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Anomalously High Lithium Storage in Three-Dimensional Graphene-Like Ordered Microporous Carbon Electrode Yonghyun Kwon, Kyoungsoo Kim, Hongjun Park, Jae Won Shin, and Ryong Ryoo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00081 • Publication Date (Web): 18 Feb 2018 Downloaded from http://pubs.acs.org on February 19, 2018

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The Journal of Physical Chemistry

Anomalously High Lithium Storage in ThreeDimensional Graphene-Like Ordered Microporous Carbon Electrode Yonghyun Kwon,†,‡ Kyoungsoo Kim,§ Hongjun Park,†,‡ Jae Won Shin,† and Ryong Ryoo*,†,‡ †

Center for Nanomaterials and Chemical Reactions, Institute for Basic Science (IBS), Daejeon

34141, Republic of Korea ‡

Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST),

Daejeon 34141, Republic of Korea. §

Department of Chemistry, Chonbuk National University, Jeonju 54896, Republic of Korea.

ACS Paragon Plus Environment

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ABSTRACT.

Zeolite-templated carbon, having 3-D graphene-like ordered microporous structure with high electrical conductivity, is a fascinating anode material for Li-ion battery (LIB). Herein, we report an extremely high Li capacity of 2,950 mAh g-1 (equivalent to Li1.3/C), which is 7.9 times the maximum capacity of graphite, Li/C6. This is equivalent to the crowded packing of 20 Li+ per pore with 0.9-nm diameter. Approximately 59% of the capacity was reversible. According to the characterizations using electron energy loss spectroscopy, 7Li NMR, and 13C NMR, most of the Li species existed as Li+ within the carbon micropores. Contrary to the often-made assumption, only a small amount of solid-electrolyte interphase layer was detected at the external surface of the carbon particles, but not inside the micropore. The anomalously high Li capacity is attributed to the extremely narrow pore environment, where Li+ would be difficult to be fully solvated. Tailoring of the carbon pores to a subnanometric range would therefore be exciting for future advancement of LIB.


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1. INTRODUCTION In response to rapidly increasing demand for mobile electronic devices and electric vehicles, it is important to find an electrode material for lithium-ion battery (LIB) that is capable of storing a large amount of electrical energy per unit mass or volume with a short charging time.1 In this context, graphite, the most common anode material for commercial LIBs, encounters relatively low capacity (less than 372 mAh g-1) and rate capability.2 Thus far, a great deal of effort has been performed toward development of carbon anode materials with energy and power density better than those of graphite.3-8 Nanostructured carbons with high porosity, such as ordered mesoporous carbons, porous carbon aerogel and MOF-derived carbons, were investigated to meet the requirements.9-12 Since a nanoporous structure increases the contact area between the electrolyte and electrode, and facilitates electrolyte transport, a large amount of Li can be easily accommodated within even under fast charging conditions. Moreover, the surface structural features in the nanostructured carbons (e.g., pore walls, surface edges, and defects) can contribute to a large Li capacity via physicochemical interactions that are different from the Li-intercalation process in graphite.13-18 The efficiency of porous carbons, however, is still in question because a significant portion of the high capacity is irreversible. Ironically, a major portion of this irreversible capacity loss is known as caused by wasting lithium for the formation of solid-electrolyte interphase (SEI) layers on the large surface area.19 It is reported that the irreversible capacity loss increases with the carbon surface area.19 However, we show in the present work that this is not the case, at least when carbon pores become sub-nanometrically small. Normally, nanoporous carbons are built of irregular arrangement of pores with various diameters and shapes. The disordered carbon structure makes it difficult to study lithium storage behaviors


