BLU9931

Engineering the Core−Shell-Structured NCNTs-Ni2Si@Porous Si Composite with Robust Ni−Si Interfacial Bonding for High- Performance Li-Ion Batteries

Ming Chen,*,†,‡ Qiang-Shan Jing,† Hai-Bin Sun,§ Jun-Qi Xu,§ Zhong-Yong Yuan,∥ Jin-Tao Ren,∥ Ai-Xiang Ding,⊥ Zhong-Yuan Huang,¶ and Meng-Yang Dong†
†College of Chemistry and Chemical Engineering and §Department of Physics and Electronic Engineering, Xinyang Normal University, Xinyang 464000, China
‡Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education) and ∥School of Materials Science and Engineering, Nankai University, Tianjin 300071, China
⊥Department of Biomedical Engineering, Case Western Reserve University, Cleveland, Ohio 44106, United States
¶Department of Chemistry, Xavier University of Louisiana, New Orleans, Louisiana 700125, United States
*S Supporting Information

1. INTRODUCTION

Silicon is one of the most promising candidates to replace commercial graphite (372 mAh g−1, 0.5 V) for high-energy- density Li-ion batteries (LIBs) due to its unparalleled theoretical capacity (3580 mAh g−1), relatively low electro- chemical potential (0.4 V vs Li/Li+), and high safety. However, practical application of Si anodes in LIBs is still restricted by the poor cycle stability and rate capability, which resulted from their severe volume change (300%) and the low electronic/ ionic conductivity during the lithiation and delithiation
automotive and stationary energy storage, such as electric vehicles, portable electronics, and energy storage for renewable energy sources.
Porosity construction in Si electrode materials is an effective way to improve the battery performance by directly accommodating the large volumetric change, enhancing the electrolyte ions diffusion, and increasing the active Si utilization. Many strategies have focused on preparing the porous Si-based materials, including the support of Si onto the porous metal framework (e.g., Cu, Ni, Cu−Al−Fe), carbon
processes.1,2 These two primary problems of the Si anode must be solved in order to meet the commercial demand of high energy density, excellent cycling stability, and high rate capability for advanced LIBs in their applications of foam or carbon sponge,3−7 introduction of nonfilling coating layer (e.g., metal, carbon, and oXide) on the Si,8−15 and construction of void-containing nanostructured Si (e.g., nanowire arrays, hollow nanospheres, pomegranate-like, tubular, and yolk−shell nanostructures).16−22 Nevertheless, creation of a porous structure for Si-based materials in the above synthetic routes was mostly implemented by the acid etching of inorganic templates (e.g., SiO , NiO, CaCO , Mg/SiO2 with Porous Structure. The raw perlite was milled in a 100 mL agate vessel filled with 50 g (Ø 20 mm), 40 g (Ø 10 mm), and 10 g (Ø 5 mm) agate balls. The milling was operated in air for 48 h with a rotation speed of 450 rpm and ball-to-perlite mass ratio of 20:1. The obtained powder was preheated at 300 °C for 25 min and then heated at 1100 °C for 5 s in a muffle furnace. Subsequently, the sample was or thermal decomposition of organic templates (e.g., PMMA, PAN, and P123).8,21,26 The sacrificial templates used in traditional porosity construction inevitably increased the production cost and also involved the tedious acid or alkali postprocessing procedure. Thus, a new strategy for porosity construction without the additional templates and acid/alkali treatment should be established, which is very critical for the economic and green synthesis of porous Si for high-performance LIBs.

2. EXPERIMENTAL SECTION
2.1. Material Synthesis. 2.1.1. Synthesis of Micrometer-Sized

In addition, considerable efforts have been devoted to keeping the Si-based materials highly conductive and mechanically robust; in particular, the carbon coating of Si is a simple but very effective approach to enhance the electronic conductivity of Si-based electrode, which could also prevent direct contact of electrolyte and Si, restrict the interfacial side reaction of Si/electrolyte, and avoid formation of unstable solid electrolyte interface (SEI) film.27 A variety of carbon-coated Si architectures have been developed, such as the graphitic carbon, amorphous carbon, or graphene-coated Si par- ticles.28−30 Unfortunately, a filling carbon-coated Si particle
with the classical core−shell structure cannot accommodate the serious volume expansion of Si and thus resulted in electrode fracture during cycling, because the coated carbon materials are normally not elastic or expandable enough.31 To buffer the mechanical stress in a better way, the nonfilling carbon-coated Si structures with rich void spaces between the inner Si core and the outer carbon shell have been reported in which the void spaces can effectively accommodate the large volume expansion of Si particles, keep the carbon shell intact, and maintain the electrode structural integrity during cycling.10 However, the nonfilling carbon-coating structure generally suffers from the poor electronic and ionic connections between the carbon shell and the Si core due to the presence of internal gaps, especially when the serious volume shrinkage of Si occurred upon delithiation, which put a new challenge in exploring the highly active Si-based materials. Thus, the rational design of a novel nonfilling carbon-coated Si architecture with a strong heterointerfacial connection is highly desirable.

Herein, we reported a new electrochemical architecture of NCNTs-Ni2Si@Si composite, which was prepared by a self- templating synthesis route for constructing the porous Si and subsequent pyrolysis of melamine and nickel acetate precursors for wrapping Ni-NCNTs on porous Si. In the NCNTs-Ni2Si@ Si composite, the porous Si core combined with the nonfilling NCNTs coating layer played an important role in accom- modating the mechanical stress of Si upon discharge/charge processes. Moreover, the interior Si and external Ni-NCNTs were bonded together via the formed Ni2Si alloy at the heterojunction of Si and Ni-NCNTs’ end points, which constructed the robust electronic and ionic connections between CNTs shell and Si core. As a consequence, the multifunctional NCNTs-Ni2Si@Si composite demonstrated extraordinary electrochemical performance and cycling stability when evaluated as the anode material for LIBs.

2.1.2. Synthesis of Micrometer-Sized Porous Si by Magnesio- thermic Reduction. Typically, 10 g of 4 M HCl treated expanded perlite and 10 g of magnesium (Mg) powder were miXed and ground for 1 h, followed by a pressing process under 20 MPa. The compacted miXture was heated from room temperature to 700 °C with a ramp rate of 2 °C min−1 and then maintained at 700 °C for 1 h in Ar flow. The obtained product was slowly added into 1 M HCl aqueous solution (this step is dangerous, and the added mass of the product should be no more than 0.01 g each time) and stirring mildly for 4 h to eliminate the byproducts of magnesium oXide (MgO) and magnesium silicide (Mg2Si). The resultant powders were further leached in diluted 0.2% HF, filtrated, and dried in vacuum at 120 °C for 24 h.

2.1.3. Synthesis of NCNTs-Ni2Si@Si Composite. In a typical synthetic procedure, 3 g of as-prepared porous Si, 3.2 g of nickel acetate, and 7.5 g of melamine were miXed and pressed under 20 MPa. The compacted miXture was further heated at 700 °C for 1 h in an Ar flow, and the black powder of NCNTs-Ni2Si@Si was finally obtained.

