (FESEM, JSM-7600F JEOL). The chemical compositions and element distribution mapping of the above-mentioned electrodes were investigated using an X-ray photoelectron spectroscope (XPS, ULVAC-PHI 1600) and an electron probe microanalyzer (EPMA, JEOL JSA8500F), respectively. The pristine and modified electrodes were assembled into CR2032 coin cells in an Ar-filled glovebox with a lithium metal counter electrode, a porous polypropylene separator, and electrolyte (1 M LiPF6 in ethlyne carbonate/dimethyl carbonate = 1:1 in vol %). The cycling performances, Coulombic efficiencies, and voltage profiles were evaluated under 0.1 C by Arbin battery tester (BT-2000). The electrochemical behaviors and the ac impedance levels of all electrodes were examined by a Potentiostat 263A for cyclic voltammetry (CV). The chemical compositions of SEI formed on the surface of electrodes underwent composition depth profiling by the FESEM and XPS.

carbon evaporation and plasma treatment provided a distinct approach which is based on the interface between the electrode and electrolyte. The founding of such plasma-enhanced C-veil (PEC-veil) undoubtedly supplied a new concept to suppress the formation of SEI from the first cycle and affect the following growth of SEI to maintain the stability of electrode, so these RWSCs-electrodes can be used in LIBs that have prolonged cyclic performance.

2. EXPERIMENTAL SECTION 2.1. Recycled Si/SiC Composites (RSCs) from Cutting Waste. The active materials, RSCs, were extracted from the silicon-wafer cutting residue of a saw-wiring process in semiconductor factories. First, the wastes were pickled in acid and then rinsed in water to clean up the contaminations and slicing fluid. Baking and physical separation were then employed to obtain the RWSCs as active materials for anode. The ingredients of the pristine electrode included 75% RWSCs, 10% carbon black (Super P), and 15% sodium alginate. 2.2. Surface Modification of Carbon Veil and PlasmaEnhanced Carbon Veil. A C-veil was deposited onto the asprepared electrode by a JEOL Smart Coater with a C rod of 5 mm diameter. The deposition time was selected from 4 to 10 s and the optimum condition for evaporating deposition was fixed at 6 s. The PEC-veil was induced by an atmospheric pressure plasma jet (APPJ) device for C-veil electrode. The plasma equipment was designed with a tungsten rod in the center and surrounded by a grounded stainless steel nozzle. Power was supplied to the APPJ by a 13.56 MHz rf power supply at 50 W and coupled with an automatic matching box. The supplying gas composed of high purity argon (99.99%, 15 slm) and nitrogen (99.99%, 40 sccm) was used as the reactant gas for plasma treatment. Plasma enhancement for C-veil was applied by putting Cveil electrodes on an x, y moving platform with 3 mm vertical distance to the jet nozzle. A clear interpretation of such surface modification was demonstrated in Figure 1. The evolved surface modification process had multiple steps including carbon evaporation and plasma treatment. First, the evaporated C-veil was unprecedentedly deposited on the microsized RSCs anode electrode. Then, the APPJ was

3. RESULTS AND DISCUSSION The XRD patterns of pristine (RSCs-based), C-veil, and PECveil electrodes are shown in Figure 2a. The RSCs powder is the scrap waste from the wafer-cutting process containing slicing fluid and contaminative metal. After chemical cleaning and physical selection, the recycled waste exhibited a composite, including cubic phase of Si (light blue), hexagonal phase of SiC (green), copper (orange), and traces of 4H-SiC (asterisk), and indexed by the ICDD database. The abrasive SiC and its prototypes are native insulators and basically inactive in reacting with Li ions to contribute capacity. The XRD patterns exhibited neither extra phase nor broadened peak, which indicated that the as deposited veils may contribute a smaller 1786


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Figure 4. EPMA analysis of PEC-veil electrode (a) images of BEI corresponding location with C-veil; mapping of (b) C signals; (c) Si signals, and (d) N signals.

