Electrostatically Assembled Silicon-Carbon Composites Employing

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Electrostatically Assembled Silicon-Carbon Composites Employing AmineFunctionalized Carbon Intra-Interconnections for Li-Ion Battery Anodes Changju Chae, Woonghee Choi, Seulgi Ji, Sun Sook Lee, Jin-Kyu Kim, Sungho Choi, Yongku Kang, Youngmin Choi, Do Youb Kim, and Sunho Jeong ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b02012 • Publication Date (Web): 13 Feb 2019 Downloaded from http://pubs.acs.org on February 18, 2019

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ACS Applied Energy Materials

Electrostatically Assembled Silicon-Carbon Composites Employing Amine-Functionalized Carbon Intra-Interconnections for Li-Ion Battery Anodes

Changju Chae,a Woonghee Choi,a Seulgi Ji,a Sun Sook Lee,a Jin-Kyu Kim,a Sungho Choi,a Yongku Kang,a,b Youngmin Choi,a,b Do Youb Kim,a,b,* Sunho Jeonga,b,*

aDivision

of Advanced Materials, Korea Research Institute of Chemical Technology

(KRICT), 141 Gajeong-ro, Yuseong-gu, Daejeon 34114, Republic of Korea.

bDepartment

of Chemical Convergence Materials, Korea University of Science and

Technology (UST), 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Korea

KEYWORDS: silicon, carbon, electrostatic, amine, flexible, battery

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ABSTRACT Recently, the development of silicon-based anodes for lithium-ion batteries has attracted tremendous attention for overcoming the disadvantages of commercial graphite-based anodes. In this paper, we suggest a chemical methodology of synthesizing silicon–carbon composite anodes, with capacity values of 763 and 182 mAh/g at current densities of 0.1 and 5 A/g, respectively. An electrostatic assembly technique is designed to be triggered by a cationic polyelectrolyte, polyethyleneimine, for negatively-charged silicon nanoparticles and graphene oxides. Amine-functionalized carbon nanotubes are synthesized in a non-destructive fashion and incorporated additionally to provide intra-connected conductive pathways between neighboring composite materials. It is revealed that the electrochemical performance of intraconnected composite materials is determined by the chemical/physical factors of constituent compartments. The applicability toward all-solid-state batteries is also suggested with a usage of a solid polymer electrolyte synthesized from a mixture of bisphenol A ethoxylate diacrylate, polyethylene

glycol

dimethyl

ether,

t-butyl

bis(trifluoromethane)sulfonimide lithium salt.


peroxypivalate

and

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INTRODUCTION Recently, tremendous attention has been devoted to exploit lithium-ion batteries (LIBs) with high energy densities to keep pace with their ceaselessly growing demand in a variety of applications ranging from portable electronics devices to electric vehicles.1–4 From the viewpoint of anodic materials, various candidate materials have been investigated extensively to determine their advantages over conventional graphite counterparts. Transition metal oxideand Si-based anodes have been regarded as promising alternatives. Metal oxide-based anodes, governed by a conversion reaction, have theoretical capacities superior to those of graphite depending on their compositions,5–8 and silicon has a high theoretical capacity of ~4200 mAh/g.9–14 However, both anodic materials have significant drawbacks that should be resolved for practical applications. Si-based anodes suffer from a low electrical conductivity and a high volumetric expansion during lithiation reaction. In particular, the uncontrolled volumetric expansion results in critically adverse results of pulverization of silicon and an undesirable growth of a solid electrolyte interphase (SEI) layer. To date, a variety of nanostructured Si-carbon composite materials have been suggested to provide electrical conduction pathways and suppress extreme volumetric expansion during repeated electrochemical reactions. The representative methodology using Si nanoparticles (NPs) is either surrounding Si NPs with conductive carbon sheath layers in a core-shell fiber form9–11 or anchoring them chemically inside hierarchically-stacked carbon layers.12–14 In the latter case, the electrochemical properties of composite electrodes are determined predominantly by a kind of interfacial chemical moiety for binding both Si and carbon materials. In this study, we propose a facile aqueous chemical scheme of synthesizing electrostatically-assembled, nanostructured Si-carbon composites. Negatively-charged graphene oxides (GOs) and Si NPs are intra-stacked with the addition of a cationic



polyelectrolyte, polyethyleneimine, in an aqueous medium. Subsequently, electrically conductive intra-pathways are established by incorporating water-dispersible aminefunctionalized multi-walled carbon nanotubes (NH2MWNTs). The surface of carbon nanotubes were modified with a synthetic molecule, N,N’-di(2-aminoethyl)-perylene-3,4,9,10tetracarboxylic diimide (AE-PTDI), in a non-destructive fashion. The factors determining the electrochemical performance are investigated for intra-connected silicon-carbon composite materials, allowing for cells exhibiting capacities of 763 and 182 mAh/g at current densities of 0.1 and 5 A/g, respectively. Apart from conventional liquid-electrolyte-based cells, we also suggest the possibility of fabricating all-solid-state Li-ion batteries, by introducing a semiinterpenetrating solid polymer electrolyte derived from a mixture of polyethylene glycol dimethyl ether, bisphenol A ethoxylate diacrylate, and bis(trifluoromethane)sulfonimide lithium salt.

