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A Revival of Waste: Atmospheric Pressure Nitrogen Plasma Jet Enhanced Jumbo Silicon/Silicon Carbide Composite in Lithium Ion Batteries Bing-Hong Chen,† Shang-I Chuang,† Wei-Ren Liu,*,‡ and Jenq-Gong Duh*,† †

Department of Materials Science and Engineering, National Tsing Hua University, No. 101, Section 2, Kuang-Fu Road, Hsinchu 30013, Taiwan ‡ Department of Chemical Engineering, Chung Yuan Christian University, 2200, Chung Pei Road, Chung Li 32023, Taiwan S Supporting Information *

ABSTRACT: In this study, a jumbo silicon/silicon carbide (Si/SiC) composite (JSC), a novel anode material source, was extracted from solar power industry cutting waste and used as a material for lithium-ion batteries (LIBs), instead of manufacturing the nanolized-Si. Unlike previous methods used for preventing volume expansion and solid electrolyte interphase (SEI), the approach proposed here simply entails applying surface modification to JSC-based electrodes by using nitrogenatmospheric pressure plasma jet (N-APPJ) treatment process. Surface organic bonds were rearranged and N-doped compounds were formed on the electrodes through applying different plasma treatment durations, and the qualitative examinations of before/after plasma treatment were identified by X-ray photoelectron spectroscopy (XPS) and electron probe microanalyzer (EPMA). The surface modification resulted in the enhancement of electrochemical performance with stable capacity retention and high Coulombic efficiency. In addition, depth profile and scanning electron microscope (SEM) images were executed to determine the existence of Li−N matrix and how the nitrogen compounds change the surface conditions of the electrodes. The N-APPJ-induced rapid surface modification is a major breakthrough for processing recycled waste that can serve as anode materials for next-generation high-performance LIBs. KEYWORDS: recycled waste, silicon lithium ion battery, atmospheric pressure plasma, surface modification, solid electrolyte interphases

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INTRODUCTION Practical solutions are being implemented for resolving green energy problems, and related technologies are entering a vigorous development stage. Demands for photovoltaic cells are growing with the advancements in renewable energy technologies, particularly for solar power industries.1,2 Most cells are produced from silicon (Si) ingots after wafer cutting by wire sawing.3 However, such processes yield numerous particle wastes including the remains of Si ingots (approximately several hundred nanometers), abraded silicon carbide (SiC) microparticles from cutting wire, and slicing fluid contaminants.4 In recent years, to reduce the sustained growth of wafer slicing wastes, waste processing has become a major topic in recent years, especially the application of purified wastes.1−4 Si, the most promising material for next-generation lithiumion batteries (LIBs), possesses a theoretical capacity of 3590 mAh/g with the formation of Li15Si4. Nevertheless, Si inherently undergoes a volumetric expansion of approximately 280% during lithiation, leading to structural pulverization and electrical disconnection of electrodes.5,6 Pulverization is strongly dependent on the particle size of Si, and the critical size in diameter is approximately 150 nm. When the particle size is larger than the critical value, such as that of microscale Si © 2015 American Chemical Society

particles from cutting waste (>150 nm), Si particles exhibit surface cracking and fracturing during cycling processes.7 Such cracks and fractures cause capacity degradation and the formation of solid electrolyte interphase (SEI),8−14 which is induced by a reaction between electrode and electrolyte. Regarding purified waste for LIBs, larger Si particles are associated with a more severe SEI in addition to electrode deterioration and diminished performance during cycling.7,15 To improve the capacity retention of Si-based LIBs, suppressing the formation of SEI is imperative, particularly for microscale Si particles. Numerous studies have endeavored to (1) shrink Si particles to nanometer scales by using various morphology-control techniques, (2) wrap or mix conductive additives onto particles or into slurries, (3) coat metals, oxides, or polymers onto electrodes, and (4) add electrolyte additives.6,16−25 Apart from the mentioned methods, nitrogen (N)-plasma enhanced chemical vapor deposition has been used to deposit a buffer layer for producing Si-based anodes.26−28 Embedded N compounds provide a Li−N conversion reaction, Received: July 1, 2015 Accepted: October 13, 2015 Published: October 13, 2015 28166

