Facile Synthesis of Hierarchical Porous Carbon Monolith: A Free

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Facile Synthesis of Hierarchical Porous Carbon Monolith: A Free-Standing Anode for Li-ion Battery with Enhanced Electrochemical Performance Shilpa *, Shishir Katiyar, Nallathamby Kalaiselvi, and Ashutosh Sharma Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02750 • Publication Date (Web): 20 Oct 2016 Downloaded from http://pubs.acs.org on October 25, 2016

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Industrial & Engineering Chemistry Research

Facile Synthesis of Hierarchical Porous Carbon Monolith: A Free-Standing Anode for Li-ion Battery with Enhanced Electrochemical Performance Shilpa,a Shishir Katiyar,a Nallathamby Kalaiselvi,b and Ashutosh Sharmaa* a

Department of Chemical Engineering, Indian Institute of Technology Kanpur, 208016, India b

Central Electrochemical Research Institute, Karaikudi-630003, Tamil Nadu, India

Abstract Hierarchical porous carbon aerogel with a continuous three dimensional framework has been synthesized via evaporation induced, co-assembly based sol-gel polymerization of organic and inorganic precursors, for application as anode in Li-ion battery. A combination of Resorcinol (R) and Formaldehyde (F) as organic precursors and hybrid silica sources comprised of TEOS (tetraethyl orthosilicate), APTES (3-aminopropyl tri-ethoxysilane) and APTEMS (3-aminopropyl tri-methoxysilane) as inorganic precursors has been used to obtain the porous carbon monolith. The porosity of the carbon aerogel has been tuned by varying the ratio of APTES/APTEMS. Post pyrolysis, the etching of the composite to remove silica network resulted in the formation of a high specific surface area (BET~ 2600 m2 g-1) carbon containing the distribution of micro, meso and macro porous domains. The electrode exhibits a high reversible capacity of 890 mAh g-1 after 100 cycles at a current density of 50 mA g-1. Keywords: Lithium ion battery, Hierarchical porous, Free-standing anode, Carbon monolith. *Corresponding author. Tel:. +91-512-259-7026. E-mail: [email protected] (Ashutosh Sharma)

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1. Introduction Environmental issues, such as global warming caused by the increased CO2 emission and rapid depletion of fossil fuels, faced with ever- growing energy demands across the world have led to an increased interest in electrochemical energy systems as next generation renewable energy sources.1,2 Lithium ion batteries, which offer the highest energy density amongst the current rechargeable technologies have already revolutionalized the field of portable electronics and are now being considered as promising power sources that could enable electric/hybrid vehicle propulsion.3 However, any major leap in this direction would require significantly improved Liion batteries with high energy/power density, durability and low-cost.4,5 Compared to the commercial graphite anode, which has a

low theoretical capacity (372 mAh g-1), certain

transition metal oxides (ZnO, SnO2, NiO, Co3O4, etc) and pure elements like (Si, Ag, Bi, Pb, Sb, Ge, etc) exhibit high theoretical capacities for Li storage and have been extensively investigated in various crystallographic forms and morphologies as probable anodes for LIBs.6-14 Unfortunately, these alloy based anodes suffer from severe capacity fading due to huge volume fluctuations (upto 360%) that occur during lithiation/delithiation cycles, resulting

in rapid

disintegration of mechanical structure and subsequent detachment from the current collector.15 Thus, the challenge to achieve a high capacity and stable cycling performance has put the focus back on carbon materials, with a special reference to porous disordered/hard carbons.16-20 Disordered carbons have regions in their structure, where the crystallite planes randomly intersect and give rise to microcavities. In such carbons, Li ions can be accommodated on both sides of the randomly oriented graphene layers, while micro cavities provide additional space for lithium storage.21 Recently, the pyrolysis of biomass products such as coconut shell, rice husk, apricot shell, cotton etc has been explored as an attractive option to obtain amorphous carbon for