2.1.4. Synthesis of NCNTs-solid Si. The synthesis process was similar to that of NCNTs-Ni2Si@Si except the fast heating treatment for evaporating crystal water from raw perlite.

2.1.5. Synthesis of NCNTs@Ni. A 3.2 g amount of nickel acetate and 7.5 g of melamine were miXed and pressed under 20 MPa. The compacted miXture was further heated at 700 °C for 1 h in an Ar flow, and the black powder of NCNTs@Ni was finally obtained.

2.2. Material Characterization. X-ray diffraction (XRD) measurements were conducted on a Rigaku SmartLab9 diffractometer (Cu Kα, λ = 1.5406 Å). Scanning electron microscopy (SEM) images were recorded using a ZEISS SUPRA 55 microscope at 5 kV, with energy-dispersive X-ray spectroscopy (EDX) operated at 20 kV. Transmission electron microscopy (TEM) measurements were performed using a Tecnai G2 F20 microscope at 200 kV. X-ray photoelectron spectroscopy (XPS) results were derived from a Kratos AXis Ultra DLD (delay line detector) spectrometer equipped with a monochromatic Al Kα X-ray source (1486.6 eV). Thermogravimetric- differential scanning calorimetry (TG-DSC) analysis was investigated on a TA SDT Q600 analyzer in air atmosphere with a heating rate of 5 °C min−1. The Raman spectrum was examined by a Renishaw 1000 spectrometer. N2 adsorption−desorption isotherms were recorded at 77 K on a Quantachrome NOVA 2000e sorption analyzer. The
contents of Si, Ni, and Ni2Si were investigated by inductive coupled plasma (ICP) emission spectroscopy on a Thermo Jarrell-Ash ICP- 9000 (N + M) spectrometer.

2.3. Electrochemical Measurements. The electrochemical tests were carried out using the coin-type half cells. The testing electrode consisted of 80 wt % active material, 10 wt % carbon black (Super−P- Li), and 10 wt % poly(acrylic acid) (PAA) binder. The above miXture was wetted by several drops of NMP (N-methylpyrrolidone) and continuously stirred for 24 h to form the homogeneous paste. The resultant paste was coated on a copper wafer (13 mm in diameter, 0.3 mm thick) with a surface density of ∼1.25 mg/cm2 to fabricate the electrode. The as-prepared electrodes were dried in vacuum at 120 °C for 24 h. Lithium foil was employed as both the reference electrode and the counter electrode (13 mm in diameter, 0.5 mm thick). A 1.0 M LiPF6 in a 1:1 (v/v) miXture of ethylene carbonate (EC) and diethyl carbonate (DEC) with 10 wt % fluorinated ethylene carbonate (FEC) additives was used as the electrolyte. Celgard 2400 membrane (25 μm thick polyethylene) was used as a separator. The assembly of coin cells (CR2032) was performed in a high-purity Ar-filled gloveboX.

Electrochemical cycling of the electrodes was conducted on a Land Battery Measurement System (Wuhan, China) for galvanostatic measurements in a broad voltage window from 10 mV to 2.0 V (vs Li/Li+). The actually adopted current density for each cell was calculated based on the theoretical capacity of Si (3580 mAh g−1). In a typical cell, the mass of anode material is 1.25 mg containing active Si of 0.45 mg. The theoretical lithium-storage capacity was calculated to be 1.611 mAh based on the theoretical capacity of 3580 mAh g−1, and the corresponding 0.1 C current value is 0.1611 mA. Thus, an approXimate current density of 358 mA g−1 is employed. For all of the tested materials, the recorded capacity was obtained based on the amount of active Si, excluding the weight of non-Si components and additional additives in the electrode. The specific capacity of the whole electrode could also be obtained by multiplying the specific capacity of the active Si component by 0.8. Electrochemical impedance spectroscopy (EIS) was conducted on a Zahner Zennium electrochemical workstation (IM6e) over the frequency range from 100 kHz to 100 mHz with 5 mV of amplitude. The Si-based electrodes were well cleaned after cycling to remove the electrolyte sample was immediately transferred from 1100 °C to room temperature for naturally cooling in air. The fast heating process was aimed to quickly release the steam by internal pressure for generating abundant blowholes, and the fast cooling process was applied for fiXing the numerous blowholes by solidification of glassy-state perlite, which finally leads to formation of expanded perlite (Figure 1b). After removing Al2O3, Na2O, K2O, and MgO from the expanded perlite by 4 M HCl treatment, the amorphous SiO2 was further generated, as evidenced by the XRD pattern in Figure 1c. Subsequently, the resultant SiO2 was reduced into Si by Mg powder at 700
°C, forming the triple phases of Si/Mg2Si/MgO (Figure S1). After the HCl etching of Mg2Si and MgO byproducts, the pure Si extracted from raw perlite was thus obtained (Figure 1d). Finally, the pyrolysis of nickel acetate and melamine precursors onto the as-prepared Si at 700 °C contributed to the composites of Si, Ni, Ni2Si, and carbon (NCNTs-Ni2Si@Si) (Figure 1e).The spontaneous morphology evolution was characterized by SEM images, as presented in Figure 2. For the raw perlite,residue and dried in the gloveboX measurements.

3.1. Material Characterization. An innovative synthetic route was proposed for fabricating the unique NCNTs-Ni2Si@ Si composite, which showed a core−shell structure of “Ni- incorporated and N-doped carbon nanotubes wrapping on porous Si with the chemically bonded Ni2Si at the interface”. The synthetic strategy involves the successive construction of porous SiO2, porous Si, and NCNTs-Ni2Si@Si composite via three crucial steps: (i) heat expansion and HCl etching on raw perlite for producing porous SiO2; (ii) magnesiothermic reduction of porous SiO2 into porous Si; (iii) pyrolysis of nickel acetate and melamine coprecursors on porous Si for producing the NCNTs-Ni2Si@Si composite. To clarify the detailed formation mechanism for NCNTs-Ni2Si@Si, the phase evolution in the whole synthetic process was particularly studied. As shown in Figure 1a, the content of crystal water in raw perlite was first adjusted to a proper level by preheating at 300 °C for 25 min. Then the raw perlite was rapidly heated at 1100 °C for 5 s to evaporate the crystal water. The heated from raw perlite by fast heat treating, the alveolate structure with a macropore size of 300−500 nm and wall thickness of 200−250 nm was yielded in the expanded perlite (Figure 2b). The subsequent HCl etching of Al2O3, Na2O, K2O, and MgO from the expanded perlite generated porous SiO2, in which the macropore size was enlarged to 500−700 nm and the wall thickness was reduced to 150−200 nm (Figure 2c). As shown in Figures 2d and 3, the magnesiothermic reduction of porous SiO2 and the HCl etching of the byproducts (MgO and Mg2Si) contributed to the formation of porous Si, with an enlarged macropore size of 600−900 nm and a thinner silicon wall thickness of 100−150 nm. The pore-to-wall volume ratio was estimated to be 6, which is twice that of lithiated Si (because the volume of lithiated Si is triple that of pristine Si), demonstrating that the honeycomb-like macroporous structure can effectively buffer the serious volume expansion of Si. The porous Si was further wrapped by a ca. 5 μm thick layer of carbon nanotube arrays with enriched void spaces, forming the ca. 20 μm sized core−shell-structured NCNTs-Ni2Si@Si composite, as shown in Figures 2e and 4a−f. The SEM mapping was performed to gain insight into the compositional and structural features of as-prepared NCNTs-Ni2Si@Si composite. As displayed in Figure 4g−k, the elemental Si was observed to be located in the central zone as the inner core, and the elemental C was presented on the surface of Si as a coating layer, indicating that the porous Si has been well wrapped by the carbon nanotube arrays through nonfilling carbon coating. Moreover, the distribution regions of Ni and N were overlapped with that of C, revealing that the Ni and N codecorated carbon nanotubes were constructed (Ni- NCNTs). Interestingly, the elemental Ni was also overlapped with Si, suggesting that the Ni2Si interlayer may be formed and coated on the Si core. The above SEM and SEM mapping analyses confirmed successful construction of core−shell- structured NCNTs-Ni2Si@Si composite in which the micro- meter-sized porous Si particles were well coated by the void- enriched carbon nanotube arrays.

