Epoxidized Natural Rubber-Chitosan Network Binder for Silicon

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Energy, Environmental, and Catalysis Applications

Epoxidized Natural Rubber-Chitosan Network Binder for Silicon Anode in Lithium Ion Battery Sang Ha Lee, Jeong Hun Lee, Dong Ho Nam, Misuk S. Cho, Jaehoon Kim, Chalathorn Chanthad, and Youngkwan Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01614 • Publication Date (Web): 24 Apr 2018 Downloaded from http://pubs.acs.org on April 24, 2018

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ACS Applied Materials & Interfaces

Epoxidized Natural Rubber-Chitosan Network Binder for Silicon Anode in Lithium Ion Battery Sang Ha Lee1, Jeong Hun Lee1, Dong Ho Nam2, Misuk Cho1, Jaehoon Kim2,Chalathorn Chanthad1,3,* and Youngkwan Lee1,* 1

School of Chemical Engineering, Sungkyunkwan University, 440-746 Suwon, Republic of

Korea. 2

School of Mechanical Engineering, Sungkyunkwan University, 440-746 Suwon, Republic of

Korea. 3

National Nanotechnology Center (NANOTEC), National Science and Technology

Development Agency (NSTDA), Pathum Thani 12120, Thailand *

To whom correspondence should be addressed

KEYWORDS: elastic binder, chitosan, natural rubber, silicon anode, lithium ion battery

ACS Paragon Plus Environment

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ABSTRACT

Polymeric binder is extremely important for Si based anode in lithium ion batteries due to large volume variation during charging/discharging process. Here, natural rubber-incorporated chitosan networks were designed as a binder material to obtain both adhesion and elasticity. Chitosan could strongly anchor Si particles through hydrogen bonding, while the natural rubber could stretch reversibly during the volume variation of Si particles, resulting in high cyclic performance. The prepared electrode exhibited the specific capacities of 1,350 mAh/g after 1,600 cycles at the current density of 8 A/g and 2,310 mAh/g after 500 cycles at the current density of 1 A/g. Furthermore, the cycle test with limiting lithiation capacity was conducted to study the optimal binder properties at varying degree of the volume expansion of silicon, and it was found that the elastic property of binder material was strongly required when the large volume expansion of Si was occurred.


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Introduction

The high theoretical capacity (~4,200 mAh/g) and low reaction potential make silicon (Si) a potentially useful anode material for lithium ion batteries, but Si anode suffer from significant intrinsic volume change (>300 %) during lithiation and delithiation.1-4 Classical PVDF binder has been found to be ineffective in supporting the stable cycling of Si anodes, because the weak adhesion of the binder is unable to sustain electrode integrity during the large volume change.1 As alternative binder materials, polymers possessing carboxyl or amine group, such as poly(acrylic acid) (PAA), carboxymethyl cellulose (CMC), alginate and chitosan, have been proposed.5-9 Their functional groups can form hydrogen- or covalent-bonding with the hydroxyl groups on the surface of Si particles, which highly enhance the sustainability of the electrode. During the expansion of Si particles, the binder suppressed electrode volume expansion by fastening the particles, resulting in the improvement of electrode stability. But unfortunately, it is generally accepted that the adhesion of the binder is not solely capable of resolving the issues arising from the volume variation of Si particles.10 To further enhance the durability of Si-based electrodes, researchers have attempted to suppress volume variation of the electrode by using a network structure with enhanced mechanical properties.11-15 The cross-linked polymer binder could further enhance the durability, because the 3D network with superb mechanical strength enables high resistance to strain, and thus tolerates the volume change of Si particles by confining the Si particles and densifying the electrode.9,11-12 Nonetheless, rigid polymer binders cannot completely relieve the stress, so that cracks often develop during repeated lithiation and delithiation cycles.13-17 Elastic polymers have been attempted to be incorporated into binder materials to relieve the stress.16-18 The elastic polymers allow volume expansion of the Si electrodes, without


