Performance Enhancement of Silicon Alloy-Based ... - ACS Publications

Mar 23, 2016 - Pankaj K. Alaboina , Jong-Soo Cho , Md-Jamal Uddin , Sung-Jin Cho ... G. Kannan , Seon Kyung Kim , Cheolho Park , Dong-Won Kim...
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Performance Enhancement of Silicon Alloy-Based Anodes Using Thermally Treated Poly(amide imide) as a Polymer Binder for High Performance Lithium-Ion Batteries Hwi Soo Yang,† Sang-Hyung Kim,† Aravindaraj G. Kannan,† Seon Kyung Kim,‡ Cheolho Park,‡ and Dong-Won Kim*,† †

Department of Chemical Engineering, Hanyang University, Seoul 133-791, Korea Next-G Institute of Technology, Iljin Electric Co. Ltd., Gyeonggi-do 425-100, Korea

‡

S Supporting Information *

ABSTRACT: The development of silicon-based anodes with high capacity and good cycling stability for next-generation lithium-ion batteries is a very challenging task due to the large volume changes in the electrodes during repeated cycling, which results in capacity fading. In this work, we synthesized silicon alloy as an active anode material, which was composed of silicon nanoparticles embedded in Cu−Al−Fe matrix phases. Poly(amide imide)s, (PAI)s, with different thermal treatments were used as polymer binders in the silicon alloybased electrodes. A systematic study demonstrated that the thermal treatment of the silicon alloy electrodes at high temperature made the electrodes mechanically strong and remarkably enhanced the cycling stability compared to electrodes without thermal treatment. The silicon alloy electrode thermally treated at 400 °C initially delivered a discharge capacity of 1084 mAh g−1 with good capacity retention and high Coulombic efficiency. This superior cycling performance was attributed to the strong adhesion of the PAI binder resulting from enhanced secondary interactions, which maintained good electrical contacts between the active materials, electronic conductors, and current collector during cycling. These findings are supported by results from X-ray photoelectron spectroscopy, scanning electron microscopy, and a surface and interfacial cutting analysis system.

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morphology,8−11 alloying with inert metals,12,13 embedding silicon in a conductive material,14,15 and applying several functional binders.16−21 In particular, the selection of a proper binder material is very important to maintain the electrode structure and provide good capacity retention because the polymer binder allows the electrode to adhere to a current collector and improves the coherence of silicon active particles.22 Improvements in cycling stability observed when using proper binders were attributed to strong adhesion and high modulus, possibly because of the presence of secondary interactions in the polymer binder or the formation of a crosslinked network during the electrode preparation.23,24 Among the various binder materials used in silicon-based electrodes, poly(amide imide) (PAI) is one of the most promising polymers because it combines the beneficial properties of polyimide and polyamide. Polyimide has exceptionally high thermal stability and mechanical strength due to its charge transfer interactions and π−π* interactions.25 Polyamide has hydrophilic functional groups, which make it soluble in polar organic solvents such as N-methyl-2-pyrrolidone (NMP). Also,

INTRODUCTION Lithium-ion batteries have become the dominant power sources for portable electronic devices due to their high energy density and long cycle life, and they are actively being developed as power supplies for electric vehicles and energy storage systems.1−6 Graphite has been widely used as an active anode material in commercialized lithium-ion batteries. The available capacity of the graphite in lithium-ion batteries is about 350 mAh g−1, which is close to its theoretical capacity of 372 mAh g−1. In order to improve the energy density of lithium-ion batteries, silicon-based anode materials have been actively studied. Silicon materials have a high theoretical capacity, a low reduction potential, are environmentally benign, and are low cost, making them attractive candidates for next-generation lithium-ion batteries.7 However, silicon materials suffer from substantial volume changes during lithiation and delithiation, which are highly detrimental to the cycling stability of lithiumion batteries. The mechanical stresses caused by repeated changes in volume can fracture the electrode, which causes poor electrical contacts between the active materials, electronic conductors, and current collector. As a result, a serious capacity decline occurs during repeated cycling. To solve these problems, many studies have been carried out by different approaches such as controlling the particle size and © 2016 American Chemical Society

Received: January 19, 2016 Revised: March 12, 2016 Published: March 23, 2016 3300

DOI: 10.1021/acs.langmuir.6b00205 Langmuir 2016, 32, 3300−3307

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Figure 1. (a) SEM image, (b) CP-SEM image, (c) XRD pattern, and (d) EDS spectrum of silicon alloy particles.