Figure 1. XRD patterns of as-prepared specific samples during the whole synthetic process: (a) raw perlite, (b) expanded perlite, (c) porous SiO2, (d) porous Si, and (e) NCNTs-Ni2Si@Si.

Figure 2. SEM images of as-prepared samples at different steps of the synthetic route: (a) raw perlite, (b) expanded perlite, (c) porous SiO2, (d) porous Si, and (e) NCNTs-Ni2Si@Si.

Figure 3. Cross-sectional SEM images (a and b) and oblique-angle SEM images (c and d) of as-prepared porous Si.

TEM image of NCNTs-Ni2Si@Si (Figure 5a) exhibited a morphology of CNT arrays wrapping on the surface of Si in which the CNTs were several micrometers in length and an average 100 nm in diameter with a wall thickness of ca. 5.6 nm (corresponding to 16 layers of graphene) (Figure 5b). The TEM mapping further verified the microstructural features of NCNTs-Ni2Si@Si by investigating the compositional distribu- tion. As shown in Figure 5c and 5d, the C mapping highly coincided with the region of tubes, demonstrating successful construction of CNTs. The Ni and N mappings showed that the Ni was incorporated at the tips of CNTs and N atoms were homogeneously doped in the whole carbon nanotubes, revealing formation of Ni-NCNTs based on the Ni-catalyzed tip-growth mechanism. In addition, two representative particles marked as 1 and 2 were both incorporated at the tips of CNTs, and both Si and Ni mappings were found to be well overlapped with the two particles. The results demonstrated that the majority of incorporated Ni in CNTs was reacted with Si to form the Ni2Si alloy at the tips of CNTs, generating an intimately connected heterojunction between the NCNTs shell and the Si core. The detailed interfacial structure at the heterojunction of NCNTs and Si was further characterized by the high-resolution TEM images. As shown in Figure 5e, a lattice spacing of ca. 0.17 nm was observed at the end point of NCNTs, corresponding to the (200) plane of Ni. At the edge of the Ni region, the graphite crystallite with an interplanar distance of ca. 0.35 nm was presented, which was ascribed to the graphitic carbon wall of CNTs. The results indicated that the incorporated Ni contributed to the formation and growth of CNTs based on the Ni-catalyzed tip-growth mechanism and was also chemically linked to the carbon wall via a Ni−C covalent bond at the interfacial nanoregion, where the graphitic structure was interrupted, and the Ni lattice defects and carbon dangling bonds occurred. At the other edge of the Ni region, two neighboring crystalline regions with lattice spacings of ca.

Figure 4. SEM images of NCNTs-Ni2Si@Si (a and b), magnified SEM images (c, d, and f) for corresponding purple square 1 and 2 (b) and 3 (c), illustration of core−shell-structured NCNTs-Ni2Si@Si (e), and SEM mapping of NCNTs-Ni2Si@Si (g−k).

The detailed chemical compositions in NCNTs-Ni2Si@Si composite were further quantified by the thermogravimetric- differential scanning calorimetry (TG-DSC) curve, which was recorded under air at a heating rate of 5 °C min−1. As presented in Figure 7, the TG curve of NCNTs-Ni2Si@Si composite exhibited a main mass loss of 19.29 wt % in the temperature range of 300−600 °C, accompanied by an obvious exothermic peak at 450 °C in the DSC curve, which was phenomenon suggested that the Ni2Si was formed at the interface of Si and Ni-NCNTs by the interfacial reaction of Ni and Si, generating the well-bonded heterojunction between Si and Ni-NCNTs via Si−Ni covalent bonds. The selected-area electron diffraction (SAED) images at the end point of CNTs 1 (Figure 5g) further revealed the compositional structures, which was identified to be the Si, Ni2Si, Ni, and graphitic carbon based on the epitaxial relations. The above TEM, TEM mapping, and SAED analysis confirmed that the Ni2Si junction has been well constructed and chemically bonded the NCNTs and Si in the core−shell-structured NCNTs-Ni2Si@Si.

Figure 5. TEM images (a−c), TEM mapping (d), high-resolution TEM images (e), structural illustration (f), and SAED patterns (g) of NCNTs-Ni2Si@Si.

XPS measurements were performed to experimentally determine the chemical state and composition of NCNTs- Ni2Si@Si. In the high-resolution Si 2p spectrum (Figure 6a), three fitted peaks were observed in which the peak at 100.0 eV corresponded to the elemental silicon, the second peak at 98.6 eV was ascribed to the intermetallic Ni2Si compounds, and the third peak at 103.0 eV was associated with the silicon oXide due to the oXidation of surface Si atoms in air.32,33 Figure 6b depicts the high-resolution Ni 2p3/2 spectrum of NCNTs- Ni2Si@Si composite, which was well deconvoluted into three distinct peaks. The first peak at 851.6 eV corresponded to the Ni, the second peak at 854.0 eV was assigned to the Ni2Si alloy, and the third peak at 856.5 eV was ascribed to the NiCx compounds.34,35 It was observed that the binding energies of Ni in Ni2Si and NiCx phases were higher than that of metallic Ni, indicating that the electron density has been slightly shifted from the metallic Ni to nonmetallic Si and C. The analysis products at 800 °C were examined to be the Si and NiO (Figure S3), and the mass percentage was about 80.71 wt %. Finally, the TG-DSC curve combined with ICP elemental analysis further identified that the weight percentages of Si, Ni2Si, Ni, and C in NCNTs-Ni2Si@Si were ca. 45.2, 20.55, 9.58, and 24.67 wt %, respectively.

