Visualization of Si Anode Reactions in Coin-Type Cells via Operando

Sep 28, 2017 - This strongly suggested that cracking caused by the breakage of the stiff binder alleviated the internal stress experienced by Si. On t...
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Letter Cite This: ACS Appl. Mater. Interfaces 2017, 9, 35511-35515

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Visualization of Si Anode Reactions in Coin-Type Cells via Operando Scanning Electron Microscopy Chih-Yao Chen,† Amane Sawamura,† Tetsuya Tsuda,*,† Satoshi Uchida,‡ Masashi Ishikawa,‡ and Susumu Kuwabata*,† †

Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan ‡ Department of Chemistry and Materials Engineering, Kansai University, 3-3-35 Yamate-cho, Suita, Osaka 564-8680, Japan S Supporting Information *

ABSTRACT: Understanding the electrochemical behavior and controlling the morphological variations of electrodes are critical for the design of high-capacity batteries. In this article, we describe a newly established operando scanning electron microscopy (SEM) to visualize the battery reactions in a modified coin cell, which allowed the simultaneous collection of electrochemical data and time-resolved images. The investigated silicon (Si)−polyimide-binder electrode exhibited a high capacity (∼1500 mAh g−1) and a desirable cyclability. Operando SEM revealed that the morphology of the Si anode drastically changed and cracks formed on the electrode because of the lithiation-induced volume expansion of the Si particles during the first charge process. Interestingly, the thickness variation in the Si composite layer was moderated in subsequent cycles. This strongly suggested that cracking caused by the breakage of the stiff binder alleviated the internal stress experienced by Si. On the basis of this finding by the operando SEM technique, patterned Si electrodes with controlled spacing were successfully fabricated, and their improved performance was confirmed. KEYWORDS: ionic liquid, operando scanning electron microscopy, Si anode, lithium-ion battery, analytical chemistry

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capacity retention. While soft and elastomeric binders are able to reversibly accommodate the large volume variations in Si,3 carboxymethyl cellulose (CMC) binders, which are brittle binders that break after a slight deformation (elasticity: ∼8%),4 far outperform poly(vinylidene fluoride) binders (elasticity: ∼25%) in Si-based electrodes.4 This finding contradicts a common belief. Polyimide (PI) binders are another interesting example.5−7 PI is an important engineering plastic that is characterized by high mechanical robustness and low ultimate elongation. Uchida and co-workers reported that untreated micron-sized Si particles incorporated with a PI binder exhibited a stable cyclability of up to 100 cycles with a Coulombic efficiency approaching 100%, whereas Si with a styrene−butadiene rubber binder showed a rapid capacity decay.8,9 The usage of micronsized Si particles provides several advantages. The particles are economical and entirely compatible with a mature slurry-coating manufacturing process for LIBs; the diverse particle size favors efficient stacking for a dense electrode preparation.2 Although their cyclability is inferior to that of their nanostructured counterparts because of the severe pulverization described above, PI binders can improve the cyclability because of their high mechanical strength.6−8 Additionally, advances in electrolytes

he increasing demand for high-performance lithium-ion batteries (LIBs) has driven researchers to explore highcapacity electrodes. Silicon (Si) has emerged as one of the most attractive active anode materials because it provides a nearly 10fold higher theoretical specific capacity than graphite, which is currently used. Additionally, Si has a moderately negative discharge potential and is abundant in natural resources.1 However, the large capacity of Si is accompanied by a tremendous volume expansion (approximately 300%), and when elemental Si transforms to its fully lithiated state, the electrode is pulverized and the interparticle electrical contact in the Si anode is broken. Another issue arising from the large volume variation in Si anodes is the instability of the solid electrolyte interphase (SEI). The SEI layer that forms on the Si surface is destroyed and regenerated upon cycling, which depletes the electrolyte. These phenomena result in fatal capacity fading in the battery and hamper the practical applications of Si anodes. Although nanostructured Si has effectively resolved some major problems, technological challenges still exist. For instance, the fine size scale results in a low tap density and high interparticle resistance.2 Moreover, these materials generally rely upon a costly synthesis process, namely, chemical vapor deposition.1 One strategy that has been proposed to resolve the above issues is to intelligently design the composite electrode. The binders contained in the composite electrode are essential for the Si anode to maintain its electrode integrity and to guarantee the © 2017 American Chemical Society