boosted dramatically. C6 exhibited the highest capacity after cycling test, and the retained capacity at 80 cycles (381.5 mAh/ g) was improved almost nine times that of the pristine case (41.5 mAh/g). The optimum deposition time was fixed at 6 s for the parameter of C-veil and PEC-veil for further investigation to understand the reasons of boosted duration of batteries. The Si particle sizes were mostly in the range of several hundred nanometers to 1 μm, and SiC particles were generally in the range of 10−15 μm as shown in Figure 3a. The examination of particle distribution is also provided in Figure S3. FESEM images clearly showed the uneven morphology of the as-prepared pristine electrode with some aggregated Si of several micrometers in size. Figure 3b displayed that carbon was deposited via evaporation and that the sheet-like C was random and inhomogeneous yet firm and compactly distributed on the electrode. The inset of Figure 3b exhibits discernibly that the sheet-like C was formed by nanoparticles of around 10−20 nm width that may fill the interval space between particles of electrode surface and play an important role in cross-linking the particles on the interface. Some portion of uncovered electrode is shown in Figure 3c with the same morphology as the pristine electrode. The inset image shows some larger carbon nanosheets embedded on the surface of the electrode,

amount of intensities as compared with the intensities of whole electrode. In addition, the decreased peak intensity of the PECveil pattern also implied that there is deposited carbon on the surface of pristine electrode. However, the results when focusing on low-angle region display that the deposited carbon exists the crystalline orientation at (002) by grazing scanning. The peak position shits to larger angle depending on the deposition time from 0 to 8 s, namely, as C2 to C8, demonstrating that the longer deposition time increases the crystallinity of carbon, as shown in Figure 2b. The deposited time dependence can also be obtained from the Raman scattering in Figure 2c. The disorder band (D-band) and graphic band (G-band) can be located at the position of 1355 and 1595 cm−1 and the domination part of total spectra turns from D- to G-band, which agrees with the results of better crystallinity in Figure 2b with the increasing of deposition time. The improved crystallinity directly reflects in the values of capacity retention and the Coulombic efficiency with applying low current at 0.1 A/g for the cycling tests in Figure 2d. To investigate the C2 case in detail, the total trend of retention can be preserved against the dropping capacities as fast as pristine electrode before 50 cycles despite the residual capacity at 80 cycles being almost the same. When the deposited time surpassed 2 s, both retention and Coulombic efficiency were 1787


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Figure 5. (a) Cycling tests and Coulombic efficiencies of pristine, C-veil and PEC-veil; the voltage profiles at cycles 1, 2, and 80 of (b) pristine, (c) C-veil,and (d) PEC-veil.

in bonding status between electrodes of pristine, C-veil and PEC-veil, indicating that the plasma treatment owns the ability to rearrange the organic bonds on the surface of electrode. In addition, the C−N bonds were doped successfully on the region with C-veil, and Si−N, SiC-N, and SiOx-N bonds were also formed on the uncovered pristine electrode. Those plasmainduced bonds may have provided a Li−N matrix after cycling to stabilize the SEI formation and increase the diffusion ability of Li ions.34−36 XPS and EPMA were applied as a strong evidence to investigate the PEC-veil with regard to the composition of chemical bonds and the homogeneity of elemental distribution. The cycling tests for pristine, C-veil, and PEC-veil electrodes were carried out under 0.1 mA/g; results are shown in Figure 5a. Without any surface modification or milling process, the pristine electrode (black line) exhibited severe capacity degradation because the particles of recycled RSCs were too large as the image in Figure 3a. When Li+ alloyed with Si in the pristine RSCs, volume expanded to more than three times the original volume and inducing surface fractures, thus encouraging the formation of SEI. The more SEI accumulated, the more surface insulation products formed, which prevented Li+ from moving between the interface of electrode and electrolyte. Production of lithium oxide and lithium carbonates resulted from decomposition of organic solvents on the surface of electrode. This obstructed the conductive route for ion transportation, worsened retention, and lowered Coulombic efficiency.11−19 The coated electrode by evaporation, C-veil

illustrating that the nucleation levels of deposited carbon, varying in size from nanoparticles to microsheets, are due to the ragged morphology shown in Figure 3a. The C-veil electrode is so named because this kind of carbon has morphology like a veil draped over the pristine electrode. The image of PEC-veil in Figure 3d presents no significant change after the plasma treatment, which suggests that energetic ions and free radicals would not damage the C-veil during subsequent plasma surface modifications. After casting APPJ on C-veil, EPMA technique for PEC-veil was used to identify the homogeneity and distribution of nitrogen on the surface of electrodes. The corresponding results of C, Si, and N elements for EPMA analyses are shown in Figure 4. The backscattering electron images (BEIs) were aimed at the locations of the PEC-veil in Figure 4a. With the mapping of as-deposited PEC-veil electrode, the position of N signals in Figure 4d were matched with the positions of C and Si signals in Figure 4b,c. The C-veil uncovered regions were also mapped in Figure S1. The mapping results showed that doped N not only existed on the surface of C-veil but also on the uncovered region of the pristine electrode and that Nrelated compounds were distributed homogeneously on the surface of PEC-veil electrode. In order to understand the chemical composition of electrodes before and after the twostep surface modification, the treated electrodes were further investigated by XPS with regard to the C 1s, Si 2p, and N 1s spectra before the electrodes were assembled into coin cells, as shown in Figure S2. All XPS spectra pointed out the differences 1788