DOI: 10.1021/acsami.5b05858 ACS Appl. Mater. Interfaces 2015, 7, 28166−28176

Research Article

ACS Applied Materials & Interfaces

Figure 1. Characteristics of purified powder. (a) XRD pattern and (b) SEM image of refined waste powder. (c) XRD pattern of treated electrode with various treatment times. (d) The particle size distribution of purified waste.

the electrode, leading to the formation of a Li−N matrix during cycling. Furthermore, mechanisms of surface reactions are proposed for investigating the effectiveness of plasma surface modification in suppressing SEI formation and consequently improving cycling performance. Plasma surface modification thus enables reviving wafer slicing wastes, which may serve as novel materials for next-generation LIBs.

which effectively suppresses the formation of SEI because of N doping. Most of the aforementioned processing methods involve high-cost vacuum systems and complicated chemical reactions, which do not meet economical manufacturing and strict environmental regulations. Instead of focusing on volume expansion problems and low-pressure processing, the novel and vacuum free method atmospheric pressure plasma jet (APPJ) processing on electrode must be investigated. Surface modifications can be achieved in a relatively short processing duration by using highly reactive particle species generated by plasma treatment processes. Although several studies have suggested that surface N doping can enhance electrochemical performance,26,28 studies that have applied atmospheric plasma technology for inducing surface modification are scant. Therefore, atmospheric pressure plasma processing is an innovative surface processing approach for suppressing the formation of SEI and has high potential for application in LIBs. In this study, a jumbo Si/SiC composite (JSC), a purified active material from waste, served as an LIB anode material. Moreover, an APPJ treatment process, a technique free of vacuum components, is presented for modifying the surface of JSC-based electrodes. Such surface modification provides energetic ions for rearranging the organic bonds on the surface of waste and forming nitride compounds at the same time on

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EXPERIMENTAL SECTION

A. Source of Purified Waste. Raw powder was obtained from cutting waste produced during saw-wiring processes in a solar-cell factory. To remove the cutting fluid and contaminations, the raw material was purified through acid pickling and water rinsing. After the drying and separating process, the JSC was extracted and used as an active material in LIBs. The phase of the recycled waste was identified using X-ray diffraction (XRD, Bruker D8 advance) with Cu Kα1 radiation, and its morphology was observed using field emission scanning electron microscopy (FESEM, JSM-7600F JEOL). The particle size distribution of recycled waste was analyzed using a laser diffraction particle size analyzer (Malvern Mastersizer2000). B. Composite Anode Formation and Nitridation by Atmospheric Pressure Plasma Jet. The active materials were mixed with 10 wt % of carbon black and 15 wt % of sodium alginate. For electrochemical characterizations, the mass loading of the active materials was 0.18−0.25 mg/cm−2 and electrode density was 0.05−0.2 28167

DOI: 10.1021/acsami.5b05858 ACS Appl. Mater. Interfaces 2015, 7, 28166−28176

Research Article

ACS Applied Materials & Interfaces

Figure 2. High-resolution XPS data and fitting analysis of N-APPJ: (a) Survey spectra, (b) N 1s of N501, (c) N 1s of N505, (d) Si 2p of N000, (e) Si 2p of N501, and (f) Si 2p of N505. g/cm3. The slurry was then casted onto a copper foil by using a cladding process, and the as-prepared electrode was subsequently treated with (N-APPJ). The APPJ comprised a centered tungsten rod surrounded by a grounded stainless steel nozzle. Power was derived from a 13.56-MHz rf power supply coupled with an automatic matching box. The supply gas was a mixture of high-purity (99.99%, 15 slm) argon (Ar) and N (40 sccm), which served as the reactant gas for the plasma ignition process. At an applied power of 50 W and with different treatment repetitions (0, 1, and 5, corresponding to N000, N501, and N505 electrodes), the plasma treatment was conducted at a rate of 5 s/cm2. The as-prepared electrode was fixed underneath a jet nozzle on an X−Y moving platform with a 3 mm distance. Subsequently, the treated electrode was assembled into a coin cell (CR2032) in an Ar-filled glovebox containing a counter electrode (Li