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related to the SEI formation. To study the problem, it is highly desirable to have ordered nanoporous carbons with uniform pore environment and well-defined structure. Among such ordered nanoporous carbons, ‘zeolite-templated carbons’ are the only currently available materials that have subnanometric pore diameters.20 The carbons have a three-dimensionally (3-D) ordered microporous structure with a very large surface area built with negatively curved single-layer graphene-like framework.21,22 The negative Gaussian surface curvature is expected to provide high binding affinity and a low energy barrier for diffusion of lithium ions, according to a recent computation work by Odkhuu et al.23 The zeolite-template carbons are now readily available through a large-scale synthesis route using La3+, Y3+ or Ca2+ catalyst in zeolite.21,22 In the present work, we measured Li storage capacity of a carbon, which was synthesized using Ca2+ ion-exchanged zeolite faujasite-Y. The zeolite-templated carbon exhibited a total capacity of 2,950 mAh g-1. The reversible portion was 1,550 mAh g-1, which was comparable to the highest capacity reported so far with carbon alone. Despite the high irreversible capacity of 1,400 mAh g1

(i.e., 2,950 - 1,550 mAh g-1), the surface of the carbon pores (diameter = 0.9 nm) did not indicate

the formation of SEI layers. It was quite puzzling how so much reversible and irreversible Li could be stored in the subnanoporous carbon. To answer the question, we investigated the chemical state (e.g., ionic or metallic) of the Li species using 7Li solid-state nuclear magnetic resonance (NMR) spectroscopy, and also the lithium location (inside pores or on external surfaces) with a highresolution transmission electron microscope (TEM) equipped with an energy-dispersive X-ray spectrometer (EDS) and an electron energy loss spectrometer (EELS).24


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2. EXPERIMENTAL Carbon synthesis. Y zeolite, used in carbon synthesis, was synthesized in a Na+ ionic form according to a procedure in the literature.25 The Si/Al molar ratio of Y zeolite was 2.3, which was obtained by inductively coupled plasma atomic emission spectroscopy. Prior to carbon synthesis, zeolites were ion exchanged with Ca2+, in the same manner, reported previously.26 After Ca2+ ion exchange, the zeolites were filtered, dried, and calcined at 550 °C in air. Carbon was synthesized following the procedure of Kim et al.22 with modifications of temperature, gas composition, and carbon deposition time due to the difference in the zeolite structure and synthesis batch size. Ethylene was used as a carbon source. 10 g of Ca2+-zeolite was placed in a vertical plug-flow fused-quartz reactor, which was equipped with a fritted disk of 75mm diameter. The reactor temperature was raised to 600 °C at a rate of 3 °C min-1 under a dry N2 flow. At 600 °C, the N2 flow was combined with ethylene gas and switched to pass through a water bubbler before reaching the zeolite bed. The temperature of the bubbler was kept at 20 °C for a constant moisture content. The N2/ethylene/H2O volumetric ratio was controlled to 86/12/2 by a mass flow controller (M2030V, Line Tech). The total gas flow rate was controlled to 820 cm3 min1

. The gas flow at 600 °C was continued for 4 h, for full carbon deposition. After the full deposition of carbon, the gas flow was switched back to dry N2. The temperature

was increased to 900 °C, and maintained there for 2 h, to crosslink the carbon structure fully. After cooling to room temperature, the resultant zeolite/carbon composite sample was treated twice with a 0.3 M HF/0.15 M HCl solution in order to remove the zeolite framework. The carbon product was dried at 100 °C after filtration and washing with 20% ethanolic water solution. The carbon yield was 0.35 g g-1 of zeolite.