EXPERIMENTAL SECTION Synthesis of Intra-Interconnected Si-RGO/MWNT Composite Materials. 40 mg of silicon nanoparticles were dispersed by sonication in de-ionized (DI) water with a concentration of 5 mg/ml. Then, the aqueous GO solution with a concentration of 2.1 mg/ml was added to the prepared Si nanoparticle solution. The compositional ratio of Si NPs to GOs was adjusted by regulating the relative amount of both aqueous solutions. The polyethyleneimine (PEI, M.W.=1,300, Aldrich) was dissolved in DI-water with a concentration of 5 wt%, and then the PEI solution was added at an injection rate of 1 ml/min. The assemblies of either GOs or Si NPs themselves can be formed with an incorporation of excessive PEIs; thus, a proper amount of PEIs was added in a dropwise fashion in this study. After reaction for 1 h, the products were collected and washed with DI-water by centrifugation at 13,000 rpm for


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10 min. The resulting Si-PEI-GO composite materials were dispersed in DI water and kept in air without further passivation processes. The amine-functionalized multi-walled carbon nanotubes (NH2MWNTs) were synthesized by a chemical synthetic methodology.15,16 N,N’di(2-aminoethyl)-perylene-3,4,9,10-tetracarboxylic diimide (AE-PTDI) was synthesized by reacting perylene-3,4,9,10-tetracarboxylic dianhydride (PTDA) with ethylenediamine. Then, it was immobilized on the surfaces of the MWNTs via a strong π–π stacking interaction. Note that we did not perform a conventional acid-based treament process to generate the surfical acitive sites; rather, we employed a non-destructive chemical synthetic methodology to preserve the superior electrical proeprty of the carbon nanotubes. The synthesized NH2MWNTs were dispersed in DI water with a concentration of 1 mg/ml. Then, the aqueous NH2MWNT solutions were mixed with the aqueous Si–PEI–GO solutions. The resulting Si– PEI–GO/NH2MWNT composite materials were collected by centrifugation at 21,000 rpm for 15 min. The precipitates were dried at 80 oC under vacuum and annealed at 200, 400 or 600 oC under an inert atmosphere to produce intra-interconnected Si–RGO/MWNT composite materials. Liquid-Electrolyte Cell Fabrication. Electrochemical tests were conducted using CR2032 coin cells with Li metal as a reference electrode. The working electrodes were prepared by casting slurries onto copper foil current collectors. The slurries were composed of active composite materials, Super-P carbon black, polyvinylidene fluoride (PVDF, Kureha KF1100) as a binder, and NMP as a solvent (active material/Super-P/PVDF = 7:1.5:1.5 by weight). The loading amount of active material was 1.4~1.7 mg/cm2. All working electrodes were pressed and vacuum-dried at 120 °C for 12 h. A Celgard 2400 was used as a separator, and 1 M LiPF6 in ethylene carbonate/diethyl carbonate (EC/DEC) mixture (1:1 v/v) was used as an electrolyte. The cells were assembled in an Ar-filled glove box.



Solid-State-Electrolyte Cell Fabrication. For solid-state Li battery cells, the electrode, semi-interpenetrating solid polymer electrolyte (SPE), and Li foil were assembled.17 The electrode was fabricated from a mixture of 67 wt% of active composite material, 6 wt% of Super-P, 19.7 wt% of polyethylene glycol dimethyl ether (PEGDME), 7.3 wt% of PVDF, and bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) in NMP. The amount of lithium salt was fixed to be 20 molar ratio for ethylene oxide (EO) unit in PEGDME and Li+ ([EO]/[Li+] = 20). A precursor solution for the SPE was prepared by mixing a cross-linker (bisphenol A ethoxylate diacrylate) and a plasticizer (PEGDME) with a weight ratio of 3:7, Li salt (LiTFSI, [EO]/[Li+] = 20), and a thermal initiator (t-butyl peroxypivalate, 2 wt% with respect to a weight of cross-linker). The slurries were cast on Cu foils and dried at 120 °C in a vacuum oven for at least 12 h before use. The cell was assembled with the composite electrode, SPE precursor solution, a microporous nonwoven separator, and Li foil, followed by heat treatment at 90 °C for 1 h to induce thermal cross-linking of the SPE precursor solution. Solid-state Li battery cells were assembled as either CR-2032 coin-type or pouch-type. Characterization. The morphology of the composites was observed by scanning electron microscopy (SEM, JSM-6700, JEOL), and the zeta potential values of the graphene oxides and amine-functionalized MWNTs were measured with a zeta-potential analyzer (ELSZ, Otsuka). The chemical state of the Si nanoparticles was analyzed by X-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo Fisher Scientific). The galvanostatic charge-discharge profile, cycling performance, and rate performance were investigated in the voltage range of 0.01 to 1.5 V vs. Li+/Li at a current density of 0.1 to 5 A/g using battery testing equipment (TOSCAT-3100, Toyo Co. Ltd.).