metal), separator (porous polypropylene), and electrolyte (1 M LiPF6 in ethylene carbonate/dimethyl carbonate = 1:1). The chemical compositions and element distribution mapping of the electrodes were investigated using an X-ray photoelectron spectroscopy (XPS, ULVAC-PHI 1600) and electron probe microanalyzer (EPMA, JEOL JXA-8500F), respectively. The retention, Coulombic efficiency, and voltage profile were cycled under 0.1 C by using Arbin battery tester (BT-2000). The electrochemical behavior of the electrodes was evaluated using cyclic voltammetry (CV), and the AC impedance of the electrodes was examined using Potentiostat (263A). A Li−N matrix formed after the first cycle was analyzed by depth profiling the electrode after one cycle by using XPS. The qualitative analysis of SEI were investigated using FESEM and Fourier transform infrared spectroscopy (FTIR) for electrodes after the first 28168

DOI: 10.1021/acsami.5b05858 ACS Appl. Mater. Interfaces 2015, 7, 28166−28176

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Figure 3. Quantitative 3D bar plots of N-APPJ (a) N−Si, (b) N−O and N−C of N 1s, (c) Si−N, Si−C, and Si−Si of Si 2p, and (d) Si−Ox, N−Si− O, and O−Si−O of Si. cycle. The results may serve as an indicator of how a Li−N matrix suppresses the formation of SEI, consequently enhancing the capacity retention of recycled Si anodes.

To understand the chemical interaction between plasma and JSC-based electrode, a compositional analysis was conducted using XPS, and the measurements were directly conducted on the as-cast electrode before the cycling process. Figure 2a illustrates the XPS spectra for all conditions; the Si, C, O and Na bonding spectra originated from the purified waste and binder. Figure 2b and 2c show the N 1s fine-scan spectra for N501 and N505, respectively, observed after the N-APPJ treatment. Because the dissociated N in the plasma reacted with indigenous materials and with the native oxide and hydrocarbon on the recycled particle, the spectra were fitted in detail by using Gaussian functions of Nx-Si (397.4 eV), N−Si0.75 (398.58 eV), N−C (399.62 eV), and N−O (400.81 eV) bonds. The N000 spectra were fitted with Si−Si (99.53 eV), Si−C (100.37 eV), Si(−OH)x (101.02 eV), Si−Ox (103.68 eV), O− Si(-OH)x (104.33 eV), and O−Si−O (105.22 eV) originating from JSC, native oxide SiOx, SiO2, and organic compounds. Broad full width at half-maximum peaks were observed for Si(−OH)x, Si−Ox, and O−Si(−OH)x and were due to the difference in the stoichiometry between organic compounds and oxide. Since JSC is highly susceptible to surface oxidation, wafer slicing waste contained numerous organic bonds because of the cleaning process. In addition to the described characteristics of the peaks, the effects of Si 2p after N-APPJ treatment were determined using Si−Nx (101.48 eV), Si−N1.33 (102.34 eV), Si−Ox (103.58 eV), and N−Si−O (104.35 eV), as shown in Figure 2e and f, indicating the replacement of organic bonds (−OH)x with the doped N, which is consistent with the

