Letter pubs.acs.org/NanoLett
Self-Rearrangement of Silicon Nanoparticles Embedded in MicroCarbon Sphere Framework for High-Energy and Long-Life LithiumIon Batteries Min-Gi Jeong,†,‡ Hoang Long Du,† Mobinul Islam,†,§ Jung Kyoo Lee,∥ Yang-Kook Sun,*,‡ and Hun-Gi Jung*,†,§ †
Center for Energy Convergence Research, Green City Technology Institute, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea ‡ Department of Energy Engineering, Hanyang University, Seoul 04763, Republic of Korea § Department of Energy and Environmental Engineering, Korea University of Science and Technology, Daejeon 34113, Republic of Korea ∥ Department of Chemical Engineering, Dong-A University, Busan 49315, Republic of Korea S Supporting Information *
ABSTRACT: Despite its highest theoretical capacity, the practical applications of the silicon anode are still limited by severe capacity fading, which is due to pulverization of the Si particles through volume change during charge and discharge. In this study, silicon nanoparticles are embedded in micron-sized porous carbon spheres (Si-MCS) via a facile hydrothermal process in order to provide a stiff carbon framework that functions as a cage to hold the pulverized silicon pieces. The carbon framework subsequently allows these silicon pieces to rearrange themselves in restricted domains within the sphere. Unlike current carbon coating methods, the Si-MCS electrode is immune to delamination. Hence, it demonstrates unprecedented excellent cyclability (capacity retention: 93.5% after 500 cycles at 0.8 A g−1), high rate capability (with a specific capacity of 880 mAh g−1 at the high discharge current density of 40 A g−1), and high volumetric capacity (814.8 mAh cm−3) on account of increased tap density. The lithium-ion battery using the new Si-MCS anode and commercial LiNi0.6Co0.2Mn0.2O2 cathode shows a high specific energy density above 300 Wh kg−1, which is considerably higher than that of commercial graphite anodes. KEYWORDS: Silicon anodes, porous carbon spheres, volumetric capacity, self-rearrangement, lithium-ion batteries
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In these regards, the silicon-based material is a promising alternative to commercial carbonaceous material due to the highest theoretical capacity. (4200 mAh g−1).8 However, due to their physical instability during the charge−discharge processes, silicon-based anode materials exhibit a short life-span, making them uncompetitive. Accordingly, there have been a large number of reports on using innovative silicon nanostructures (i.e., nanoparticles,9−11 nanowires,12−14 and nanotubes15−17)
o keep pace with the rapid developments in automobile and energy industries, such as hybrid electric vehicles (HEV), electric vehicles (EV), and grid-scale energy storage systems (ESS), the safety, energy and power density, and cycle life of lithium-ion batteries (LIBs) all need to be improved.1,2 The performance of LIBs is strongly affected by materials of both the cathode and anode. The specific capacities of existing cathode materials have already approached their theoretical limit.3−7 Meanwhile, most conventional anode materials are still hindered by low specific capacity, cycle life, and rate capability. Therefore, they need significant innovation to match the improvements achieved by the cathode materials for LIBs. © 2017 American Chemical Society
Received: June 8, 2017 Revised: August 2, 2017 Published: August 28, 2017 5600
DOI: 10.1021/acs.nanolett.7b02433 Nano Lett. 2017, 17, 5600−5606
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Scheme 1. Schematic Illustration of Silicon Nanoparticle Rearrangement Inside Micron-Carbon Sphere during Cycling
Figure 1. FE-SEM images of embedded micro-carbon spheres (Si-MCS) at (a) low and (b) high magnification. (c) XRD patterns of Si nanoparticles, MCS, and Si-MCS. (d) TEM image of one individual Si-MCS. (e) Magnified TEM image for the same Si-MCS as that in panel d. (f) Latticeresolved high-resolution TEM micrograph for Si-MCS; corresponding selected area diffraction patterns (SAED). Comparison of the N2 adsorption− desorption isotherms (g) and corresponding BJH pore size distribution curves (h) of MCS and Si-MCS.