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RESULTS AND DISCUSSION Scheme 1 shows the sequential procedures conducted to synthesize the silicon-carbon composites. The Si nanoparticles (NPs) used in this study were 70 to 300 nm in diameter (Figure 1a). The nano-sized morphology is beneficial to overcome the critical drawbacks of silicon as an anodic material for lithium-ion batteries, namely, an inferior electrical conductivity and an undesirable volume expansion during repeated electrochemical reactions. The X-ray photoelectron spectroscopy (XPS) Si 2p spectrum data revealed that a subtle oxide phase is formed along the surface of silicon nanoparticles (Figure 1b). The sub-peaks positioned at 98.7, 101.2, and 102.7 eV are attributable to pure Si, SiOx, and stoichiometric SiO2 phase, respectively. Taking into consideration of measurement depth in XPS analysis, it can be speculated that subtle oxide phases are present to the surface of Si nanoparticles. The formation of oxide phase is unavoidable along the surfaces of silicon nanoparticles due to thermodynamic stability in an ambient atmosphere; the surficial SiOx phase acts as another active site where lithium ions can undergo lithiation/delithiation reactions18. The silicon NPs and GOs were mixed in DI water with a neutral pH, forming a well dispersed phase. As shown in Figure 1c, both constituent materials have negative zeta potentials in the neutral aqueous media. Graphene oxides have a negative surface potential due to the presence of inherent surface defects. The Si NPs show a pH-dependent variation in zeta potential due to the chemical contribution of surficial oxide phase. It is well known that the silicon oxide phase exhibits a slightly-positive zeta potential through a protonation reaction in very acidic aqueous media, and a negative potential is generated through a deprotonation reaction in overall pH range, resulting in an isoelectric point of approximately 2. Thus, for the Si NPs used in this study, the surface charge increases negatively with increasing the pH of surrounding medium and a high negative surface charge with a value as high as -48 mV evolves at pH of 11.



Subsequently, polyethyleneimine (PEI) was added to the Si-GO aqueous solutions, which triggers an electrostatic assembly reaction between silicon and GOs. The PEI possesses a sufficient amount of amine groups that can be transformed into positively charged chemical moieties at a neutral pH. Thus, the incorporation of PEIs enables a surface charge inversion in negatively-charged materials, as confirmed in the PEI-GO suspension (Figure 1c). This instantaneous electrostatic reaction generates the formation of electrostatically-assembled SiPEI-GO composite materials (Figure S1). The role of PEIs can be limited as an intra-bridging chemical moiety between Si NPs and GOs, not forming undesirably enlarged bulk composite materials. As shown in Figure 1d, the overall zeta potential of Si-PEI-GO composite materials prepared under optimal synthetic conditions was 55 mV at a neutral pH. The Si-PEI-GO suspensions preserved their excellent dispersion phase even after reaction for 1 h, without forming bulk aggregates. The well-dispersed Si-PEI-GO composites were collected by a centrifugation separation method. To provide an electrical conduction pathway between neighboring composite materials, amine-functionalized carbon nanotubes (NH2MWNTs) were incorporated into an aqueous composite solution with a neutral pH, and the resulting composite materials were finally collected by further centrifugation. Pristine MWNTs are not capable of being dispersed in aqueous phases due to the absence of polar surface groups.19 In this study, we functionalized the surfaces of MWNTs with amine-derivative molecules through a nondestructive surface modification method. N,N′-di(2-aminoethyl)-perylene-3,4,9,10-tetracarboxylic diimide (AEPTDI), which was synthesized by reacting perylene-3,4,9,10-tetracarboxylic dianhydride (PTDA) with ethylenediamine as shown in Figure S2, was immobilized on the surfaces of the MWNTs via a strong π–π stacking interaction of aromatic groups.15 The surface functionalization of the MWNTs was clarified by the evolution of the zeta potential around 40 mV in aqueous media with neutral and


