Zeolite-Templated Mesoporous Silicon Particles for Advanced Lithium

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Zeolite-Templated Mesoporous Silicon Particles for Advanced Lithium-Ion Battery Anodes Nahyeon Kim, Hyejeong Park, Naeun Yoon, and Jung Kyoo Lee* Department of Chemical Engineering, Dong-A University, Busan 49315, Republic of Korea S Supporting Information *

ABSTRACT: For the practical use of high-capacity silicon anodes in high-energy lithium-based batteries, key issues arising from the large volume change of silicon during cycling must be addressed by the facile structural design of silicon. Herein, we discuss the zeolite-templated magnesiothermic reduction synthesis of mesoporous silicon (mpSi) (mpSi-Y, -B, and -Z derived from commercial zeolite Y, Beta, and ZSM-5, respectively) microparticles having large pore volume (0.4−0.5 cm3/g), wide open pore size (19−31 nm), and small primary silicon particles (20−35 nm). With these appealing mpSi particle structural features, a series of mpSi/ C composites exhibit outstanding performance including excellent cycling stabilities for 500 cycles, high specific and volumetric capacities (1100−1700 mAh g−1 and 640−1000 mAh cm−3 at 100 mA g−1), high Coulombic efficiencies (approximately 100%), and remarkable rate capabilities, whereas conventional silicon nanoparticles (SiNP)/C demonstrate limited cycle life. These enhanced electrochemical responses of mpSi/C composites are further manifested by low impedance build-up, high Li ion diffusion rate, and small electrode thickness changes after cycling compared with those of SiNP/C composite. In addition to the outstanding electrochemical properties, the low-cost materials and high-yield processing make the mpSi/C composites attractive candidates for highperformance and high-energy Li-ion battery anodes. KEYWORDS: mesoporous silicon, magnesiothermic reduction, zeolites, energy storage, Li-ion battery to that of metallic lithium (3865 mA h g−1), and has relatively low voltage for delithiation (approximately 0.4 V Li/Li+). Silicon has also demonstrated its promise as an anode not only for high-energy LIBs5,9−11 but also for new electrochemical couples such as Li−S12−16 or Li−O2,17 or all solid-state18 batteries free of dendrite-forming lithium−metal anodes. With these opportunities and the evolution of nanotechnology in material synthesis, remarkable progress has been recently achieved in addressing the inherent issues associated with silicon-based anodes: large volume changes (approximately 300%)8,19 during cycling causing electrode pulverization, electrical contact loss, and uncontrolled growth of solid electrolyte interphase (SEI) layers leading to short cycle life.4,20,21 Among the many strategies reported to date, nanoengineered Si/C composites enabling long cycle life by effectively mitigating the mechanical strains due to volume

A

s an integral approach to addressing global carbon and climate issues, the use of energy-efficient electric vehicles (EVs) and energy storage systems (ESSs) equipped with lithium-ion batteries (LIBs) have been increasing at a rapid rate. However, the energy density (a measure of the drive range of EVs per charge) of LIBs based on the combination of lithium metal oxide (approximately 200 mAh g−1) cathodes and graphitic carbon anodes (theoretical capacity = 372 mAh g−1) is significantly less than those of internal combustion vehicles with a drive range of approximately 400 miles.1 In addition to advanced cell engineering with improved cell subcomponents, the adoption of high capacity active materials in electrodes is essential to boosting the energy density of LIBs.1−6 In particular, new anode materials offering high capacity at low working voltage have long been pursued as the energy and power density of LIBs have approached their limit in terms of the inherent performance limit of graphitic carbon anodes. In view of the capacity and working voltage as anodes for advanced LIBs, silicon is an ideal material because it exhibits high capacity (3570 mA h g−1 in the form of Li15Si4),7,8 closest © 2018 American Chemical Society

Received: February 10, 2018 Accepted: March 29, 2018 Published: March 29, 2018 3853

DOI: 10.1021/acsnano.8b01129 ACS Nano 2018, 12, 3853−3864

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Cite This: ACS Nano 2018, 12, 3853−3864

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organic-template. Therefore, zeolite Y could be potentially a low-cost precursor for porous silicon structures. Previously, zeolite NaY was used to prepare porous amorphous silicon structures using a sodiothermic reduction method for supercapacitors.51 However, it has never been used in MR to prepare porous silicon structures for lithium-ion battery anodes. Herein, we report the MR synthesis of mesoporous silicon (mpSi) microparticles from zeolite Y crystals and their use as high performance lithium-ion battery anodes. We conducted a systematic MR of zeolite Y under different conditions (types of reactors: open or closed system, reaction temperature, reaction time, with or without heat scavenger (NaCl)) to identify a pseudo-optimal condition leading to porous silicon structures with high yield and purity, small primary silicon particle size, and large open pore volume. We also extended the MR synthesis process to different types of zeolites such as Beta and ZSM-5. The mpSi particles were coated with conducting carbon to yield mpSi/C composites for LIB anodes. The mpSi microparticles retained the original microparticle morphology of zeolites and possessed large pore volume (0.39−0.53 cm3/g) in the particles owing to the evenly distributed mesoporous (approximately 19−31 nm in diameter) structures. The mpSi particles were composed of interconnected small primary crystalline silicon particles of 20−35 nm in diameter. With these appealing features of mpSi, the mpSi/C composites demonstrated an excellent cycling stability for 500 cycles at Coulombic efficiency (CE) approaching 100.0% and at high capacities (1000−1700 mAh g−1 at 100 mA g−1 dependent upon carbon content). Owing to the highly porous structure and small primary Si particles in mpSi, mpSi/C also exhibited high rate capability with a considerably higher Li diffusion rate, and significantly less impedance build-up and smaller electrode thickness increase than those in the conventional SiNP/C composites. Finally, a high capacity mpSi/C was hybridized with graphite to demonstrate a reliable strategy for costeffective anodes attaining important industrial requirements such as medium level of capacity (800−1000 mAh g−1) yet with high first cycle efficiency (85%), excellent cycling stability (97.5% capacity retention at 500 mA g−1 for 150 cycles), and high volumetric capacity (150% higher than that of graphite).