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RESULTS AND DISCUSSION Figure 1a shows the XRD pattern of recycled waste after the pickling and rinsing processes. The purified powder was indexed as a mixture of cubic Si (ICDD: 27-1402) and hexagonal SiC (ICDD: 49-1426) formed from Si ingot debris and piano wire abrasion. Moreover, 4H-SiC (ICDD: 29-1127), which is marked with an asterisk in the figure, was traceable as a SiC prototype, and such a prototype has low conductivity and no capacity contribution.29 As illustrated in Figure 1b, the purified powder had an uneven morphology, consisting of a JSC comprising Si with a particle size of several hundred nanometers and SiC measuring 10 μm. Figure 1c shows the XRD patterns of the fabricated electrode (N000) and plasmatreated electrodes (N501 and N505). The XRD patterns exhibited no extra phase or amorphous properties, even when the treatment repetition was increased, indicating that plasma treatment may be effective on only the surface of a structure rather than affect the integrity of a bulk structure. Figure 1d illustrates the particle size and content of the recycled Si/SiC, indicating that the size of the Si particles mainly ranges from several hundred nanometers to several micrometers because of particle aggregation and that the dominant size of the SiC particles is 10 μm. 28169

DOI: 10.1021/acsami.5b05858 ACS Appl. Mater. Interfaces 2015, 7, 28166−28176

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Figure 4. EPMA analysis of electrodes after N-APPJ: (a) image of BEI (note that the displayed BEI shows a slightly electron charged up because of poor conducting polymer binders), (b, d) Si mapping, (c, e) N mapping image, and (f) schematic plot for the function of N-APPJ.

results of the N spectra and those of previous studies.17,28,30−36 Table S1 shows a summarized table of XPS data for all electrodes for N 1s and Si 2p. Chemical bonding was quantitatively analyzed using integrated 3D-bar plots to examine the effectiveness of the N-APPJ treatment (Figure 3). For the N 1s spectra, the Ncontribution was clearly determined from the Si and C or O bonds. N−Si bonding was deconvoluted into Nx-Si and N− Si0.75 bonds, and the amounts of N−C and N−O bonds demonstrated an inverse relationship with the Si bonds when the treatment repetition was increased (Figure 3a and b). Furthermore, for the Si 2p spectra (Figure 3c), the percentage of Si−Si bonds dropped slightly because of the oxidization of Si and the N-doping of Si−N and N−Si−O. The Si−C bonds remained almost constant under all treatment conditions, and the variation of the Si−N bonds was inversely proportional with the increase in treatment repetition, which is consistent with the findings in the N 1s spectrum. The Si−O bond was divided

Figure 5. Cycling test and Coulombic efficiency of N000, N501, and N505.

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DOI: 10.1021/acsami.5b05858 ACS Appl. Mater. Interfaces 2015, 7, 28166−28176

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Figure 6. Charge−discharge profiles of (a) N000 and (b) N501 during the first two and 40th cycles.

with broken − H from hydrocarbons. Furthermore, N doping was achieved through the reaction between N ions and the tangling bonds on the surface. Therefore, the function of NAPPJ was determined to be a result of N doping of SiONx, CNx, SiNx, and SiCNx with suggested interstitial sites.37−39 All the reactions of tangling bonds and the nitridation of Si, O, C, and SiC were resolved by low-pressure plasma applications as follows:17,28,34,35

into three components: (1) Si−Ox, (2) N−Si−O, and (3) O− Si−O; each integrated area and the total amount of Si−O related bonds were based on the treatment repetition. As illustrated in Figure 3d, N501 and N505 exhibited a higher percentage of Si−O bonds among the three components, and the percentages of the remaining bonds remained almost constant in N000, except for the aforementioned attenuation in the amount of Si−Si bonds. Overall, the results of N 1s and Si 2p, decrease in N bonds, and increase in O bonds indicated the interdependence of plasma-surface reaction and the treatment duration. In general, a longer plasma treatment duration generated higher energy through particle collision, achieving excited and ionized gas states (e−, Ar, and N). Surface modification occurs because of energy transfer generated during particle collisions. However, the N 1s and Si 2p spectra indicated that the percentage of N bonds was independent of the treatment time. This result revealed that the N doping process was dependent on the surface potential energy of reactants. According to ab initio studies conducted for determining the potential energy of Si, C, and SiC, vacancies are available on surface interstitial sites,37−39 indicating the possibility of executing doping processes with energetic ions. In addition, Jantschner et al.37 demonstrated that Si sites exhibited preferential attraction to oxygen; they reported that the highest negative energy value was associated with the potential surface energy of oxygen, signifying the priority reaction between Si and O. Therefore, in the longer plasma treatment process, the bonds dominating the chemical reaction switched from N−Si to N−C and N−O. The N−C bond exhibited a major contribution in N 1s spectra because SiC was the dominant component in the waste. Moreover, the increased magnitude of N−O bonds after plasma treatment may have resulted from the collision between energetic N ions with native oxide SiOx, which then recombined into N−Si−O. Hence, a longer plasma treatment may provide continuous chain reactions that involve rearranging organic bonds and doping processes on electrode surfaces, leading to the possible formation of stable SiO2. Before the interstitial sites were determined for reaction, the rearrangement of energetic ions was initiated reacting at weaker van der Waals, hydrocarbon, and covalent bonds on the surface; consequently, the ascending bonding energy was approximately several 10−1 to 101 eV. The broken −O bond from the native oxide recombined with SiOx or evaporated into water vapor