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Materials characterization. Powder X-ray diffraction (XRD) data were collected on a Rigaku Multiplex instrument using Cu Kα radiation (30 kV, 40 mA). Ar adsorption-desorption isotherms were measured at liquid argon temperature (87 K), using a Micromeritics ASAP 2020 volumetric adsorption analyzer. The Brunauer-Emmett-Teller (BET) equation was used to calculate the specific surface area, using the adsorption data points in a relative pressure range of 0.05 ~ 0.15. Pore size distribution and pore volume were determined by quenched solid densityfunctional theory (QSDFT), assuming a slit-shaped pore geometry. Micropore volume was determined from the DFT cumulative volume in the pore diameter range of d < 2 nm. Total pore volume was determined at P/P0 = 0.97. Raman spectrum was recorded on a Horiba Jobin Yvon ARAMIS spectrometer with a laser excitation wavelength of 514 nm. X-ray photoelectron spectroscopy (XPS) was performed using a Thermo Fischer Scientific Sigma probe with a monochromated Al Kα X-ray source operated with an emission current of 10 mA, pass energy of 30 eV, and beam area of 400 μm. The elemental contents were determined using a Thermo Fischer Scientific Flash 2000 series elemental analyzer. Scanning electron microscope (SEM) images were taken with an FEI Verios 460 instrument at a landing voltage of 1 kV in deceleration mode (stage bias voltage = 5 kV). TEM and scanning TEM (STEM) images were collected with an FEI Titan E-TEM G2 instrument operated at an acceleration voltage of 300 kV. EDS and EELS analyses were performed in diffraction mode using EDAX #PV97-61850-ME and Gatan US1000XP-P imaging spectrometers equipped in the FEI Titan E-TEM G2 instrument. For EELS, K-edges were taken for both C and Li. The lithiated and delithiated samples for electron microscopy were recovered and washed with anhydrous dimethyl carbonate (DMC) in an Ar-filled glove box, and then placed onto a lacey carbon Cu grid mounted


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on a TEM sample holder. The sample-loaded TEM holder was quickly transferred to the TEM while being sealed in an air-proof container. The time of exposure to air was less than 2 s. 7

Li and 13C solid-state NMR spectra were acquired with a magic-angle spinning (MAS) rate of

12 kHz using a Bruker Digital Avance HD 400WB spectrometer at 9.4 T and at room temperature. The observed frequencies for 7Li and 13C were 92.41 and 100.613 MHz, respectively. The 7Li and 13

C chemical shifts were calibrated indirectly using LiCl and glycine as external references. 7Li

MAS NMR spectra were recorded with a 90° pulse of 3.0-μs width, a relaxation delay of 5 s, and 100 scans. 13C MAS NMR spectra were recorded with a 90° pulse of 3.8-μs width, a relaxation delay of 5 s, and 30,000 scans. The lithiated and delithiated samples for NMR spectroscopy were retrieved from the coin cell, and washed with anhydrous DMC in an Ar-filled glove box. After washing, the samples were dried in a vacuum chamber equipped in a glove box at room temperature. The dried sample was transferred into a 4 mm MAS NMR rotor and gas-tightly closed with a rotor cap.

Electrode fabrication and electrochemical measurement. Electrodes were prepared by slurry coating. Zeolite-templated carbon was used as an active material. Carbon black (Super-P, TIMCAL Graphite & Carbon) was used as a conducting agent. Poly(vinylidene fluoride) (SigmaAldrich) was added as a binder. These materials were blended homogeneously in N-methyl-2pyrrolidone (99.0%, Tokyo Chemical Industry) solvent using an agate mortar. The optimized weight ratio between the active material, conducting agent, and binder was 85:5:10. Using a doctor blade, the blended slurry was coated to a thickness of 25 μm on a Cu foil (Wellcos Cor.) and dried at 80 °C under vacuum for 12 h. Areal loading weight of the active material was around 0.5 mg cm-2. For electrochemical measurements, the film was punched out to make discs with 16 mm diameter. Coin cells (CR2032, Wellcos Cor.) were assembled in a high purity Ar-filled glove box


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with low contents of O2 and H2O (< 1 ppm). Li foil (450 μm of thick, Wellcos Cor.) was used as the counter electrode. 1 M LiPF6 in ethylene carbonate (EC)/DMC (1:1 in volume ratio, MTI Cor.) was employed as the electrolyte. A porous polypropylene film (Celgard 2400, Celgard) was used as the separator for the coin cells. Galvanostatic charge/discharge tests were carried out at a voltage window of 0.005 ~ 3.0 V vs. Li/Li+ on a battery cycle tester (TOSCAT-3000, TOYO).