changes and thus maintaining the electrical integrity of the electrodes for many hundreds to thousands of cycles have provided inspiration for the rational design of silicon-based anode materials. For example, nanostructured Si/C composites where empty spaces were generated between the silicon and conducting carbon matrix allowing silicon to freely expand and contract demonstrated promising cycling performances.22−27 Rather than using preformed silicon nanoparticles, more recently, great attention has been addressed to porous silicon structures as a bottom-up approach.28−45 The open and porous structure bears sufficient free space in the particles to absorb the large volume expansion,46 thereby maintaining electrical integrity in the electrode and enhancing the cycling stability. For the preparation of porous silicon replicas from 3D silica assemblies, Bao et al.47 demonstrated a low-temperature (650 °C) magnesiothermic reduction (MR) reaction 1 below, as an alternative to the conventional high-temperature (≥2000 °C) carbothermal reduction. Since then, several types of silica structures such as mesoporous silica,28−34 spherical silica particles35,36 and capsules,37 silica foam,38 silica nanowires,39 and silica purified from natural resources including sand,40,41 rice husks,42,43 and even plants44,45 have been used to prepare corresponding porous silicon replicas using reactions 1 and 2 for their use as anodes in LIBs. However, it has become extremely difficult to obtain porous silicon replicas with small grain size in high purity and high yield owing to the inhomogeneity of silica and magnesium particles and sintering by the large reaction heat generated from the highly exothermic reaction (ΔH = −586.7 kJ/mol silica) and the side reactions 3 and 5 below. To mitigate the sintering due to large MR heat approaching 1720 °C,48 heat-scavengers such as NaCl powders were employed to facilitate the formation of porous silicon replicas without severe structure collapse and the aggregation of the silicon domains.49 2Mg(g) + SiO2 (s) → 2MgO(s) + Si(s)

(1)

MgO + 2HCl → MgCl2 + H 2O

(2)

2Mg + Si → Mg 2Si

(3)

4Mg + SiO2 → Mg 2Si + 2MgO

(4)

2MgO + SiO2 → Mg 2SiO4

(5)

RESULTS AND DISCUSSION Structure and Properties of mpSi and mpSi/C Samples. Scheme 1 illustrates the synthetic process of

According to previous reports on the preparation of porous silicon structures by MR reaction, porous silica precursors are favored because (1) Mg vapor can readily access the surfaces deep inside the silica particles without forming secondary phases such as magnesium silicide (Mg2Si), and (2) the reaction heat can be more effectively dissipated thus preventing excessive sintering of silicon nanocrystals.34,42,43 Continuing this line of thought, zeolites, crystalline aluminosilicates, could be an attractive silica source for porous silicon replicas. The global production of zeolites amounted to more than 4400 kilo tons in 2016, natural and synthetic zeolites being 2800 kilo tons and more than 1600 kilo tons, respectively,50 and their demand is expected to increase owing to their wide range of applications in numerous industries including water treatment, refinery, nuclear, biogas, detergents, construction, medical, and agriculture. Zeolite Y accounts for the largest share of the synthetic zeolite market owing to its application as catalysts and adsorbents in the oil refining and petrochemical processes. Further, its hydrothermal synthesis does not require any costly

Scheme 1. Synthetic Process of the mpSi/C Composite with Its Structural Features on Cycling

mesoporous silicon (mpSi) by the MR of zeolites with magnesium and the structural benefit of mpSi/C in accommodating the large volume expansion of silicon leading to stable cycling as LIB anodes. In this study, we employed zeolite Y (USY, CBV 780, Zeolyst International), which has a high SiO2/Al2O3 molar ratio of 80 as a precursor for the mpSi particles. We also extended the MR synthesis of mpSi samples 3854