N2 + C → CNx + (1 − x)N2

(1)

2SiOx + N2 → 2SiONx

(2)

2Si + N2 → 2SiNx + (1 − x)N2

(3)

2SiC + N2 → 2SiCNx + (1 − x)Nx

(4)

As mentioned, plasma surface modification induces a conductive layer on a surface comprising various nitride compounds, which is beneficial for Li-ion transfer.6,26,28 Moreover, longer plasma treatment increases the insulating properties of Si−N and Si−Ox bonds as the formation of Si3N4 and SiO2, which are not beneficial in ensuring cycling stability. The exact treatment duration is thus a critical condition for balancing the proportion of insulators (Si3N4 and SiO2) and conductors (CNx, SiONx, SiNx, and SiCNx). Therefore, to verify the effectiveness of plasma, the electrodes were mapped with Si and N signals to obtain their distribution information. EPMA compositional maps were examined for determining the electrode element distribution after N-APPJ treatment (Figure. 4). Figure 4a shows the backscattering electron image (BEI) of the N-APPJ electrode for the corresponding mapping position. Carbon and oxygen are relatively inessential mainly because the analysis is biased by carbon coatings (which improve conductivity) and organic compounds existing in binders. Figure 4b and c illustrate the results of the Si and N mapping. As shown in these figures, N was mainly distributed at the positions where Si and SiC were observed, and this result is consistent with the SEM image depicted in Figure 4a. In addition, places with relatively weak Si signals exhibited a N trace, indicating that N was doped not only onto Si but also onto carbon black (Figure 4d and e). Through the mapping results, we can confirm that the nitride compounds are distributed homogeneously on the surface of electrode. 28171

DOI: 10.1021/acsami.5b05858 ACS Appl. Mater. Interfaces 2015, 7, 28166−28176

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and N505 at 100 mA/g (Figure 5). N000, the JSC-based electrode, exhibited capacity decay because the size of recycled Si (i.e., several hundred nanometers) induced severe volume expansion with continue formation of SEI during cycling. Conversely, the N501 and N505 electrodes exhibited considerable enhancement after the N-APPJ treatment, implying that the surface-modified electrode may have a peculiar reaction with the interface between electrode and electrolyte: the Li−N matrix.26,28 Irreversibility disclosed certain clues at the first cycle; the reversible capacity values for the N000, N501, and N505 electrodes were 86.96%, 81.25%, and 71.53%, respectively, and the lower value corresponds to the more consumption of Li ions for L−N and L−O compounds during charge/discharge.26,28 To check the reproducibility of N501 and N505, more cells for cycling test and voltage plot were displayed in Figures S1 and S2. The phase transformation induced by the N-APPJ products draining more Li ions that can be expressed as follows reactions on the surface of JSC-based electrodes: SiNx + y Li ↔ Si + x Li y / xN

(5)

CNx + y Li ↔ C + x Li y / xN

(6)

SiONx + y Li ↔ SiO2 − x + x Li y / xN

(7)

SiCNx + y Li ↔ SiC + x Li y / xN

(8)