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Figure 6. CVs comparison of (a) pristine, (b) C-veil, and (c) PEC-veil; (d) Nyquist plots with fitted electronic circuits and values after the first charged process.

profiles proved that the C-veil reduced the draining of Li ions and stabilized the growth of continuous SEI. The cycling reversibility of the PEC-veil exhibited a surprising 79.79% irreversibility, which indicates an even lesser consumption of Li+ at the first cycle. Moreover, the trait of SEI formation in voltage profile exhibited totally different behavior and reduced more than the case of C-veil, proving that APPJ treatment also prevented SEI formation, as shown in Figure 5d. The capacity of the PEC-electrode had a smooth growth trend, unlike the typical slopes of SEI formation in Figure 6b,c. Both charge/ discharge processes in C-veil and PEC-veil promoted the lithiation/delithiation and provided the stability of SEI formation. So far, an extraordinary coating, C-veil, on the electrode was demonstrated to achieve the effect of stabilizing SEI formation, and the combination of APPJ improved its effect to reach the requirement of higher capacity and longer duration. Such multiple treatments proposed a rapid method without complex chemistry processes (e.g., nanostructured Si) that promoted the electrochemical performance of RSCs as a renewable material source. CV measurements were carried out with a low scanning rate of 0.025 mV/s to understand the redox reaction behavior and the internal differences of SEI formation caused by the C-veil and PEC-veil, as shown in Figure 6a−c, respectively. The CV of the pristine cell displayed a broad reduction peak at the first charging process, which was attributed to the formation of SEI

(red line), displayed better retention (∼9 times of pristine electrode) and more efficiency in lithiation/delithiation reactions, which indicated that the C-veil may have enhanced the reaction kinetics of Li and Si and may have reduced the growth rate of SEI formation. Additional plasma modification of the C-veil (blue line) provided a homogeneous nitride mask on the interface between electrode and electrolyte, which resulted in outstanding performance in not only cycling performance but also Coulombic efficiency when compared with both the pristine electrode and C-veil-treated electrode. The irreversibility of electrode was also improved by either C-veil or PEC-veil. The pristine electrode had an irreversibility of 70.12%; the evaporated C-veil electrode had an irreversibility of 74.97%, showing that the C-veil reduced the consumption of inserted Li+ at the first cycle, as reflected in the voltage profile in Figure 5b,c. The potential profiles exhibited familiar plateau regions except the first cycle and the capacity degradation differed. When the pristine electrodes were first charged, predominantly high potentials of SEI formation were obviously observed via examining the charging profile (from 1.2 to 0.5 V). To inspect the charging process of C-veil case, the window of SEI was lessen its formation obviously, and such phenomena were consistent with the corresponding values of irreversibility. In addition, the typical plateau of alloying/dealloying reactions was evident; it began around 0.3 V for charging and ended around 0.5 V for discharging. The cycling tests and voltage 1789


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Figure 7. FESEM images: top views and crosssectional views of (a, e) unassembled, (b, f) pristine, (c, g) C-veil, and (d, h) PEC-veil after first cycle.

Figure 8. XPS depth etching profiles of C 1s spectrum (a) pristine, (b) C-veil, and (c) PEC-veil; (d) N 1s spectrum of PEC-veil; and Si 2p spectra of (e) pristine, (f) C-veil, and (g) PEC-veil.

as the result of voltage profile. The characteristic peaks of Lialloying on the surface, and the extraction of Li, were noted at 0.206, 0.356, and 0.499 V for the pristine cell. In Figure 6b, the C-veil electrode presented a lower polarization, which indicated a smaller energy barrier for lithiation/delithiation processes, and there were notable readings at 0.222, 0.306, and 0.491 V with lower polarization. Moreover, an extra reduction peak after a few cycles of cell activation was observed at 0.095 V in the CV curve of the C-veil electrode, which indicated the lithiation of amorphous Si with deep charging. During the activation

process, the stress of volume expansion cracked the extracted Si into small pieces; then, the shattered Si transformed from crystalline bulk to amorphous fragments. Furthermore, the peculiarity of SEI appeared less severe at the first cycle, which illustrated that the C-veil had the ability to modify the SEI formation. The results of Figure 6a,b echo the voltage profile results in Figure 6b,c. For the PEC-veil electrode in Figure 6c, all the traits of lithiation/delithiation were traced to positions similar to the positions of the C-veil electrode but instead of 0.084, 0.221, 0.307, and 0.488 V with lower polarization. 1790