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→ 2MgCl2 + SiO2 + 4H2 (↑). Figure 1b (iii) shows that the XRD pattern of mpSi-Y and the primary particle size estimated by the line broadening of the Si(111) peak at 2θ = 28.4° with the Debye−Scherrer formula was approximately 28.5 nm. When the same MR was performed under the same condition except in a tube furnace under Ar flow (entry 6), the HClwashed sample indicated broad and intense XRD peaks owing to amorphous SiO2 (at approximately 2θ = 23°) together with crystalline silicon peaks (see Figure S1a). Hence, the yield of mpSi-Y after removal of the SiO2 by HF etching was only 12.2 wt %, possibly owing to the local depletion of Mg in the open flow system. The MR of zeolite Y without NaCl as a heat sink at low MR temperature (650 °C) (entry 1) resulted in a low conversion of SiO2 and thus considerably low yield of mpSi-Y. Conversely, all the other MR tests without NaCl at low MR temperature (entry 2 and 5) for a longer reduction time (entry 2) or at high MR temperature (entry 3 (resulting XRD in Figure S1b) and 4) resulted in large primary Si particles and a low purity of Si owing to the large reaction heat and the side reaction 5 above, even though the MR conversion and thus yield of mpSi-Y were reasonably high (see Figure S1 and Table S1). After HF treating to remove the Mg2SiO4, MgF2 was formed by Mg2SiO4 + 10HF → 2MgF2 + H2SiF6 + 4H2O.52 Figure 2a compares the XPS spectra of the Si 2p of mpSi-Y and that of zeolite Y in the inset. The single Si 2p peak for zeolite Y at 103.5 eV due to Si4+ virtually disappeared in the spectrum of mpSi-Y where two deconvoluted peaks at 99.4 and 98.8 eV are ascribed to the metallic Si 2p3/2 and Si 2p1/2, respectively. The atomic percentage of metallic Si 2p amounts to as high as 94.6%; the remainder is the Si 2p Si−O due to surface oxidation. The Fourier-transform infrared (FTIR) spectrum of mpSi-Y in Figure 2b indicated significantly stronger bands at 2246 and 2090 cm−1 for ν (H−Si)O3 and ν (HSi), respectively, whereas it demonstrated weaker bands for ν (HO−Si) and ν (Si−O−Si) than those of commercial SiNPs. Thus, the HSi bond is rich in composition on the surface of mpSi-Y. The small peaks at 2800−3000 cm−1 on the mpSi-Y are due to the C−H stretching on the surface SiOCH2CH3. The surface ethoxy-bonds were formed by the reaction between the Si−H surfaces and ethanol,53 which was mixed with DI water and used as washing solvent after chemical etching of reduced zeolites. The ethoxybonds, however, will be pyrolyzed during carbon coating process at high temperature. The two small peaks at 625 and 875 cm−1 can be assigned to the Si−H wagging and bending modes, respectively.54 The N2 adsorption/desorption isotherms and pore size distributions of parent zeolite Y and mpSi-Y are compared in Figures 2c and 2d, respectively, and the resulting textual properties are summarized in Table S2. The amount of N2 adsorbed on mpSi-Y was substantially reduced compared to that on zeolite Y. Zeolite Y displayed a distinct hysteresis in the isotherm over the pressure range of P/P0 = 0.4−1.0, indicating the presence of mesopores generated by the dealumination process to yield ultrastable Y (USY) zeolite with a high SiO2/ Al2O3 molar ratio. Zeolite Y demonstrated bimodal pore size distribution owing to its microporous and mesoporous nature. On mpSi-Y, the amount of N2 adsorbed remained extremely low to P/P0 = 0.85 followed by a sudden increase in the range of P/P0 = 0.85−1.0, where a distinct hysteresis developed owing to condensation in the mesopores. Although the Brunauer−Emmett−Teller (BET) surface area of mpSi-Y (90.7 m2/g) is considerably less than that of parent zeolite Y (871.0 m2/g), mpSi-Y has a considerably greater pore diameter

with zeolite Beta (B) and ZSM-5 (Z). The carbon-coated mpSi samples are denoted as mpSi-x/C-y (where, x = zeolite type and y = carbon content in wt %). For example, mpSi-Y/C-40 indicates that mpSi is derived from zeolite Y and the carbon content in the composite is 40 wt %. Figure 1a displays photos of the samples before and after MR of zeolite Y. The light gray mixture of zeolite Y, NaCl, and Mg

Figure 1. (a) Photographs of samples taken during MR reaction processes and (b) XRD patterns of (i) mixture of zeolite Y and Mg powder, (ii) after water washing, and (iii) after subsequent acid washing of MR reaction product.

became homogeneous dark brown powders after MR at 750 °C for 5 h (entry 9 in Table S1) in a sealed metal tube reactor. They then became light brown powders of mpSi-Y after acid washing with dilute HCl and HF. In a typical MR of zeolite Y (1.0 g), the recovered mpSi-Y was approximately 0.26 g corresponding to a yield of 56.1 wt % based on the SiO2 content in the dry zeolite Y. For MR with zeolite Beta (entry 11) and ZSM-5 (entry 12) under the same conditions as the MR with zeolite Y, the recovered yields of mpSi-B and mpSi-Z were 54.9 and 60.7 wt %, respectively. In the MR with zeolites in a sealed reactor, zeolite samples, which tend to adsorb water in air, must be completely dried before MR otherwise Mg is oxidized to MgO leading to a low yield of mpSi. As indicated in Figure 1b, the intense X-ray diffraction (XRD) peaks of crystalline zeolite Y and metallic Mg (Figure 1b (i)) completely disappeared after MR whereas strong XRD peaks assignable to Si and MgO were observed with small peaks due to Mg2Si (Figure 1b (ii)). Mg2Si, the only byproduct, can be formed by reactions 3 and 4 when the local concentration of Mg exceeds that of the SiO2. After HCl washing, both MgO and Mg2Si peaks completely disappeared. The formation of Mg2Si reduces the final yield and purity of the silicon because it is reconverted into SiO2 after HCl washing by Mg2Si + 4HCl (dilute) + 2H2O 3855


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Figure 2. (a) XPS spectrum of mpSi-Y with that of zeolite Y in the inset, (b) FT-IR spectra of commercial SiNPs and mpSi-Y, (c) N2 adsorption/desorption isotherms, and (d) BJH pore size distributions of zeolite Y and mpSi-Y.