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challenging physical integrity or developing cracks, inducing enhanced stability. A study of using elastic binder with Si particles was first presented in 2007, where Li et al. attempted to use CMC/Styrene butadiene rubber (SBR) as a binder material.18 But the CMC/SBR-based electrode exhibited inferior cyclic performance to the CMC-based electrode due to the absence of the functional groups in SBR. After repeated volume changes of Si particles during charging/discharging, SBR part might be physically separated from Si particles due to insufficient adhesion. Gum arabic or pectin has recently been used,16,17 which are more elastic than CMC or PAA. They also have abundant carboxyl or hydroxyl groups to provide robust interactions with the Si particles, inducing the electrode with enhanced cyclic stability. More recently, Choi’s group suggested a highly elastic binder material adopting a novel structure, polyrotaxane-incorporated polyacrylic acid, where the ring sliding motion of polyrotaxane induced highly elastic properties, resulting in highly stable Si anode. This study confirmed that high elasticity of binder materials could induce the enhancement of cyclic stability of Si anodes, however, its complex structure and preparation processes are the limitation for the commercial utilization. In this study, natural rubber-incorporated chitosan network structure was designed to obtain both sufficient adhesion and elasticity. The chitosan could strongly anchor the Si particles through hydrogen bonding, due to the presence of amine and hydroxyl groups, while the rubber part could stretch and contract reversibly during charging/discharging process (Fig. 1b). So, the elastic property of rubber in the network could stabilize the adhesion between chitosan and Si particles during the volume variation of Si,10 and the electrode film could expand and contract reversely without mechanical failure of the electrode, resulting in high sustained capacitive performance (Fig. 1c).


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We conducted conventional peracetic acid process to allow natural rubber be epoxidized (ENR), and prepared chitosan-ENR networks via a crosslinking reaction between epoxy and amine (Fig. S1). Natural rubber was used in this study because of its high fracture toughness, abundance, inexpensive cost, and facile process for epoxidization.19,20 Mechanical properties of the prepared materials were characterized by varying the rubber contents, and their electrochemical performance as a Si-based anode binder in lithium ion battery was investigated. Furthermore, the influence of the elasticity for binder material on capacitive properties of its electrode was carefully discussed with morphological and electrochemical analysis results.

Figure 1. (a) Design of the binder material, (b) working mechanism of binder and (c) the variation of electrode during charging/discharging process.


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EXPERIMENTAL SECTION

Materials: Silicon nanoparticles (average particle size of ~100 nm) were purchased from Alfa Aesar. Chitosan derived from crab shell chitin and Igepal CO-890 (nonionic surfactant) were purchased from Aldrich. High ammonia natural rubber latex (60 % dry rubber content) was purchased from the Thai Rubber Latex Corp., Thailand. Formic acid, and hydrogen peroxide (30 %W/V) were purchased from Fisher Chemical, and glacial acetic acid was purchased from Carlo Erba reagents. Epoxidization of natural rubber: The 50 g of natural rubber latex was stirred with 5 g of Igepal CO-890 and 50 ml of deionized water. Then 10 g of formic acid and 15 g of H2O2 were added dropwise. The mixture was heated to 70 °C for (0.5 and 1) h, respectively, to attain different degrees of epoxidization, resulting in epoxidized natural rubber (ENR) of (15 and 30) mol%, respectively. The products were washed via dialysis tube to achieve a concentration of ~20 wt% ENR in water. The 15 mol% ENR was used in all experiments, except for the electrochemical performance experiments. Film and electrode fabrication: The chitosan was dissolved in a 2 wt% acetic acid aqueous solution to form a 2 wt% chitosan solution. Then, the ENR solution was added into the chitosan solution, and stirred over 12 h. The ratios for chitosan and ENR were set as 70:30, 50:50, and 30:70, which are coded as CE73, CE55, and CE37, respectively. To prepare polymer films for tensile testing, the mixture was cast on a petri dish, and dried at 80 °C for 8 h to obtain the polymer films. The polymer films were cut into dumbbell shape with dimensions of 20 mm by 4 mm (length × width). The thickness varied for each sample, ranging from about (0.08 to 0.13) mm. The exact values for each specimen were recorded, to calculate the stress and strain from


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tensile tests. To prepare the electrode, the Si was mixed with Super P and the prepared binder material, in a weight ratio of 70:15:15, to form a homogeneous slurry (Si and binder materials in a weight ratio of 70:30 without Super P were used for the peeling test and indentation test). The slurries were spread onto copper foil with a doctor blade, and dried in the oven at 80 °C for 12 h to completely remove the water, and complete the epoxy-amine reaction. The loading mass of silicon on the copper foil was about 0.6 mg/cm2. Characterization: The morphology of the products was observed by scanning electron microscopy (SEM; JSM-6390A), and the functional groups on the surface of the samples were characterized by Fourier-transform infrared spectrometry (FT-IR; Bruker tensor 27).