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polyamide has good binding properties because it has amide groups that allow for hydrogen bonding, which is much stronger than any other secondary interactions that occur in polyimide. Choi et al. reported that PAI could be successfully applied as a polymer binder in silicon-based negative electrodes by improving the electrochemical reversibility of the active silicon material.16 In a silicon-based electrode employing a PAI binder, the secondary interactions in PAI can play an important role in determining the structural stability of the electrode and the binding characteristics of electrode/current collector interface. However, the presence of silicon alloy particles and conductive carbon spatially confines the PAI polymer chains, thereby restricting the possibility of secondary interactions. This can be overcome by thermally treating the PAI polymer above its glass transition temperature to increase the polymer chain mobility, which permits enhanced intra- or interchain secondary interactions through polymer chain rearrangement. To the best of our knowledge, there have been no systematic studies related to tailoring the secondary interactions of PAI using thermal treatment to improve its binding characteristics and cycling performance of silicon-based electrodes. In this work, we synthesized silicon alloys as an active anode material, which was composed of silicon nanoparticles embedded in Cu−Al−Fe matrix phases. Here, the inactive metal matrix helped reduce the volume expansion of silicon and enhanced the electronic conductivity.26 Using these high capacity silicon alloy materials, we prepared the electrodes employing PAI binders thermally treated at different temperatures, and their electrochemical performances were investigated. Our objective was to improve the cycling performance of silicon alloy electrodes by identifying an efficient thermal treatment that gives improved interfacial adhesion between the active materials, electronic conductors, and current collector through the strong secondary interactions of the polymer binder.

EXPERIMENTAL SECTION

Synthesis of Silicon Alloy Materials. Silicon alloy materials were synthesized using arc melting followed by the single roll solidification method (SRSM), as previously reported.26 Si (50 at. %), Cu (22.5 at. %), Al (22.5 at. %), and Fe (5 at. %) metals were used as precursors to synthesize the silicon alloy. Alloy buttons were obtained by arc melting in a Cu hearth using a nonconsumable tungsten electrode under an argon atmosphere. The buttons were remelted three times to ensure homogeneity. SRSM ribbons were produced by a graphite nozzle single-roll method under an inert atmosphere to prevent oxidation. The obtained ribbons were mechanically crushed into powders in an attrition mill at a rotating speed of 150 rpm with zirconia beads. Finally, silicon alloy powders with an average particle size of 3.8 μm were obtained. Electrode Preparation and Cell Assembly. PAI (HV 4000 T, Mw = 18 000−25 000) was purchased from Solvay Advanced Polymer. The silicon alloy electrodes were prepared by coating a viscous slurry containing 86.6 wt % silicon alloy, 3.4 wt % Ketjen black, and 10 wt % PAI binder dissolved in NMP onto copper foil. The cast slurry was dried in a vacuum oven at 80 °C for 1 h to evaporate the NMP solvent. The dried electrodes were then thermally treated at different temperatures (200, 300, and 400 °C) in a tube furnace for 1 h in an Ar atmosphere. A silicon alloy electrode without any thermal treatment was used as a control sample. Also, the silicon alloy electrodes employing other commercial binders such as poly(vinylidene fluoride) (PVdF) and poly(acrylic acid) (PAA) were prepared as control samples. The active mass loading in the electrodes was about 1.4 mg cm−2 based on weight of Si alloy, which corresponded to a capacity of approximately 1.6 mAh cm−2. In order to evaluate the cycling characteristics of silicon alloy electrodes, a CR2032-type coin cell was assembled by sandwiching the polyethylene separator (ND420, thickness: 20 μm, Asahi Kasei E-materials) between the metallic lithium counter electrode and the silicon alloy working electrode. The lithium electrode consisted of a 100 μm thick lithium foil (Honjo Metal Co. Ltd.) pressed onto a copper current collector. The cell was then injected with an electrolyte solution consisting of 1.15 M LiPF6 in ethylene carbonate (EC)/ethylmethyl carbonate (EMC)/diethyl carbonate (DEC) (3/5/2 by volume) containing 5 wt % fluoroethylene carbonate (FEC) (battery grade, 3301

