Strategy for Boosting Li-Ion Current in Silicon Nanoparticles - ACS

Aug 27, 2018 - Improvement in the rate capability needs to be addressed for utilization of a Si anode in high-power Li-ion batteries. Regarding the ra...
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Strategy for Boosting Li-Ion Current in Silicon Nanoparticles Min-Sang Song,†,‡,∇ Geewoo Chang,§,∇ Dae-Woong Jung,∥ Moon-Seok Kwon,†,∥ Ping Li,§,∥ Jun-Hwan Ku,† Jae-Man Choi,† Kan Zhang,§,⊥ Gi-Ra Yi,*,∥ Yi Cui,*,‡,# and Jong Hyeok Park*,§ ACS Energy Lett. 2018.3:2252-2258. Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SANTA BARBARA on 09/15/18. For personal use only.

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Energy Material Lab, Material Research Center, Samsung Advanced Institute of Technology, Samsung Electronics, 130 Samsung-ro, Yeongtong-gu, Suwon-si, Gyeonggi-do 16678, Republic of Korea ‡ Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States § Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Republic of Korea ∥ Department of Chemical Engineering, Department of Energy Science and SKKU Advanced Institute of Nano Technology (SAINT), Sungkyunkwan University, Suwon 440-746, Republic of Korea ⊥ MIIT Key Laboratory of Advanced Display Materials and Devices, Institute of Optoelectronics and Nanomaterials, School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, People’s Republic of China # Stanford Institute of Materials and Energy Sciences SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States S Supporting Information *

ABSTRACT: Improvement in the rate capability needs to be addressed for utilization of a Si anode in high-power Li-ion batteries. Regarding the rate capability, its improvement by Si−C nanocomposites seems to be somewhat saturated, thus indicating that the other method should be tried for further enhancement of the rate capability. Here, we introduce Si nanoparticles uniformly coated with nanometer-thick polyacrylonitrile (PAN) with better wettability to liquid electrolytes and minimizing electronic resistance, which might result from a thick PAN coating: the effective contact surface area made by the contact of Si nanoparticles and liquid electrolyte is increased for larger Li-ion current, leading to ultrafast rate capability retaining 62% of the 0.2C rate discharge capacity at 100C. In addition, a strong adhesive property of PAN provides highly mechanically robust Si anodes for multielectrode-stacked flexible lithium-ion batteries, which show no physical damage after 30 000 bending cycles with a bending radius of 25 mm.

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in the cycle performance. As a result, the cyclability of silicon has been remarkably improved by adopting nanostructured Si−C composites in various shapes of nanoparticles, nanotubes, nanowires, and their derivative hollow or porous forms to minimize the induced strain and pulverization of Si without losing the electronic pathway.12,19−31 Along with the remarkable progress in the cyclability, the rate capability of silicon should be improved for its successful practical application in lithium-ion batteries. Although most studies regarding silicon have been aimed at improvement in the cyclability as aforementioned, there have also been a considerable number of works to improve the rate capability of silicon based on Si−C nanocomposites where the Li-ion

uring the past decades, astonishing advances in portable electronics and hybrid/full electric vehicles during the past decades have awakened the persistent demand for higher energy density lithium-ion batteries to power them longer. Due to the limited specific capacity of traditional graphite anode materials (370 mAh g−1),1−3 attractive features of silicon as an anode material, a much higher theoretical specific capacity of 4200 mAh g−1, a low discharge potential of around +0.5 V versus Li/Li+, and natural abundance,4−16 have made silicon one of the most promising alternatives to be able to fulfill the demand. However, a large volume expansion/shrinkage (∼400%) of silicon during repeated lithium insertion/extraction tremendously deteriorates its cycle performance by inducing fractures both inside of Si and between Si particles and a conductive additive or current collector that can result in the loss of electronic paths and the instability of solid electrolyte interphase,17,18 which have thus driven numerous studies focusing on improvement © 2018 American Chemical Society

