Nanoscale Electrical Degradation of Silicon–Carbon Composite

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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 24549−24553

Nanoscale Electrical Degradation of Silicon−Carbon Composite Anode Materials for Lithium-Ion Batteries Seong Heon Kim,* Yong Su Kim, Woon Joong Baek, Sung Heo, Dong-Jin Yun, Sungsoo Han, and Heechul Jung* Samsung Advanced Institute of Technology, Gyeonggi-do 443-803, Republic of Korea

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S Supporting Information *

ABSTRACT: High-performance lithium-ion batteries (LIBs) are in increasing demand for a variety of applications in rapidly growing energy-related fields including electric vehicles. To develop high-performance LIBs, it is necessary to comprehensively understand the degradation mechanism of the LIB electrodes. From this viewpoint, it is crucial to investigate how the electrical properties of LIB electrodes change under charging and discharging. Here, we probe the local electrical properties of LIB electrodes with nanoscale resolution by scanning spreading resistance microscopy (SSRM). Via quantitative and comparative SSRM measurements on pristine and degraded LIB anodes of Si−C composites blended with graphite (Gr) particles, the electrical degradation of the LIB anodes is visualized. The electrical conductivity of the Si−C composite particles considerably degraded over 300 cycles of charging and discharging, whereas the Gr particles maintained their conductivity. KEYWORDS: Li-ion battery, Si−C composite, LIB degradation mechanism, electrical degradation, scanning spreading resistance microscopy, SSRM

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INTRODUCTION With the rapid growth of energy-related fields including electric vehicles, high-performance lithium-ion batteries (LIBs) with a high capacity and stability are in high demand.1−3 Therefore, numerous materials have been studied as anode and cathode materials to improve the performance of LIBs. Among these, Si, which provides an extremely high Li capacity, is widely recognized as the most promising anode material, because it can compensate for the low energy density of graphite (Gr), which is the common commercially used anode material. Si has considerable strengths as an anode material for LIBs. First it has a high theoretical Li capacity of ∼4200 mA h/ g in the Li4.4Si phase. This is almost 10 times higher than that of Gr (approximately 372 mA h/g).4,5 In addition, Si is advantageous with regard to safety, cost, and abundance.6−11 However, Si has a critical intrinsic weakness due to its excessive volume expansion during lithiation.12,13 The volume of fully lithiated Si increases up to 400% compared to that of the original material.12,13 This excessive volume expansion induces fracture of the Si active materials during repeated charging/discharging cycles, leading to rapid deterioration in the performance of the LIB, in particular, the cyclic stability. To overcome this intrinsic weakness of Si, numerous studies have been performed. One effective solution for the fracture issue is the use of Si nanostructures including nanowires, nanotubes, porous Si, and Si−C composites, since nanosize Si has been demonstrated to be free from fracture.12,14−19 However, for LIBs with the optimal cyclic stability, it is © 2018 American Chemical Society

advantageous to blend Si materials with other anode materials of greater cyclic stability, such as Gr. The performance of LIB cells depends on various factors such as the anode and cathode materials, configuration of the electrodes, and transport path.20−23 One critical barrier limiting the commercial use of Si as an LIB anode material is its cyclic degradation, which mainly leads to the disconnection of ionic or electrical paths. This disconnection in the ionic or electric paths can be induced via the formation of a thick solid−electrolyte interphase (SEI) layer, which acts as the ionic or electric insulator, as well as by mechanical disconnection. In particular, it is essential to maintain the electrical paths during charging/discharging cycles, because the SEIs and electrolyte that can occupy the newly formed empty gaps inside the anode are electrically insulating, although most of them are Li-ion conductors. Further, although numerous studies have been performed to verify the degradation mechanisms of LIBs, there have been few studies on the electrical degradation of LIB anodes, in particular, with nanoscale resolution.24,25 Since the invention of scanning tunneling microscopy and atomic force microscopy (AFM), scanning probe microscopy (SPM) has been one of the most powerful techniques to analyze physical and chemical properties of various nanoscale materials.26 In particular, because of its easy usage and Received: April 30, 2018 Accepted: June 26, 2018 Published: June 26, 2018 24549