the increased surface area of these nanostructures also means a larger interface with the electrolyte, which can cause excess side reactions and high irreversibility in the Si-based anode.25,26 Moreover, the ample interstices between nanoparticles in the packing powder lowers the tap density as well as volumetric capacity compared to particles with microstructures.27−29 In addition, recent studies found that even applying surface
and the surface modification by coating strategy to prolong the cycle life of Si-based anodes.18−22 Typically, the nanostructures can shorten the ionic transport pathway and relieve the loss of electrical contact resulting from Si pulverization,23,24 and the coating strategy with carbon on the surface of Si nanoparticles shows the benefits in relieving the large volume expansion of the Si anode.19−21 Unfortunately, 5601
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Figure 2. Electrochemical performances of lithium-ion half cells with Si-MCS electrode: (a) charge/discharge potential profiles, cycled between 0.01 to 1.5 V vs Li/Li+, at 0.2 C-rate (1 C-rate = 4 A g−1), and (b) corresponding capacity retention. (c) Potential profile of rate capability measured at various current densities and (d) corresponding rate capability. (Note that all the capacity values were calculated based on the total mass of Si NPs and carbon.) The first five CV curves of the (e) MCS and (f) Si-MCS anode.
modification, such as carbon coating, the fracture of Si nanoparticles still occurs during the lithiation step in the early cycles.30 In other words, a thin carbon layer on the Si surface can be destroyed by repeated volume change during cycling. It is proved that encapsulation of the nanometer-sized metal or metal oxide particles, such as Sn−Co31 and NiO32 in a carbon matrix, is an effective approach to overcome the pulverization issue in LIBs. Concurrently, it can be speculated that the encapsulated Si nanoparticles are free to change their morphologies, in the so-called self-rearrangement process within domains bounded by the porous carbon. This speculation inspires us to investigate carbon encapsulated Si nanoparticles as the anode for LIBs. To illustrate the mechanism of our proposed electrode prototype, the selfrearrangement of the silicon nanoparticles during repeated lithiation/delithiation is schematically shown in Scheme 1. Generally, the silicon nanoparticles, whether embedded in the porous carbon spheres or not, undergo volume change upon lithiation/delithiation. During repeated lithiation/delithiation, the embedded silicon nanoparticles can break into smaller pieces and continuously redistribute (i.e., self-rearrangement) in a domain within the micro-carbon spheres. Therefore,
despite the pulverization and self-rearrangement of the silicon nanoparticles in the pores, the structural integrity of the electrode can be maintained even after extremely long cycles. Herein, we designed Si nanoparticles encapsulated by micron-sized carbon spheres (Si-MCS) using a straightforward hydrothermal technique, which has the advantages of low energy consumption and high scalability. The structural features of the new silicon-based electrode material were examined by various methods. The morphology of the synthesized Si-MCS powders was observed by scanning electron microscopy (SEM, Figure 1a,b). These images show that the carbon particles are spherical in shape (diameter ≈ 2 μm) and highly monodispersed. The structures of silicon nanoparticles before and after embedding in micro-carbon spheres were investigated by X-ray diffraction (XRD). In Figure 1c, both samples display well-defined peaks for the (111), (220), (311), (400), and (331) planes of Si crystallites (JCPDS no. 27−1402). In addition, broad peaks at 22−25° and 43° are due to the amorphous nature of the carbon spheres in the SiMCS sample ascertained by the MCS XRD pattern displayed at the bottom. However, these two analyses cannot conclusively determine the existence of Si nanoparticles confined by carbon microspheres. Therefore, transmission electron microscopy 5602