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LIB anodes due to the low cost involved and their renewable nature. 22-24 However, such carbons usually exhibit low surface area and an undeveloped pore structure resulting in poor diffusion of Li-ions and hence a poor rate performance.25, 26 Hence, the synthesis of disordered carbons with porous structure continues to constitute an active area of research for development of high capacity LIB anodes. Various methodologies have been adopted over the years to provide a simplified approach towards fabricating porous carbons which include i) activation by physical/chemical methods 27-29

, ii) carbonization of polymer blends composed of one carbonizable and one thermally

unstable component30,31, iii) catalyst-assisted activation using metallic or organometallic compounds32,33, iv) use of pre-synthesized organic/inorganic hard templates as moulds for replication34 and (v) soft template approach based on self-assembly of

organic

molecules.35However, all of these approaches suffer from some limitation or other and none of them produce a truly hierarchical structure. For e.g., in the case of hard templates, the resulting pore size is dependent on the pore structure of the template with no freedom on its tunabilty, whereas in case of soft templates, the porosity is determined by the size of surfactant micelles, which are usually monodisperse in nature and hence cannot generate hierarchical porosity. The activation by physical/chemical methods although generate high surface area, but the activation is mostly uneven and results in high degree of microporosity, which is not very beneficial for Li-ion batteries. The use of polymeric blends, generate mesopores, but the overall surface area is usually low. In this work, we demonstrate an easy and scalable approach to fabricate a high surface area carbon aerogel (CA) with tunable porosity through a simple one step sol-gel process, wherein the solvent phase can be removed by drying in ambient pressure. Since the first report on the 3 ACS Paragon Plus Environment


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synthesis of CA by Pekala in 1989, several articles have appeared in literature illustrating different synthesis and processing techniques for preparing aerogels and studying their effect on the final aerogel nanostructure, thus relating it to its macroscopic performance in various applications.36-45 Usually, the synthesis of aerogel requires a supercritical dying step in order to avoid the pore collapse which arises due to the built up of capillary pressure at the liquid -vapor interface.36 As this is an extremely expensive step, it often limits the commercial feasibility of the synthesized aerogels. Herein, we demonstrate a low cost fabrication method, where the solvent phase has been removed by simple drying in ambient pressure without affecting the pore structure of the aerogel, made possible due to the mechanical stability of the system originating from its hierarchically arranged structured. Another novel feature of our synthesis is the use of hybrid silica precursors, which not only act as reactants but also catalyze the sol-gel reaction, thereby playing an important role in tuning the aerogel porosity. Further, etching out the silica framework serves the purpose of activation and is a much easier and effective method for generating porosity compared to the use of KOH/ZnCl2 or activating gases (CO2/steam), which usually require high temperature and often produce uneven activation within the system. 27, 28, 46, 47

The as-synthesized CAs not only exhibit very high specific surface area, but also an

interconnected 3D hierarchical porous structure which provides a continuous electronic pathway yielding sufficiently high electrical conductivity in the monolith. As a result, the carbon aerogel shows an excellent electrochemical performance displaying a high reversible capacity of 890 mAh g-1 after 100 cycles with coulombic efficiency of nearly 99% at current density of 50 mAg1

. Further it demonstrates an excellent rate capability by maintaining a capacity of ~ 350 mAh g-1

even after 50 cycles at a current density of 1 A g-1.