According to the Coulombic efficiency, the electrodes subjected to the N-APPJ treatment demonstrated more effective recovery during the lithiation/delithiation process compared with those that did not undergo this treatment process. This result confirms that the L-N matrix provides higher Li-ion conductivity.40,41 When the N-APPJ process was not applied, N000 had the lowest Coulombic efficiency; nevertheless, N505 exhibited a lower efficiency than did N501. This phenomenon was explained by the XPS analysis results (Figure 3d), which revealed that the Si−O bond comprised two components: Li-ion conductors (e.g., Si−Ox and N−Si−O) and insulators (e.g., O−Si−O). In addition to the magnitude of the Si−Ox bond, the magnitude of the N−Si− O bond was based on the treatment duration, and this bond can be converted into a Si−Ox bond according to eq 7. Si−Ox bond is likely to react with electrolyte at the first cycle according to following equations:6,8−10,13,14 SiOx + y Li + ye → Si + Li yOx yLi + SiOx → Li ySiOx

Figure 7. CV comparison of (a) N000 and (b) N501 with the characteristic peaks. (c) Nyquist plots after the first charging process involving a fitted electronic circuit.

(9) (10)

The production of LiyOx results in an irreversible capacity that hinders Li-ion conductivity, implying that the N505 electrode, which contained a higher amount of N−Si−O and Si−Ox bonds, retained lower irreversibility at the first cycle compared with the other electrodes. Therefore, a higher amount of LiyOx generated on the interface indicates a reduction in the number of transfer routes, diminishing the Coulombic efficiency. Noticeably, the distributions of N−Si−O and Si−Ox were lower for the overall interface compared with the Li−N matrix. According to the mentioned reactions, insulators, such as Si3N4, SiO2, and LiyOx, formed during the first cycle and occupied some conductive paths used for ion transportation. Nevertheless, the presence of the Li−N compound provided paths for transporting Li ions and blocked cracked Si, leading to a chain reaction forming a thicker SEI. To

To conclude the XPS and EPMA results, Figure 4(f) illustrates the effectiveness of the N-APPJ induced surface treatment in modifying the electrodes. The functions of the NAPPJ treatment approach include (I) rearrangement of organic bonds, (II) oxidation of SiO2 and (III) doping of N on the interface between the electrode and electrolyte. Such products may suppress SEI formation, which may solve the problem of poor capacity retention by resolution of existing conflicts between forming a conductive layer and creating insulating yet stable compounds. After the interaction of N-APPJ with recycled waste was characterized, the low rate cycles were tested for N000, N501, 28172

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Figure 8. XPS Ar etching of the first cycle: (a) Si 2p of N000, (b) Si 2p of N501, and (c) N 1s of N501.

that the phase transformation occurred according to eqs 5−8. The slope of the typical alloying plateau of N501 demonstrated a smoother growing trend (almost linear) than did that of the N000 electrode during the charge (approximately 0.3 V) and discharge (approximately 0.5 V) processes, indicating that the phase transformation of the Li−N matrix was reversible with higher stability. For the second cycle of the N501 electrode, limited diminished capacity was observed because of battery activation. Moreover, the capacity of the N501 electrode for the 40th cycle remained almost constant, unlike that observed at the beginning, which was considerably enhanced compared with that of the N000 electrode. To further investigate the electrochemical behavior of the NAPPJ electrode, the CV curves measured at the first two and fifth cycles at a rate of 0.025 mV/s were observed for N000 and N501, as shown in Figure 7a and b, respectively. The N000 differential capacity curve exhibited characteristic peaks at 0.21, 0.35, and 0.50 V, corresponding to the insertion, intercalation on the surface, and extraction of Li, respectively. A broad peak was observed at the first charging process, and this peak was attributable to the formation of SEI and the blunt pattern fluctuation was due to the inertia abundant organic bonds on the surface of recycled Si. After the N-APPJ treatment, the sharp peaks shifted to a lower potential of lithiation/delithiation at 0.22, 0.31, and 0.48 V, demonstrating easier insertion/ extraction processes for Li ions with lower polarization.42−44 Furthermore, a smooth pattern was observed in the first charging process, implying radical change of SEI formation, and an extra peak was observed in N501 at 0.1 V. This peak was attributable to the lithiation of amorphous Si with deep charging, illustrating that the Li−N matrix could provide an amorphous medium for achieving a highly complete charging process for JSC-based LIBs. The N000 and N501 electrodes were subjected to electrochemical impedance spectroscopy (Figure 7c) for demonstrating the effectiveness of the Li−N matrix in promoting capacity retention. The Nyquist plots were detected at 0 V for the charged state with the formed SEI, and the equivalent circuit was defined as one series resistance connected with two parallel resistance-constant phase elements. The two semicircles are separated by the series resistance, SEI, and charge transfer,