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nanoparticles and 70 wt % carbon according to elemental analysis (EA). This weight ratio was furthermore supported by TGA analysis (Figure S4). Figure 2a shows the discharge (delithiation) and charge (lithiation) voltage profiles of the SiMCS electrode operating at 0.2 C-rate (0.8 A g−1) during 500 cycles. The excellent cycling stability is revealed by the approximately overlapping voltage profiles among specific cycles. All voltage profiles exhibit the same type of gentle slope between 0.2 and 0.01 V, which corresponds to the typical electrochemical behavior of the lithium alloy of amorphous Si.38 Generally, the Si anode has an increasing plateau after the first cycle because of changing the phase from crystalline to amorphous.40,41 However, before assembling the cell, the prelithiation step was performed to be in the amorphous state earlier by forming an alloy with Li,42−45 resulting in the voltage profile of the first cycle and the following cycles being similar in this study. Moreover, the initial Coulombic efficiency could be increased up to 99.6% from 75% of the Si-MCS without prelithiation (Figure S5). The prelithiation step maximized the initial Coulombic efficiency in this study. Figure 2b shows the cycle retention with Coulombic efficiency. In accordance with the voltage profiles, the Si-MCS electrode displays good cycling stability, with high discharge (delithiation) capacity retentions of 99.5%, 98%, 97%, and 96% after 100, 200, 300, and 400 cycles, respectively. Especially, after the 500th cycle, the specific discharge capacity (about 1070 mAh g−1) is still 94% of the initial cycle, indicating high anode stability in the Li-ion battery. Conversely, rapid capacity decay is observed for an electrode constructed by a Si nanoparticle, and the MCS can contribute a little to the entire capacity of the Si-MCS electrode, as demonstrated in Figure S6. Furthermore, the Coulombic efficiency of Si-based electrodes is a critical parameter to evaluate their electrochemical performance in practical Li-ion full batteries.46 Besides the prelithiation step, the high Coulombic efficiency may be due to the perfectly encapsulating carbon sphere, which can reduce the direct contact between Si and the electrolyte and promote the formation of a stable solid electrolyte interface (SEI) layer on the surface of Si nanoparticles. The porous structures of MCS can also increase the accessibility of lithium ions and electrolytes to maintain a high Coulombic efficiency. With regard to the rate capability, the Si-MCS electrode was evaluated at various discharge (delithiation) current densities from 0.1−10 C-rates (0.4−40 A g−1) with a fixed charge (lithiation) current density of 0.1 C-rate. As reported in Figure 2d, the specific capacity drop is negligible between 0.1−3 Crates. The resulting specific capacities are 1202 mAh g−1 from 0.1−3 C, 1137 mAh g−1 at 5 C, and 1039 mAh g−1 at 7.5 C. Even at the high rate of 10 C, the discharge capacity is still 880 mAhg−1, or 73% of the initial capacity at 0.1 C-rate. This excellent rate capability is believed to be the combined result of the nanosized silicon particles and the appropriate pore structure in carbon spheres. First, the Si nanostructure facilitates Li+ mobility by shortening the ionic transport pathways through the encapsulated Si nanoparticles. Second, the BET analysis confirmed that the Si-MCS powder includes mesoporous structures, which have been reported to improve the ionic diffusivity in electrodes and hence benefit the highrate performances of a Si-MCS electrode.47,48 Moreover, due to the electrical conductive network of MCS, a higher electrical conductivity (2.2 × 10−3 S cm−1) was obtained for the Si-MCS electrode (see Table 1), while that of pure Si nanoparticles is 50 nm) originating from the interspace created between Si nanoparticles and MCS. These results point out that the pores were generated and occupied by Si nanoparticles inside the MCS. Once again, the BET surface area reduction from MCS to SiMCS (from 404.1 to 212.9 m2 g−1) indicates the successful insertion of Si NPs into a carbon microsphere.38,39 These results for SEM, TEM, XPS, and BET analyses clearly demonstrate that the Si nanoparticles are successfully confined inside the micro-carbon spheres in the current synthesis approach. In order to test their electrochemical performance as anodes in Li-ion batteries, the synthesized Si-MCS electrodes were examined by galvanostatic charge and discharge cycling at various rates in the 0.01−1.5 V (vs Li/Li+) potential range. All the specific capacities are based on the total Si-MCS active material mass, which is composed of 30 wt % silicon 5603