3. RESULTS AND DISCUSSION 3.1. Ordered microporous carbon of single-layer 3-D graphene-like framework. Figures 1a,b show electron microscopic images of the Y zeolite-templated carbon used in the present work. As shown in the SEM image in Figure 1a, the carbon has crystal morphologies, which were inherited from the zeolite template. Each crystal diameter in this powdery sample ranges from 100 to 600 nm. The TEM image in Figure 1b exhibits well-defined lattice fringes with a spacing of 1.33 nm, indicating that the carbon could inherit the highly ordered microporous structure from the zeolite template (Figure 1b). The lattice fringes are consistent with a sharp peak appearing at 2θ = 6.5° in the powder XRD pattern (Figure S1a). The structure of this carbon can be represented by the electron density map in Figure 1c, which was obtained via an X-ray singlecrystal diffraction analysis in a previous study.21,22 The structure is built of a single-layer 3-D graphene-like framework with periodic surface curvatures. The sp2-hybridized carbon nature can be confirmed by 13C solid-state MAS NMR spectroscopy (Figure S1b).21,,27 In this structure, the carbon layer separates surfaces on both sides. The outer surface indicated in Figure 1c is open for adsorbates and electrolytes, but the inner surface is not accessible because of narrow pore necks. The accessible surface area determined by the BET method is 2,770 m2 g-1. The accessible micropore diameter is 0.92 nm (Figure S2).


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Figure 1. Structure of 3-D graphene-like ordered microporous carbon. (a) SEM image of the Y zeolite-templated carbon. (b) TEM image (main picture) and Fourier diffractogram (inset) of the carbon. (c) 3-D electron-density map of carbon framework formed within pore channels of FAUtype zeolite. The electron-density map reproduced with permission from ref. 21. The iso-surface level of the electron density is set to 0.25 electrons per Å 3 (green) and 0.35 electrons per Å 3 (magenta). Light gray areas represent cross-sectional cuts.

3.2. LIB anode performance. The electrochemical performance of the Y zeolite-templated carbon as an LIB anode was investigated using a coin-type half cell with Li foil as a counter electrode. Figure 2a shows the galvanostatic charge/discharge profiles of the carbon electrode, which were repeatedly measured in cycles with a voltage window of 3.0 ~ 0.005 V at a current density of 0.1 A g-1. The profiles showed a very high initial discharge capacity (i.e., total lithiation capacity) of 2,950 mAh g-1, but


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the subsequent charge capacity (i.e., reversible capacity) decreased to 1,750 mAh g-1. As the cycles were repeated five times, the reversible capacity became to stabilize at a level of 1,550 mAh g-1. Thereafter, the carbon maintained the same capacity over 100 cycles with a high coulombic efficiency of nearly 100% (Figure 2b). Upon increasing the current density to 0.2, 1.0 and 5.0 A g-1 successively, the reversible capacity decreased to 1,290, 970 and 680 mAh g-1 (Figure 2c and Figure S3). The capacity loss at high current density was somewhat larger than that of mesoporous carbon electrodes.28 This can be attributed to slow diffusion of electrolytes through small pores in the case of the zeolite-templated carbon.29 Nevertheless, the value of 680 mAh g-1 is still remarkable for the capacity of a carbon electrode at a high current density of 5.0 A g-1. The cyclability at high current is also quite high (530 mAh g-1 even after 500 cycles in Figure 2d). The high cyclability indicates that the 3-D graphene-like microporous structure was stable during the repeated lithiation and delithiation processes. When the carbon sample after 5 cycles was retrieved from a discharged coin cell in an argon glove box, SEM images of the sample did not show any distinct changes in the particle morphology, compared with the pristine carbon (Figure S4a). The TEM image of the 5-cycled sample still exhibited ordered lattice fringes with a spacing of 1.35 nm, similar to the pristine sample (Figure S4b). The spacing of the lattice fringes was in good agreement with an XRD peak appearing at 2θ = 6.4° (Figure S4c).