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Figure 2. (a) N 1s and (b) O 1s XPS spectra of PAI binders thermally treated at different temperatures. PANAX ETEC Co. Ltd.). All cells were assembled in a drybox filled with argon gas. Characterization and Measurements. The morphologies of the silicon alloy powders were examined using field emission scanning electron microscopy (FE-SEM, JEOL JSM-7600F) equipped with energy dispersive X-ray spectroscopy (EDS). In order to examine the cross-sectional morphologies of the silicon alloy materials and the silicon alloy electrodes, the samples were cut using an argon-ion beam polisher (JEOL, IB-09010CP) at a constant power under an inert Ar atmosphere to avoid chemical damage. The samples were transferred into the SEM chamber without any air exposure, and their crosssectional SEM images were obtained. X-ray diffraction (XRD) pattern was recorded on a Rigaku DMAX/2500 using Cu Kα radiation. The chemical bonding energy of PAI in the different electrodes was investigated using X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific Co.). Tensile measurements were carried out using a universal testing machine (Instron E3000LT). Charge and discharge cycling tests of the silicon alloy electrodes were carried out at a current density of 0.32 mA cm−2 (0.2 C rate) over a voltage range of 0.005− 1.5 V using battery test equipment (WBCS 3000, WonA Tech Co., Ltd.). We hereafter refer to lithiation as the charge and delithiation as the discharge, based on the practical applications of lithium-ion batteries. During the charging cycles, lithiation was performed at a 0.2 C rate to a set voltage of 0.005 V. This was followed by charging at constant voltage until the final current reached 10% of the charging current. The specific capacities were calculated based on the mass of silicon alloy in the electrode. In order to investigate the mechanical strength of the PAI binder in the silicon alloy electrodes, the shear stress and adhesion strength in the electrodes were measured using a surface and interfacial cutting analysis system (SAICAS, Daipla Wintes Co. Ltd.).20 In the SAICAS measurements, a boron nitride blade fixed at a 45° shear angle was used. The blade moved in a horizontal direction at 0.2 μm s−1, maintaining a vertical force of 0.4 N during the test.