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all SEI of pristine, C-veil, and PEC-veil are dominated with ROCO2Li, Li2CO3, and numerous organic compounds, implying that the compositions in all electrodes have no difference after the surface modification. The etching spectra of C 1s and Si 2p from 0 to 10 min are displayed in Figures 8a− c,e−g, respectively, and the N 1s spectra are presented individually in Figure 8d. Compared with the C 1s and Si 2p spectra before cycling in Figure 3a, the cycled pristine electrode exhibited mainly OC−O bonds (289 eV) with a slight accompaniment of C−Si (283.65 eV) and C−C (284.8 eV) bonds. These bonding signals gradually decreased from the surface to the inner electrode, which implied that after the first cycle carbon-related bonding materials on the surface such as SiC and super P reacted with the electrolyte to contribute to SEI partly. In addition, the Si 2p spectra also changed from the double peaks of Si−Si and Si−organic (as shown in Figure 3d) to a broad peak that involved mainly Si−organic bonds, which corresponded to another part of the SEI reaction. With the Cveil, the C 1s spectrum (Figure 8b) presented similar results, but stronger C−C bonds were signaled because the C-veil itself had bonding. The Si 2p spectrum (Figure 8f) showed a dominance of Si−Si bonds during the etching period of 3−5 min. These results indicated the C-veil was able to slow the spread of SEI formation and prevent the Si from reacting with the electrolyte during the first cycle. Although the inhomogeneity of the C-veil limited its ability to prevent SEI from spreading, the finding of C-veil was potential to present a distinct path to effect the growth of SEI. The C 1s and Si 2p bonding conditions on the surface of the PEC-veil electrode were quite different than conditions on the pristine and C-veil electrodes, and are shown in Figure 8c,g, respectively. When the electrode was etched initially (0 and 1 min), the C 1s spectrum of the PEC-veil electrode shed its nitrogen doping, and resembled the state of the C-veil electrode; this corresponded to changes from Figure 3c to those in Figure 3b. This phenomenon provided direct evidence of Li−N matrix formation (∼400 eV) on the surface after the first cycle; this was consistent with the etching results of N 1s in Figure 8 d. Moreover, Li−N bonding signal vanished after 3 min of plasma etching, which indicated that the Li−N matrix was formed only on the surface of the electrode. The C 1s spectrum in Figure 8c depicted a purer domination of C−C bonding (284.8 eV) after 3 min of etching. The Si 2p bonding in Figure 8g showed no trace of Si bonding at the first minute of surface etching. Then, Si bonds emerged gradually, with a majority Si−Si bonding (99.53 eV) and with some Si-related bonds. Both results provided the evidence to prove that the Li−N matrix was only present on the surface of electrode, which had direct effects on the formation of SEI. Overall, C-veil and the further plasma enhancement were able to modify the surface states of electrodes and to slow the growth of SEI on the interface between electrode and electrolyte, especially at the first cycle.