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Figure 2. LIB anodic performances of the Y zeolite-templated carbon. (a) Galvanostatic charge/discharge profiles measured at a current density of 0.1 A g-1. (b) Cycling performance measured at a current density of 0.1 A g-1. (c) Rate performance measured at various current densities increasing from 0.1 to 5.0 A g-1. (d) Cycling performance measured at a current density of 5.0 A g-1.

A notable characteristic of the zeolite-templated carbon as an LIB anode is the absence of a constant voltage region during the reversible lithiation/delithiation periods. The charge/discharge profiles after the initial cycle exhibited continuous changes of voltage against the uptake or release of lithium throughout the entire range of capacity. This is in a distinct contrast to the behavior of graphite exhibiting a well-defined voltage plateau around 0.1 V over a wide range.30 The charge/discharge characteristic of the zeolite-templated carbon electrode providing varying cell voltage would be a drawback in practical LIB applications. It is analogous to the electrical doublelayer capacitive behavior, accommodating ions on the surface of the charged electrode via electrostatic attraction. Similar phenomena were observed in carbon nanotubes and graphene-


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based porous carbons.31-33 It is also noteworthy that the zeolite-templated carbon exhibited a large irreversible loss of capacity (i.e., 2,950 - 1,750 = 1,200 mAh g-1 during the initial cycle). The irreversible portion amounted to 41% of the total capacity, despite the structural stability of the carbon. Such an irreversible capacity loss is usually very small in graphite electrodes. In contrast, it can account for more than 60% of the total capacity in carbon nanotubes, reduced graphene oxides and nanoporous carbons.31,34,35 Generally, the capacity loss in a carbon LIB anode is attributed to the formation of SEI at surfaces, which is known to occur through the decomposition of electrolytes and solvents.36,37 In a case with a significant amount of SEI, the SEI formation is indicated by a constant voltage plateau or a shoulder around 0.7 V during the initial discharge cycle. The capacity loss corresponding to the plateau region (i.e., 950 - 500 = 450 mAh g-1 in Figure 2a) does not seem to be enough to account for the loss of 1,200 mAh g-1. The SEI formation and other possible causes for the irreversible capacity will be investigated in more detail in the following sections.

3.3. Location and chemical state of Li 3.3.1. EELS and EDS elemental analyses. Figure 3 shows STEM-EELS-mapping images for Li and C elements in the zeolite-templated carbon sample. The images in Figure 3a-c were taken from the carbon that was completely lithiated along the initial discharging curve in Figure 2a. Figure 3d-f was obtained after full delithiation along the subsequent charging curve. For each of the mapping experiments, the carbon sample after discharging or charging was retrieved from the coin cell and loaded onto a TEM holder inside a glove box to prevent migration of the Li species (see EXPERIMENTAL for details). According to the resultant mapping images, the lithium distribution in the carbon particles was always uniform, no matter whether lithiated or delithiated. The concentration of the lithium was approximately twofold higher in the lithiated case when


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estimated from the EELS intensity (see also Figure S5). This indicates that both the lithiation and delithiation occurred uniformly within the microporous carbon particles, and that approximately half the lithium still remained inside the carbon particles even after full delithiation. Hence, the remaining lithium is regarded the cause of the irreversible portion of the initial discharge capacity, i.e., (2,950 - 1,750)/2,950 = 0.41.


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Figure 3. STEM-EELS elemental analysis of the Y zeolite-templated carbon electrode. (a) STEM image of the carbon sample obtained after the first full lithiation. (b,c) EELS mapping images for C (b) and Li (c) of the lithiated sample. (d) STEM image of the carbon sample obtained after the subsequent full delithiation. (e,f) EELS mapping images for C (e) and Li (f) of the delithiated sample.