proven to act as inactive matrix during charge/discharge cycling from ex situ XRD and ex situ HRTEM analyses in our previous literature.26 The EDS results presented in Figure 1d reveal that the silicon alloy consisted of 49.1 at. % Si, 22.2 at. % Al, 22.9 at. % Cu, and 5.8 at. % Fe. The binding energy in the PAIs thermally treated at different temperatures was investigated by XPS analysis. Figures 2a and 2b show the N 1s and O 1s XPS spectra of the PAI binders thermally treated at different temperatures. In Figure 2a, the peak observed at around 400.0 eV could be resolved into two components, as illustrated in Figure S1. The peak at 399.8 eV in the PAI without thermal treatment was ascribed to the nitrogen atoms of amide groups, and the peak at 400.5 eV was assigned to the nitrogen atoms of the imide group.27 With increasing thermal treatment temperature, the peaks shifted to higher binding energies at 400.2 and 400.9 eV, respectively, in the PAI thermally treated at 400 °C. The peak shifts to higher binding energies indicated that inter- and intramolecular secondary interactions between functional groups in PAI became stronger due to molecular chain rearrangement during thermal treatment.28,29 In particular, the peak shift was more pronounced at 400 °C, which was well above the glass transition temperature of PAI (∼285 °C) and therefore allowed a higher degree of chain mobility during heat treatment.30 The enhanced mobility allowed facile molecular chain rearrangement; therefore, the functional groups in amides and imides interacted more effectively. The electrons in the oxygen or nitrogen were positively influenced by the neighboring protons, resulting in the increase of binding energy. In the case of the O 1s XPS spectra of the PAI, the peak observed in the range of 529−535 eV were also deconvoluted into two components, as presented in Figure S2. The peak that appeared at 531.4 eV in the PAI without thermal treatment was assigned to the oxygen atoms of amide group, and the peak at about 532.6 eV corresponded to the oxygen atoms of the imide group.27,31 These two peaks also gradually shifted to higher binding energy with increasing thermal treatment temperature, and the peaks appeared at 531.8 and 533.0 eV, respectively, in the PAI thermally treated at 400 °C. On the basis of these results, we expected that the temperature of the thermal treatment played a vital role in forming strong interactions in the silicon alloy electrodes and provided the electrodes with mechanical strength to withstand the mechanical stresses induced by the volume changes of the silicon alloy particles. To demonstrate the enhanced mechanical property of PAI binder upon thermal treatment, tensile measurements were carried out using PAI films thermally treated at different temperatures, and the results are presented in Figure S3. As expected, the untreated PAI film showed the

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RESULTS AND DISCUSSION Silicon alloy particles were obtained by mechanically crushing the ribbon-type alloy material. A SEM image of the silicon alloy particles is shown in Figure 1a. The silicon alloy particle had a flat surface and sharp edges. Its average particle size was measured to be about 3.8 μm. Cross section polished (CP)SEM was performed using an argon-ion beam polisher to observe the microstructure of the silicon alloy material. As shown in Figure 1b, the alloy was composed of two different phases. The distribution of silicon in the alloy was found to be very uniform. According to EDS analysis, the dark regions were silicon crystallites, and the light parts corresponded to inert metal phases composed of Al4Cu9 and AlFe.26 XRD pattern of silicon alloy particles (Figure 1c) also confirms that the alloy material is composed of crystalline silicon, Al4Cu9, and AlFe. The Al4Cu9 and AlFe phases were 3302

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Figure 3. Preconditioning charge and discharge curves of silicon alloy electrodes thermally treated at different temperatures: (a) first cycle and (b) second cycle (0.1 C constant current (CC) and constant voltage (CV) charge, 0.1 C CC discharge, cutoff voltage 0.005−1.5 V).

Figure 4. Cross-sectional SEM images of (a) the silicon alloy electrode without thermal treatment and (b) the silicon alloy electrode thermally treated at 400 °C. The SEM images were obtained before cycling (left), at fully charged (lithiated) state (middle), and at fully discharged (delithiated) state (right).

400 °C showed the highest initial charge and discharge capacities of 1375 and 1177 mAh g−1 with an initial Coulombic efficiency of 85.6%. Higher charge and discharge capacities in the electrode thermally treated at 400 °C were ascribed to the enhanced mechanical strength of the PAI binder, which accommodated the mechanical stress due to volume changes during cycling. The irreversible capacity observed in the first cycle was attributed to SEI layer formation due to the reductive decomposition of liquid electrolyte as well as the irreversible lithiation of native oxides present on the silicon surface.32 Also, the CO groups present in PAI may irreversibly react with lithium ions during the first charge process, thereby partially contributing to the irreversible capacity.16 Silicon materials undergo large volume expansion during the lithiation process, resulting in continuous formation and breakdown of a SEI layer on the exposed silicon alloy surface through electrolyte decomposition.33 In the silicon alloy electrode thermally treated at 400 °C, the strong binding of PAI reduced the cracking and pulverization of active silicon alloy materials, and therefore improved the Coulombic efficiency by suppressing electrolyte decomposition on the surface of the silicon alloy particles. The Coulombic efficiencies increased during the second cycle for all the electrodes, as shown in Figure 3b. Higher Coulombic efficiency at the second cycle indicated that