The

mechanical and adhesive properties were measured by universal test machine (Lloyd/LR30K), using a load cell of 250 N at room temperature. The films were cut into 50 × 10 mm rectangles, and then clamped onto the grips at a distance of 25 mm, and loaded at a constant strain rate of 0.5 mm min-1, until failure. 180-degree peel test was conducted, to measure the adhesion of the prepared binder. The Cu side of the Si/binder electrodes (30 × 50 mm) was fixed vertically to the bottom sample holder. The adhesive side of a 3M tape (of 20 mm width) was firmly applied onto the electrode laminate side. By pulling the tape at a constant displacement rate of 5 mm/s, a layer of the Si/binder laminate was peeled off, and adhered to the moving tape. The nanoscale mechanical property measurements were performed by JPK nanowizard 3 with N-type Si SPM probe, AppNano model ACSTA. All measurements were conducted at room temperature under ambient condition. The elastic modulus maps were evaluated from Quantitative Imaging (QI) contact mode, with constant contact force at 30 nN. The elastic modulus images were collected with the size of 0.5 µm × 0.5 µm at a resolution of 32×32 pixels, with motion speed of 15 µm/s. The swellability of the binder was studied through electrolyte absorption testing. Dry films were


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initially weighed (Wbefore), then immersed in electrolyte at room temperature for 6 h, and after excess electrolyte was wiped from their surfaces, weighed (Wafter) again. The swellability (S) was calculated as:

S=

× 100 %

Measurement of Electrochemical Properties: Electrochemical properties of the prepared samples were measured using a 2032 coin-type cell. The electrode film was punched into 12-mmdiameter discs, and weighed. The cell was fabricated in a glove box filled with high-purity argon gas, and the electrolyte consisted of 1 M LiPF6 in a mixture of ethylene carbonate/diethyl carbonate with 5 wt.% fluorethylene carbonate and 1 wt.% vinyl carbonate (volume ratio of EC/DEC = 1:1). The charge and discharge behaviors were monitored using a battery test system (WonATech Corp., Korea) with a voltage range of (0.005–2.0) V (vs. Li/Li+) at room temperature. Electrical impedance spectroscopy (EIS) measurements were conducted using a potentiostat (VSP, Princeton Applied Research, USA), applying a 10 mV amplitude sine wave in the frequency range of 0.1 Hz to 100 kHz.


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RESULTS AND DISCUSSION

Natural rubber could easily form the epoxy groups when peracetic acid was applied,21 which was confirmed by 1H NMR analysis. In the NMR spectrum, peaks at 2.7 and 5.1 ppm are contributed by the C–C in epoxy group and C=C, respectively,22 and their intensity ratio varies according to the composition (Fig. S2). The peak intensity ratio of 2.7 to 5.1 ppm is 15 %, indicating that the molecular content for epoxy group is 15 %. Then, the prepared ENR was mixed with chitosan in acetic acid solution (2 % in water), where the ratios for chitosan and ENR were set as 70:30, 50:50, and 30:70 (coded as CE73, CE55, and CE37). The prepared solution was spread on the copper foil, and dried. During the drying process, water was evaporated, and the crosslinking reaction between epoxy in ENR and amine in chitosan occurred. To verify the crosslinking reaction, Fourier transform infrared spectroscopy (FT-IR) analysis and dissolution test were conducted (Fig. S3). In the FT-IR spectra, the peak located at 1645 cm-1 was assigned to the N-H bending of primary amine,23,24 in which the intensity of chitosan/ENR was clearly decreased compared to that of chitosan, indicating that the primary amine successfully reacted with epoxide group. It was also observed that the chitosan/ENR film could not dissolve in the acetic acid solution, while the chitosan film was completely dissolved. These phenomena provide good evidence that the chitosan and ENR were chemically cross-linked, through the amineepoxide reaction. The mechanical properties and adhesive force are important physical properties of binder materials. To investigate the mechanical properties, films of chitosan, CE73, CE55, CE37, and ENR were prepared, and their stress-strain curves were obtained by using a universal testing machine (Fig. 2a). The chitosan film showed high modulus. While undergoing strain, chitosan chains are rearranged along the loading direction with their hydrogen bonds being cleaved,