Figure 3. SEM images of (a) zeolite Y, (b, c) mpSi-Y, (d, e) TEM images of mpSi-Y, and (f) SAED pattern of mpSi-Y.

reduced BET surface area compared to mpSi-Y, whereas their pore volumes and pore sizes were considerably or less comparable to those of mpSi-Y (see Table S2 and Figure S2). The scanning electron microscopy (SEM) image of zeolite Y in Figure 3a indicates a multihedron-shaped crystal morphology with a rather uniform particle size distribution in the range of 0.5−1.0 μm. The SEM images of mpSi-Y in Figures 3b and 3c

(24.3 nm) compared to that of zeolite Y (4.4 nm). The total pore volume of mpSi-Y (0.49 cm3/g) is comparable to that of zeolite Y (0.52 cm3/g). As compared in Table S2, mpSi-Y obtained from MR without NaCl in an open system (entry 3 in Table S1) demonstrated a low BET surface area and pore volume owing to severe sintering of the silicon by the large reduction heat. mpSi-B and mpSi-Z demonstrated marginally 3856


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Figure 4. (a) TEM image of the mpSi-Y/C-40 with the elements (Si and C) mapping images of the selected area, (b) magnified TEM images of the white-dotted square in (a), and (c) high resolution TEM image of the black-dotted square in (b).

Figure 5. (a) Voltage profiles for the initial two cycles at 100 mA g−1, (b) CV profiles, (c) rate responses with varying both charge and discharge currents at the same time, (d) discharge rate responses at a fixed charge rate at 100 mA g−1, and (e) cycling performances of mpSiY/C-50 cycled over 0.01−1.5 V Li/Li+.

reveal that each particle had similar size dimensions to the parent zeolite Y particles yet were highly porous with a

considerable roughened surface. The SEM images of mpSi-B and mpSi-Z (Figure S3) also indicated highly porous particular 3857


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Figure 6. (a) Voltage profiles for the initial two cycles at 100 mA g−1, (b) CV profiles, (c) rate responses with varying both charge and discharge currents at the same time, (d) continued cycling performance after the rate test in (c), and (e) cycling performance of mpSi-Y/C-40 cycled over 0.01−1.5 V Li/Li+.

aggregates with a rather wide size distribution of 0.5−3 μm. The transmission electron microscopy (TEM) images of the single mpSi-Y particle in Figures 3d and 3e clearly indicate its porous structure with pore sizes in the range of 20−40 nm. The primary silicon particles are 20−30 nm in diameter and are highly interconnected, possibly due to sintering by high exothermic heat during MR to form 0.78 × 0.91 μm microparticle aggregates as seen in Figure 3e. The selected area electron diffraction (SAED) pattern in Figure 3f further verifies that mpSi-Y is comprised of crystalline silicon. The TEM image of mpSi-Y (entry 3) obtained without NaCl as a heat sink (Figure S2c) clearly indicates large aggregates of silicon owing to severe sintering in accordance with the BET results (Figure S2 and Table S2). Carbon coating must be conducted to enhance the electrical conductivity of the mpSi-Y samples for their use as LIB anodes. Because the mpSi-Y particles are not miscible in aqueous solvent, we used petroleum pitch dissolved in an aprotic solvent, tetrahydrofuran, as the carbon source. By varying the relative amount of mpSi-Y and pitch during the carbon-coating process, the mpSi-Y/C-30, mpSi-Y/C-40, and mpSi-Y/C-50 samples containing 30.0, 39.5, and 50 wt % carbon by thermal gravimetric analysis (TGA) (see Figure S4a), respectively, were prepared. In a similar manner, mpSi-B/C-40 and mpSi-Z/C-40 samples containing 40.6 and 39.5 wt % carbon, respectively, were prepared (Figure S4b). A TEM image of mpSi-Y/C-40 is displayed in Figure 4. Discrete mpSi-Y/C-40 particles with the same size distribution

as the zeolite Y particles are observed in Figure 4a. In the element mapping images for the selected area, the carbon map is exactly superposed onto the silicon map indicating that carbon was evenly coated on the mpSi-Y particles. A magnified TEM image of the white dotted square in Figure 4a is displayed in Figure 4b, where a substantial fraction of the porous structure remains in mpSi-Y/C-40. Its BET surface area and pore size remained virtually unaffected whereas the total pore volume was decreased by carbon coating (Table S2). A highresolution TEM image of the black dotted square in Figure 4c clearly indicates a thin carbon layer of at least 2 nm thickness covering the crystalline silicon particle. Even carbon coating on the mpSi-Y particle was also observed in the TEM image with Si and C mapping images for mpSi-Y/C-50 in Figure S5. Electrochemical Responses of mpSi/C and Si/C Composites. Figures 5a−5e display the electrochemical responses of mpSi-Y/C-50 in a half-cell. In the voltage profile in Figure 5a, the first discharge (lithiation in a half cell configuration) and charge capacities were 1399 and 1031 mAh g−1 giving the first Coulombic efficiency (CE) of 73.7%. The sloping voltage plateau at 0.9−0.35 V vs Li/Li+ disappeared and the long voltage plateau near 0.01 V increased marginally in the second discharge owing to the solid electrolyte interface (SEI) formation accompanied by electrolyte decomposition and electrode polarization, respectively. In the CV profile in Figure 5b, a broad peak was observed in the 0.9−0.35 V range in the first lithiation scan and then disappeared in the subsequent cycles owing to the SEI formation in accordance with the first 3858


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Figure 7. (a) Equivalent circuit for the silicon−carbon composite electrodes, (b) Nyquist plots of various electrodes, (c) Li ion diffusion coeffecients (DLi) obtained after 50 cycles, and (d) Nyquist plots of Si/C-40 and mpSi-Y/C-40 obtained after 300 and 500 cycles, respectively.