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acidic pH (Figure 1e). A uniform dispersion of Si-PEI-GO and NH2MWNT is achievable with a repulsive interaction between positively-charged ones in aqueous medium, and then, uniformly-distributed precipitates were obtainable by a forced centrifugation separation out of solutions. After drying the resulting precipitates, a polar-polar interaction evolves between amine groups from Si-PEI-GO and NH2MWNT in a dried state. The formation of uniformlysynthesized Si-PEI-GO/NH2MWNT composite materials is confirmed in the SEM image of the resulting precipitates (Figure 1f). The homogenously-distributed Si-PEI-GO/NH2MWNT composite materials were annealed at 400 oC under an inert atmosphere to convert the insulating graphene oxides into moderately-conductive reduced graphene oxides (RGOs) and to remove the interfacial amine groups present at heterogeneous junctions in composite materials, resulting in the formation of intra-interconnected Si-RGO/MWNT composite materials. The obtained intra-interconnected Si-RGO/MWNT composites were assembled into CR2032-type coin cells with a lithium foil as a counter/reference electrode. At first, we investigated the electrochemical performance of the composite materials with various compositional ratios of Si NPs to GOs. The 10 μm-long NH2MWNTs were used as an intrainterconnection moiety, and the composition of NH2MWNTs relative to Si-PEI-GO was 3 wt%. The voltage profiles of the 1st, 2nd, 4th, and 70th cycling reactions are shown in Figure 2a-c. The first 3 cycles and the next 67 cycles were tested at current densities of 0.1 and 0.3 A/g, respectively. As the chemical compositions of Si NPs increased, the 2nd discharge capacities improved with values of 707, 873, and 1050 mAh/g, for the composite materials with Si/GO ratios of 20/80, 30/70, and 40/60, respectively. With this composition-dependent trend, the irreversibility, which can be represented by Coulombic Efficiency in the 1st voltage profile, was improved accordingly with the values of 56, 63, and 65%. However, the retention in



capacity after 67 cycles at a current density of 0.3 A/g was measured to be 78, 66, and 49% for the composite materials with chemical compositions of 20/80, 30/70, and 40/60, respectively. In our intra-interconnected Si-RGO/MWNT composite materials, the surface amine chemical groups allow a facile interaction between them, but after the formation of the composite materials, they hamper the efficient charge carrier conduction at heterogeneous interfaces. To clarify the importance of removing those functional groups from composite materials, we annealed the composite materials at different temperatures of 200 and 400 oC, respectively. Figure 2d shows the voltage profiles of the 2nd cycling reaction at a current density of 0.1 A/g for cells employing composite materials annealed at 200 and 400 oC. The compositional ratio of Si NPs to GO was 40/60, and the composition of NH2MWNTs was 3 wt%. As seen in Figure 2d, the capacity was improved significantly from 676 to 1050 mAh/g by annealing at the higher temperature. Graphene oxide tends to be reduced thermally by a partial reduction of surface defects at temperatures above 200 oC, being transformed into relatively conductive reduced graphene oxides.20 However, the thermal decomposition of amine functional groups of NH2MWNTs is almost complete at 400 oC, as confirmed by thermogravimetric analysis (TGA) data under Ar atmosphere (Figure S3). Thus, in developing the composite materials with well-established intra-conduction pathways, the elimination of chemical groups incorporated for designated electrostatic interactions is a prerequisite to produce high-performance silicon-carbon composite anode materials. With increasing the annealing temperature up to 600 oC, a significant improvement in performance was not observed (Figure S4), indicative of a complete thermal decomposition of PEIs and impurities in both of GOs and NH2MWNTs at 400 oC.21,22 The critical role of NH2MWNTs was confirmed for the cell employing the NH2MWNT-free composite materials. As shown in Figure S5, by incorporating the


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NH2MWNTs in composite materials with a compositional ratio of 40/60, the discharge capacity in the 2nd voltage profiles (at a current density of 0.1 A/g) increased significantly from 489 to 676 mAh/g. In graphene-based composite materials, the improved cycling stability is obtainable by increasing the volumetric fraction of the stress-releasing, flexible graphenederivate compartment. However, the reduction in capacity is inevitably accompanied by a limited contribution of high-capacity constituent materials. A proper addition of highlyconductive one-dimensional materials compensates such an impediment of graphene-based composite materials, as more anodic composite materials take part in electrochemical reactions by establishing the electrical conduction pathways for those that are not connected electrically to underlying current collectors. One-dimensional MWNTs are a viable candidate that can develop electrical pathways in composite materials in terms of both cost-effectiveness and electrical conductivity. However, they tend to be entangled uncontrollably by virtue of their high surface area, which gives rise to restriction in the formation of spatially-distributed conductive networks interconnecting electrochemically-active materials. This morphological limitation in the composite materials would affect the capacity value in voltage profile and the rate performance at high current density. In investigating the rate capability, the composite material with lower Si content (Si/GO=20/80 by weight) was assembled into the cells for more improved retention in capacity at high current density conditions. As shown in Figure 3a, when the 1 μm-long NH2MWNTs were used rather than their 10 μm-long counterparts, the capacity at a current density of 0.1 A/g was increased from 580 to 763 mAh/g. The capacity of 182 mAh/g was preserved even at a high current density of 5 A/g, and the capacity returned well to the value of 678 mAh/g at a current density of 0.1 A/g. Taking into consideration that the graphene oxides used in this study are a few micrometer in size and the excessively-stacked morphology is restricted to some