Received: July 2, 2018 Accepted: August 27, 2018 Published: August 27, 2018 2252

DOI: 10.1021/acsenergylett.8b01114 ACS Energy Lett. 2018, 3, 2252−2258

Letter

Cite This: ACS Energy Lett. 2018, 3, 2252−2258

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ACS Energy Letters diffusion length in Si is significantly reduced by a nanostructure with electron conduction facilitated by carbon.11,29−35 However, further improvement in the rate capability by the Si−C nanocomposites now seems to be somewhat difficult because of the fully minimized dimensions of their nanostructures and the maximized utilization of conductive carbon, indicating that another new strategy is essentially required to achieve further advancement in the rate capability. Aware of this limited condition of the Si−C nanocomposites, we turned our attention toward the other factor determining the rate capability, the Li-ion current (i.e., the amount of Liions that pass through in a specified time) into/from Si nanoparticles to attain a higher rate capability instead of electron current and the lithium-ion diffusion length, which has been already addressed in Si−C composites. Li-ion current into/from Si is strongly dependent on the Liion flux (i.e., areal current density) and the total cross section area that Li ions pass through.36,37 In terms of Li-ion flux, it is affected by Li-ion conductivity, which is in proportion to the Li-ion mobility and concentration.38 In fact, the Li-ion mobility in Si cannot be tuned because it is basically dependent on the atomic structure of Si. Besides, the viscosity and Li-ion concentration of the commonly used liquid electrolyte, which govern the Li-ion mobility and concentration around Si particles in the electrolyte, have been already optimally set for the highest Li-ion conductivity. In the case of the total cross section area for the Li-ion current, it has almost been optimized by increasing the specific surface area of Si from nanostructural designs. Accordingly, there would be few factors for a Si-based anode to increase the Li-ion flux and total cross section area. However, at this point, we should note that the effective contact surface area for the Li-ion current corresponds to the contact area between Si particles and the electrolyte, meaning that the better wettability of the electrolyte to the surface of Si particles can bring about a larger contact surface area to increase the Li-ion current. None can confirm that the entire exposed surface area of Si nanoparticles in an electrode is covered with liquid electrolyte even after enough time. Moreover, a conventional polymer binder simply mixed with Si nanoparticles cannot maximize the wettability, which inherently affects the effective contact surface area for Li-ion current. Here, we introduce Si nanoparticles uniformly coated by nanometer-thick polyacrylonitrile (PAN) (Si NPs@PAN) with better wettability to liquid carbonate electrolytes for the purpose of further improvement in the rate capability of silicon, as shown in Figure 1. Similar to our strategy focusing on Li-ion conductivity, a few works have reported improvement in the rate capability of Si nanoparticles by using a Li-ion conductive polymer binder.39,40 To the best of our knowledge, few works have been reported to improve the rate capability of silicon by controlling the effective contact surface area of the Li-ion current. PAN is known as one of the most suitable polymer materials for Li-ion batteries because of its high electrochemical stability, excellent resistance to oxidative degradation and flame, and easy processability.41−44 Most importantly for our work, PAN has been demonstrated to have better electrolyte wettability than poly(vinylidene fluoride) (PVdF) and carboxymethyl cellulose (CMC), which are the representative polymer binders used in Li-ion batteries, owing to the high affinity of the strongly polar nitrile group in PAN to carbonate.41−43 Consequently, the

Figure 1. Schematic of the Si NP@PAN with an increase in Li-ion current for ultrafast rate capability: Nanometer-thick PAN coating leads to a larger effective cross section area for Li-ion transfer between Si and liquid electrolyte, which can enable a higher rate capability by increasing the Li-ion current into/from Si nanoparticles.