respectively, giving the first CE of 73.1%. The obvious irreversible Li ion consumption due to the SEI layer formation was also observed in the range of 0.9−0.35 V in the first lithiation CV profile in Figure 6b. The rate responses of mpSiY/C-40 obtained by changing both charge and discharge currents simultaneously are given in Figure 6c. The capacity faded with current increase; however, it recovered to 1352 mAh g−1 when the current decreased stepwise back to 100 mA g−1 over 45 cycles. At the high currents of 1 and 2 A g−1 corresponding to approximately 0.75 and 1.5 C rates (1 C = 1350 mA g−1), the recovered capacities were approximately 915 and 633 mAh g−1, respectively, corresponding to the capacity retention of 76.3% and 52.8%. The corresponding capacity retention of graphite at 1.3 C was as small as 21.1% in our previous study.26 When the delithiation current was varied in the range of 0.1−20 A g−1 at a fixed lithiation current of 0.1 A g−1 (Figure S6), mpSi-Y/C-40 delivered virtually constant capacities up to 2 A g−1 (1.5 C) and continued to deliver capacities of 1186 mAh g−1 (88.7% retention), 1157 mAh g−1 (86.5% retention), and 1088 mAh g−1 (81.4% retention) at delithiation currents of 5 A g−1 (3.7 C), 10 A g−1 (7.4 C), and 20 A g−1 (14.8 C), respectively, indicating the high power capability of mpSi-Y/C-40. After the rate test in Figure 6c, mpSi-Y/C-40 was further cycled up to 300 cycles at a current of 1 A g−1. As indicated in Figure 6d, mpSi-Y/C-40 exhibited stabilized and reversible capacity of 888 mAh g−1 with 99.80% CE at the 60th cycle and continued to deliver a capacity of 816 mAh g−1 with 99.96% CE after 300 cycles. The capacity retention was 91.9% at 1.0 A g−1 corresponding to a capacity fading rate of −0.034% per cycle. Figure 6e displays a different set of the cycling test with mpSi-Y/C-40. After the initial several activation cycles at 0.1 A g−1, the current was increased to 500 mA g−1. After 210 cycles at 500 mA g−1, it exhibited the capacity 1015 mAh g−1 with 99.5% CE. When the current was reduced to 100 mA g−1 after the cycle run at 500 mA g−1, the recovered capacity was 1165 mAh g−1, giving a capacity retention of 96.0% (fading rate = −0.019% per cycle) over the average initial capacity of 1213 mAh g−1 at 100 mA g−1. Then,

discharge voltage profile in Figure 5a. The CV peaks corresponding to the typical lithiation (0.20 V) and delithiation (0.33 and 0.51 V) of Si increased in intensity over the first ten cycles owing to the initial electrode activation process including the gradual electrolyte infiltration into the densely coated electrode and mpSi-Y/C-50 particles. Figure 5c indicates the rate responses of mpSi-Y/C-50 obtained by changing both charge and discharge currents simultaneously. The capacity of mpSi-Y/C-50 decreased with current increase; however, its capacity was fully recovered when the current decreased stepwise back to the initial condition over 45 cycles. At the high currents of 1 and 2 A g−1 corresponding to approximately 0.9 and 1.8 C rate (1 C = 1130 mA g−1), the recovered capacities were greater than 678 and 400 mAh g−1, respectively, which are greater than the theoretical capacity of graphite (372 mAh g−1). When the delithiation current was varied in the range of 100− 5000 mA g−1 at a fixed lithiation current of 100 mA g−1 (Figure 5d), mpSi-Y/C-50 delivered virtually constant capacities up to 2000 mA g−1 (1.8 C) and 988 mAh g−1 (97% capacity retention) at the current of 5000 mA g−1 (4.4 C) indicating its high discharge (delithiation in a full cell configuration) power capability. Figure 5e presents the cycling behavior of mpSi-Y/ C-50 cycled at 100 mA g−1 for the initial five cycles followed by 200 cycles at 500 mA g−1. During the initial five cycles at 100 mA g−1, the capacity gradually increased owing to the electrode activation process as observed in the CV profiles in Figure 5b; the CE also increased from approximately 74% (1st cycle) to 98.4% (5th cycle). When the current was increased to 500 mA g−1, the capacity faded in ten cycles; however, the CE leached to >99.8% at the 15th cycle. When mpSi-Y/C-50 was further cycled at 500 mA g−1, it delivered a capacity of 729 mAh g−1 after 200 cycles with an excellent cycling stability and very high CE > 99.8%. Figures 6a−6e display the corresponding electrochemical responses of mpSi-Y/C-40. The voltage and CV profiles of mpSi-Y/C-40 in Figures 6a and 6b, respectively, are similar to those of mpSi-Y/C-50 in Figure 5. mpSi-Y/C-40 delivered the first discharge and charge capacities of 1620 and 1184 mAh g−1, 3859


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Figure 8. Electrode specific and volumetric capacities, and 1st cycle Coulombic efficiencies of various silicon-based composites.