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Figure 2. Mechanical and adhesive properties of the prepared binder: (a) strain-stress curve, and (b) comparison of the modulus and elongation; (c) Modulus mapping, and (d) distribution of the chitosan and CE55 electrode; (e) peeling test results, and (f) adhesive force comparison; (g) photograph of the chitosan and CE55 electrode after peeling test.

where the hydrogen bonding might resist strongly to the rearrangement, resulting in the high modulus.10 But the film was broken at a strain of ~2 %, where it is anticipated that interchain hydrogen bonds rupture. In the case of CE films, CE exhibited analogous behavior with the chitosan film, but exhibited lower modulus and higher strain at break. In the CE films, the stretching of the ENR chains entirely contributes to the strain being increased. The rotation about bonds and straining of bond angles during the stretching is almost entirely intramolecular, rather than intermolecular, which reduces the stress accumulation with respect to an increase in strain, resulting in lower modulus of the CE film than that of the chitosan film (Fig. 2b).25 The


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stretching ability of the rubber also enhanced the strain behavior of the chitosan film, and thus maximum strain of the CE films increased as the ENR content increased. To confirm the microscopic distribution of chitosan and ENR in the electrode, indentation test using atomic force microscopy (AFM) was conducted, following the coating of Si/binder (3:1 wt.%) on Cu foil. Figure 2c shows the Young’s modulus distribution images of the chitosan and CE55 electrodes. The image for CE55 shows various entangled colors from dark to white, where light-brown dots indicate a modulus of 300–500 MPa and dark dots indicate the lower modulus of 0–300 MPa. In the image for the chitosan electrode, dots with light-brown color dominate the electrode surface. It is clearly seen that ENR chains was well distributed between chitosan chains. To accurately analyze, modulus distributions of the electrodes were investigated (Fig. 2d). The pixels in the images extracted from the images, and modulus values were assigned to each extracted pixel. Then, the number of obtained modulus values was calculated to obtain the distribution. The modulus distribution for the chitosan electrode shows a narrow peak centered at 350 MPa, which might be from the presence of chitosan. In the case of the CE electrode, the modulus exhibits a wide and bimodal distribution, where peaks at 40–200 MPa and 300–450 MPa are from the presence of ENR and chitosan, respectively. As a result, ENR chains were distributed between chitosan chains, maintaining their mechanical properties, and thus it could effectively reduce the stress from the volume expansion of Si nanoparticles. To investigate the adhesive properties of the binder, a conventional 180° peeling test was carried out (Figs. 2f and 2g). It is clear that the bare ENR electrodes exhibited much weaker adhesive force compared to those of the CE electrodes, giving an indication of enhanced binder functions of the CE networks. The amine group in the chitosan and hydroxyl group on the Si can form hydrogen bonding, while ENR could not induce strong interactions. Thus the adhesive


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force can increase as the chitosan content increases.9 However against expectation, the chitosan electrode exhibited weak adhesion force. The reason was found in the photograph after peeling test (Fig. 2e). In the case of CE55 electrode after the peeling test, Si-CE55 film remained on the Cu foil, indicating that the force between CE and Cu foil is sufficient. However in the case of chitosan electrode, Si-chitosan film was perfectly removed from Cu foil, suggesting low adhesive force between them. Generally, for the adhesive forces to be significant, intimate contact must be present between the two surfaces.26 The adhesion length scale of a material describes the length over which a material is able to deform to make adhesive contact with a surface, which is the reciprocal proportion of the modulus of the contacting material.26,27 The high modulus of chitosan corresponds to low adhesion length, resulting in extreme difficulty for the chitosan to independently adhere to solid surfaces. It was found that the Si-chitosan film with lower chitosan content (20%) maintained adherence, while that with higher content (30%) was detached from the Cu foil during the drying (Fig. S4). This might be due to low adhesion length from high modulus of chitosan. When the binder consisted of ENR, it likely possessed significantly high adhesion length scales from the decrease of modulus, and they could make intimate contact with the surface to adhere on the Cu foil. To characterize the electrochemical performance of the prepared materials as anodes for lithium ion battery binder, a galvanostatic charge-discharge test was conducted in the voltage range of 0.01–2.0 V. For this test, the electrode was fabricated using the prepared binder material, Si nanoparticles, and conducting agents, followed by the preparation of coin-type half-cells with Li metal as both the reference and counter electrodes. To clarify the rate capability of the electrodes, the capacitive behavior at increasing current density from 400 to 8,000 mA/g was