The structural characteristics of NCNTs-Ni2Si@Si compo- site were also evaluated by Raman spectroscopy. As shown in Figure 8, the distinctive peak at 510 cm−1 corresponded to the crystalline Si, and the peaks at ∼1384 and 1582 cm−1 were attributed to the disordered carbon (D band) and graphitic carbon (G band), respectively.15 Generally, the intensity ratio of the D band and G band (ID/IG) was applied to estimate the graphitic degree of carbon materials. The ID/IG value of NCNTs-Ni2Si@Si was calculated to be 0.98, reflecting that the graphitic carbon framework was constructed by Ni-catalyzed growth of CNTs, which is helpful to enhance the electronic conductivity of NCNTs-Ni2Si@Si composite.

The porous features for the raw perlite, SiO2, Si, and NCNTs-Ni2Si@Si composite were investigated by N2 adsorption−desorption isotherms (Figure 9). The isotherm of raw perlite revealed the nonporous structure with a small specific surface area of 3.06 m2 g−1, while the isotherm of SiO2 exhibited a porous structure with a much higher surface area of 94.50 m2 g−1, reflecting that evaporation of crystal water from perlite created abundant pores in the resultant SiO2 and remarkably enhanced its surface area. In comparison with the raw perlite and porous SiO2, the isotherms of Si and NCNTs-Ni2Si@Si composite displayed distinct hysteresis loops at relative pressures (P/P0) of 0.45−0.90, indicating an obvious mesoporous structure with large specific surface areas of 128.76 and 112.23 m2 g−1, respectively. The hierarchical porosity, including mesopores and macropores (demonstrated by the SEM analysis), was expected to accommodate the severe volume changes, enhance the electrolyte penetration, and enlarge the Li+ accessible active sites when NCNTs-Ni2Si@Si served as the anode material in LIBs.

Figure 6. High-resolution XPS spectra of NCNTs-Ni2Si@Si composite: (a) Si 2p, (b) Ni 2p, (c) C 1s, and (d) N 1s.

Figure 7. TG-DSC curve of NCNTs-Ni2Si@Si composite in the air atmosphere.

3.2. Electrochemical Performance. The NCNTs-Ni2Si@ Si was adopted as an anode material for LIBs to investigate the electrochemical performance. For comparison, the porous Si and NCNTs-solid Si were also prepared and evaluated as the anode materials in LIBs. Figure 10a shows the first galvanostatic discharge−charge profiles for the three electrodes at a current density of 358 mA g−1 (0.1 C) within the voltage range of 0.01−2.00 V vs Li+/Li. All of the Si-based materials showed distinctive discharge and charge plateaus at 0.10 and
0.45 V vs Li+/Li, which resulted from the electrochemical alloying of crystalline Si and dealloying of amorphous LixSi, respectively. In the case of porous Si and NCNTs-solid Si electrodes, the initial discharge and charge specific capacities are 2254/1535 and 2067/1014 mAh g−1 under a current density of 358 mA g−1, corresponding to the initial Coulombic efficiencies of 68.2% and 49.0%, respectively. The rapid capacity decay was observed for NCNTs-solid Si in the sequential cycling, and the reversible capacity was only 210 mAh g−1 at the 100th cycle, though the corresponding Coulombic efficiency increased to 97.8% (Figure 10b). It has been commonly recognized that the electrochemical decom- position of organic electrolytes through passivating reactions to form a stable ion conductive but electron insulative SEI layer on Si upon cycling is extremely crucial for Si electrode reaction in the lithium-ion system.41,42 The remarkable irreversible capacity loss in the early cycling period and the explicit irreversibility in the prolonged cycles for NCNTs-solid Si anode were mainly attributed to the collapse and reformation of an SEI film and the loss of electronic contacts, which were caused by the huge morphology changes of Si electrode without the porosity, as explained by the top-view SEM images of the electrode (Figure S4a,b). It was found that the inner Si without porous structure suffered from the severe volume expansion and expanded out to destroy the coated NCNTs buffering layer at the 100th cycling, yielding the exposed Si particles in NCNTs-solid Si electrode. The strain-induced cracking upon lithiation and delithiation made more fresh Si exposed to the electrolyte and promoted the SEI formation and infiltration until it fully coated the newly formed Si particles. The phenomenon accelerated the pulverization and electronic insulation, which finally caused the continuous loss of capacity retention in NCNTs-solid Si electrode.

Figure 8. Raman spectrum of NCNTs-Ni2Si@Si composite.

Figure 9. N2 adsorption−desorption isotherms of raw perlite, porous SiO2, porous Si, and NCNTs-Ni2Si@Si.

Figure 10. First galvanostatic discharge−charge voltage profiles for NCNTs-Ni2Si@Si, porous Si, and NCNTs-solid Si electrodes at a current density of 358 mA g−1 (0.1 C) (a), cycling performance and Coulombic efficiency of three electrodes at a current density of 358 mA g−1 over 100 cycles (b), cycling performance of the NCNTs-Ni2Si@Si electrode at a current density of 358 mA g−1 over 600 cycles (c), and 100th discharge− charge voltage plateaus (d) and rate capabilities (e) of NCNTs-Ni2Si@Si electrode at various current densities of 716 (0.2 C), 1790 (0.5 C), 3580 (1 C), and 7160 (2 C) mA g−1.