Figure 4. STEM-EDS elemental analysis of the Y zeolite-templated carbon electrode. (a) STEM image of the carbon obtained after the first lithiation/delithiation cycle. (b) EDS line scan profiles of the C, O, F, and P K-edge signal measured along the red line shown on the left side image.


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In a number of previous LIB studies using graphite and nanoporous carbons, TEM images often showed SEI layers of about 10 ~ 50 nm thickness deposited at the carbon surfaces.38,39 The SEI layers were attributed to a major cause of irreversible capacity. In the present work, it was difficult to detect such SEI layers on the zeolite-templated carbon, even by using high-resolution STEM. For a more accurate investigation, the delithiated carbon sample was EDS line-scanned for C, O, F and P elements, which were known as major components of SEI. The EDS scanning was carried out from the exterior toward the core of a carbon particle along the line shown in Figure 4a. In Figure 4b, the resultant scan profiles show a layer of 15-nm thickness containing O and F at the exterior of the carbon particle. The relative atomic ratios of O and F with respect to C in this layer were significantly higher than those in the interior of the carbon particle. The XPS analysis indicates that the exterior layer consisted of mostly Li2CO3 with minor components of Li2O, LixPOyFz, and LiF (Figure S6). On this account, the 15-nm layer is attributed to SEI. However, considering the very small thickness on the external surfaces alone, the amount of SEI cannot account for the large irreversible capacity. Much of the irreversible capacity is attributed to the residual lithium remaining inside the carbon particles after delithiation. Based on the inductively coupled plasma analysis of the delithiated carbon, the irreversible, internal lithium can be calculated as 7 Li per carbon micropore (calculation details in Supporting Information). The micropore diameter is 0.92 nm according to the QSDFT analysis of Ar adsorption. The micropore would be too narrow to accommodate so many Li+ if every Li+ were associated with solvent molecules (the EC solvent is 0.57 nm in molecule diameter, and DMC is 0.80 nm)40 and accompanied by a bulky counter anion (i.e., PF6-).41 Furthermore, when fully lithiated, the lithium loading should increase to 20 Li+ per carbon cage. It would then be unreasonable to put all the lithium into the very narrow pore, unless the lithium existed as completely or partially desolvated


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without a counter anion. The desolvation possibility can be ascribed to the extremely narrow pore diameter, smaller than a fully solvated Li+. Lithium ions, driven by the electrochemical potential, seem to be desolvated in order to diffuse into the narrow pores. A similar desolvation phenomenon of ions within subnanometer pores was proposed for anomalously high electrical double-layer capacitance by Chmiola et al.42 If this is the case, the Li+ in the subnanometer pores would become solvent-deficient. The solvent deficiency would be important in preventing SEI formation, for which the solvent molecules are required. This agrees with the aforementioned EDS line-scanning result, indicating that a SEI is formed dominantly on the external surface. A tiny amount of the SEI elements existing inside the carbon would be the result of solvated Li+ penetration into the defective mesopores, of which volume is only ca. 7% of the total pore volume (see Figure S2). The subnanometer porosity appears to be critical. Even in other carbon materials with 4-nm mesopores, the irreversible portion of the lithiation capacity was more than 60% due to the formation of the SEI on the mesopore wall (Figure S7, and see also ref. 43).

3.3.2. 7Li and 13C NMR investigation. The chemical state of the lithium in the zeolite-templated carbon was investigated with 7Li solid-state MAS NMR spectroscopy. The NMR spectrum was taken in a gas-tight rotor, into which the sample was transferred after lithiation or delithiation inside a deoxygenated glove box. The 7Li NMR spectrum of the carbon taken after the first full lithiation in Figure 2a exhibited a sharp peak centered at 1.1 ppm (Figure 5a). This peak can be assigned to a Li+ state existing on the carbon surface.44,45 No NMR peaks indicating metallic lithium were detected. After full delithiation, the chemical shift changed slightly to 0.1 ppm. The NMR peak intensity decreased to 47%, which indicated that 53% of Li was removed during the delithiation process. The reversibly removable fraction of lithium, estimated from the 7Li NMR data, was very similar to the ratio of the galvanostatic capacities of the carbon (i.e., reversible