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2. Experimental 2.1. Synthesis of carbon aerogel The synthesis of carbon aerogel involved two main steps 1) synthesis of carbon/silica composite aerogel by a novel single step sol-gel process, 2) etching of silica phase in order to get a hierarchically structured high surface area carbon. Resorcinol (R), Formaldehyde (F), Tetraethyl orthosilicate (TEOS), APTES (3-aminopropyl tri-ethoxysilane), APTEMS (3aminopropyl tri-methoxysilane) and acetone were used as raw materials. The porosity in micro, meso and macro domains has been tuned by varying the ratio of APTES/APTEMS (RAE/AM), which not only act as reactants but also as catalysts for the sol-gel reaction. Four types of solutions were prepared namely, AE-100 (only APTES), AE-75 (RAE/AM = 75/25), AE-50 (RAE/AM = 50/50) and AM-100 (only APTEMS). All chemicals were obtained from Sigma Aldrich (USA), and were used as received without further purification. For all the samples, the molar ratio of R: F: TEOS was kept constant as 1:2:3. Firstly, 0.8 gm of resorcinol was added to 5 ml of acetone. After mixing of resorcinol, 2.4 ml of TEOS, 350 µl of APTES and 250 µl of APTMS were added into the solution. Finally, 1.6 ml of formaldehyde was added to the solution mixture to start the sol-gel reaction. This solution was kept in a closed beaker for 3-4 hrs to allow the formation of gel, followed by drying in oven at 60°C for 12 h at ambient pressure. The dried RF/silica monolith was then carbonized in argon atmosphere by heating up to 800°C at a ramp rate of 5oC/min with a holding time of 1 hour. The resulting C/silica monolith was then dipped in a 2M NaOH solution for 12 hrs to etch out the silica phase, thus yielding a hierarchically porous carbon aerogel. Afterwards, the carbon aerogel was washed several times with DI water, followed by overnight drying in vacuum at 80°C. Silica removal activates the monolith uniformly leading to an increased surface area. This is a much easier and effective 5 ACS Paragon Plus Environment


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way for porosity generation compared to the use of activating gases, which often result in uneven generation of porosity. The as-prepared CAs are denoted as CA-100, CA-75, CA-50 and CA-M, corresponding to the precursor solutions AE-100, AE-75, AE-50 and AM-100, respectively. 2.2. Material Characterization The surface morphology and microstructure of the carbon aerogel was analyzed using a field emission scanning electron microscope (FE-SEM, Quanta 200, Zeiss, Germany) and transmission electron microscope (TEM, JEM-2100F JEOL, Japan). The crystal structure of the aerogels was characterized by XRD (X’Pert PRO, PANanalytical, Netherlands) using Cu Kα (λ = 1.5416 Å) radiation. The Nitrogen adsorption-desorption isotherms were recorded using an Autosorb-1C machine (Quantachrome, USA) at 77K. The specific surface area and the pore size distribution of the carbon aerogels were estimated using the Brunauer-Emmett-Teller (BET) Barrett-Joyner-Halenda (BJH) and Density functional theory (DFT) methods. The degree of graphitization of the carbon aerogels and carbon/silica aerogel composite was studied by Raman spectroscopy (WiTec, Germany) using laser light of 532 nm wavelength. X-ray photoelectron spectroscopy (XPS) measurements were conducted using Omicron ESCA Probe spectrometer with unmonochromatized Mg KαX-rays (hν=1253.6 eV). Casa XPS software was used to deconvolute the obtained spectra to their component peaks. 2.3. Electrochemical Characterization The electrochemical performance of the monolithic carbon aerogels (CA-100, CA-75, CA-50 and CA-M) was evaluated using 2032 coin cells assembled inside an argon filled glove box. The carbon monoliths, being mechanically stable and electrically conducting were used directly as electrodes without any binder and Cu collector. Lithium foil of ~750 µm (Sigma Aldrich) thick 6 ACS Paragon Plus Environment

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was used as the counter electrode with polypropylene membrane (Celgard 2320) as separator and a 1 M LiPF6 solution in a solvent mixture of ethylene carbonate and diethyl carbonate (EC/DEC, 1:1 (v/v), Merck) as the electrolyte. After allowing 12 hours of ageing time, the cell was galvanostatically cycled between 0.05 to 2.5V at current densities 50, 100, 200, 300, 500 and 1000 mA g-1 using a battery analyzer (MTI Corporation, USA). Cyclic voltammetry was carried out at a slow scan rate of 0.1 mV s-1 whereas impedance measurements were performed in the frequency range 100 kHz to 10 mHz with a perturbation a.c. amplitude of 10 mV using a PGSTAT 302N electrochemical workstation (Metrohm Autolab, Netherlands).



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3. Results and Discussion

Figure 1: (a), (b) and (c) FE-SEM micrographs of the carbon monolith, CA-100 at different magnifications. The inset of (a) shows the monolith in a free-standing form used directly as anode. 8 ACS Paragon Plus Environment

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Figure 2: FE-SEM images of (a) CA-75 carbon monolith, (b) magnified image of ‘a’ showing the surface morphology, (c) CA-50 carbon monolith, (d) magnified image of ‘c’, (e) CA-M carbon monolith, (f) magnified image of ‘e’.