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several other advantages that contribute to their outstanding electrochemical performance. First, the carbon framework functions as an electronic highway for the silicon nanoparticles to increase the electronic conductivity. The interconnected network of mesopores and micropores in the carbon framework also significantly facilitates the Li+ diffusion. Third, while individual silicon nanoparticles form unstable SEI upon contacting the electrolyte and cause high irreversible capacities during charge (lithiation) and discharge (delithiation) processes, in Si-MCS the carbon spheres encapsulating the Si nanoparticles promote more stable SEI formation on the outer surface of the embedded Si nanoparticles, resulting in high reversible capacities. Last but not the least, this embedded structure in microcarbon successfully overcomes the problems of low tap density and electrode density owning to the nanosized Si powder in practical applications, making Si a feasible material for commercial LIB anodes. The volumetric capacity of the electrode, which is a crucial parameter in battery applications, is intimately related to the tap density of active materials and its gravimetric capacity. As summarized in Table 1, the tap density of the Si-MCS powder (0.68 g cm−3) is more than four times that of Si nanoparticles alone. The volumetric capacity of our Si-MCS electrode was calculated by multiplication of both the tap density and gravimetric capacity, and the results are summarized in Table 1. Remarkably, the volumetric capacity of the Si−MCS anode (814.8 mAh cm−3) is 1.7 times higher than that of MCMB graphite anodes.53 Therefore, the noticeably enhanced tap density and the resulting volumetric capacity further supported that this approach (embedding Si nanoparticles in micro-carbon spheres) can potentially overcome the most chronic problems of Si nanoparticles in anodes. To demonstrate the use in practical battery systems, full LIBs were fabricated with our Si-MCS anode and LiNi0.6Co0.2Mn0.2O2 (NCM622) cathode. As shown in Figure 4, the batteries were cycled at the current density of 90 mA g−1 (0.5 C-rate vs the cathode limiting capacity) in the voltage window of 1.8 to 4.15 V. The cathode initial voltage profile along with cycling data in a half-cell mode is included in the Supporting Information (Figure S8) even though it had already been reported in an earlier study.54 The typical voltage profiles of the full-cell initial cycle are presented in Figure 4a, well in accord with that of the cathode half-cell (Figure S8a). As expected from their respective voltages, the combination of Si with NCM622 gives rise to a battery operating in the 3.7 V range. The initial specific capacities of charge and discharge are 144 and 136 mAh g−1, respectively, showing a 95% Coulombic efficiency. Even after 100 cycles, the full battery shows the steady specific capacity of 134 mAh g−1 with an average
Table 1. Comparison of the Tap Densities, Specific Capacities, Associated Volumetric Capacities, and Electrical Conductivity of Si Nanoparticles, Si−MCS, MCS, and Commercial MCMB materials Si-MCS MCMB53 MCS Si nanoparticles
tap density (g cm−3) 0.68 1.42 0.39 0.14
volumetric capacity (mAh cm−3)
specific capacity (mAh g−1)
814.8 1202 484.4 341 134.2 213 rapid capacity fading
electrical conductivity (S cm−1) 2.2 × 10−3 8.1 2 × 10−2 4.8 × 10−5
of the MCS and Si-MCS electrodes (without prelithiation) in the first five cycles, respectively. In the first cathodic scan, a splay cathodic peak at 0.5−0.9 V is observed for both MCS and Si-MCS, which could be attributed to the irreversible reduction and the formation of the SEI layer on the carbon sphere surface, as it is absent in the following cycles.49−52 In addition, two anodic peaks at ∼0.35 and ∼0.52 V correspond to delithiation of a lithiated silicon. From the second cycles onward for Si-MCS, a cathodic peak at ∼0.18 V occurred, and the anodic peaks around 0.35 and 0.52 V indicate a series of Li−Si alloy/dealloy formations.51,52 Existence of these characteristics peaks related to reversible transformation between Si to LixSi further identifies the successful construction of the SiMCS electrode. Further insights into the high performance of the Si-MCS anode were obtained via energy dispersed X-ray spectroscopy (EDS) mapping, which revealed the distribution and structural change of Si, and allowed us to identify the interplay of Si nanoparticles in the pores of MCS during charge (lithiation) and discharge (delithiation) processes. The