nanometer-thick PAN coating strategy on Si nanoparticles is greatly beneficial to increase the effective contact surface area of the Li-ion current while minimizing the electronic resistance resulting from the insulating nature of PAN. Besides, PAN can provide a strong adhesive force to keep Si particles in contact with the carbon conductive additive and Cu current collector to ensure excellent cyclability of Si.45,46 In the context of developing a flexible battery for future wearable electronic devices that goes through physical deformation of bending and twisting that can lead to detachment of the electrode layer from the current collector,47,48 the expected strong adhesion strength of Si NPs@PAN would prevent such detachment. To evaluate this, for the first time, we use a multielectrode-stacked flexible lithium-ion battery where not only tensile and compressive stresses in the electrode but also the friction force between the electrodes simultaneously occur. The abovementioned coupled advantageous characteristics of Si NPs@ PAN are of great use to develop flexible lithium-ion batteries for future applications as well as current conventional lithiumion batteries. Figure 2 shows the contact angles measured with a droplet of liquid electrolyte and the proposed scheme for synthesizing Si NP@PAN. In Figure 2a, a contact angle of 27° was observed between a nonoxidized Si wafer and a droplet of the liquid electrolyte used for the charge/discharge test. In the case of an intrinsic oxide layer existing on Si wafers (Figure 2b), the surface of SiO2/Si exhibited a slightly lower contact angle of 22.3°. It can be presumed from this result that both Si NPs with and without a very thin oxide layer may form poor interfaces with an organic electrolyte, which could reduce the Si−electrolyte contact area and hinder efficient transport of more Li-ions to the Si surface. However, the PAN-coated Si wafer exhibits a contact angle of 6.3° because of the better wetting of the electrolyte, resulting in an increased contact surface area for Li-ion transfer and possibly improved rate capability of Si NP@PAN. To synthesize Si NP@PAN, we adapted the emulsion drying method to obtain a uniform nanometer-thick PAN layer coating on Si NPs. Note that appropriate selection of solvents is critical for producing Si NP@PAN. First, Si NPs were dispersed in the organic solvent toluene. Second, PAN was dissolved in another organic solvent, dimethylformamide (DMF). These two solutions were mixed to serve as the oil phase (hydrophobic solution). The water phase (hydrophilic solution) was F108 (poly2253

DOI: 10.1021/acsenergylett.8b01114 ACS Energy Lett. 2018, 3, 2252−2258

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Figure 2. Contact angle measurement and synthesis technique: (a) contact angle measurement with a droplet of liquid electrolyte on a SiO2/Si wafer, (b) the Si wafer, and (c) the PAN-film-coated Si wafer. (d) Schematic illustration of the synthesis technique: (1) Homogenization: The oil (hydrophobic) phase and the water (hydrophilic) phase are mixed. (2) Toluene/DMF evaporation: The oil phase is evaporated to collapse Si/PAN, during which various Si@PAN shapes are formed. (3) Sedimentation: Solid phases containing Si and PAN drop to the bottom as the oil solvents evaporate. (4) Collection: Submicron- and micron-sized Si@PAN powder is collected. The nanosized primary particles of Si@PAN.

Figure 3. Characterization of Si NP@PAN. (a) SEM images of Si NPs. (b) SEM images of Si NPs@PAN. (c−e) SEM images of various points in Si NPs@PAN clusters with different assembled shapes. (f) XRD pattern of pure Si NP, PAN, and as-synthesized Si NPs@PAN.

rate performance compared to the carbon-coated Si NP, the Si NP@PAN powders (Figure S1a) were thermally annealed at 600 °C under oxygen-free (argon) conditions to carbonize the PAN shells (Si NP@C). As shown in Figure S1b, there were no noticeable morphology changes after the carbonization step. Thermogravimetric analysis (TGA) of pure PAN under oxygen-free conditions confirmed that more than 40 mass % of initial PAN still remained after carbonization at 600 °C (Figure S2). As an indication of successful carbonization of PAN, a TEM image (Figure 4c) of Si NPs@C carbonized from Si NPs@PAN showed a 2−3 nm thick carbon layer on the Si NP, meaning that the original PAN shell on the Si NP was successfully carbonized without deteriorating the initial morphology. EDX mapping data (Figure 4d−f) also confirmed that the outer shell was mainly composed of carbon. Before preforming the charge/discharge test, cyclic voltammetry (CV) was performed to examine the effect of PAN on the charge/discharge behavior of Si NPs. Figure S3 shows CV curves of Si reference and Si NPs@PAN electrodes during three cyclic loops. The transition from crystalline Si NPs to an amorphous structure during charging (i.e., lithium insertion) was confirmed by the representative CV curves in Figure S3a. Lithium insertion results in two peaks at approximately 0.07 and 0.21 V that represent the formation of an amorphous LixSi phase.50 In the discharge (Li-extraction) process, the main peaks at 0.37 and 0.50 V correspond to the formation of amorphous Si. All reduction and oxidation peaks in the Si reference and Si/PAN electrodes were observed at almost the same positions, confirming that the charge/