lowest tensile strength among the tested samples, whereas the PAI film treated at 400 °C exhibited the highest tensile strength. The improved mechanical property of PAI was attributed to the enhanced secondary interactions such as hydrogen bonding and π−π* interactions. The effect of the thermal treatment on the electrochemical performance of silicon alloy electrodes was investigated by galvanostatic measurements. The silicon alloy electrodes were initially subjected to two preconditioning cycles at 0.1 C rate. The current rate (0.1 C) in the preconditioning cycle was lower than current rate (0.2 C) for cycling test, in order to effectively form solid electrolyte interphase (SEI) layer on the electrode surface during initial cycles. Figures 3a and 3b show the charge and discharge curves of the silicon alloy electrodes thermally treated at different temperatures, for the first and second cycles, respectively. All the electrodes exhibited typical charge and discharge profiles corresponding to the lithiation of Si and the delithiation of LixSi, respectively. The initial charge and discharge capacities for the silicon alloy electrode without any thermal treatment were 1205 and 957 mAh g−1 based on the silicon alloy material in the electrode, respectively, showing a Coulombic efficiency of 79.4%. Both reversible capacities and Coulombic efficiencies for the first cycle increased with increasing thermal treatment temperature. The silicon alloy electrode thermally treated at 3303

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Figure 5. (a) Charge and discharge curves of the silicon alloy electrode thermally treated at 400 °C and (b) discharge capacities of the silicon alloy electrodes thermally treated at different temperatures. (0.2 C CC and CV charge, 0.2 C CC discharge, cutoff voltage 0.005−1.5 V).

Figure 6. Cross-sectional and surface SEM images of different silicon alloy electrodes after 100 cycles: (a, b) silicon alloy electrode without thermal treatment; (c, d) silicon alloy electrode thermally treated at 400 °C.

silicon alloy electrode thermally treated at 400 °C expanded to 148% after lithiation and maintained its robust structure; furthermore, the active materials were relatively well connected after delithiation. This result may be attributed to the high mechanical stability of the PAI binder, which withstands the volume changes of silicon during cycling due to the strong secondary interactions in the electrode thermally treated at 400 °C. The strong binding properties of PAI arose from strong interactions between the functional groups of PAI, which reduced the electrode expansion and maintained the electrical contact between active materials, conducting carbons, and current collector. After two preconditioning cycles, the cells were cycled in the voltage range of 0.005−1.5 V at a 0.2 C rate. Figure 5a shows the charge and discharge curves of the silicon alloy electrode thermally treated at 400 °C. The silicon alloy electrode initially delivered a discharge capacity of 1084 mAh g−1 with a Coulombic efficiency of 98.6%. A delithiation plateau was

the reductive decomposition of the electrolyte solution was suppressed by the SEI layer formed during the first cycle. In order to investigate the effect of thermal treatment on the interfacial contacts and adhesive properties in the silicon alloy electrode, we obtained cross-sectional SEM images of the silicon alloy electrodes without thermal treatment and after thermal treatment at 400 °C after preconditioning cycles. The cross-sectional SEM images were obtained before cycling, in a fully charged state and in a fully discharged state, respectively, as shown in Figures 4a and 4b. As charging or discharging progressed, the cross-sectional morphology changed distinctly, depending on the thermal treatment. The silicon alloy electrode without thermal treatment expanded about 260% after the full charging process relative to its initial thickness. It contracted again by about 19% after the full discharging process relative to its thickness in a fully charged state. The electrode showed some voids and cracks between the electrode and current collector after the preconditioning cycles. In contrast, the 3304

DOI: 10.1021/acs.langmuir.6b00205 Langmuir 2016, 32, 3300−3307

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Figure 7. SAICAS results of (a) shear stress and (b) adhesion strength of the silicon alloy electrodes without thermal treatment and after thermal treatment at different temperatures after 100 cycles.