Figure 1 shows the FE-SEM image of the carbon monolith, CA-100, at different magnifications revealing its hierarchical porous structure composed of three different pore size distributions, i.e,



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micropores, mesopores and macropore. It can be observed in Figure 1(a) that the monolith is composed of uniform carbon particles forming aggregates which are interconnected in different directions through macropores, constituting a 3-D continuous carbon framework. Micropores can be observed on the surface of carbon particles. This type of hierarchical porous structure is considered to be beneficial for LIB electrodes, as it facilitates better electrolyte diffusion and electron transport. The inset of Figure 1(a) shows the carbon monolith in a free-standing form, which is used directly as anode without any binder and current collector. Figure 2 (a-b), (c-d) and (e-f) show the SEM micrographs of the carbon aerogels, CA-75, CA-50 and CA-M, respectively, revealing their microstructure.


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Figure 3: TEM images of (a) CA-100 carbon monolith, (b) magnified image of ‘a’ showing the microstructure, (c) CA-75, showing the microstructure, (d) magnified image of ‘c’.



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Figure 4: TEM images of (a) CA-50 carbon monolith, (b) magnified image of ‘a’ showing the microstructure, (c) TEM image of CA-M (d) magnified image of ‘c’.

Figure 3 shows the TEM images of CA-100 and CA-75, whereas Figure 4 shows the TEM images of CA-50 and CA-M carbon aerogels. The mesoporous structure of the aerogels, CA100, CA-75, CA-50 and CA-M, can be observed in Figures 3(a), 3(c), 4(a) and 4(c), 12 ACS Paragon Plus Environment

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respectively. Further, the micropores can be observed on the surface of carbon aerogels (Figures 3(b), 3(d), 4(b) and 4(d)), and are mainly generated by the removal of the silica framework. (In order to take the TEM images, a small amount of the monolith was crushed and dispersed in ethanol, which was then drop cast onto a carbon coated copper grid for visualization). The carbon monoliths showed high mechanical strengths with Young’s modulus in the range of 3-15 MPa, which were calculated from their compressive stress-strain curves (See Supporting Information Figure S1 for stress-strain curves, and Table S1 for the Young’s modulus values of the carbon monoliths). The Young’s modulus obtained for our monoliths is higher than that reported earlier for nanocast and soft template carbon monoliths.48,

49

The high mechanical

integrity possessed by the carbon monoliths allowed them to be used directly as anode without any binder.

Figure 5: Nitrogen adsorption-desorption isotherm of CA-100 and its pore size distribution estimated by BJH method.



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Figure 5 shows the N2 adsorption-desorption isotherm and pore size distribution of the carbon aerogel, CA-100. The N2 adsorption-desorption isotherms and the pore size distributions of the carbon aerogels, CA-75, CA-50 and CA-M are shown in Figure S2, S3 and S3, respectively, of the Supporting Information. All the isotherms exhibit a hysteresis loop in the high pressure region, which is usually associated with capillary condensation in mesopores.50 The BET surface area and pore size distribution of the carbon aerogels are summarized in Table 1. The specific surface areas of the aerogels have been estimated using the multipoint BET method in the normalized pressure range of 0.05 to 0.35. The total pore volume has been calculated from the amount of N2 adsorbed at a normalized pressure close to unity (∼0.994). The mesopore volume is calculated using the Barrete–Joyner–Halenda (BJH) method, whereas for calculating micropore volume, the density functional theory (DFT) method has been used. The specific surface area of the CA-100 aerogel is estimated to be 2620 m2 g-1, with a total pore volume of 1.558 cc g-1, meso and micro pore contributions being 0.365 and 1.101 cc g-1, respectively. The pore size distribution of the carbon aerogels depict the existence of a pore system extending from microporous to the macroporous (Table 1). During the pyrolysis, decomposition of the RF component results in the generation of mesopores and micropores. After carbonization, NaOH etching facilitates the removal of silica framework from the composite and activates the aerogel further, increasing its surface area. Table 1: BET specific surface area and pore size distribution of carbon aerogels. Sample Surface area Total pore Mesopore Micropore 2 -1 -1 -1 (m g ) Volume (cc g ) volume (cc g ) volume (cc g-1) CA-100