(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer) dissolved in formamide. The oil phase solution was poured into the water phase solution to make the emulsion solution, and subsequent evaporation of the oil phase solvents led to sedimentation of the final product: Si NP@ PAN composites. Field emission scanning electron microscopy (FE-SEM) images of Si NPs@PAN prepared via the technique in Figure 2c are shown in Figure 3a−e; a comparison with pristine Si NPs shows that individual Si NPs are coated with a PAN polymer layer. The PAN-coated Si NPs were held together to form various shapes of agglomerated particles due to van der Waals force during the evaporation of the oil-in-water emulsion. The existence of PAN in Si NP@PAN was clearly confirmed by X-ray diffraction, as shown in Figure 3f. The sharp 2θ peaks at approximately 28.18, 47.2, 55.98, 69.02, and 76.18° correspond to the (111), (210), (311), (400), and (331) reflections, respectively, of Si NPs (JCPDS card number: 27-1402). In addition to those peaks, the 2θ peak at approximately 16.96° for Si NPs@PAN arises from the PAN layer; the same peak is observed in pure PAN and represents the crystalline orientation of PAN molecules.49 Transmission electron microscopy (TEM) images of Si NP@PAN are displayed in Figure 4a,b, in which an approximately 2 nm thick PAN layer coats the Si NP surface. In the magnified image (Figure 4b), Si lattice fringes indicate the discrete interface between PAN and Si, further confirming the existence of the PAN layer on the surface of the Si NPs. To prove the effectiveness of Si NP@PAN in the context of higher 2254

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voltage cutoff range between 0.01 and 1.0 V vs Li/Li+. Figure 5a shows the charge/discharge voltage profiles of Si NP@PAN at different C rates. Surprisingly, Si NP@PAN exhibited outstanding performance even at 100C. At a 100C discharge condition, the Si NP@PAN achieved 1193 mAh g−1 (62.1% of the discharge capacity at first cycle (1922 mAh g−1)) with a discrete voltage plateau, indicating a considerable faradaic reaction unlike capacitor-like behavior; to our knowledge, this value is the largest one ever reported. Furthermore, in comparison with the discharge capacities of Si NP, Si NP@ C, and Si NP@PAN at different C rates and current densities (Figures 5b and S4), Si NP@PAN showed even better rate performance than Si NP and Si NP@C, proving that increasing the Li-ion current is an effective approach to further improve the rate capability of Si, which has been barely tried so far. A decrease in charge transfer resistance (Rct) for Si NP@PAN is also confirmed in electrochemical impedance spectroscopy (EIS) spectra (Figure S5), which demonstrates that increased Li-ion current reduces Rct. Generally, under the fast charge/ discharge condition, the specific capacity might be influenced by both electron transport in the electrodes and ionic transfer from the electrolyte. However, especially for Li-ion batteries with high specific capacities such as Si anodes, more Li ions need to be inserted in the anode electrode compared with conventional carbonaceous materials. The insertion or extraction process of Li ions in Si anodes is not strongly affected by ionic resistance during the slow charge/discharge process. However, it should be noted that when the cell is operated under a high C rate condition of over 20C (i.e., an ultrafast charge/discharge condition) the ion concentration at redox sites can be decreased much more than the electron concentration because of the lower intrinsic ionic conductivity (Figure S6). This effect of ion depletion around Si NPs causes a significant decrease in performance by acting as a ratedetermining step. As previously mentioned in the introduction, one of the effective ways to relieve this ion depletion is to supply more Li ions to the silicon at the same C rate by