Received: August 17, 2017 Accepted: September 28, 2017 Published: September 28, 2017 35511

DOI: 10.1021/acsami.7b12340 ACS Appl. Mater. Interfaces 2017, 9, 35511−35515

Letter

ACS Applied Materials & Interfaces will help to enhance the cycling stability of Si anodes. Ionic liquids (ILs), which have a suite of unique properties, such as nonflammability, negligible vapor pressure, and good thermal and chemical stabilities, have recently been shown to be valuable, advantageous electrolytes for Si anodes.10 These approaches deserve attention because they hold great promise for effectively utilizing affordable raw materials while still achieving a satisfactory battery cycling performance. In addition to these studies, extensive efforts have been devoted to determining the critical factors that are related to the performance enhancement of Si anodes. Because the physical and chemical states of the electrode are sequentially renewed (nonequilibrium states) during the reaction, in situ or operando studies on battery reactions have been attempted to gain more indepth information on the time-resolved changes in battery components. Various in situ or operando analytical techniques, e.g., X-ray diffraction, neutron depth profiling, Fourier transform infrared spectroscopy, atomic force microscopy, electrochemical dilatometry, scanning electron microscopy (SEM), and transmission electron microscopy, have been established.11−14 The combined use of these methods ought to provide invaluable insight into the design of high-performance batteries. However, because of potential compatibility problems between the cell and certain analytical methods, the existing electrochemical setup may need to be modified. Because the battery is a multifaceted system in which complicated physical and chemical phenomena occur simultaneously, researchers should design the cell setups featuring practical cases in every way possible.14 Tracking the structural/compositional information in a realistic operating battery as the cycling proceeds is highly desirable. Here, an unprecedented operando SEM approach was developed to simultaneously acquire reliable electrochemical data and information on the morphological variations in a conventional coin cell with IL electrolytes. The extremely low vapor pressure and antistatic effect of ILs allow for direct SEM observations of the ILs and electrode reactions in the ILs. The sufficient working space in the SEM chamber makes it an open possibility to employ various electrochemical cells, as demonstrated by our group.15−19 In this article, we focused on coin-type LIBs because they are a convenient and rapid testing platform in laboratories. This new operando SEM observation technique was applied to investigate the cycling of microscale Si anodes with a PI binder in a full cell configuration with bis(fluorosulfonyl)amide-based IL electrolytes. On the basis of real-time observation results, a dot-shaped, PI-binder Si anode was designed to achieve a better battery performance. A schematic of the electrochemical setup used in this article is illustrated in Figure 1a. The coin-type cell was almost the same as a conventional one except the cell was cut off at the edge so the operando SEM observations could be conducted. Parts b and c of Figure 1 show typical photographs of the modified coin-type cell and a comprehensive SEM image of the battery components, respectively. The experiments were conducted using Li[TFSA]/ [C2mim][FSA] (1:5 molar ratio) and Li[FSA]/[C2mim][FSA] (1:1 molar ratio) IL electrolytes [TFSA = bis(trifluoromethanesulfonyl)amide; C2mim = 1-ethyl-3-methylimidazolium; FSA = bis(fluorosulfonyl)amide]. For the operando SEM experiments, the beam energy and probe currents were optimized. Additionally, the wetting conditions of the IL electrolytes on the working electrode, which largely affect the electrode performance and image quality, were optimized. The presence of a sufficient quantity of the electrolyte allows the electrode to function well, but a thick IL layer can disturb the

Figure 1. (a) Schematic diagram showing the concept of operando SEM observation for a practical coin cell using IL-based electrolytes. (b) Photograph of the experimental setup located in the SEM specimen chamber. (c) Low-magnification SEM image showing a Si−PI anode in a full cell configuration captured in the area indicated by the red rectangle in part b.