observed at approximately 0.45 V during the first 20 cycles. This plateau can be attributed to delithiation of Li15Si4 that formed during the previous lithiation process, as reported earlier.17,26 After 100 cycles, the discharge capacity decreased to 927 mAh g−1, which corresponds to 85.5% of the initial discharge capacity. Coulombic efficiency steadily increased with cycling, and it was maintained at >99.5% throughout cycling after stabilization. Figure 5b shows the discharge capacities of the silicon alloy electrodes thermally treated at different temperatures during 100 cycles at a 0.2 C rate. As shown, the thermal treatment significantly affected the electrochemical performance of the electrodes. The silicon alloy electrode without thermal treatment exhibited large capacity fading in the first few cycles. On the other hand, the silicon alloy electrodes thermally treated at higher temperatures (300 and 400 °C) showed stable cycling characteristics and relatively higher discharge capacities during cycling. The differences in the capacity retention behaviors in the electrodes thermally treated at different temperatures were due to the differences in binding force of the PAI binders in the electrodes. As mentioned earlier, the strong binding due to secondary interactions in the electrodes thermally treated at higher temperatures reduced cracks or pulverization of the silicon alloy active material and maintained good electrical contact between the silicon alloy active materials, which led to good capacity retention. In comparison, the discharge capacity of silicon alloy electrodes with widely used commercial binders such as PVdF and PAA rapidly decayed, as shown in Figure S4a, which reveals that thermally treated PAI binder exhibits superior binding characteristics. The electrochemical performance of silicon alloy electrodes was also compared with some of the reported systems based on silicon and silicon alloy electrodes employing various binders (Table S1). It shows that the performance of the thermally treated silicon alloy electrode in this study is comparable to or better than the reported results. Furthermore, the difference in the cycling performance of silicon alloy electrodes treated at different temperatures was more pronounced at a higher current rate (2.0 C) (Figure S4b). As shown in figure, the silicon alloy electrode treated at 400 °C showed the best cycling performance in terms of discharge capacity and capacity retention. These results demonstrate that the use of thermally treated PAI binder remarkably enhanced the cycling performance of the silicon alloy electrode. After 100 cycles, the cross-sectional and surface FE-SEM images of silicon alloy electrodes were examined in the lithiated

state. As shown in Figures 6a and 6b, significant electrode pulverization and electrode detachment from the current collector were observed in the silicon alloy electrode without thermal treatment, as would be expected from large volume changes during the charge and discharge cycles. The loss of electrical conduction paths in this electrode may be a main cause for severe capacity degradation shown in Figure 5b. On the other hand, the electrode expansion was remarkably reduced in the silicon alloy electrode thermally treated at 400 °C, as presented in Figures 6c and 6d. Furthermore, the electrical contacts between active materials and copper foil were well maintained in the electrode. This result suggested that the good cycling stability of the silicon alloy electrode thermally treated at 400 °C was closely related to the strong binding ability, which provided a robust electrode structure during cycling. To measure the shear stress and adhesive strength of the electrodes, SAICAS experiments were performed for the silicon alloy electrodes thermally treated at different temperatures after 100 cycles. As reported in previous studies using SAICAS on electrodes, the shear stress and adhesion strength at a specific interface in the electrode was obtained by this method.34,35 SAICAS was used to measure the horizontal and vertical force by dragging a blade across the middle of the electrode layer on a copper current collector. Once the blade penetrated the middle of the electrode, the cutting mode was complete, and the peeling mode began. Figure S5 shows the horizontal and vertical force while the blade cut and peeled the middle layer of the electrodes. Overall, both horizontal and vertical forces increased with thermal treatment temperature, indicating that the silicon alloy electrode thermally treated at higher temperature had strong adhesion and was more difficult peel out. The shear stress and adhesion strength of the electrodes were calculated as a function of horizontal force during the cutting mode and peel mode. Figures 7a and 7b show the shear stress as a function of depth in cutting mode and the adhesion strength as a function of peel test time, respectively. As shown in Figure 7a, the shear stress corresponding to the force needed to cut through the electrode layer increased with thermal treatment temperature. Similar to the shear strength behavior, the adhesion strength was higher in the silicon alloy electrode thermally treated at higher temperatures. On the basis of these SAICAS results, we concluded that the high mechanical strength of the PAI binder in the electrode thermally treated 3305