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investigated (Fig. 3a). The capacity of all electrodes decreased as the current density increased, and the

Figure 3. Battery test of the electrodes prepared by using chitosan, CE73, CE55, and CE37 as a binder: (a-b) rate capability test results; (c) plots of the real impedance and the reciprocal square root of the angular frequencies; (d) Specific capacity according to cycle number; (e) Specific capacity of CE55 electrode according to cycle number (1600 cycles).


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capacity was recovered when the current returned to 400 mA/g. It was clearly observed that the capacity for the CE37 electrode quickly reduced as the current density increased compared to the other electrodes. The chitosan, CE73, and CE55 electrodes showed a similar trend, and thus the capacity retentions according to the current density were calculated for an accurate comparison (Fig. 3b). When 1, 2 and 4 A/g current densities were applied, the chitosan electrode showed capacity retentions of 84, 77, and 69%, respectively. However, when the ENR was incorporated into the binder composition, the rate performance became poor. The CE73 and CE55 electrodes exhibited capacity retention near 66, 65, and 61% at these current densities. To identify the influence of binder composition on rate performance, the diffusion coefficient of lithium ions was measured by electrochemical impedance spectroscopy (EIS). The relationship between the real impedance and the reciprocal square root of the angular frequencies was plotted, and the diffusion coefficient values were calculated using the Warburg coefficient obtained from the slope of the straight line in the plot (Fig. 3c).28,29 The diffusion coefficient of the chitosan electrode was 4.7 × 10-13 cm2/s, and it reduced as the ENR was incorporated (Table S1), which agrees with the results from the rate capability test. Due to the large coverage of the Si particle surface by the binder, the rate capability of the electrodes was determined by the conductive property of their binder.30 The binder is required to provide access for lithium ions to reach the Si surface, in order to achieve a high rate capability. Regarding the ionic conductivity of the polymer binder, the electrolyte uptake is important for allowing facile Li+ migration through the polymer binder to the Si.31,32 Concerning the electrolyte uptake of the electrodes, it was observed that chitosan electrode exhibited the highest uptake value, which is due to the


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abundant concentration of polar functional groups in chitosan (Fig. S5).33 As a result, when the chitosan was incorporated into the binder material, higher electrolyte uptake of chitosan resulted in the rate capability improvement. However, note that excess electrolyte uptake could weaken the adhesion, and induce the problem in cyclic performance.34 To compare the cyclic performance of the electrodes, the specific capacities of the prepared electrodes were investigated according to cycle number at a current density of 8,000 mA/g, after activation at 400 mA/g for the first 10 cycles (Fig. 3d). The specific capacity of the chitosan electrode decreased from 3,811 to 2,841 mAh/g during the activation. The specific capacity of the CE73, CE55, and CE37 electrodes exhibited 2,762, 2,630 and 2,429 mAh/g after the activation, respectively, which is comparable with the chitosan electrode. As the current density increased from 400 to 8,000 mA/g, the capacity dramatically decreased due to the limitation of rate capability. The specific capacity of the CE73 and CE55 electrodes showed a slight decrease with additional cycles, and reached 1,338 and 1,621 mAh/g after 300 cycles, respectively. Furthermore, the CE55 electrode exhibited a specific capacity of 1,350 mAh/g even after 1,600 cycles (Fig. 3e). The first columbic efficiency of CE55 electrode was 81.4% (Fig. S6), which increase after few cycles and maintained a columbic efficiency of over 99.5 % after 1,600 cycles (Fig. 3e). In the case of chitosan electrode, the specific capacity reached to 712 mh/g only after 200 cycles with substantial decay. When the current density of 1,000 mA/g was applied, the CE55 electrode exhibited outstanding cyclic performance, specific capacity of 2,310 mAh/g after 500 cycles, compared to CE73, CE37 and chitosan electrodes (Fig. S7). The tendency of capacity variation according to cycle number was similar with the result obtained from the current density of 8,000 mA/g, but difference of cyclic stability of the electrodes was much clearly observed.