Compared with the NCNTs-solid Si, the porous Si electrode displayed a much higher remnant capacity of 909 mAh g−1 and superior Coulombic efficiency of 99.6% at the 100th cycle. The possible reason was further analyzed from the eletrochemical behavior of porous Si. Without the protective carbon coating layer, the porous Si was irreversibly reacted with the electrolyte to form a stable SEI film in the initial 30 cycles, which consumed excessive lithium involved in electrode reaction and resulted in the decline of capacity retention. It was interesting that the capacity retention showed a slight decrease in the following 30−100 cycles, which may benefit from the advanced porous system in Si. As indicated in Figure S4c-d, the alveolate open-pore structure was well maintained after the 100th cycle, proving that the huge macropore volume with a pore-to-wall volume ratio of 6 successfully accommodated the serious volume expansion of the Si wall upon lithiation, which avoided breaking of the SEI film and contributed to the reliable cycling performance in the sequential cycles.
In terms of the NCNTs-Ni2Si@Si electrode, the first discharge and charge specific capacities were 2418 and 1956 mAh g−1, corresponding to an initial Columbic efficiency of 80.9%. The Columbic efficiency was then quickly restored to 95.3% at the 10th cycle, and the average Columbic efficiency approached 99.7% over 100 electrochemical cycles. More importantly, the NCNTs-Ni2Si@Si electrode delivered a significant long-term stability during 600 cycles in which a high reversible capacity of 1547 mAh g−1 was achieved in the first 500 cycles and the residual capacity was still maintained at 1311 mAh g−1 after 600 cycles (Figure 10c). Nevertheless, deterioration of capacity after 500 cycles for NCNTs-Ni2Si@ porous Si may be attributed to the quickly increased resistance originating from the thickened SEI passivation layer at the anode/electrolyte interface. Besides, the volumetric energy density of NCNTs-Ni2Si@Si was calculated to be 1006 mAh cm−3 based on the tap density of 1.44 g cm−3 (Si: 0.65 g) and the mass specific capacity of 1547 mAh g−1. Generally, the theoretical volumetric energy density of graphite was 837 mAh cm−3, and the actual volumetric energy density was normally less than 650 mAh g−1. The higher volumetric energy density of NCNTs-Ni2Si@Si to that of graphite demonstrated the superior electrochemical performance for the practical application of LIBs. The morphological structure of NCNTs- Ni2Si@Si electrode after 500 discharge−charge tests was also investigated by SEM images (Figure 11a−d). It was found that the NCNTs with intact morphology were still intimately wrapped on the active Si (Figure 11a−c) and attached on the current substrate (Figure 11d), forming a stable SEI film on their surface. The observation verified that the flexible and retractable one-dimensional NCNTs-wrapped porous Si successfully kept the electrode integrity by buffering the volume changes and stress-induced fracturing of Si, allowing formation of SEI film in the initial cycles and reaching the steady status over extended cycles. Meanwhile, TEM mapping (Figure 11e and 11f) and the Ni 2p XPS spectrum of NCNTs- Ni2Si@Si (Figure S5) further supported that the Ni2Si interfacial bonding was still remained for linking NCNTs to the Si surface after 500 discharge−charge tests. Therefore, the NCNTs constructed robust conductive networks between Si particles and Si/Cu substrate, and the Ni2Si junctions maintained good electrical contacts between NCNTs and Si core, effectively enhancing the charge transport and transfer in the Si electrode upon cycling. The above-mentioned advanced structural features contributed to the rapider growing of Columbic efficiency and higher cycling retention for the NCNTs-Ni2Si@Si electrode than those of porous Si. In addition, the rate performance of the NCNTs-Ni2Si@Si electrode was also evaluated from 716 (0.2 C) to 7160 mA g−1 (2 C). It was observed that the NCNTs-Ni2Si@Si electrode delivers a high Columbic efficiency of 99.5% (Figure 10d) and an impressive rate capability of 1365, 1176, 974, and 778 mAh g−1 after 100 cycles at the current densities of 716 (0.2 C), 1790 (0.5 C), 3580 (1 C), and 7160 (2 C) mA g−1, respectively (Figure 10e), demonstrating the excellent kinetic advantages for reversible electrochemical reactions within the electrode. Moreover, the comparison of the electrochemical performance of previously reported Si and their composite electrodes (e.g., bulk nanoporous Si, Mesoporous Si sponge, Si/carbon Si/graphite, Si/graphene, Si/SWCNT, MWCNT@ Si, TiSi2@porous Si, and C@porous Si) with this work further demonstrated the superior electrochemical architecture of the NCNTs-Ni2Si@Si electrode for LIBs (Table 1).

Figure 11. Top-view SEM images of NCNTs-Ni2Si@Si electrode after 500 cycles (a−c), cross-sectional SEM image of the 500th lithiated NCNTs-Ni2Si@Si electrode/Cu current collector (d), and TEM mapping of NCNTs-Ni2Si@Si electrode (e and f).

Electrochemical impedance spectroscopy (EIS) is a powerful tool for analyzing the interfacial kinetics of NCNTs-Ni2Si@Si, porous Si, and NCNTs-solid Si electrodes, which were obtained after the 20th electrochemical cycle (Figure 12a). The equivalent circuit is given in Figure 12b for fitting EIS plots by using the software ZSimpWin. In an equivalent circuit, Rs represents the series resistance, which arises from the Li+ migration through the electrolyte and the electrons trans- portation through electrodes. Rf represents the resistance caused by the Li+ migration through the SEI film, and Cf corresponds to the capacitance of the SEI film. Rct and Rw represent the resistance caused by charge transfer on the electrode and Li+ diffusion through the electrode, respectively, and a constant phase element (CPE) corresponds to the double-layer capacitance of the interface.58 The fitted EIS values have been mass normalized and are listed in Table 2. As indicated in Table 2, the Rs of NCNTs-Ni2Si@Si (3.86 Ω) and NCNTs-solid Si (4.23 Ω) were much lower than 11.18 Ω of porous Si, which was mainly attributed to their well-developed conductive network by coating NCNTs on the Si core. The Rw of NCNTs-Ni2Si@Si was 85.15 Ω, similar to 78.36 Ω of porous Si but much lower than 167.39 Ω of NCNTs-solid Si. The result indicated that the hierarchical porous system in
NCNTs-Ni2Si@Si and porous Si electrodes enhanced the Li+ diffusion when compared to the bulk NCNTs-solid Si. The NCNTs-Ni2Si@Si with the integrated structural advantages delivered a much lower Rct of 35.67 Ω than 91.96 Ω of porous Si and 156.32 Ω of NCNTs-solid Si, revealing the faster charge transfer on the NCNTs-Ni2Si@Si electrode. The smaller Rf and larger Cf also demonstrate the better film kinetics for NCNTs-Ni2Si@Si. In addition, impedance spectra of the NCNTs-Ni2Si@Si electrode presented similar profiles at the 20th, 100th, 300th, and 500th cycles (Figure 12c), confirming that the formed SEI film holds dynamic equilibrium at the interface of Si and electrolyte and maintained the steady film kinetics in extended cycles.

In order to better understand the interfacial kinetics of the SEI film, the critical components in the SEI film were also investigated by analyzing the XPS results of the Si-based electrode that were extracted from the 100th cycled cells (Figure 13). The C 1s and Li 1s spectra showed that the components at the surface of three Si-based electrodes were divided into the organic species (e.g., RO−COOLi) and the inorganic species (e.g., LixO and LiF) upon cycling in 1 M LiPF6 EC-DEC-FEC electrolyte. It is generally understood that the polymeric RO−COOLi is the decomposition product of EC, DEC, and FEC organic molecules, and the LiF is the decomposition product of LiPF6 and FEC. The concentration of polymeric Li salts in SEI films of porous Si and NCNTs- solid Si is much higher than that of NCNTs-Ni2Si@Si, suggesting the excessive decomposition of EC, DEC, and FEC molecules at the structurally and chemically unstable surface of the Si electrode. The observation well explained their distinct SEI resistance, while for the NCNTs-Ni2Si@Si electrode the higher concentration of inorganic species of LiF and LixO stabilized the SEI layer and enhanced Li+ diffusion, which contributed to the better electrode kinetics and improved Si electrode performance.

On the whole, the discharge−charge behaviors and EIS results well supported that the NCNTs-Ni2Si@Si with the unique architecture of “NCNTs-wrapped hierarchical porous Si with the robust Ni−Si bonding” could achieve outstanding electrochemical performance. The following structural features were primarily responsible for the good electrochemical behavior of NCNTs-Ni2Si@Si. First, the macro- and mesopores in the Si core facilitated electrolyte penetration and increased the surface area of NCNTs-Ni2Si@Si, which shortened the Li+ diffusion length and increased the utilization of active Si materials. Second, the nonfilling CNTs coating layer and the hierarchical porous system in the Si core worked together to buffer the volume expansion and avoid the break of the SEI layer. Finally, the NCNTs formed the robust conductive pathways between neighboring Si particles and Si/substrate, and the Ni2Si junction provided the good electrical contacts between Si and NCNTs and also effectively prevented the NCNTs’ detachment from the Si core, constructing a well-organized and stable framework for charge transport and transfer. Consequently, the NCNTs-Ni2Si@Si achieved superior electrochemical performance and cycling stability, which has great potential to be used as the anode candidate for high energy density LIBs.