extent in our synthetic scheme, it can be presumed that the morphological dimensions of the composite materials do not significantly exceed those of pristine GOs. Thus, the relativelyshort, 1 μm-long NH2MWNTs would provide a sufficient amount of intra-electrical-pathways between neighboring composite materials, reducing the possibility of their forming aggregates in comparison to 10 μm-long NH2MWNTs. The composition of silicon in optimized composite material was measured to be ~30 wt% from TGA analysis result in air (Figure S6), which increased slightly from the value, 20 wt%, in composition for as-prepared one (prior to annealing process) owing to a thermal decomposition of residual substances. This importance of sophisticatedly-designed composite materials is supported by the fact that excessive incorporation of even short 1-dimensional carbons would also have an adverse impact on electrochemical performance. As shown in Figure 3b, after the 40th cycle, the capacity at a current density of 0.3 A/g was 571 mAh/g for the composite materials with 3 wt% of 1 μm-long NH2MWNTs; however, when 5 wt% of 1 μm-long NH2MWNTs was added, the capacity at a current density of 0.3 A/g decreased with values of 505 mAh/g. In fact, in formulating the aqueous suspensions with 1 μm-long NH2MWNTs, subtle agglomerates are observable in concentrated ones. The cycling performance of the optimized composite materials (the ratio of Si NPs to GO and the composition of 1 μm-long NH2MWNTs were 20/80 and 3 wt%, respectively) is shown in Figure 3c, with the values of 745 and 606 mAh/g in initial capacities at current densities of 0.1 and 0.3 A/g, and of 388 mAh/g capacity after 100 cycles at a current density of 0.3 A/g. A stable voltage profile after 100 cycles is shown in Figure S7. All-solid-state lithium batteries including solid polymer electrolytes (SPEs) have been regarded as a promising post LIB system that can avoid the critical safety issue of current liquid-electrolyte-based Li-batteries.23,24 We fabricated all-solid-state cells using intra-


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interconnected Si-RGO/MWNT composite electrodes and semi-interpenetrating network SPEs. Figure S8a shows photographs of the semi-IPN SPE before and after polymerization, indicating a solid-state formation reaction in a glass vial. The solid-state SPEs possess a self-supporting mechanical strength and excellent flexibility such that they can be bent into arbitrary angles (Figure S8b). The SPEs synthesized in this study have a high ionic conductivity of 2.0 × 10-4 S/cm at 25 °C and a relatively wide and stable electrochemical window up to 5.55 V (vs. Li/Li+) (Figure S8c). Figure 4a and 4b show the voltage profiles and cycling performance of all-solidstate CR2032 cells at a current density of 0.1 A/g at room temperature. The all-solid-state cells exhibited relatively inferior specific capacity and cycling performance in comparison to the performance of liquid-electrolyte-based cells; this is attributable to the limited inter-penetration of viscous SPE precursor solution inside the composite electrodes as well as the moderate ionic conductivity of SPE at room temperature.25 Both factors are representative constraints limiting the electrochemical performance of all-solid-state lithium batteries. To mediate the wettability of electrolyte along active materials, we deliberately incorporated a part of electrolyte in the composite electrodes. The chemical structural modification of SPE would further improve the electrochemical performance of the resulting all-solid-state batteries. Recently, several attempts have been made to use of gel-polymer electrolytes26,27; however, to the best of our knowledge, all-solid-state battery cells employing a Si-based anode and SPEs have not been reported so far. Encouraged by these results, an all-solid-state aluminum pouch cell was fabricated as shown in Figure 4c. It was demonstrated successfully that the battery cell maintained its performance in flat, bent, and folded configurations, powering a blue lightemitting diode (Figure 4d). This indicates that the intra-interconnected Si-RGO/MWNT composite materials facilitate its potential for use in flexible and safe energy storage devices.