mpSi-Y/C-40 was further cycled at a current of 1000 mA g−1 up to 500 cycles. The initial capacity was approximately 840 mAh g−1 at a current of 1 A g−1. After 500 cycles, the capacity remained as high as 697 mAh g−1 (82.9% capacity retention at 1.0 A g−1) and the CE attained 100.0%. Conversely, the control composite of Si/C-40, which has similar carbon content as mpSi-Y/C-40, exhibited extremely poor capacity retention delivering only 182 mAh g−1 after 300 cycles at 500 mA g−1 as compared in Figure 6e. The mpSi-Y/C-30 samples prepared with mpSi-Y obtained from different MR conditions (Table S1) also exhibited a significant difference in their electrochemical responses (see Figure S7). mpSi-Y (entry 9)/C-30 delivered considerably higher reversible capacity than mpSi-Y (entry 3)/C-30, for which mpSi-Y was prepared without NaCl as a heat sink in the MR, to yield a considerably larger average Si particle size of 50 nm owing to severe thermal sintering (see Figure S2c). mpSiB/C-40 and mpSi-Z/C-40 also displayed stable cycling behavior up to 150 cycles at 500 mA g−1 with rather high CE values of 84.9% and 80.7%, respectively, in the first cycle (see Figure S8). To better understand the different cycling performances of Si/C-40, mpSi-Y/C-40, and mpSi-Y/C-50, the Nyquist plots of the EIS measurements on the cycled electrodes are compared in Figure 7. The impedance data were obtained on electrodes after 50 cycles and can be well fitted with the equivalent circuit in Figure 7a to estimate the different resistances in Table S3. The main difference in the Nyquist plots of the samples in Figure 7b was observed in the two distinct semicircles in the high and medium-frequency regions due to the SEI (RSEI) and charge-transfer (Rct) resistances, respectively. The RSEI values of the Si/C-40, mpSi-Y/C-40, and mpSi-Y/C-50 were estimated to be 7.0, 4.5, and 1.1 Ω, respectively. The corresponding Rct values were 14.1, 6.6, and 4.9 Ω, respectively. Both the RSEI and Rct values of the mpSi-Y/C samples were significantly less than those of Si/C-40 suggesting a thinner SEI layer and higher charge-transfer rates at the electrode−electrolyte interface, providing superior electrochemical performances of the mpSiY/C series than those of Si/C-40. The Li+ diffusion coefficients (DLi) (Figure 7c) were estimated from the Warburg region in Figure 7b by using the parameters in Table S3 and the Warburg factor determined in Figure S9. The DLi values were approximately 2−3 times greater on the mpSi-Y/C electrodes than that on the Si/C-40 owing to much smaller Si particles in mpSi-Y. In the Nyquist plots of the samples obtained after 300 cycles in Figure 7d, the total resistance on the Si/C-40 became significantly greater than that of mpSi-Y/C-40 owing to the instability and uncontrolled growth of SEI layers on the Si/C-

40 electrode. Furthermore, electrode thickness changes before and after cycling were measured by SEM and are compared in Figure S10. The mpSi-Y/C-40 electrode demonstrated only a 5.7% thickness increase after 50 cycles at 500 mA g−1, whereas the thickness of the control Si/C-40 electrode increased more than 190%. Therefore, it is evident that volume expansion of silicon can be better accommodated in mesoporous silicon structures than in conventional solid silicon nanoparticles. The above electrochemical responses and characterization results support the statement that mpSi-Y/C-50 and mpSi-Y/ C-40 are promising anode materials with excellent long-term cycling stability, high electrochemical reversibility, and efficiency. These appealing performances can be attributed to (1) the effective absorption of the mechanical strains of silicon into its porous structure coupled with, (2) the small primary silicon particle sizes (20−30 nm) minimizing the extent of volume expansion, and (3) the complete coating of silicon with highly conducting carbon derived from pitch, which is beneficial to promote electron transfer and prevent silicon surfaces from direct contact with the electrolyte. Compared with other types of porous silicon-based anodes in the literature, mpSi/C derived from zeolite Y outperformed in terms cycling stability and efficiency (Table S4). Figure 8 compares the electrode specific and volumetric capacities based on the electrode weight and volume, inclusive of active material, conductive additive, and binder. The electrode specific capacities of mpSi-Y/C-40 and mpSi-Y/C50 were 1082 and 907 mAh g−1electrode, respectively, which are approximately 3.7 and 3.1 times greater than that of graphite (296 mAh g−1electrode). With the electrode thicknesses measured by SEM, the electrode coating densities (de, g cm−3) can be obtained, and thereby the electrode volumetric capacities were obtained by (electrode specific capacity, mAh g−1electrode) × de (g cm−3). The de values were 0.67, 0.75, and 1.63 g cm−3 for mpSi-Y/C-40, mpSi-Y/C-50, and graphite, respectively, indicating that the mpSi/C samples had very low electrode coating density owing to their porous nature. Consequently, the electrode volumetric capacities of mpSi-Y/C-40 and mpSi-Y/C50 were 721 and 676 mAh cm−3electrode, respectively, which are still approximately 50% and 40% larger than that of graphite (483 mAh cm−3electrode). Overall, the mpSi/C samples exhibited 3.1−3.7 times higher electrode specific capacities than graphite while their electrode volumetric capacities remained 40−50% larger than that of graphite. Electrochemical Responses of Hybrid Composites of mpSi/C and Graphite. Despite many appealing electrochemical properties, the mpSi/C samples presented rather low CE values (73−74%) in the first cycles compared with 3860


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Figure 9. (a) Voltage profiles for the first cycle at 100 mA g−1, (b) discharge rate responses at a fixed charge rate at 100 mA g−1, and (c) cycling performance of mpSi-Y/C-30@Gr cycled over 0.01−1.5 V Li/Li+.