Figure 13. XPS spectra of the 100th lithiated Si-based electrodes in EC:DEC:FEC electrolyte: C 1s (a) and Li 1s (b).

Figure 12. Nyquist plots of NCNTs-solid Si, porous Si, and NCNTs- Ni2Si@Si electrodes (a), equivalent circuit for fitting EIS data (b), and Nyquist plots of NCNTs-Ni2Si@Si measured at the 20th, 100th, 300th, and 500th cycles (c).

4. CONCLUSIONS

In summary, the hierarchical porous Si particles were innovatively constructed by evaporation of crystal water from perlite and subsequent magnesiothermic reduction reaction. The advanced hierarchical porous system and high surface area from the interior Si remarkably accommodated the volume expansion of Si, increased the Si utilization, and enhanced the Li+ diffusion during the discharge−charge process. The further wrapping of NCNTs on the interior Si also played a positive role in stabilizing the electrode structure and improving the electronic contacts among neighboring Si particles. Impor- tantly, the Ni2Si heterojunction, which chemically linked the Ni-NCNTs and Si via the Si−Ni and Ni−C covalent bonds, not only provided the conductive bridges between Si core and carbon shell but also effectively prevented the CNTs’ detachment from the Si core. Consequently, the NCNTs- Ni2Si@Si delivers promising Li storage properties with high specific capacities, stable cycling abilities, and good rate performance, which shows a specific capacity of 1311 mAh g−1 after 600 cycles at 358 mA g−1, with a high capacity retention of 85% relative to the stabilized capacity of 1547 mAh g−1.Therefore, the work successfully provided an accessible strategy to industrially prepare a novel structured Si/C composite with high capacity and good cycling stability in an economic and environmentally friendly way.

ACKNOWLEDGMENTS

This research was financially supported by the National Natural Science Foundation of China (No. 21802116), Nanhu Scholars Program for Young Scholars of XYNU, the Doctoral Start-up Research Fund of Xinyang Normal University (15006), the 111 project (B12015), and the Natural Science Foundation of Henan Province of China (182300410218).