CONCLUSION In summary, we have synthesized electrostatically-assembled silicon-carbon composite anode materials by introducing a cationic polyelectrolyte, polyethyleneimine, in an aqueous mixture of negatively-charged graphene oxides and silicon nanoparticles. To produce facile conductive pathways, non-destructively amine-functionalized carbon nanotubes were incorporated additionally into the composite materials. It was revealed that the electrochemical properties of the resulting composite materials are adjustable depending on the relative composition of the silicon nanoparticles, the composition of the amine-functionalized carbon nanotubes, and the annealing temperature of the composite materials. This comprehensive study allowed the synthesis of composite materials with the capacity values of 763 and 182 mAh/g at current densities of 0.1 and 5 A/g, respectively. The solid polymer electrolyte, synthesized from a mixture of bisphenol A ethoxylate diacrylate, polyethylene glycol dimethyl ether, t-butyl peroxypivalate, and bis(trifluoromethane)sulfonimide lithium salt, was also evaluated in combination with silicon-carbon composite materials, showing good potential possibility for application in all-solid-state batteries.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication websites at DOI: SEM image of composite materials, chemical reaction of AE-PTDI, TGA result of


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NH2MWNTs under inert atmosphere, voltage profiles of the cells employing composite materials, and basic data of SPE film.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (D. Y. Kim); [email protected] (S. Jeong)

ACKNOWLEDGMENTS This research was supported by the Global Research Laboratory Program of the National Research Foundation (NRF) funded by the Ministry of Science, Information and Communication Technologies and Future Planning (NRF-2015K1A1A2029679) of Korea. It was also partially supported by the Nano·Material Technology Development Program through the National Research Foundation of Korea funded by the Ministry of Science, Information and Communication Technologies and Future Planning (NRF-2015M3A7B4050306). References (1) Etacheri, V.; Maron, R.; Elazari, R.; Salitra, G.; Aurbach, D. Challenges in the Development of Advanced Li-Ion Batteries: A Review. Energy Environ. Sci. 2011, 4, 3243-3262. (2) Scrosati, B.; Hassoun, J.; Sun, Y.-K. Lithium-Ion Batteries. A look Into the Future. Energy Environ. Sci. 2011, 4, 3287-3295. (3) Blomgren, G. E. The Development and Future of Lithium Ion Batteries. J. Electrochem. Soc. 2017, 164, A5019-A5025. (4) Roy, P.; Srivastava, S. K. Nanostructured Anode Materials for Lithium Ion Batteries. J. Mater. Chem. A 2015, 3, 2454-2484.



(5) Lee, D.; Wu, M.; Kim, D.-H.; Chae, C.; Cho, M. K.; Kim, J.-Y.; Lee, S. S.; Choi, S.; Choi, Y.; Shin, T. J.; Chung, K. Y.; Jeong, S.; Moon, J. Understanding the Critical Role of the Ag Nanophase in Boosting the Initial Reversibility of Transition Metal Oxide Anodes for Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2017, 9, 2171521722. (6) Chae, C.; Kim, K. W.; Yun, Y. J.; Lee, D.; Moon, J.; Choi, Y.; Lee, S. S.; Choi, S.; Jeong, S. Polyethyleneimine-Mediated Electrostatic Assembly of MnO2 Nanorods on Graphene Oxides for Use as Anodes in Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2016, 8, 11499-11506. (7) Kim, S. J.; Yun, Y. J.; Kim, K. W.; Chae, C.; Jeong. S.; Kang, Y.; Choi, S.-Y.; Lee, S. S.; Choi, S. Superior Lithium Storage Performance using Sequentially Stacked MnO2/Reduced Graphene Oxide Composite Electrodes. ChemSusChem 2015, 8, 1484-1491. (8) Chae, C.; Kim. K. W.; Kim, S. J.; Lee, D.; Jo, Y.; Yun, Y. J.; Moon, J.; Choi, Y.; Lee, S. S.; Choi, S.; Jeong, S. 3D Intra-Stacked CoO/Carbon Nanocomposites Welded by Ag Nanoparticles for High-Capacity, Reversible Lithium Storage. Nanoscale 2015, 7, 10368-10376. (9) 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. (10) Wu, H.; Zheng, G.; Liu, N.; Carney, T. J.; Yang, Y.; Cui, Y. Engineering Empty Space between Si Nanoparticles for Lithium-Ion Battery Anodes. Nano Lett. 2012, 12, 904909. (11) Zhou, X.; Guo, Y.-G. A PEO-Assisted Electrospun Silicon-Graphene Composite as an Anode Material for Lithium-Ion Batteries. J. Mater. Chem. A 2013, 1, 9019-9023. (12) Luo, J.; Zhao, X.; Wu, J.; Jang, H. D.; Kung, H. H.; Huang, J. Crumpled GrapheneEncapsulated Si Nanoparticles for Lithium Ion Battery Anodes. J. Phys. Chem. Lett. 2012, 3, 1824-1829. (13) Agyeman, D. A.; Song, K.; Lee, G.-H.; Park, M.; Kang, Y.-M. Carbon-Coated Si Nanoparticles Anchored between Reduced Graphene Oxides as an Extremely Reversible Anode Material for High Energy-Density Li-Ion Battery. Adv. Energy Mater. 2016, 6, 1600904-1600913. (14) Lee, W. J.; Hwang, T. H.; Hwang, J. O.; Kim, H. W.; Lim, J.; Jeong, H. Y.; Shim, J.;