electrode reaction observations because the secondary electrons that are generated cannot escape from the IL layer to reach the detector.20 The appropriate amount of the IL electrolyte was approximately 80 μL under our experimental conditions. Figure S1 shows the typical PI-binder Si microparticle anode performance obtained with the IL electrolytes in this cell setup for the operando SEM observation. The electrochemical characteristics were similar to those measured by a standard coin cell (Figure S2), which suggested that our method can reproduce the behavior of the real electrode. In both cells, substantial capacity losses were identified in the first cycle along with formation of the SEI.1,2 The reversible discharge capacity was slightly lower than that recorded in a previous report using the same electrode in 1 mol dm−3 LiPF6 in ethylene carbonate/dimethyl carbonate,8 and this is mostly due to the higher viscosity of the IL electrolyte. Figure 2a shows a series of operando SEM images of lithiation/ delithiation on a PI-binder Si anode for selected cycles in Li[TFSA]/[C2mim][FSA] (1:5 molar ratio; the LiI concentration is ca. 1.0 M.). The uncycled Si layer has a smooth surface and a thickness of approximately 20 μm. Upon the first lithiation, the surface becomes noticeably rougher, and the thickness increases to ∼29.3 μm. Concurrently, the observed Si anode part protrudes somewhat, indicating the volume variation in the Si active materials contained in the anode. The figure also shows that the electrode morphology did not return to its original state after the first delithiation. Nevertheless, a nearly reversible variation in the electrode thickness for the subsequent charge/ discharge cycles was observed (Figure 2b). Because the Si volume change upon full lithiation can be up to 300%, even the stiff PI binder is likely to fracture during the cycling. Uchida et al. showed through ex situ SEM observations of the anode that cracks form on the PI-binder Si anode.8 Ex situ SEM observation of the electrode recovered from the coin cell, however, causes an overestimation of the electrode thickness, i.e., an artifact, due to the absence of the stacking pressure.21 To probe the variations in the PI-binder Si anode surface in a coin cell, we deliberately 35512

DOI: 10.1021/acsami.7b12340 ACS Appl. Mater. Interfaces 2017, 9, 35511−35515

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ACS Applied Materials & Interfaces

Figure 2. (a) Operando SEM images of a Si−PI anode cycled in Li[TFSA]/[C2mim][FSA] (1:5 molar ratio) at different states and selected cycles. Evolution in the electrode thickness as a function of the (b) cycle number and (c) lithiation depth (quantified at the 17th cycle). The electrode was first cycled at a rate of C/20 for a single conditioning cycle and at C/4 for the next 19 cycles (1 C = 4000 mA g−1 Si).

Figure 3. (a) Schematic illustration for the explanation of the observation zone. (b) Operando SEM images during the lithiation/delithiation processes at the edge of the separator.

electrode thickness is virtually unchanged up to 400 mAh g−1. Then, a linear increase in the thickness begins, and the thickness reaches ca. 150%. The two distinct steps in the thickness variation are explained as follows: the lithiation-induced expansion of Si particles first fills up the cracks that are generated by the binder breaking, and further lithiation mainly causes an increase in the electrode thickness. This implies that the selfformed, flexible structure can alleviate the volume variation caused by the lithiation/delithiation of Si to some extent. A similar nonlinear thickness variation of a Si anode was also revealed by an electrochemical dilatometry method.13 Recently, upon the importance of the binder as a component in a Si-based

misaligned the electrode and separator (Figure 3). The lithiation process readily proceeded, even for the area that was not in contact with the separator, upon cycling at a moderate rate of C/ 4 (1000 mA g−1), and the electrode surface became rough after both charge/discharge cycles. Notably, crack formation was observed in this exposed area during the first charge process, as indicated by the arrows in the figure. Accordingly, the irreversible thickness expansion (∼24%) after the first lithiation is attributed to the generation of space. Figure 2c depicts the thickness variations in a PI-binder Si anode as a function of the lithiation depth for the 17th cycle (movie 1 is available in the Supporting Information). The 35513