Figure 9. FESEM images: (a, c, and e) top view and (b, d, and f) cross section of the unassembled, N000, and N501 electrodes.

elucidate the formation of the Li−N matrix, the first two and 40th charge/discharge potential profiles for both N000 and N501 were compiled (Figure 6). The plateau regions, which represent potential ranges of alloying/dealloying reactions, began at 0.3 V for charging and at 0.5 V for discharging. As illustrated in Figure 6a, a predominantly high potential of SEI formation was observed in the first charge process. This phenomenon is consistent with the irreversibility shown in Figure 5, indicating reduced ionic conductivity. After the Li−N matrix was incorporated, the N501 potential was slightly greater than 0.5 V during the first two charging processes, indicating 28173

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Figure 10. Schematic plot for the difference in the amount of waste with/without N-APPJ (recycling waste).

namely, Rs, RSEI, and Rct, respectively, defined as the battery interfaces.45,46 When the Rs value remains constant, the RSEI value drops from 96.01 to 57.93 Ω and the Rct value decreases from 20.59 to 13.02 Ω. The fitting results indicate that the Li− N matrix suppresses the formation of SEI and improves the charge transfer ability of Li ions. To analyze SEI with Li−N matrix thoroughly, XPS depth profiling was conducted by subjecting the N000 and N501 electrodes to Ar etching after the first cycle (Figure 8). As shown in Figure 8a, the Si−O bonds in the N000 electrode were constantly present over an etching durations of 0−30 min. This proved the formation of cracks, which resulted in the production of internal SEI (i.e., S−O bonds) during the charge/discharge process. In addition, the peak in Figure 8(a) represents the shifts of the Si−O bonds to higher binding energy levels, revealing the formation of O−Si−O bonds. The depth profiling results indicated that the N000 cycling performance degradation was mainly induced by cracked Si waste, leading to the propagation of SEI on the fresh fractures. Moreover, the N000 electrode in Figure 8b was compared with the N501 electrode, and the results revealed that the Si 2p profile illustrated a completely different SEI trait. For an etching duration of 0−3 min, the signals did not show any Si; however, the Si−O bond emerged during an etching duration of 3−5 min. This observation indicated that the plasma surface modification introduced a nitride layer on the electrode surface. After 10 min of etching, the XPS characteristic observed was similar to that of the uncycled electrode shown in Figure 2e, illustrating that the nitride layer maintained the completeness of fractural particles and isolated the organic bonds on the surface of cracked Si to prevent a continual SEI reaction as N000 electrode. Furthermore, a N compound was observed at 400 eV during the first 5 min of Ar etching, signifying the