DOI: 10.1021/acsami.8b07012 ACS Appl. Mater. Interfaces 2018, 10, 24549−24553

Research Article

ACS Applied Materials & Interfaces

morphological analysis and elemental-mapping of the pristine Si−C/ Gr anode were performed via Auger electron spectroscopy (Figure S1). In the cross-sectional SEM images of the Si−C/Gr anode samples, the bright and the dark areas correspond to Si−C composites and Gr particles, respectively. For the electrochemical evaluation, an 18650-cylindrical cell was assembled with a separator (polyethylene sheet), a NMC (LiNixMnyCozO2, x + y + z = 1) cathode, and an electrolyte (1.3 M LiPF6 dissolved in ethylene carbonate/diethyl carbonate/fluoroethylene carbonate). For cycling, the cells were tested via constant current (CC)−constant voltage charging and CC discharging in the voltage range of 2.8 to 4.3 V (vs Li/Li+) using a TOSCAT-3100 battery cycler (Toyo System). The electrochemical performance of the Si−C/Gr anode investigated in this study is shown in Figure S2. For analytical measurements, such as SSRM, scanning electron microscopy (SEM), and electron probe microanalysis (EPMA), the anode electrodes were detached from the cell, washed with dimethyl carbonate, and dried in overnight in vacuum. To obtain the morphological and electrical properties of the interior part of the anode, cross-sectional samples were prepared using an Ar ion beam (5 keV) under cryogenic conditions at −140 °C. To measure the electrical properties using SSRM, an electrical connection was made to the cross-sectional sample and the electrical resistance was probed with nanoscale resolution using a conductive AFM tip.

modification, AFM has been used in a wide range of research fields. Many kinds of AFM-based measurement techniques have been developed, including SPM techniques using conductive probes, such as conductive AFM (C-AFM), scanning spreading resistance microscopy (SSRM), electrostatic force microscopy, Kelvin probe force microscopy, and scanning capacitance microscopy. Among them, C-AFM and SSRM directly measure the electrical conductivity of various samples and provide two-dimensional (2D) conductivity maps on a nanometer scale. In particular, SSRM can be used to study a variety of samples with properties ranging from insulating through semiconducting to metallic because it employs a logarithmic current amplifier which can widen the measured resistance range (typically 104−1011 Ω), as shown in Figure 1a.27,28 Furthermore, in a previous study, we demonstrated

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RESULTS AND DISCUSSION The Si−C composite anode materials are gradually degraded by repeated charge/discharge cycles. Figure 1c and d reveals the morphological differences between the pristine and the 300-cycled Si−C/Gr anode samples. In the cross-sectional image of the sample cycled 300 times, inhomogeneous phases in the Si−C composite particle are observed owing to the degradation of the active materials. Figure 2 shows the results of EPMA measurements for the 300-cycled Si−C/Gr anode sample. The Si−C composites tended to become dark in the imaging contrast with repeated charging/discharging. The Si− C composite circled in green exhibits a darker SEM contrast and stronger O, P, and F signals than the ones encircled in red, as shown in the SEM image (Figure 2a) and the corresponding EPMA elemental maps (Figure 2b−f). The large amounts of O, P, and F detected in the Si−C composite circled in green indicate that it suffered more severe degradation. These EPMA results support the currently accepted degradation mechanism: Si−C composites are degraded by the penetration of the electrolyte and the subsequent formation of a thick SEI layer.24 To investigate the electrical degradation caused by repeated charging/discharging, we performed quantitative and comparative SSRM measurements on the pristine and 300-cycled anode samples. In the SSRM measurements, a conductive diamond-coated AFM tip (CDT-NCHR, NanoWorld) was used to scan the sample surface in the contact mode and the local electrical resistance (current) values were simultaneously measured while applying an appropriate bias voltage (3 V in this study), as shown in Figure 1a. In the typical AFM topographic image of a pristine Si−C/Gr anode shown in Figure 3a, the Si−C composites and Gr particles cannot be distinguished clearly. Figure 3b shows the corresponding SSRM results for the pristine Si−C/Gr sample. SSRM measures the local resistance at each position of the AFM tip and maps the logarithmic values of the measured local resistance. Figure 3c showing the magnified SSRM Log(Resistance) images of a pristine Si−C composite confirms the nanoscale resolution of SSRM, because the nanoscale resistance protrusions originating from Si nanoparticles within the composite were obviously imaged. In Figure 3b, the SSRM