results and the corresponding illustration are displayed in Figure 3. At the initial lithiation step of charging, the embedded Si nanoparticles expand as they alloy with lithium (see Figure 3a). After a few cycles, the expanded silicon nanoparticles generally undergo dramatic pulverization, which is one drawback restricting the cycle life of Si-based anodes. However, from the results of EDS analysis, all the pulverized silicon particles in our Si-MCS electrode are confined and become more evenly distributed inside the pores of the carbon sphere, without breaking the structure of the latter. A TEM image shows the well-preserved carbon spheres, verifying structural integrity of the electrode after cycling as shown in Figure S7. Therefore, the rigid carbon structure successfully retains the pulverized and broken Si nanoparticles, instead of allowing them to be removed after undergoing the volume change. With this unique self-rearrangement of Si nanoparticles during the few initial cycles, our Si-MCS electrodes possess
Figure 3. Schematic illustration and EDS image of the Si-MCS particle after the first lithiation process (a) and during repetitive delithiation (b) and lithiation (c) processes. 5604
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Figure 4. Performance of the Si-MCS electrode in a full-cell using a LiNi0.6Co0.2Mn0.2O4 cathode: (a) the voltage profiles of the initial charge− discharge cycle of the battery run at 0.5 C-rate (1 C-rate = 180 mA g−1) and (b) the capacity retention curve.
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working voltage of 3.7 V. The capacity retention of the NCM622 electrode after 100 cycles in a full-cell is about 93%, comparable to that of a half-cell (Figure 4b and Figure S8b). In addition, the full-cell exhibits a high specific energy density of about 504 Wh kg−1. Using the accepted 40% penalty factor to account for the weights of the electrolyte and ancillary components (case, current collectors, etc.),55,56 a practical energy density of 302 of Wh kg−1 is anticipated, which is considerably higher than that from the presently available LIBs (230 Wh kg−1).57 We believe that all these features make batteries using the Si-MCS electrode excel over other present competitors in the field of Li-ion batteries. In conclusion, we have used a straightforward hydrothermal approach to synthesize Si nanoparticles encapsulated in microcarbon spheres (Si-MCS) as an anode material, which demonstrates reasonably high capacity for next-generation lithium-ion batteries. In the active material, the fractured and broken pieces of Si nanoparticles undergo unique selfrearrangements within the stiff carbon sphere framework during repeated lithiation and delithiation processes. This feature enables the Si-MCS electrode to achieve a much longer extended cycle life than materials using the conventional carbon coating method: a high reversible capacity of 1070 mAh g−1 at 0.2 C-rate (0.8 A g−1) and an excellent cycle retention of 93.5% after 500 cycles. Even at the high current rate of 10 C (40 A g−1), the discharge capacity remains at 880 mAh g−1. The high tap density of the Si-MCS powder also contributes to the high volumetric capacity of 814.8 mAh cm−3, a very significant achievement toward commercialization. As further demonstrated for practical battery systems, lithium-ion full batteries using the Si-MCS anode and LiNi0.6Co0.2Mn0.2O2 (NCM622) cathode have a reversible capacity of around 135 mAh g−1 at 0.5 C-rate (90 mA g−1) and a cycle retention of >93% of the initial cycle. These results suggest potential future avenues to commercialize silicon-based anodes for lithium-ion batteries with high energy density.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Yang-Kook Sun: 0000-0002-0117-0170 Hun-Gi Jung: 0000-0002-2162-2680 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Korea Institute of Science and Technology (KIST) institutional program (2V05540). This work was also supported by the National Research Foundation of Korea (NRF) grant (No. 2017R1A2B2006275 and 2017M1A2A2044477) and by the Global Frontier R&D Program (2013M3A6B1078875) on Center for Hybrid Interface Materials (HIM) funded by the Korea government (MSI).
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.7b02433. Experimental methods and additional data for SEM, TEM, TGA, and electrochemical characterization of the materials (PDF) 5605
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