presence of Li−N bonds. These results validated the existence of the Li−N matrix on the electrode surface. To further investigate the SEI formation, SEM images for unassembled, N000, and N501 electrodes were examined (Figure 9). Figure 9a and b depicts typical top and side views of the unassembled electrode, respectively. The surface morphology exhibited randomly distributed Si particles, and the electrode thickness ranged from approximately 13 to 14 μm after the cladding process. As depicted in Figure 9c and d, excessive cracks emerged and propagated on the surface and cross section of the N000 electrode (after the first cycle) because the volume expansion of recycled Si (several hundred nanometers) induced severe particle fractures. The thickness expansion was nearly 20 μm, which can be considered another indicator of volume change. By contrast, the morphology of recycled Si changed from angular particles to spherical pieces, which were obviously wrapped by the SEI. Moreover, comparing Figure 9a and c revealed that the formed SEI of N000 was rough and uneven that the fractured N000 electrode could not bear the strain during the cycling process. Unexpectedly, with the plasma treatment, the morphology of the SEI and degree of volume expansion were considerably different (Figure 9d and e). For the N501 electrode, after the first cycle, the SEI topology exhibited a flat and smooth layer on the electrode, which is consistent with the results show in Figure 8b and c, clearly indicating the existence of a nitride layer with surface modification. In addition, the thickness expansion was 16.825 μm, and this thickness value is between the value of the unassembled electrode (approximately 13−14 μm) and that of the N000 electrode (approximately 20 μm), signifying that the plasma treatment not only produces a nitride layer to reduce SEI formation, as the electrochemical data had clearly showed but also advantages to the topology of SEI on the surface of electrode. Furthermore, the measurement of 28174

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FTIR was also examined for the qualitative analysis of SEI with N000 and N501 electrode after the first cycle. However, the absorbed pattern (Figure S3) exhibit that both SEI of N000 and N501 are dominated with ROCO2Li, Li2CO3, and numerous organic compound, indicating the composition in both samples have no difference. On the basis of these comprehensive investigations and additional comparisons in Figures 8 and 9, this study obtained evidence according to the XPS etching profile and FESEM after the first cycle. The nitride layer formed in the N-APPJ treatment demonstrated a relatively stable SEI, which enabled retaining the stability of JSC-based electrode. Figure 10 illustrates the waste JSC reuse strategies implemented in this study. The JSC was obtained from cutting waste, and it was subjected to a simple purification process and used as an LIB anode material. Moreover, Si waste particles measuring several hundred nanometers incur severe volume expansion, and such expansion produces fresh surfaces through particle fracture during charge/discharge. Therefore, the disadvantageous capacity retention effect is due to SEI formation through the reaction of cracked Si and electrolyte. A N-APPJ treatment approach is proposed for pretreating JSC electrodes. The electrodes were modified using N-doped compounds, and the Li−N matrix was constructed during charge/discharge processes. Such surface modification provides a nitride layer to suppress the formation of SEI, leading to improved capacity retention.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The authors are grateful for the financial support from the National Science Council of Taiwan (NSC 103-2622-E-007001-CC1). The assistance of XPS, EPMA analysis in Precision instrumentation center at National Tsing Hua University are also appreciated.

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CONCLUSIONS This study presents a novel N-APPJ technique for recycling solar power industry cutting waste by modifying the surface of recycled jumbo Si particles. According to the XPS analysis, the chemical composition of the recycled waste was different from that of typical nano-Si, which does not have abundant organic bonds on the surface. The functions of the N-APPJ treatment include the rearrangement of organic bonds, oxidation of SiO2, and doping of N on the electrode surface. The EPMA results revealed that the distribution of N was interspersed on the electrode surface, where N reacted not only with the recycled waste but also with carbon black. The cycling performance of the JSC-based electrode was investigated using an electrochemical analysis, and L−N matrix was discovered and addressed after CV and AC measurements. According to the results of XPS involving Ar etching and the FESEM images before/after cycling, the Li−N matrix supplied a flat nitride layer to suppress the formation of SEI and prevented the continual propagation of SEI from the reaction between cracks Si and electrolyte. This study therefore extends existing knowledge about the application of atmospheric pressure plasma technology for recycling JSC anode materials for highperformance LIBs, providing a new lease for waste Si.

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Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b05858. XPS spectra with fitting, a table of percentage and fitting parameters for each different duration of plasma treatment, two figures of information on the repetition tests of long cycles and voltage profile, and FTIR measurement before/after first cycle (PDF) 28175

DOI: 10.1021/acsami.5b05858 ACS Appl. Mater. Interfaces 2015, 7, 28166−28176

Research Article

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