Figure 4. Characterization of Si NP@PAN and Si NP@C. (a) TEM image of Si NP@PAN. (b) Magnified TEM image of Si NP@PAN. (c) TEM image of Si NPs@C. (d−f) EDX mapping data of Si NPs@C.

discharge mechanism in Si NPs is independent of the existence of PAN. To investigate the high rate charge/discharge properties of the electrodes, a galvanostatic charge/discharge process was carried out using various current densities (C rates) and a

Figure 5. Electrochemical characterization. (a) Charge/discharge profiles of Si NP@PAN anodes. (b) Rate performance of Si NP (0.5 mg cm−2), Si NP@C (0.7 mg cm−2), and Si NP@PAN anodes (1.0 mg cm−2). (c) Cyclability of Si NP and Si NP@PAN anodes. 2255

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ACS Energy Letters increasing the total contact surface area of ionic transfer through the electrolyte; this area is influenced by interfacial characteristics between the organic electrolyte and the Si NP surface (i.e., wettability). As observed in Figure 2c, the PANcoated Si wafer exhibits a much lower contact angle of 6.3°, indicating better wettability of the electrolyte on the Si surface. In agreement with the contact angle data, the PAN-coated Si NP electrode also exhibited a liquid electrolyte uptake that was ∼2 times higher. Therefore, the use of Si NPs surrounded with a PAN layer can reduce ion depletion under high C rate conditions. In the case of a pure Si NP anode, this ion depletion might increase the electrode resistance strongly and thus affect capacity values under high C rate conditions. The outstanding rate performance of Si NP@PAN comes from the increased liquid electrolyte wettability and uptake by PAN molecules, which result in sufficient ion species around Si NPs to enable a larger Li ion current during ultrafast charge/ discharge. However, it could not be achieved without the help of carbon black as a conductive additive that was already added enough to the electrode because electron conduction is also an important factor for high rate capability. It should be noted that the Si NP reference anodes and Si@PAN anodes have the same electrode composition with a 20 wt % carbon conductive additive, which excludes any differential contribution of electronic conductivity to the high-rate performance. Therefore, it can be concluded that enhanced Li-ion current under sufficient electron conduction leads to significant improvement in the rate capability of Si. Figure 5c shows the discharge capacity retention of the cells with Si NP and Si@PAN anodes during galvanostatic cycling at 1C. In Figure 5c, the large improvement in cyclability of Si NP@PAN (>91.4% retention after 100 cycles) is distinctly highlighted. Moreover, the Si NP@PAN anode retained more than 65% of its initial capacity after 1000 cycles. This stable cyclability can be attributed to robust structure retention arising from the strong adhesion of Si NPs to Cu the foil and carbon-conductive additive, enabled by the adhesive property of PAN, during long-term cycling. After 100 cycles, the Si NP reference electrode exhibited no voids that would allow ionic diffusion (Figure S7a), which could be a result of the successive growth of unstable SEI layers. As shown in Figure S7b, however, the Si NP@PAN electrode retained its porous nature even after 1000 cycles, enabling its excellent long-term cyclability by relatively suppressing SEI formation on Si NPs. On the basis of the adhesive property of Si NP@PAN, 3 × 9 cm2 sized pouch-type flexible batteries with Si NP or Si NPs@ PAN multielectrodes stacked were assembled at an industrial facility (Samsung Advanced Institute of Technology) to investigate the mechanical durability against bending fatigue (i.e., bending durability). To obtain the same stress conditions as those in an industrially fabricated flexible battery, four Si anodes and three dummy cathodes were stacked layer-by-layer with six polyethylene separator sheets containing liquid electrolyte by using the method inspired by book binding (Figure 6a). As shown in Figure 6b, the initial anodes were very smooth with uniform surfaces. After undergoing 30 000 bending cycles with a bending radius of 25 mm (industrial condition, Figure S8), reference Si NP anodes exhibited severe cracks, and some fragments were detached from the Cu foil. However, there was no noticeable change in the Si NP@PAN anodes after the same bending test. In the magnified images (Figure S8), the damage on Si NP anodes after the bending test is more clearly seen in comparison with Si NP@PAN