Figure 1. (a) Schematic of the SSRM measurement setup. (b) Crosssectional view of the Si−C composite particle. (c, d) Cross-sectional SEM images of (c) pristine Si−C composite particles and (d) those charged/discharged over 300 cycles.

that SSRM could be a powerful tool to study the electrical degradation of LiNi0.8Co0.15Al0.05O2 cathodes.25 In this study, we utilized SSRM as the main analytical technique to study the local electrical properties of Si−C composite anodes. Using the SSRM technique, we successfully probed and visualized the electrical degradation of Si−C composite anodes.

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EXPERIMENTAL SECTION

Figure 1b depicts the structure of the Si−C composite particles used in this study. Si nanoparticles were prepared via ball-milling of micron-sized Si powder. For the C-coating, coal tar pitch used as the C source was mixed with the milled Si nanoparticles in a planetary centrifugal mixer, and the mixture was heated at 900 °C for 3 h to complete the carbonization reaction. The size of the synthesized Si−C composite particles ranged from approximately 5 to 15 μm. Figure 1c and d shows the SEM images of a pristine Si−C particle and a particle severely degraded after 300 cycles of charging/discharging. To evaluate the electrochemical performance, Si−C/Gr anodes were prepared by coating slurries of a mixture of Si−C composite, Gr, and lithium polyacrylate used as the binder on Cu foil substrates. The pressed anode was dried in vacuum at 120 °C for 2 h. The 24550

DOI: 10.1021/acsami.8b07012 ACS Appl. Mater. Interfaces 2018, 10, 24549−24553

Research Article

ACS Applied Materials & Interfaces

the Si−C composite and Gr are colored in red and bright blue, respectively. The differences in the resistance are more obvious in the line profile for a line across the Si−C composite and Gr regions. The profile of Log(Resistance) in Figure 3d was extracted from the dotted line in Figure 3b. The Si−C composite and Gr regions are indicated by green and black arrows, respectively. The resistance values of the Gr region are almost monotonic at ∼8.5 Log(Ω), while those of the Si−C composite fluctuate in the range of 9 to 9.5 Log(Ω). The fluctuation in the resistance of the Si−C composites is expected, because the Si−C composite is a mixture of two materials: Si nanoparticles and C. Figure 4a shows an SEM image of the 300-cycled Si−C/Gr anode, which exhibited a reduced capacity retention of ∼80%

Figure 2. (a) Cross-sectional SEM and (c−f) the corresponding EPMA elemental mapping images for a Si−C/Gr anode subjected to 300 charge/discharge cycles.

Figure 4. (a) SEM image of 300 cycled Si−C/Gr anode and the corresponding (b) topographic, (c) Log(Resistance) images acquired in the same sample area. (d) Log(Resistance) profile extracted from the dotted line in panel c.

(Figure S2). Figure 4b and c shows the corresponding AFM topographic and SSRM Log(Resistance) images acquired for the same sample area, respectively. Notably, the resistance of the degraded Si−C composites increased significantly, and they exhibited diverse resistance values depending on the extent of electrical degradation. For example, the Si−C composite indicated by the red arrow in Figure 4c exhibits a resistance approximately 3 orders of magnitude higher than that of the Si−C composite indicated by the green arrow, as clearly shown in the Log(Resistance) profile of Figure 4d. These experimental findings were reproducibly observed in our SSRM measurements. Other SSRM data acquired for different pristine and 300-cycled anode samples are shown in Figures S3 and S4, respectively, and similar results to those in Figures 3 and 4 are obtained. To confirm the increase in the resistance of the degraded anode sample, we performed statistical data processing using the acquired SSRM data. The distributions of the resistance values for the pristine and 300-cycled samples, which were

Figure 3. (a) Topographic and (b) the corresponding Log(Resistance) images of a pristine Si−C/Gr anode. (c) Magnified Log(Resistance) image of a pristine Si−C composite. (d) Log(Resistance) profile extracted from the dotted line in (b).