DOI: 10.1021/acsami.7b12340 ACS Appl. Mater. Interfaces 2017, 9, 35511−35515

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ACS Applied Materials & Interfaces anode, several new binder concepts, e.g., dynamic cross-linking,22 three-dimensional polymer network,23 self-healing,24 and molecular pulley principle,25 have been introduced to improve the electrode property. This operando SEM observation would provide useful information for the further study and improvement of the anode with such binders, like a PI-binder Si anode. The LIB performance strongly depends on the lithium salt fraction in conventional electrolytes,26 but this has rarely been studied for IL electrolytes.27,28 When the lithium transference number increases, the energy efficiency of a battery improves, although the conductivity and viscosity values may become unfavorable because of the intensified ionic interactions. As seen in Figure S1, under the same charge/discharge conditions (C/4 rate), the PI-binder Si anode in the operando cell delivers a higher discharge capacity in the IL electrolyte with a higher lithium content (Li[FSA]/[C2mim][FSA], 1:1 molar ratio) than the less concentrated Li[TFSA]/[C2mim][FSA] (1:5 molar ratio). Figure S3a shows the operando SEM images of the morphology variation in a PI-binder Si anode tested in Li[FSA]/ [C2mim][FSA] for selected cycles. Drastic changes in the morphology due to (de)lithiation were confirmed. The electrode thickness gradually increased as the cycling proceeded and reached ca. 175% for lithiation and ca. 150% for delithiation (Figure S3b). This irreversible expansion (∼50%, nearly 2 times greater than that observed in the less concentrated electrolyte) was indicative of the crack formation due to the fracturing of the PI binder, and the larger expansion suggested that more free space was generated in the Si anode. As expected, the ex situ SEM images of the Si anode after the 20-cycle test showed more extensive cracks with broader widths of ca. 1 μm throughout the electrode (Figure S4). Similar ex situ SEM images are observed when Si anodes are made of other stiff binders such as CMC and poly(acrylic acid).21,25,29 Nonetheless, the two-step thickness variation is very similar to that recorded in the less concentrated electrolyte, indicating the identical crack-filling mechanism of Si anodes (Figures 2c and S3c). Especially, in the PI-binder Si anode, the free space spontaneously generated by the cracks seems to enhance the electrode performance. The free space would contribute to the internal stress relaxation of the lithiated Si anode, the increase in the surface area of the active material, and the enhancement of lithium-ion transport. To validate the effect of the free space on the Si anode, we tailored the slurry-coating process to fabricate a patterned, PIbinder Si microparticle anode with a certain degree of space on the electrode. Figure 4a shows the schematic diagram and typical SEM image of the as-prepared Si anode. The PI-binder Simicroparticle dot-shaped islands on the electrode were ca. 1 mm in diameter, and the intervals between each island were 400−500 μm. Figure 4b shows the charge/discharge results for standard coin cells. The patterned electrode with the reserved uncoated space displayed a stable cyclability, a much higher capacity, and well-defined voltage plateaus in the charge/discharge profiles [Figure 4b (inset)] compared to those of the normal electrode. Figure S5 corroborates the formation of extensive cracks within each of the PI-binder Si islands, and the PI binder remained welladhered even after delivering such a high capacity. Further optimization of the patterning and loading amount of the active material will undoubtedly further enhance the electrochemical properties. In summary, a unique operando SEM observation technique was developed to characterize electrode materials in modified coin cells and comprehensively observe the interplay between the Si active material, the binder, and the electrolyte. This

Figure 4. (a) Schematic diagram and SEM image of a patterned Si−PI anode. (b) Cyclability of the (□) patterned and (■) normal Si−PI anodes in Li[TFSA]/[C2mim][FSA] (1:5 in molar ratio) measured by standard coin cells. The inset shows the typical charge/discharge profiles of these two electrodes.

operando technique revealed that the free space on the anode is a critical factor in enhancing the electrochemical properties of the PI-binder Si-microparticle anode. By control of the factor, a highperformance PI-binder Si-microparticle anode was successfully fabricated. Although applicable electrolytes are limited to nonvolatile ones such as ILs, our imaging methodology using a readily available SEM instrument should help to deepen the understanding of current- and next-generation rechargeable batteries under actual operating conditions.

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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.7b12340. Experimental methods, electrochemical characterization results, and ex situ SEM images (PDF) Movie of the operando SEM observation of a Si−PI anode during the 17th charge process (AVI)

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AUTHOR INFORMATION

Corresponding Authors

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

Tetsuya Tsuda: 0000-0001-9462-8066 Notes

The authors declare no competing financial interest. 35514

DOI: 10.1021/acsami.7b12340 ACS Appl. Mater. Interfaces 2017, 9, 35511−35515

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ACS Applied Materials & Interfaces

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ACKNOWLEDGMENTS This research was partially supported by JSPS KAKENHI Grants JP15H03591, JP15K13287, JP15H02202, and JP16K14539 and by the Advanced Low Carbon Technology Research and Development Program for Specially Promoted Research for Innovative Next Generation Batteries, Japan Science and Technology Agency. Li[FSA] was provided by Nippon Shokubai Co., Ltd. (Japan).

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DOI: 10.1021/acsami.7b12340 ACS Appl. Mater. Interfaces 2017, 9, 35511−35515