Figure 6. Evaluation of strong adhesive property of Si NP@PAN. (a) Schematic of a pouch-type flexible Li-ion battery (prepared at the Samsung Advanced Institute of Technology) sized 3 × 9 cm2. (b) Photographs of anodes before and after the bending test at a bending radius of 25 mm. (c) Voltage profiles for Si NP and Si NP@PAN anodes before and after the bending test. (d) Schematic of the Si NP@PAN anode undergoing tensile and compressive stress during the bending test.

anodes. In addition, we collected anodes from the flexible batteries after the bending test and investigated their charge/ discharge performances using coin-type half-cells with Li metal as a reference electrode. As shown in Figure 6c, there was no performance degradation in Si NPs/PAN anodes, whereas the reference Si NP anodes maintained only 12.5% of their initial specific capacity. Therefore, as shown in Figure 6d, the strong adhesive nature that originates from the PAN surrounding Si NPs is confirmed to withstand large mechanical compressive and/or tensile stresses during a rigorous bending test. To the best of our knowledge, it is the first time that maintaining the performance and integrity of the anode comprising a conventional smooth 2D Cu foil even after 30 000 times bending at a 25 mm radius has been achieved by applying a practical-level multielectrode-stacked flexible battery. In conclusion, we achieved ultrafast rate capability of Si by newly focusing on the Li-ion current instead of electron conduction, which has been conventionally pursued as a representative strategy for improving the rate capability of Si. Nanometer-thick PAN layers coated with Si nanoparticles are successfully synthesized by the emulsion drying method to minimize the electronic resistance able to result from PAN coating and show the ultrafast rate capability that is even superior to that of Si−C nanoparticles while still having excellent cycle life, first proving that our devised concept is quite valid for further improving the rate capability of Si beyond the current state of the art, which cannot be attained only by Si−C nanocomposites. Moreover, the strong adhesive property of PAN as a potential binder material enables not only excellent cycle life but also mechanically robust Si anodes for multielectrode-stacked flexible lithium-ion batteries, which are newly shown in this work. On the basis of the outstanding performance of Si NPs@PAN, our new strategy will help us move forward to the realization of advanced Li-ion batteries with higher power and energy density as well as practical futuristic flexible Li-ion batteries. 2256

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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/acsenergylett.8b01114.

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Experimental section (materials and methods, characterization, measurements and analysis) and supplementary Figures S1−S8 showing SEM analysis, TGA data, electrochemical characterization, and a schematic of the test apparatus (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G.-R.Y.). *E-mail: [email protected] (Y.C.). *E-mail: [email protected] (J.H.P.). ORCID

Kan Zhang: 0000-0003-1121-3426 Gi-Ra Yi: 0000-0003-1353-8988 Yi Cui: 0000-0002-6103-6352 Jong Hyeok Park: 0000-0002-6629-3147 Author Contributions ∇

M.S.S. and G.C. contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Samsung Electronics. J. H. Park acknowledges the support by the National Research Foundation of Korea (NRF) funded by Ministry of Science and ICT (2018M3D1A1058624 (Creative Materials Discovery Program), 2016R1A2A1A05005216, 2017M3A7B4041987).

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DOI: 10.1021/acsenergylett.8b01114 ACS Energy Lett. 2018, 3, 2252−2258

Letter

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DOI: 10.1021/acsenergylett.8b01114 ACS Energy Lett. 2018, 3, 2252−2258