Log(Resistance) image clearly distinguishes the Si−C composites and Gr particles via the contrast in their resistances. The Si−C composites have a slightly higher resistance than the Gr particles. The areas corresponding to 24551

DOI: 10.1021/acsami.8b07012 ACS Appl. Mater. Interfaces 2018, 10, 24549−24553

Research Article

ACS Applied Materials & Interfaces

Figure 5. (a, b) Distributions of Log(Resistance) values that were statistically processed from two Log(Resistance) images in Figures 3c and 4d. (c) 3D plot of the data in panels a and b in logarithmic scale.

statistically processed from two Log(Resistance) images in Figure 3b and 4c, are shown in Figure 5a,b, and the two sets of distribution data are plotted on a logarithmic scale in Figure 5c. As shown in Figure 5, areas of high resistance significantly increased after 300 cycles of charging/discharging. The highly increased electrical resistance indicated that electrons hardly move through the anode materials, although Li ions might pass through. This severe problem in the electrical transport through the anode can be a critical issue causing a reduction in its capacity. Figure 6 shows the SSRM results for two Si−C composites that appear to have undergone two different types of degradation; these were acquired in the same sample area corresponding Figure S4. For the first Si−C composite shown in Figure 6a and b, the growth of a thick SEI layer within and around the whole composite particle, rather than mechanical fracture, could be the main origin of the degradation, because a distinct crack-like boundary is not observed in the topographic image (Figure 6a). In contrast, mechanical fracture appears to be the main reason for the degradation of the second Si−C composite shown in Figure 6c and d. Comparing the Log(Resistance) image of the second Si−C composite (Figure 6d) with the corresponding AFM topographic image (Figure 6c) reveals a strong possibility that the highest resistance part in yellow in Figure 6d had been connected to the neighboring (red-colored) parts of the original Si−C composite; however, these parts were mechanically separated by the repeated volume changes during charging/discharging processes, because the crack-like boundary is clearly observed in the

Figure 6. (a, c) Topographic and (b, d) the corresponding Log(Resistance) images of two degraded Si−C composites.

AFM topographic image (Figure 6c). On the basis of our observations, the first type of degradation (Figure 6a and b) is the major cause of the anode degradation, while the second one (Figure 6c and d) occurs rarely. 24552

DOI: 10.1021/acsami.8b07012 ACS Appl. Mater. Interfaces 2018, 10, 24549−24553

Research Article

ACS Applied Materials & Interfaces

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CONCLUSIONS We performed quantitative and comparative SSRM measurements on pristine and cycled (degraded) Si−C/Gr anodes to examine the local electrical degradation of the LIB anodes with nanoscale resolution. Using these measurements, we successfully probed and visualized the electrical degradation of the cycled LIB anodes. The electrical conductivity of the Si−C composite particles was found to degrade considerably after 300 charge/discharge cycles, although the Gr particles maintained their conductivity. The results indicated that SSRM is an effective technique for measuring the local electrical properties of a variety of LIB electrodes with nanoscale resolution and understanding their degradation mechanism, which is imperative for the development of the LIB industry, as well as other fields of research.

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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/acsami.8b07012. Auger electron mapping result for a pristine Si−C/Gr anode sample, electrochemical performance result for the 300-cycled Si−C/Gr anode, and additional SSRM results for pristine and 300-cycled Si−C/Gr anodes (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Seong Heon Kim: 0000-0001-6054-2109 Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acsami.8b07012 ACS Appl. Mater. Interfaces 2018, 10, 24549−24553