commercial graphite, which indicated the first CE as high as 93.1% (Figure S11). Although the pitch used as the carbon source was not only cost-effective55 but also primarily free of oxygen-species, the first CE values of the mpSi/C samples required improvement by, for example, reducing surface fractions such as SiOx, and surface-area and heteroatom (especially oxygen residues) contents in carbon coatings, which are highly reactive with electrolytes and cause high irreversible lithium consumption in the first cycles.56 Thus, the first CE of mpSi-Y/C composite can be increased by reducing the reactive surface fractions in the composites. As a first attempt, we prepared mpSi-Y/C-30, which contained a lower surface carbon fraction than mpSi-Y/C-40 and mpSi-Y/ C-50. As indicated in Figure S7, mpSi-Y/C-30 delivered a high reversible capacity of 1730 mAh g−1 at 100 mA g−1 with the first CE of 81.3%. mpSi-Y/C-30 exhibited a reversible capacity of 1110 mAh g−1 at the current of 500 mA g−1 with acceptable cycling stability up to 60 cycles. Because there is considerable room in the specific capacity of mpSi-Y/C-30 (1730 mAh g−1) compared with that of graphite, one can further reduce the reactive surface fraction by employing a hybrid sample composed of a physical mixture of mpSi-Y/C and graphite at the expense of specific capacity.26,57 For this purpose, as a second attempt, two hybrid samples of mpSi-Y/C-40@Gr and mpSi-Y/C-30@Gr were prepared by physical mixing of the mpSi-Y/C composites and graphite with a weight percent of 60:40 wt %, respectively. Figure 9 displays the electrochemical responses of mpSi-Y/C-30@Gr, whose capacity is estimated to be approximately 1182 mAh g−1 (approximately 1730 mAh g−1 × 0.6 + 360 mAh g−1 × 0.4). Actually, this delivered the first discharge and charge capacities of 1199 and 1018 mAh g−1 at a current of 100 mA g−1, giving the first CE of 84.9% (Figure 9a). Figure 9b presents the rate capability of mpSi-Y/C-30@Gr evaluated at different discharge (delithiation) current densities from 0.1−10 C rates (1 C = 1000 mA g−1) with a fixed charge current of 0.1 C rate. The discharge capacity at the current range of 0.5−2.0 C was approximately 93−99% of that at 0.1 C, and it remained at 87% at a current of 5.0 C, indicating an

excellent rate capability for the hybrid sample. As indicated in Figure 9c, the hybrid sample also exhibited an extremely stable cycling stability (capacity retention of 97.5% up to 150 cycles) at a current of 500 mA g−1 with a capacity level of 700 mAh g−1. At the 150th cycle, the CE was approximately 99.7%. As indicated in Figure S12, mpSi-Y/C-40@Gr also demonstrated higher first cycle CE (79.5%) than that (73.1%) of mpSi-Y/C40 at the expense of the specific capacity. It also indicated an excellent rate capability up to 5.4 C and no capacity fading at 500 mA g−1 up to 150 cycles where the CE was as high as 99.8%. Its electrode thickness change after cycling was less than 10% (see Figure S9c). As compared in Figure 8, the hybrid samples delivered virtually comparable volumetric capacities to those of mpSi-Y/C-40 and mpSi-Y/C-50 owing to the increased electrode coating densities of the hybrid samples (see Figure S13).

CONCLUSIONS For a facile process for the preparation of mesoporous silicon (mpSi) structures for next generation high-performance LIB anodes, we conducted a systematic study on MR with commercial zeolites. A pseudo-optimal MR condition leading to high yield and high purity mpSi was proposed with commercial zeolites (Y, Beta, and ZSM-5), which could be potentially low cost and ideal precursors owing to their microporous nature (facile heat dissipation), high thermal stability, and readily tunable crystal size and morphology. In favor of the many appealing properties of the mpSi samples such as small primary Si particles and mesoporous structure with large pore volume (approximately 0.5 cm3 g−1) and microparticulate morphology, the mpSi/C series samples demonstrated high reversible capacities with excellent cycling stabilities and Coulombic efficiencies. In particular, mpSi-Y/C40 delivered reversible capacities of greater than 800−1200 mAh g−1 at 500−1000 mA g−1 with outstanding capacity retention up to 500 cycles, whereas conventional Si/C-40 3861


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mA, Cu Kα radiation, λ = 1.5418 Å). The surface chemical compositions of the samples were characterized by X-ray photoelectron spectroscopy (XPS) using a Thermo Electron Corporation spectrometer with Al Kα (1486.6 eV) radiation. FTIR spectra were obtained on a Thermo Scientific Nicolet 380 spectrometer using KBr pellet methods. The specific surface area and pore size distribution of the samples were measured by BET and Barrett−Joyner−Halenda (BJH) methods, respectively. The structure and morphology of the samples were investigated using field-emission scanning electron microscopy (FE-SEM, JEOL JSM-35CF operated at 10 kV) equipped with energy-dispersive X-ray spectroscopy (EDS) element-mapping functionality and transmission electron microscopy (TEM, JEOL JEM2010 operated at 200 kV). The Si and C contents of the composite samples were determined by TGA run to 800 °C at a ramp rate of 10 °C min−1 under air flow. Electrochemical Measurements. Electrochemical properties were measured using CR2032 coin-type half-cells assembled in an argon-filled glovebox with Li foil as the counter electrode. The working electrode was prepared by casting of the active material (mpSi/C or Si/C): carbon black (Super P Li, Timcal Ltd.):poly(vinyl alcohol) (5 wt %, dissolved in dimethyl sulfoxide) binder at a mass ratio of 8:1:1 onto a copper foil using a Meyer-bar coating device (Kipae E&T, Korea). The coated electrodes were dried at room temperature for 1 h and followed by drying at 80 °C for 2 h. Then, the electrodes were roll-pressed several times and further dried under vacuum at 80 °C for 2 h. The resulting electrode thickness was approximately 15−40 μm with a total mass loading of 1.0−2.0 mg cm−2. The separator was a microporous polypropylene membrane (Celgard 2400) and the electrolyte was 1 M LiPF6 in a mixture of ethylene carbonate(EC)/ethyl methyl carbonate(EMC)/diethylcarbonate (DEC) with a volume ration of 3:4:3 containing 10 vol % of fluoroethylene carbonate (FEC) (Panax Etech Co., Ltd., Korea). Electrochemical cycling and rate tests were performed in the cutoff voltage range of 0.01−1.5 V vs Li/Li+, and cyclic voltammetry (CV) tests were conducted at a scanning rate of 0.1 mV s−1 on a galvanostat/ potentiostat system (WonATech Co., Ltd., Korea). For rate responses, the 1 C capacity of an electrode was based on the actual reversible capacity obtained at the current of 0.1 A g−1. The electrode thicknesses were estimated by SEM of electrode cross sections before and after the cycling test. Electrochemical impedance spectroscopy (EIS) measurements were conducted in a frequency range of 100 kHz to 0.01 Hz with an AC amplitude of 10 mV on a ZIVE SP2 (WonATech Co., Ltd., Korea) analyzer.