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Han, T. H.; Kim, J. Y.; Choi, J. W.; Kim, S. O. N-Doped Graphitic Self-Encapsulation for High Performance Silicon Anodes in Lithium-Ion Batteries. Energy Environ. Sci. 2014, 7, 621-626. (15) Kim, J. Y.; Ji S.; Jung, S.; Ryu, B.-H.; Kim, H.-S.; Lee, S. S.; Choi, Y.; Jeong, S. 3D Printable Composite Dough for Stretchable, Ultrasensitive and Body-Patchable Strain Sensors, Nanoscale 2017, 9, 11035-11046. (16) Chae, C.; Kim, J.; Kim, J. Y.; Ji, S.; Lee, S. S.; Kang, Y.; Choi, Y.; Suk, J.; Jeong, S. Room-Temperature, Ambient-Pressure Chemical Synthesis of Amine-Functionalized Hierarchical Carbon-Sulfur Composites for Lithium-Sulfur Battery Cathodes. ACS Appl. Mater. Interfaces 2018, 10, 4767-4775. (17) Suk, J.; Lee, Y. H.; Kim, D. Y.; Kim, D. W.; Cho, S. Y.; Kim, J. M.; Kang, Y. SemiInterpenetrating Solid Polymer Electrolyte Based on Thiol-Ene Cross-Linker for AllSolid-State Lithium Batteries. J. Power Sources 2016, 334, 154-161. (18) Chen, T.; Wu, J.; Zhang, Q.; Su, X. Recent Advancement of SiOx Based Anodes for Lithium-Ion Batteries. J. Power Sources 2017, 363, 126-144. (19) Son, G.-C.; Chee, S.-S.; Jun, J.-H.; Son, M.; Lee, S. S.; Choi, Y.; Jeong, S.; Ham, M.H. Chemically Functionalized, Well-Dispersed Carbon Nanotubes in Lithium-Doped Zinc Oxide for Low-Cost, High-Performance Thin-Film Transistors. Small 2016, 12, 1859-1865. (20) Wang, Z.-L.; Xu, D.; Huang, Y.; Wu, Zhong.; Wang, L.-M.; Zhang, X.-B. Facile, Mild and Fast Thermal-Decomposition Reduction of Graphene Oxide in Air and Its Application in High-Performance Lithium Batteries. Chem. Commun. 2012, 48, 976978. (21) Roy, S.; Tang, X.; Das, T.; Zhang, L.; Li, Y.; Ting, S.; Hu, X.; Yue, C. Y. Enhanced Molecular Level Dispersion and Interface Bonding at Low Loading of Modified Graphene Oxide To Fabricate Super Nylon 12 Composites, ACS Appl. Mater. Interfaces 2015, 7, 3142. (22) Renteria, J. D.; Ramirez, S.; Malekpour, H.; Alonso, B.; Centeno, A.; Zurutuza, A.; Cocemasov, A. I.; Nika, D. L.; Balandin, A. A. Strongly Anisotropic Thermal Conductivity of Free-Standing Reduced Graphene Oxide Films Annealed at High Temperature, Adv. Funct. Mater. 2015, 25, 4664. (23) Jung, Y.-C.; Park, M.-S.; Kim, D.-H.; Ue, M.; Eftekhari, A.; Kim, D-W. RoomTemperature Performance of Poly(Ethylene Ether Carbonate)-Based Solid Polymer



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Si NPs simple mixing

Well-dispersed aqueous Si NPs/GO solution graphene oxides (GOs)

adding polyethyleneimine (PEI)

PA

PD

O

O

O

O

O

H 2N

NH2

O

H 2N

O

O

N

N

O

O

NH2

mixing with NH2NWNTs annealing at 400 oC

intra-interconnected Si-RGO/MWNT composites

Si-PEI-GO composites

Scheme 1. Synthetic procedures of electrostatically-driven, intra-interconnected Si– RGO/MWNT composite materials. The NH2MWNT denotes the amine-functionalized multiwalled carbon nanotubes synthesized non-destructively in this study.