2620

1.558

0.3657

1.101

CA-75

1920

1.079

0.1566

0.8647

CA-50

1830

1.210

0.2978

0.7602

CA-M

940

0.510

0.0506

0.4276


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The porosity in the aerogels has been tuned by varying by the ratio of APTES/APTEMS (RAE/AM) during synthesis. It has been reported that increase in APTES content decreases the gelation time.51 APTES acts as a silica source and also as a catalyst for the gelation process. The amine groups present in APTES catalyze the condensation reaction between resorcinol and formaldehyde.52 When only APTES is used (AE-100), gelation occurs fast, which prevents the micropores and mesopores from coalescing, thus inhibiting the formation of macropores. This results in the development of a porous structure dominated by micropores and mesopores. The addition of APTEMS eliminates the mesoporosity and results in a structure predominated by macro and micropores. APTEMS slows down the gelation process due to which a major fraction of micro and mesopores gets converted to macropores due to phase separation. The high specific surface area of the carbon aerogels, resulting from a well developed hierarchical porous structure leads to an enhanced electrode/electrolyte contact. This results in an increased Li storage capability due to the availability of large number of Li active sites.

Figure 6: X-ray diffraction pattern of carbon aerogels CA-100, CA-75, CA-50 and CA-M, showing their amorphous nature. 15 ACS Paragon Plus Environment


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The X-ray diffraction pattern of CAs exhibit a distinct peak around 2θ = 23°, which corresponds to the (002) reflection of the hexagonal graphitic domains (Figure 6). However the relatively broader nature of the peaks and the calculated d-spacing value of 3.9 Å between the (002) planes are in favor of amorphous nature of carbon with low degree of graphitization. As reported in various studies, Li insertion in amorphous carbons occur not only between the graphene layers but also in the nanocavities present in the amorphous region resulting in a potentially higher Li storage compared to graphitic carbons.21

Figure 7: Raman spectra of carbon aerogels CA-100, CA-75, CA-50 and CA-M. The phase composition and the extent of graphitization in the aerogel samples were further studied by performing Raman spectral analysis. Two major characteristic peaks at around 1340 and 1580 cm-1, corresponding to the fundamental D and G bands for carbon are observed in all the CA samples (Figure 7). The appearance of G band is attributed to the vibrations in the 16 ACS Paragon Plus Environment

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graphitic region of carbon whereas the D band originates due to the presence of disorders/defects in the carbon. The peaks were deconvoluted using the Gaussian-Lorentzian function to determine the D/G band intensity ratio (ID/IG) for assessing the graphitic nature of aerogels. The ID/IG for the carbon aerogels CA-100, CA-75, CA-50 and CA-M were calculated to be 1.24, 1.21, 1.19 and 1.26 respectively, indicating partial graphitization found with the largely amorphous carbon framework. 53

Figure 8: (a) XPS scan of the carbon aerogel, CA-100, de-convoluted XPS spectra of (b) C 1s and (c) O 1s. 17 ACS Paragon Plus Environment


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X-Ray photoelectron spectroscopy (XPS) was performed to further analyze the chemical state of carbon and oxygen in the synthesized carbon aerogels. Figure 8 shows the XPS spectrum of carbon aerogel (CA-100) consisting of distinct carbon (C 1s) and oxygen (O 1s) peaks. It can be observed that the C1s peak is asymmetric with a tail towards the high binding energy region of 285-295 eV, suggesting the presence of oxygen containing functional groups on the surface. The C1s spectrum was deconvoluted using Gaussian Lorentzian function to fit three peaks which could be assigned to C-C (sp2) or the aliphatic carbon at 284.8 eV, C-O-R at 286.6 eV and C=O (carbonyl) group at 288.5 eV. The deconvoluted O1s spectrum was fitted as two peaks, corresponding to the presence of single bond oxygen at 532.2 eV and adsorbed water molecule at 535.5 eV.