REFERENCES

(1) Magasinski, A.; DiXon, P.; Hertzberg, B.; Kvit, A.; Ayala, J.; Yushin, G. High-Performance Lithium-Ion Anodes Using a Hierarchical Bottom-Up Approach. Nat. Mater. 2010, 9, 353−358.
(2) Jin, Y.; Zhu, B.; Lu, Z.; Liu, N.; Zhu, J. Challenges and Recent Progress in the Development of Si Anodes for Lithium-Ion Battery.
Adv. Energy Mater. 2017, 7, 1700715.
(3) Gowda, S. R.; Pushparaj, V.; Herle, S.; Girishkumar, G.; Gordon, J. G.; Gullapalli, H.; Zhan, X.; Ajayan, P. M.; Reddy, A. L. Three- Dimensionally Engineered Porous Silicon Electrodes for Li Ion Batteries. Nano Lett. 2012, 12, 6060−6065.
(4) Yang, H. S.; Kim, S. H.; Kannan, A. G.; Kim, S. K.; Park, C.;
Kim, D. W. Performance Enhancement of Silicon Alloy-Based Anodes Using Thermally Treated Poly(amide imide) as a Polymer Binder for High Performance Lithium-Ion Batteries. Langmuir 2016, 32, 3300−3307.
(5) Roy, A. K.; Zhong, M.; Schwab, M. G.; Binder, A.; Venkataraman, S. S.; Tomovic, Z. Preparation of a Binder-Free Three-Dimensional Carbon Foam/Silicon Composite as Potential Material for Lithium Ion Battery Anodes. ACS Appl. Mater. Interfaces 2016, 8, 7343−7348.
(6) Hu, L.; Wu, H.; Gao, Y.; Cao, A.; Li, H.; McDough, J.; Xie, X.;
Zhou, M.; Cui, Y. Silicon-Carbon Nanotube Coaxial Sponge as Li-Ion Anodes with High Areal Capacity. Adv. Energy Mater. 2011, 1, 523− 527.
(7) Zhao, C. S.; Li, S. W.; Luo, X.; Li, B.; Pan, W.; Wu, H. Integration of Si in a Metal Foam Current Collector for Stable Electrochemical Cycling in Li-Ion Batteries. J. Mater. Chem. A 2015, 3, 10114−10118.
(8) Hwang, T. H.; Lee, Y. M.; Kong, B.-S.; Seo, J.-S.; Choi, J. W. Electrospun Core-Shell Fibers for Robust Silicon Nanoparticle-Based Lithium Ion Battery Anodes. Nano Lett. 2012, 12, 802−807.
(9) Yu, Y.; Gu, L.; Zhu, C.; Tsukimoto, S.; van Aken, P. A.; Maier, J.
Reversible Storage of Lithium in Silver-Coated Three-Dimensional Macroporous Silicon. Adv. Mater. 2010, 22, 2247−2250.
(10) Cho, Y. J.; Kim, H. S.; Im, H.; Myung, Y.; Jung, G. B.; Lee, C.
W.; Park, J.; Park, M.-H.; Cho, J.; Kang, H. S. Nitrogen-Doped Graphitic Layers Deposited on Silicon Nanowires for Efficient Lithium-Ion Battery Anodes. J. Phys. Chem. C 2011, 115, 9451−9457.
(11) Zhang, L.; Rajagopalan, R.; Guo, H.; Hu, X.; Dou, S.; Liu, H. A
Green and Facile Way to Prepare Granadilla-Like Silicon-Based Anode Materials for Li-Ion Batteries. Adv. Funct. Mater. 2016, 26, 440−446.
(12) Fang, S.; Shen, L.; Xu, G.; Nie, P.; Wang, J.; Dou, H.; Zhang, X.
Rational Design of Void-Involved Si@TiO2Nanospheres as High- Performance Anode Material for Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2014, 6, 6497−6503.
(13) Wu, H.; Chan, G.; Choi, J. W.; Ryu, I.; Yao, Y.; McDowell, M.
T.; Lee, S. W.; Jackson, A.; Yang, Y.; Hu, L.; Cui, Y. Stable Cycling of Double-Walled Silicon Nanotube Battery Anodes through Solid- Electrolyte Interphase Control. Nat. Nanotechnol. 2012, 7, 310−315.
(14) Lu, Z.; Liu, N.; Lee, H. W.; Zhao, J.; Li, W.; Li, Y.; Cui, Y.
Nonfilling Carbon Coating of Porous Silicon Micrometer-Sized Particles for High-Performance Lithium Battery Anodes. ACS Nano 2015, 9, 2540−2547.
(15) Zhu, J.; Wang, T.; Fan, F.; Mei, L.; Lu, B. Atomic-Scale Control
of Silicon EXpansion Space as Ultrastable Battery Anodes. ACS Nano
2016, 10, 8243−8251.
(16) Yu, Q.; Ge, P.; Liu, Z.; Xu, M.; Yang, W.; Zhou, L.; Zhao, D.;
Mai, L. Ultrafine SiOx/C Nanospheres and Their Pomegranate-Like Assemblies for High-Performance Lithium Storage. J. Mater. Chem. A 2018, 6, 14903−14909.
(17) Chan, C. K.; Peng, H.; Liu, G.; McIlwrath, K.; Zhang, X. F.;
Huggins, R. A.; Cui, Y. High-Performance Lithium Battery Anodes Using Silicon Nanowires. Nat. Nanotechnol. 2008, 3, 31−35.
(18) Lin, L.; Xu, X.; Chu, C.; Majeed, M. K.; Yang, J. Mesoporous
Amorphous Silicon: A Simple Synthesis of a High-Rate and Long-Life Anode Material for Lithium-Ion Batteries. Angew. Chem., Int. Ed. 2016, 55, 14063−14066.
(19) Liu, N.; Wu, H.; McDowell, M. T.; Yao, Y.; Wang, C.; Cui, Y. A
Yolk-Shell Design for Stabilized and Scalable Li-Ion Battery Alloy Anodes. Nano Lett. 2012, 12, 3315−3321.
(20) Yao, Y.; McDowell, M. T.; Ryu, I.; Wu, H.; Liu, N.; Hu, L.; NiX,
W. D.; Cui, Y. Interconnected Silicon Hollow Nanospheres for Lithium-Ion Battery Anodes with Long Cycle Life. Nano Lett. 2011, 11, 2949−2954.
(21) Yoo, J.-K.; Kim, J.; Jung, Y. S.; Kang, K. Scalable Fabrication of
Silicon Nanotubes and their Application to Energy Storage. Adv. Mater. 2012, 24, 5452−5456.
(22) Kim, N.; Park, H.; Yoon, N.; Lee, J. K. Zeolite-
TemplatedMesoporous Silicon Particles for Advanced Lithium-Ion Battery Anodes. ACS Nano 2018, 12, 3853−3864.
(23) Huang, X.; Yang, J.; Mao, S.; Chang, J.; Hallac, P. B.; Fell, C.
R.; Metz, B.; Jiang, J.; Hurley, P. T.; Chen, J. Controllable Synthesis of Hollow Si Anode for Long-Cycle-Life Lithium-Ion Batteries. Adv. Mater. 2014, 26, 4326−4332.
(24) An, Y.; Fei, H.; Zeng, G.; Ci, L.; Xiong, S.; Feng, J.; Qian, Y.
Green, Scalable, and Controllable Fabrication of Nanoporous Silicon from Commercial Alloy Precursors for High-Energy Lithium-Ion Batteries. ACS Nano 2018, 12, 4993−5002.
(25) He, W.; Tian, H.; Xin, F.; Han, W. Scalable Fabrication of
Micro-Sized Bulk Porous Si from Fe-Si Alloy as a High Performance Anode for Lithium-Ion Batteries. J. Mater. Chem. A 2015, 3, 17956− 17962.
(26) Jaumann, T.; Herklotz, M.; Klose, M.; Pinkert, K.; Oswald, S.; Eckert, J.; Giebeler, L. Tailoring Hollow Silicon-Carbon Nano- composites As High-Performance Anodes in Secondary Lithium-Based Batteries through Economical Chemistry. Chem. Mater. 2015,27, 37−43.
(27) Zhang, H.; Zhao, H.; Khan, M. A.; Zou, W.; Xu, J.; Zhang, L.;
Zhang, J. Recent Progress in Advanced Electrode Materials, Separators and Electrolytes for Lithium Batteries. J. Mater. Chem. A 2018, 6, 20564−20620.
(28) Holzapfel, M.; Buqa, H.; Scheifele, W.; Novaḱ, P.; Petrat, F.-M.
A New Type of Nano-Sized Silicon/Carbon Composite Electrode for Reversible Lithium Insertion. Chem. Commun. 2005, 1566−1568.
(29) Ng, S. H.; Wang, J. Z.; Wexler, D.; Chew, S. Y.; Liu, H. K.
Amorphous Carbon-Coated Silicon Nanocomposites: A Low- Temperature Synthesis viaSpray Pyrolysis and Their Application as High-Capacity Anodes for Lithium-Ion Batteries. J. Phys. Chem. C 2007, 111, 11131−11138.
(30) Ji, J.; Ji, H.; Zhang, L. L.; Zhao, X.; Bai, X.; Fan, X.; Zhang, F.;