Obviously, the feature of SEI was replaced by a smooth plateau was at the first cycle, which was relevant to the results in Figure 6d. The Nyquist plots of ac impedance shown in Figure 6d were carried out at 0.05 V to imitate the status of interfaces at complete charged state. The equivalent circuit was defined as one series resistance connected with two parallel resistanceconstant phase elements; then, the two semicircles were separated by the series resistance (Rs), SEI (RSEI), and charge transfer (Rct). After delicate fitting, the Rs remained almost constant; the R SEI and R CT were found to decrease progressively, as shown in the inset table of Figure 6d. As a result, the C-veil not only reduced the formation of SEI but also improved the ionic conductivity of interfaces, and the PEC-veil inherited the benefits of C-veil with the additional benefit by introducing nitrogen compounds on the electrode. Both fitting results of C-veil and PEC-veil were desirable for the scalable performance of RSCs-based LIBs. To verify the effect of suppressing the formation of SEI, a FESEM observation of electrodes after one cycle was done to examine the difference in expanded level and SEI morphology of C-veil and PEC-veil, as shown in Figure 7. Top and side views of an unassembled electrode without cycling are displayed in Figure 7a,e. The surface morphology exhibited randomly distributed recycled waste containing Si, SiC, Super P, and binders, and the electrode after the cladding process was estimated to be between the thickness of 13 and 14 μm. In Figure 7b,f, it can be seen that excessive cracks emerged and propagated both on the surface and in the inner pristine electrode, which was mainly caused by the serious volume expansion of RSCs during cycling, especially at the first cycle. Moreover, the particles were clearly wrapped by SEI, and the surface of the electrode was obviously chapped by inner forces through the alloying process. The expanded thickness was evaluated as approximately 20 μm when the electrode was on the verge of collapse; this can be attributed to the reason for capacity degradation during cycling. Under the stern condition of intrinsic expansion during lithiation, the techniques of C-veil and PEC-veil were provided to soothe the issues by surface modification. Surprisingly, the C-veil coating produced an entirely different surface morphology after the first cycle, which was flatter than the pristine electrode yet with some fissures on the surface of electrode, as depicted in Figure 7c. The distinctly different surface conditions of the C-veil electrode indicated milder SEI formation, and the thickness of C-veil electrode expanded to only 18.34 μm, as shown in Figure 7g. However, the C-veil was inhomogeneous; thus, the C-veil electrode still had minor cracks, which limited its effect in SEI suppression (consistent with the results in its voltage profile and CV/ac measurements). The application of C-veil did have a remarkable potential to reduce the magnitude of SEI formation and loose the inner stress to stabilize the electrode during cycling. The top view of the PEC-veil electrode revealed an extraordinarily smooth surface, as shown in Figure 7d. Such topography suggested that the PEC-veil retarded severe interactions between electrode and electrolyte and inhibited the formation of SEI. Therefore, the PEC-veil electrode displayed less expansion, with a measured thickness of 15.31 μm. Nitride-based compounds may have protected the electrode and slowed the growth of SEI formation.34−36 To analyze SEI chemical composition, after the pristine, Cveil, and PEC-veil electrodes had undergone the first cycle, measurements were taken via FTIR spectra (Figure S4) and XPS depth profiling (Figure 8). The FTIR patterns exhibit that

4. CONCLUSIONS In this study, we demonstrated a distinct approach to modify the RSCs-based electrodes for lithium ion batteries via developing the technique of C-veil and its further extended application with APPJ. The particular electrodes were made with renewable material, RSCs, which was extracted from the cutting waste in solar panels industry. The size of RSCs contained Si/SiC composites in microscale and expected to have severer capacity degradation as active material of anode. With the C-veil, the electrode surface was filled by the carbon 1791


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nanoparticles, and particles on the surface were also crosslinked. This C-veil prevented the disconnection of conductive routes after cycling and enhanced the ability of Li+ transportation. Through the investigation of XPS and EPMA, the results revealed random carbon decoration and homogeneous nitride compounds overall C-veil and PEC-veil electrode. The effect of gradual suppression of the formation of SEI was verified through the voltage profile, CV and ac measurement. The characteristic SEI peaks were lessened, and the RSEI and Rct were also reduced by the integrated treatment. The performance and irreversibility of RSCs were improved gradually by Cveil-based techniques from severe capacity degradation (70.12%) to flat cyclic retention (79.79%) under 80 cycles of testing with 0.1 A/g. The FESEM images of top and side views exhibited an entirely different SEI morphology from ragged to smooth. Futhermore, the expanded thickness of electrodes was also improved by developed treatment from 20.51 to 15.31 μm after cycling. This illustrated that the spread of SEI can be stabilized by C-veil and its further application with combining APPJ. The XPS depth profile gave evidence for the existence of Li−N matrix and the SEI reaction at first cycle. All results suggest that a novel approach to stabilize SEI formation by Cveil can make possible to RSCs as a low-cost electrode material for LIBs.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02522. EPMA analyses, fitted XPS spectra, particle distribution of recycled silicon/silicon carbide composites, and FTIR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jenq-Gong Duh: 0000-0002-7364-4283 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from the National Science Council of Taiwan (NSC 103-2622-E-007-001-CC1). We also appreciate the efforts of supporting resources from Prof. WeiRen Liu in Chung Yuan Christian University and the assistance of Precision Instrumentation Center at National Tsing Hua University with XPS and EPMA analyses.

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REFERENCES

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