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Figure 9: Galvanostatic charge/discharge profile of carbon aerogel anodes, (a) CA-100, (b) CA75, (c) CA-50 and (d) CA-M, at current density of 50 mA g-1.



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Figure 10: Cyclic voltammogram of (a) carbon aerogel, CA-100 and (b) carbon aerogel, CA-75. Figure 9 displays the galvanostatic charge/discharge profiles exhibited by the carbon anodes, CA-100, CA-75, CA-50 and CA-M corresponding to their behavior at 1st, 2nd and 100th cycles at 50 mAg-1 current density. As the carbon is amorphous in nature, the process of Li insertion/extraction usually starts at around ~1 V, following a sloping voltage curve.54 In the first discharge cycle, a plateau is observed between 1.0 to 0.5 V, which is attributed to the electrolyte decomposition and formation of a solid electrolyte interface (SEI) layer on the electrode surface. This is consistent with the reduction peak observed in CV in the first cycle between 1.0-0.5 V (Figure 10), performed at a slow scan rate of 0.1 mV/s in the 0.01-3V potential window. As can be seen in Fig 7, the initial discharge/charge capacities of the carbon aerogel electrodes CA-100, CA-75 CA-50 and CA-M are 1000/1540, 750/1240, 645/1140 and 520/890 mAh g-1 respectively at 50 mA g-1. The irreversible capacity loss (ICL) exhibited by the aerogels is mainly due to the partially irreversible consumption of Li in SEI formation on a large specific area carbon surface. Also, the Li entrapment in the nanocavities present in the disordered carbon structure, which is a common phenomena associated with such type of carbons, contributes towards the irreversible capacity loss.55, 56


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Figure 11: Cycling performance of the carbon aerogels: CA-100, CA-75, CA-50 and CA-M at current density of 50 mA g-1.

The electrochemical stability of carbon aerogel anodes has been evaluated by running the cells for 200 cycles at 50 mA g-1 (Figure 11). After displaying an initial columbic efficiency of ~ 65 %, CA-100 electrode exhibits a highly stable cycling performance maintaining a coulombic efficiency of ~99 % from 2nd cycle onwards. The CA-100 electrode delivers a capacity of 890 mAh g-1 at the end of 100 cycles with an overall capacity fade of only 12%. This remarkably large reversible specific capacity of CA-100 and the corresponding stable cycling performance can be attributed to the high specific surface area of carbon containing hierarchically arranged porous framework, which effectively buffers the volume changes during charge-discharge cycles to form a stable SEI. Usually the repeated expansion/contraction of a high capacity anode generates mechanical strain within the SEI layer that results in the formation of solvent



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permeable cracks within SEI, thus leading to undesirable SEI growth which irreversibly consumes Li from the cell and reduces its capacity upon cycling. The reduced volume expansion of CA-100 electrode upon lithiation and delithiation results in good mechanical and electrochemical stability of the SEI. As a result, low capacity fade has been observed in CA-100 anode compared to the other anodes, i.e., CA-75, CA-50 and CA-M which deliver specific capacities of 540, 440 and 380 mAh g-1 at the end of 100 cycles with a corresponding capacity fade of 28%, 30% and 26%. All the observed capacities are however higher than that of the graphite electrode, which is quite interesting. 57,58 As the overall capacity is mainly dependent on the accessible Li storage sites in the host material, the currently discussed CA electrodes with high porosity and a large surface area allow good penetration of electrolyte into the deep and inner areas of the electrode, giving rise to better contact between the electrode and electrolyte, thus demonstrating a high capacity behavior.

Figure 12: Rate performance of the carbon electrode CA-100 at current densities: 50, 100, 200, 300, 500 and 1000 mA g-1. 22 ACS Paragon Plus Environment

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As the CA-100 aerogel displayed an excellent cycling performance, we further investigated its electrochemical performance by conducting the rate capability experiment. Figure 12 displays the rate performance of CA-100 electrode, conducted at different current densities. The aerogel delivers capacities of 850, 730, 640, 480 and 350 mAh g-1 at current densities 100, 200, 300, 500 and 1000 mA g-1 respectively. On lowering the current back to 50 mA g-1, the electrode resumes a capacity of 950 mAh g-1, demonstrating excellent resilience to current change. The remarkable rate performance can be attributed to the highly porous structure that results in nanometer sized Li diffusion lengths, which facilitate nearly complete lithiation even at high charge/discharge rates. Further, the interconnected 3D carbon framework provides a continuous electronic pathway ensuring reasonably high electrical conductivity in the electrode, necessary for fast charge transfer kinetics.