Ruoff, R. S. Graphene-Encapsulated Si on Ultrathin-Graphite Foam as Anode for High Capacity Lithium-Ion Batteries. Adv. Mater. 2013, 25, 4673−4677.
(31) Choi, S.; Jung, D. S.; Choi, J. W. Scalable Fracture-free SiOC
Glass Coating for Robust Silicon Nanoparticle Anodes in Lithium Secondary Batteries. Nano Lett. 2014, 14, 7120−7125.
(32) Chen, X.; Zhao, A. Q.; Shao, Z. F.; Li, C.; Williams, C. T.;
Liang, C. H. Synthesis and Catalytic Properties for Phenylacetylene Hydrogenation of Silicide Modified Nickel Catalysts. J. Phys. Chem. C 2010, 114, 16525−16533.
(33) Lin, L.; Xu, X.; Chu, C.; Majeed, M. K.; Yang, J. Mesoporous
Amorphous Silicon: A Simple Synthesis of a High-Rate and Long-Life Anode Material for Lithium-Ion Batteries. Angew. Chem., Int. Ed. 2016, 55, 14063−14066.
(34) Chen, X.; Wang, X.; Xiu, J.; Williams, C. T.; Liang, C. Synthesis
and Characterization of Ferromagnetic Nickel-Cobalt Silicide Catalysts with Good Sulfur Tolerance in Hydrodesulfurization of Dibenzothiophene. J. Phys. Chem. C 2012, 116, 24968−24976.
(35) Chen, M.; Zhao, G.; Shao, L.-L.; Yuan, Z.-Y.; Jing, Q.-S.;
Huang, K.-J.; Huang, Z.-Y.; Zhao, X.-H.; Zou, G.-D. Controlled Synthesis of Nickel Encapsulated into Nitrogen-Doped Carbon Nanotubes with Covalent Bonded Interfaces: The Structural and Electronic Modulation Strategy for an Efficient Electrocatalyst in Dye- Sensitized Solar Cells. Chem. Mater. 2017, 29, 9680−9694.
(36) Shin, D. H.; Lee, J. S.; Jun, J.; Jang, J. Fabrication of Amorphous
Carbon-Coated NiONanofibers for Electrochemical Capacitor Applications. J. Mater. Chem. A 2014, 2, 3364−3371.
(37) Yu, J.; Guo, M.; Muhammad, F.; Wang, A.; Zhang, F.; Li, Q.;
Zhu, G. One-Pot Synthesis of Highly Ordered Nitrogen-Containing Mesoporous Carbon with Resorcinol-Urea-Formaldehyde Resin for CO2 Capture. Carbon 2014, 69, 502−514.
(38) Yang, H. D.; Luo, S.; Li, X. Z.; Li, S. W.; Jin, J.; Ma, J. T.
Controllable Orientation-Dependent Crystal Growth of High-Index Faceted Dendritic NiC0.2Nanosheets as High-Performance Bifunctio- nalElectrocatalysts for Overall Water Splitting. J. Mater. Chem. A 2016, 4, 18499−18508.
(39) Okpalugo, T. I. T.; Papakonstantinou, P.; Murphy, H.;
McLaughlin, J.; Brown, N. M. D. OXidative Functionalization of Carbon Nanotubes in Atmospheric Pressure Filamentary Dielectric Barrier Discharge (APDBD). Carbon 2005, 43, 2951−2959.
(40) Shrestha, A.; Batmunkh, M.; Shearer, C. J.; Yin, Y.; Andersson,
G. G.; Shapter, J. G.; Qiao, S.; Dai, S. Nitrogen-Doped CNx/CNTs Heteroelectrocatalysts for Highly Efficient Dye-Sensitized Solar Cells. Adv. Funct. Mater. 2017, 7, 1602276.
(41) Mukhopadhyay, A.; Sheldon, B. W. Deformation and Stress in Electrode Materials for Li-Ion Batteries. Prog. Mater. Sci. 2014, 63, 58−116.
(42) Nie, M.; Abraham, D. P.; Chen, Y.; Bose, A.; Lucht, B. L.
Silicon Solid Electrolyte Interphase (SEI) of Lithium Ion Battery Characterized by Microscopy and Spectroscopy. J. Phys. Chem. C 2013, 117, 13403−13412.
(43) Ryu, J.; Hong, D.; Choi, S.; Park, S. Synthesis of Ultrathin Si Nanosheets from Natural Clays for Lithium-Ion Battery Anodes. ACS Nano 2016, 10, 2843−2851.
(44) Wada, T.; Ichitsubo, T.; Yubuta, K.; Segawa, H.; Yoshida, H.; Kato, H. Bulk-Nanoporous-Silicon Negative Electrode with EXtremely High Cyclabilityfor Lithium-Ion Batteries Prepared using a Top- Down Process. Nano Lett. 2014, 14, 4505−4510.
(45) Li, X.; Gu, M.; Hu, S.; Kennard, R.; Yan, P.; Chen, X.; Wang,
C.; Sailor, M. J.; Zhang, J. G.; Liu, J. Mesoporous Silicon Sponge as an Anti-Pulverization Structure for High-Performance Lithium-Ion Battery Anodes. Nat. Commun. 2014, 5, 4105.
(46) Hwang, T. H.; Lee, Y. M.; Kong, B. S.; Seo, J. S.; Choi, J. W. Electrospun Core-Shell Fibers for Robust Silicon Nanoparticle-Based Lithium Ion Battery Anodes. Nano Lett. 2012, 12, 802−807.
(47) Cui, L. F.; Ruffo, R.; Chan, C. K.; Peng, H.; Cui, Y. Crystalline-
Amorphous Core-Shell Silicon Nanowires for High Capacity and High Current Battery Electrodes. Nano Lett. 2009, 9, 491−495.
(48) Iwamura, S.; Nishihara, H.; Kyotani, T. Effect of Buffer Size
around Nanosilicon Anode Particles for Lithium-Ion Batteries. J. Phys. Chem. C 2012, 116, 6004−6011.
(49) Hwang, S. S.; Cho, C. G.; Kim, H. Polymer Microsphere
Embedded Si/Graphite Composite Anode Material for Lithium Rechargeable Battery. Electrochim. Acta 2010, 55, 3236−3239.
(50) Chou, S. L.; Wang, J. Z.; Choucair, M.; Liu, H.-K.; Stride, J. A.;
Dou, S.-X. Enhanced Reversible Lithium Storage in a Nanosize Silicon/Graphene Composite. Electrochem. Commun. 2010, 12, 303−
306.
(51) Chou, S. L.; Zhao, Y.; Wang, J. Z.; Chen, Z. X.; Liu, H. K.; Dou,
S. X. SiliconSingle-Walled Carbon Nanotube Composite Paper as a Flexible Anode Material for Lithium Ion Batteries. J. Phys. Chem. C 2010, 114, 15862−15867.
(52) Kim, H.; Huang, X.; Wen, Z.; Cui, S.; Guo, X.; Chen, J. Novel
Hybrid Si Film/Carbon Nanofibers as Anode Materials in Lithium- Ion Batteries. J. Mater. Chem. A 2015, 3, 1947−1952.
(53) Shao, L.; Shu, J.; Wu, K.; Lin, X.; Li, P.; Shui, M.; Wang, D.;
Long, N.; Ren, Y. Low pressure preparation of spherical Si@C@ CNT@C anode material for lithium-ion batteries. J. Electroanal. Chem. 2014, 727, 8−12.
(54) Chen, Y.; Du, N.; Zhang, H.; Yang, D. Facile Synthesis of
Uniform MWCNT@SiNanocomposites as High-Performance Anode Materials for Lithium-Ion Batteries. J. Alloys Compd. 2015, 622, 966− 972.
(55) Yoo, J. K.; Kim, J.; Jung, Y. S.; Kang, K. Scalable Fabrication of Silicon Nanotubes and Their Application to Energy Storage. Adv. Mater. 2012, 24, 5452−5456.
(56) Kim, Y. M.; Ahn, J.; Yu, S.-H.; Chung, D. Y.; Lee, K. J.; Lee, J.-
K.; Sung, Y.-E. Titanium Silicide Coated Porous Silicon Nanospheres as Anode Materials for Lithium Ion Batteries. Electrochim. Acta 2015, 151, 256−262.
(57) Han, X.; Chen, H.; Liu, J.; Liu, H.; Wang, P.; Huang, K.; Li, C.;
Chen, S.; Yang, Y. A Peanut Shell Inspired Scalable Synthesis of Three-Dimensional Carbon Coated Porous Silicon Particles as an Anode for Lithium-Ion Batteries. Electrochim. Acta 2015, 156, 11−19.
(58) Shin, J.; Cho, E. Agglomeration Mechanism and a Protective
Role of Al2O3 for Prolonged Cycle Life of Si Anode in Lithium-Ion Batteries. Chem. Mater. 2018, 30, 3233−3243.
(59) Yoon, T.; Bok, T.; Kim, C.; Na, Y.; Park, S.; Kim, K. S.
Mesoporous Silicon Hollow Nanocubes Derived from Metal-Organic BLU9931 Framework Template for Advanced Lithium-Ion Battery Anode. ACS Nano 2017, 11, 4808−4815.