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capacity/total capacity = 0.59). The 7Li NMR results were reproduced during the initial 5 cycles of the lithiation/delithiation processes (Figure S8). In graphite LIB anodes, Li+ is intercalated between adjacent graphene layers with a narrow interlayer spacing of 0.37 nm.46 Under the tightly intercalating environment, the Li+ can share the electron density in the 2-dimensional carbon conduction band via delocalization with the empty Li 2s orbital, to a significant extent. The C-Li electronic interaction makes the Li+ possess a partially metallic character. Due to this effect, the graphite-intercalated Li+ is known to exhibit a fairly large chemical shift (e.g., 40 ppm for the LiC6 composition), similar to the Knight shift in metals.47 Contrary to the graphite-intercalated lithium, our zeolite-templated carbon exhibited a negligible chemical shift. The NMR results indicate that the lithium in the carbon existed in a nonmetallic +1 oxidation state, no matter whether reversibly or irreversibly stored. The lithiation did not affect the ordered microporous structure of the carbon, as indicated by the lattice fringes in the TEM image (Figure S4b). In the ordered microporous structure, the pore is single-walled by a curved graphene-like layer containing defects.21,22 There is not a significant amount of multiwalled graphitic domains that can accommodate Li+ via intercalation. The Li+ is supposed to be adsorbed onto the pore-wall surface wall rather than intercalated. This seems to explain why the 7

Li NMR spectrum exhibited a chemical shift similar to that of the lithium ion.


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Figure 5. Solid-state NMR characterization of the Y zeolite-templated carbon electrode. (a) 7Li and (b) 13C solid-state MAS NMR spectra of the Y zeolite-templated carbon electrode obtained after the first full lithiation and subsequent delithiation. The unlithiated sample was additionally compared in (b). Asterisks in the 7Li and

13

C NMR spectra indicate spinning sidebands. The

inverted triangles in the 13C NMR spectra of the lithiated and delithiated samples mark the peaks of alkyl and carbonate groups of the SEI layer components.48

As calculated in the previous section, there were approximately 20 Li+ per carbon pore of 0.92nm diameter when the carbon was fully lithiated. The Li+ would require the same number of negative charges to balance. If the negative charges were provided by the anions existing in the electrolyte solution, i.e., 20 PF6-, the pore would be far too narrow to host all these ions. A rational possibility for the presence of 20 Li+ is that the carbon framework could be negatively charged. Regarding this possibility, the chemical state of the carbon framework was investigated with solidstate 13C MAS NMR spectroscopy. The 13C NMR spectrum was measured in a gas-tight rotor in the same manner as used for the aforementioned 7Li NMR. According to the

13

C NMR results

(Figure 5b), the pristine carbon sample exhibited a broad signal over 115 ~ 145 ppm, with a peak at 123 ppm and a shoulder at 130 ppm. This NMR signal could be assigned to sp2 carbon atoms


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constituting the curved 3-D graphene-like framework.21,49 After lithiation, the intensity of the 123ppm peak decreased markedly while a new peak appeared at 108 ppm. The appearance of the upfield-shifted peak can be interpreted as a result of negative charge built-up at the carbon by galvanostatic discharging for lithiation.50 It is known that negative charges can be localized on defects of graphene sheets.51,52 Compared with the planar graphene, the zeolite-templated carbon possesses very narrow micropores with severe surface curvatures, and probably contains 5-, 7- or 8-membered ring defects.21 This seems to make the present carbon accommodate negative charges effectively, and accordingly, much Li+ could be stored via the coulombic interactions with the negatively charged carbon framework. As judged from the intensity of the 108-ppm

13

C NMR

peak, a fairly large portion of the negative charges seemed to remain on the framework after delithiation. This is consistent with the aforementioned elemental analysis and EELS mapping data indicating 41% irreversible lithium.