indicated significantly short cycle life with high impedance build-up during cycling and low Li-ion diffusion rate. The characterizations and electrochemical responses of the mpSi-Y/C composites suggest that the internal void spaces and nanosized primary Si particles in mpSi-Y can effectively damper the huge volume expansion of silicon and maintain electrical integrity of the electrode for prolonged cycles. Moreover, the mpSi-Y/C composites exhibited approximately 3.7 times higher electrode specific capacities at approximately 50% larger electrode volumetric capacities than commercial graphite anodes. Hybrid anode samples comprised of high capacity mpSi/C and high efficiency graphite successfully demonstrated an economic and reliable strategy for advanced anodes to exhibit a high first-cycle Coulombic efficiency (approximately 85%) at a high capacity (>1000 mAh g−1) with excellent cycling stability. Thus, the mpSi/C composites derived from the MR of commercial zeolites could be highly attractive candidates for high-performance and high-energy Li-ion battery anodes.

EXPERIMENTAL SECTION Preparation of mpSi and mpSi/C Composites. 1. Preparation of Mesoporous Si (mpSi). A series of mpSi/C samples were prepared according to Scheme 1 with commercial zeolites such as zeolite Y, zeolite Beta, and ZSM-5 as precursors. In a typical preparation, zeolite Y (1.0 g, SiO2/Al2O3 = 80, Zeolyst) as the silica precursor was mixed with sodium chloride (NaCl) (99%, Duksan Chemical) in deionizaed (DI) water (Zeolite Y:NaCl = 1:10 wt %) under stirring. After 1 h, the DI water was evaporated under 100 °C and the sample was further dried at 80 °C in air overnight to obtain a homogeneous mixture of zeolite Y/NaCl. The obtained powders were thermal treated at 700 °C in air to remove the adsorbed water and surface hydroxyl-groups on the zeolite. Then, the zeolite Y/NaCl mixture was physically admixed with magnesium powder (0.8 g) and vacuum-dried at 200 °C for 2 h to completely remove any moisture. The mixture was transferred into a stainless-steel chamber and sealed under an argon-filled glovebox. The chamber was initially heated to 400 °C at a ramp rate of 10 °C/ min in an electrical furnace under inert gas flow. It was further heated to 750 °C at the rate of 5 °C/min, and held at 750 °C for 5 h. After cooling to room temperature, the obtained powders were initially washed in DI water several times to remove NaCl followed by a 1 M HCl solution for 5 h to eliminate reduction byproducts such as MgO and Mg2Si. For the complete removal of the unreacted SiO2, the sample was exposed in a dilute hydrofluoric acid solution (HF, 49− 52%, Duksan) for 3 h followed by a washing with an H2O/EtOH solution several times. Finally, the solution was filtered and dried at 80 °C in air overnight to obtain mpSi-Y (0.26 g corresponding to the recovered yield of 56 wt %). The mpSi-B and mpSi-Z samples were also prepared by the same process described above with zeolite Beta (SiO2/Al2O3 = 300, Zeolyst) and ZSM-5 (SiO2/Al2O3 = 150, Zeolyst) with the experimental recovery yields of approximately 55% and 61%, respectively. 2. Preparation of mpSi/C Composites. To render high electrical conductivity, the mpSi samples were coated with carbon to obtain mpSi/C composites. The generic preparation is described herein for mpSi-Y/C-40 (representing that mpSi is derived from zeolite Y and the carbon content in the composite is 40 wt %). The mpSi-Y powders (0.1 g) and pitch carbon (0.154 g) were dispersed in tetrahydrofuran (THF, 99.5%, Junsei) under stirring and ultrasonication. Then, the mixed solution was maintained at 75 °C in a water bath and continuously stirred to evaporate the THF solvent. The obtained dark brown sample was further dried under an 80 °C oven overnight and finally carbonized at 800 °C in a tube furnace under Ar flow to obtain mpSi-Y/C-40. For comparison, a conventional Si/C sample (Si/C-40 with approximately 40 wt % of carbon content) was prepared using the same method as above with commercial SiNPs (Alfa Aesar, 50 nm in diameter). Characterizations. XRD patterns of the samples were obtained using a Rigaku model Miniflex 600 X-ray diffractometer (40 kV, 15

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b01129. Preparation of various mpSi samples; Comparison of electrochemical properties of mpSi-Y/C in the literature; Details on the prepared samples analyzed by XRD, BET, TEM, SEM, TGA and electrochemical responses; additional electrode properties (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel: +82-51-200-7718. Fax: +8251-200-7728. ORCID

Jung Kyoo Lee: 0000-0003-1673-7587 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Human Resource Training Program for Regional Innovation and Creativity through the 3862


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Ministry of Education and the National Research Foundation of Korea (NRF-2014H1C1A1073093), by the National Research Foundation of Korea Grant funded by the Ministry of Education (NRF-2017R1A2B4002940).

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