b SiO2

Intensity (a.u.)

a

Si SiOx

200 nm

106

104

102

100

98

96

Binding Energy (eV)

d

PEI-treated GOs Si NPs GOs

60

Zeta Potential (mV)

Zeta Potential (mV)

c

30 0 -30

60 30 0 -30 -60

-60

2

4

6

8

10

2

12

e

4

6

8

10

12

pH

pH

Zeta Potential (mV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60

f

30

0

-30 500 nm

-60

2

4

6

8

10

12

pH

Figure 1. (a) SEM image and (b) XPS Si 2p spectrum of silicon nanoparticles used in this study; The zeta-potential values of (c) graphene oxides (GOs), silicon nanoparticles (Si NPs), PEI-treated GOs, (d) Si-GO-PEI composite materials, (e) NH2MWNTs. The inset in Figure 1d is a photograph of well-dispersed Si-PEI-GO aqueous solution; (f) SEM image of SiGO/NH2MWNT composite materials.


st

1 nd 2 th 4 th 70

1.0

0.5

b

1.5

st

1 nd 2 th 4 th 70

+

1.5

+

Voltage (V vs. Li/Li )

a

1.0

0.5

0.0

0.0 0

300

600

900

1200

0

1500

Specific Capacity (mAh/g)

st

+

1 nd 2 th 4 th 70

1.0

0.5

0.0 0

300

600

900

1200 1500


600

900

1200

1500

d

1.5

+

1.5

300


Voltage (V vs Li/Li )

c Voltage (V vs Li/Li )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



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1.0

200℃ 400℃

0.5

0.0

0

400

800

1200


Figure 2. Voltage profiles of cells employing intra-interconnected Si-RGO/MWNT with different compositional ratios of Si NPs to GOs, (a) 20/80, (b) 30/70, and (c) 40/60. 3 wt% of 10 μm-long NH2MWNTs was added, and the composite materials were annealed at 400 oC. The first 3 cycles and next 67 cycles were tested at current densities of 0.1 and 0.3 A/g, respectively. (d) Voltage profiles of the cells employing intra-interconnected Si-RGO/MWNT composite materials annealed at 200 and 400 oC. The compositional ratio was 40/60, and 3 wt% of 10 μm-long NH2MWNTs was added. The voltage profiles were recorded during the 2nd cycle at a current density of 0.1 A/g.




a

1500 1℃ -long NH2MWNT 10℃ -long NH2MWNT

1200 900

0.1 A/g 0.2 0.3

600

0.1 0.5

1

2

300 0

3 5

0

10

20

30

40

Cycle Number

NH2MWNT 3 wt%

1.5

NH2MWNT 5 wt%

+


b

1.0

0.5

0.0 0

c Specific Capacity (mAh/g)

200

400

600

Specific Capacity (mAh/g) 100 1200

Discharge Charge 80 Coulombic Efficiency

800 0.1 A/g

60

0.3 A/g 40 400 20 0

0

20

40

60

80

Coulombic Efficiency(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 24

0 100

Cycle Number

Figure 3. (a) Rate performance of the cells employing the intra-interconnected SiRGO/MWNT composite materials with 1 and 10 μm-long NH2MWNTs. The composition of NH2MWNTs was 3 wt%. (b) Voltage profiles of the cells employing the intra-interconnected Si-RGO/MWNT composite materials with various amounts of 1 μm-long NH2MWNTs. The voltage profiles were recorded at the 40th cycle at a current density of 0.3 A/g. (c) Cycling performance of the cell employing the optimized intra-interconnected Si-RGO/MWNT composite materials with 3 wt% of 1 μm-long NH2MWNTs. The first 3 cycles and next 67 cycles were tested at current densities of 0.1 and 0.3 A/g, respectively. For all composite materials, the compositional ratio was 20/80, and the composite materials were annealed at 400 oC.


Page 23 of 24

b

1.5

+


a Voltage (V vs. Li/Li )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


st

1 th 10 th 20 th 30 th 40 th 50 th 100

1.0

0.5

0.0 0

100

200

300

400

500 400 300 200 100 0

0


c

25

50

75

100

Cycle Number

d

Figure 4. (a) Voltage profiles and (b) cycling performance of all-solid-state coin cells employing intra-interconnected Si-RGO/MWNT composite electrodes and SPEs. The current density was 0.1 A/g. (c) Illustration of an all-solid-state pouch cell comprising the composite electrode and SPE. (d) Photographs showing the illumination of blue LEDs connected to a flat, bent, or folded pouch cell.



Table of Contents (TOC)


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Electrostatically Assembled Silicon-Carbon Composites Employing

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