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Figure 8. (a) Voltage profiles of the silicon alloy electrode thermally treated at 400 °C as a function of the C rate and (b) discharge capacities of the silicon alloy electrodes thermally treated at different temperatures, with the C rate increasing from 0.1 to 5.0 C every five cycles.

of silicon-based electrodes for high energy density lithium-ion batteries.

at high temperature enhanced the adhesion between silicon alloy particles, conducting carbon, and current collector, which enabled the electrode to withstand mechanical stresses during cycling. We evaluated the rate capability of the silicon alloy electrodes thermally treated at different temperatures. The cells were charged to 0.005 V at a constant current rate of 0.2 C, followed by a constant voltage charge, and they were then discharged at different current rates ranging from 0.1 to 5.0 C. Figure 8a shows the charge and discharge curves of the silicon alloy electrode thermally treated at 400 °C as a function of the C rate. Discharge capacities slightly decreased with increasing the C rate due to the polarization. The electrode exhibited good high rate performance, showing a discharge capacity of 887 mAh g−1 at 5 C rate. Figure 8b compares the discharge capacities of the silicon alloy electrodes thermally treated at different temperatures for C rates increasing from 0.1 to 5.0 C every five cycles. The effect of thermal treatment temperature on the rate performance of the silicon alloy electrodes was noticeable as the current rate was increased from 0.1 to 5.0 C rate. The rate capability was greatly enhanced by increasing thermal treatment temperature to 400 °C. When the volume of the silicon alloy changed during repeated cycling, the interfacial contacts deteriorated. In the silicon alloy electrode thermally treated at 400 °C, the PAI binder had good mechanical strength, and the strong secondary interactions in the electrode prevented the silicon alloy from being pulverized and detaching from the electrode at high rate cycling, which resulted in good high rate capability. As the current rate decreased to 0.1 C again, it recovered its high discharge capacity of 1056 mAh g−1, indicating its stable cycling behavior.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b00205. Table comparing the performance of silicon-based electrodes employing various binders, deconvoluted N 1s and O 1s XPS spectra of PAI binders thermally treated at different temperatures, load−extension curves of PAI films thermally treated at different temperatures, cycling performance of silicon alloy electrodes with various binders, cycling performance of silicon alloy electrodes with thermally treated PAI at different temperatures at 2.0 C rate; peeling test profiles of SAICAS for silicon alloy electrodes (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (D.-W.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Iljin Electric Co., Ltd., provided the silicon alloy materials. This work was supported by the green industry leading secondary battery technology development program of KEIT (10046341, Development of a high capacity, low cost silicon based anode material for lithium secondary batteries) and by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (2014R1A2A2A01002154).

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CONCLUSIONS We showed that the thermal treatment of PAI binder in silicon alloy electrodes had a profound effect on the mechanical stability and cycling performance of the electrodes. The thermal treatment of the electrodes at high temperatures improved the mechanical integrity of the electrodes by maintaining good contacts between the active materials, the conducting carbon, and the current collector, resulting in a great improvement in cycling performance such as discharge capacity, capacity retention, and rate capability. Our results demonstrate that the thermal treatment of PAI binder at optimal temperature is an efficient strategy to preserve the electrode integrity and ensure good capacity retention during charge/discharge cycling

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REFERENCES

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DOI: 10.1021/acs.langmuir.6b00205 Langmuir 2016, 32, 3300−3307

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DOI: 10.1021/acs.langmuir.6b00205 Langmuir 2016, 32, 3300−3307