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Note that the cycle performance of the CE55 electrode was comparable to those of the reported electrodes (Table S2).


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Figure 4. (a) Resistance, and (b) thickness variation of the electrode prepared with chitosan, CE73, and CE55 according to cycle number; (c) Cross-sectional SEM image after 100 cycle; (inset of b) Thickness variation of the electrode as prepared, after 1st lithiation and 1st delithiation.


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Additionally, the performance of CE electrodes prepared with higher loading mass or lower binder content was investigated (Fig S8 and S9). When the loading mass was increased from 0.6 mg/cm2 to 1.2 mg/cm2, 2011 mAh/g of capacity was obtained after 100 cycles, which is quite good cyclic stability. However, the rate performance of the electrode is considerably poor, which might be due to low diffusion coefficient of ENR. When lower binder content (10%) was used for an electrode preparation, the electrode exhibited 1801 mAh/g of capacity after 100 cycles. To understand the variation in the electrodes according to the cycle number, the resistance and thickness of chitosan, CE73, and CE55 electrodes after cycle test was investigated (Fig. 4). The resistance was measured by using EIS test with fully-delithiated electrodes (Fig. S10), and it was found that the entire pristine electrodes showed a charge transfer resistance near 40 Ω (Fig. 4a). As the cycle number increased, the resistance values for chitosan and the CE73 electrode gradually increased, and approached 320 and 180 Ω after 300 cycles, respectively. Meanwhile, the resistance for CE55 electrode increased rapidly to 140 Ω after 50 cycles, and maintained this value even after more than 250 cycles. When the thickness variation was investigated, it was found that the variation tendency for the electrodes thicknesses according to cycle number was similar to that of the electrodes resistances (Fig. 4b). The thicknesses of the pristine chitosan and CE73 electrodes were near 13 µm, which gradually increased as cycle number increase. After 300 cycles, the thickness of chitosan and CE73 electrode reached 38 and 31 µm, respectively; while the CE55 electrode reached 21 µm. The dramatic capacity decay of the Si electrode was due to the destruction of the initial morphology during cycling, as a result of the volume change of Si, which caused Si nanoparticles to lose electrical contact with the electrode.35 The morphological variation of the Si electrode also induced the additional formation of a SEI layer, which also increased the electrical resistance.36 In fact, the chitosan electrode and its particles


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showed a dense surface with large particles after cycling test (Fig S11), indicating the coverage of thick SEI layers.12,37 On the other hand, CE55 electrode maintained its porous morphology with small-size particle. This result might induce a difference of electrical resistance. In the cross-sectional SEM image of the chitosan electrode after cycling test, a number of large cracks were observed unlike that before cycling test (Fig. 4c), which might have formed during morphological destruction. These cracks increased the thickness of electrode, and simultaneously impeded the electron transfer, resulting in a decay of capacity. In contrast with the chitosan electrode, the CE55 electrode showed ignorable crack formation, which might induce the retention of thickness and resistance. In the case of the CE electrodes, uniformly dispersed natural rubber in the network resulted in an elastic framework that could stretch reversibly. The elastic framework could accommodate the stress from the volume change of Si nanoparticles, while maintaining its structure without the formation of cracks. This postulate was demonstrated by the thickness variation of the electrodes during lithiation/delithiation in a cycle. The CE73 and CE55 electrode recovered 90 and 95 % of the thickness increase from the lithiation at the following delithiation, whereas the chitosan electrode recovered only 32 % (Inset of Fig. 4b). The CE37 electrode exhibited poor thickness recovery, less than 33 %, which might be from the low modulus and the adhesive force of the binder, resulting in inferior cyclic performance compared to the CE73 or CE55 electrodes. Even though the modulus and adhesive force of binder are significant considerations to obtain cyclic performance, it should be noted that when the binder is used for Si-based anodes in lithium ion battery, the maximum strain of the binder is also important. The CE73 electrode has a higher modulus and adhesive force compared to the CE55 electrode; however, it was found that the CE55 electrode exhibited better cyclic performance. We conducted cycle tests with limited