Figure 13: Nyquist plot of carbon electrode CA-100 between the frequency range of 100 kHz to10 mHz: in the as fabricated state, and at the end of 5th, 50th, 80th, 100th and 200th discharge cycles.



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To investigate upon the structural evolution and impedance variation in the CA-100 electrode during cycling, electrochemical impedance spectroscopy (EIS) was performed on the half cell in the as-fabricated state, and at the end of 5th, 50th, 80th , 100th and 200th discharge cycles (Figure 13). All the measurements were recorded in the fully relaxed state of the cell, when the open circuit potential had reached 0.05 V. In each Nyquist plot, a semicircle was observed in the HF region which represents the impedance to Li-ion migration through the SEI layer and the charge transfer resistance encountered at the electrode/electrolyte interface. The intercept on the real axis in the HF region denotes the ohmic resistance due to the electrolyte solution. The sloping beeline in the low-frequency region is attributed to the Li diffusion in the bulk of the electrode. The equivalent circuit (shown as inset of Figure 13) constructed is comprised of series of two resistors and a constant phase elements in parallel with a Warburg diffusion element. As can be observed in Nyquist plots, the impedance of the cell first increases initially from the as fabricated state (due to SEI formation). With the gradual thickening of SEI, the resistance to charge transfer increases, which is reflected in the initial enlargening of semicircles. However, as can be seen in the Nyqusit plot, stable SEI is formed after a few cycles, through which Li ions can move easily. Further, it can be observed that there is a change in the slope of the beeline in the low frequency region from 50th to 80th cycle. This is mainly due to an uneven diffusion through SEI during the early cycles when SEI is still forming. However, with the formation of a stable SEI, nearly identical slopes are observed in the low-frequency region for 80th, 100th and 200th cycles. The hierarchical porous structure of the electrode effectively buffers the volumetric strains generated during long cycling, leading to the formation of a stable, defect free SEI. The decreasing Rct value with the increasing cycle number is in favor of cycling and structural stability of chosen CA-100 electrode.


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4. Conclusions In summary, a mechanically stable carbon monolith with a high specific surface area and an interconnected hierarchical porous structure has been fabricated by an efficient, low-cost, one step sol-gel process with potential for up scaling. The monolith is used directly in the freestanding form as anode in Li-ion half cell, without any binder, current collector or conducting agent, which are inactive components, responsible for weight penalty issues. The continuous porous carbon framework with interconnected walls ensures high electrical conductivity in the monolith as well as facilitates faster Li diffusion in the electrode. Further, the high specific surface area provides a large number of accessible Li storage sites. The aerogel thereby exhibits a remarkable electrochemical performance with high reversible capacity (890 mAh g-1 after 100 cycles), a low capacity fade (only 12% after 100 cycles) and an excellent rate capability (350 mAh g-1 at 1A g-1). The optimal combination of porosity, specific surface area, electrical and mechanical properties, make the monolith potentially attractive for a variety of applications like energy storage, catalysis, double layer capacitors, fuel cells, etc.

Acknowledgement The support received from Department of Science and Technology (DST), New Delhi to Nanoscience Center at IITK is gratefully acknowledged.



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Supporting Information Available: Compressive stress-strain curves of the carbon aerogels: CA-100, CA-75, CA-50 and CA-M (Fig S1) ; Tabulated Young’s moduli value of the monoliths calculated from their respective stressstrain curves (Table S1) ; Nitrogen adsorption-desorption isotherms of CA-75, CA-50 and CA-M and their pore size distribution estimated by BJH method (Figure S2-S4).


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Facile Synthesis of Hierarchical Porous Carbon Monolith: A Free

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