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Figure 5. Cyclic test with the limitation of lithiation capacities at a current density of 1,000 mA/g. lithiation capacity = (a) 1,000 mAh/g, (b) 1,500 mAh/g, and (c) 2,000 mAh/g.

lithiation capacities of 1,000, 1,500 and 2,000 mAh/g to study the cyclic performance of the electrodes according to the volume expansion of Si particles (Fig. 5). By limiting the lithiation charge, it was possible to control the degree of volume expansion of the Si particles.30,38 When the lithiation charge was allowed to high values, the Si became fully lithiated with large volume expansion. Meanwhile, lithiation of Si particle could be restricted when the lithiation charge was


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limited to lower values, and the expansion of Si particle became smaller. It was found that the result for the stability test differs depending on the lithiation capacity. When the lithiation capacity was limited to 2,000 or 1,500 mAh/g, the CE55 electrode maintained over 600 or 1,000 cycles, respectively, which is better cyclic performance than the CE73 electrode. When the volume of Si largely expanded, the CE55 electrode exhibited better cyclic performance, which might be due to the higher maximum strain of the CE55 binder. However, when the lithiation capacity was limited to 1,000 mAh/g, the CE73 electrode exhibited better cyclic performance. After 2,000 cycles, both the CE73 and CE55 electrode maintained a delithiation capacity near 1,000 mAh/g; but it was clearly observed that the delithiation capacity after 1,200 cycles of the CE55 electrode was slightly lower than that of the CE73 electrode. To simulate the variation of the electrode in the test with the limited lithiation of 1,000 mAh/g, the end-potential for lithiation according to cycle number was investigated (Fig. S12). It was found that the end-potential for the CE55 electrode gradually decreased. When higher lithiation state of Si was required due to the deterioration of the electrode, the end-potential for lithiation was decreased, and thus a gradual decrease of end-potential for CE55 electrode with increasing cycle number indicates the continuous deterioration of the electrode. In the case of CE73 electrode, after a dramatic decrease during the first 300 cycles, the end-potential slowly decreased. The rapid decrease of endpotential in the initial cycles might be due to the tremendous morphological variation of the electrode, because of the failure to tolerate the stress from volume expansion of Si. After the electrode underwent these variations, it could stabilize with the assistance of the high modulus and adhesive force of the CE73 binder. The CE55 binder had a lower modulus and adhesive strength compared to the CE73 binder, which might have induced more severe continuous degradation of the electrode materials, and resulted in lower cyclic stability. As a result, it was


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found that all of binder properties, including maximum strain, modulus and adhesive force, is very important to realize the cyclic stable electrode, but the most important binder property for enhancing cyclic stability significantly differs depending on volume expansion degree of active material. In conclusion, the chitosan-ENR network was successfully prepared by crosslinking, following the epoxidization of natural rubber. The properties were controlled by varying ENR contents, and the optimized network provides both sufficient elasticity and adhesive interaction with Si particles. With aids of the prepared binder, the Si electrode could be expanded and contracted reversely without mechanical failure of the electrode, resulting in high sustained capacitive performance. The electrode exhibited the specific capacities of 1,350 mAh/g after 1,600 cycles at the current density of 8 A/g and 2,310 mAh/g after 500 at the current density of 1 A/g. It was also found that the most important binder property for enhancing cyclic stability significantly differs depending on volume expansion degree of silicon. High modulus and adhesive force of binder material were required to obtain cyclic stability for the lower degree of volume expansion, while high elasticity of binder material was required for higher degree of volume expansion.


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ASSOCIATED CONTENT Supporting Information. Additional characterization data, electrochemical performances and comparison table. This material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION Corresponding Author *Give contact information for the author(s) to whom correspondence should be addressed. [*] Prof. Y. Lee Email: [email protected]; Tel: +82 31 290 7326 [*] Dr. C. Chanthad Email: [email protected]; Tel: +66 21176500

ACKNOWLEDGMENT This research was supported by Basic Science Research Program through the National Research Foundation

of

Korea(NRF)

funded

by

the

Ministry

of

Education(NRF-

2016R1D1A1B03930806). This work was also supported by Ministry of Science, ICT and Future Planning of Korea (NRF-2016R1A2B40008389).


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TOC


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Epoxidized Natural Rubber-Chitosan Network Binder for Silicon

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