4. CONCLUSIONS In summary, the zeolite-templated single-wall 3-D graphene-like, microporous carbon exhibited a galvanostatic Li storage capacity of 2,950 mAh g-1. This is a huge capacity of 7.9 times the maximum capacity of graphite. Of the total lithium capacity, approximately 60% was reversible and 40% irreversible. Our STEM-EELS-NMR results indicated that the lithium storage occurred mostly inside micropores. This was not only for the reversible portion of the lithium, but also even for most of the irreversible portion. All the lithium was in a distinct +1 oxidation state, unlike the pseudometallic lithium state in the case of graphite intercalation. The irreversible formation of SEI at carbon surfaces, which was often assumed in LIB studies, was difficult to account for the irreversible lithium located in the internal micropores. If we neglect the SEI formation at exterior


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surfaces, the total lithium capacity becomes 20 Li+ per micropore of the carbon. Even if we consider the 60%-reversible amount only, each of the very narrow carbon micropores, 0.92 nm in diameter, should contain as many as 13 Li+. This anomalously high loading of lithium led us to conclude that the lithium could exist as completely or at least partially desolvated Li+, without the PF6- counter anions. The micropore apertures could be too narrow for free diffusion of the solvated Li+ and even for the solvent molecules, which are required for the SEI formation. Compared with graphite, where Li+ is intercalated between graphene layers, the zeolite-templated carbon seems to store a large amount of Li+ by adsorption on the negatively charged carbon surface of the pore walls. Although the reversible lithium capacity (1,550 mAh g-1) is very high for carbon, a disadvantage for practical LIB application is its varying cell voltage, as in other porous carbons. Nevertheless, the LIB anodic characteristics of the present zeolite-templated carbon shed new light on the Li storage phenomena in subnanometric porous carbon environments. The present information would be useful for future applications of nanoporous carbons to lithium-ion hybrid capacitors, if not immediately to LIB.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: xx.xxxx / acs.jpcc.xxxxxxx. XRD pattern and 13C NMR spectrum of Y zeolite-templated carbon (Figure S1); Ar sorption isotherm and pore size distribution of Y zeolite-templated carbon (Figure S2); Galvanostatic charge/discharge profiles of Y zeolite-templated carbon (Figure S3); Electron microscopic images of Y zeolite-templated carbon retrieved after initial 5 cycles (Figure S4); STEM-EELS spectra of Y zeolite-templated carbon obtained after full lithiation and subsequent delithiation (Figure S5); XPS spectra of the fully lithiated Y zeolite-templated carbon (Figure S6); LIB anode performance and SEI analysis of mesoporous carbon CMK-3 (Figure S7); 7Li NMR spectra of Y zeolite-templated carbon obtained after the initial 5 cycles (Figure S8); Quantification details on Li accommodated in Y zeolite-templated carbon (Supplementary Note).

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (R.R.) ORCID Yonghyun Kwon: 0000-0002-2703-3692 Kyoungsoo Kim: 0000-0002-2042-0913 Hongjun Park: 0000-0002-8978-7342


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Ryong Ryoo: 0000-0003-0047-3329 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by IBS-R004-D1. REFERENCES (1) Etacheri, V.; Marom, 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) Shu, Z. X.; McMillan, R. S.; Murray, J. J. Electrochemical intercalation of lithium into graphite. J. Electrochem. Soc. 1993, 140, 922-927. (3) Wu, Y. P.; Rahm, E.; Holze, R. Carbon anode materials for lithium ion batteries. J. Power Sources 2003, 114, 228-236. (4) Yoshio, M.; Wang, H.; Fukuda, K.; Hara, Y.; Adachi, Y. Effect of carbon coating on electrochemical performance of treated natural graphite as lithium-ion battery anode material. J. Electrochem. Soc. 2000, 147, 1245-1250. (5) Yan, Y.; Yin, Y. X.; Guo, Y. G.; Wan, L. J. A sandwich-like hierarchically porous carbon/graphene composites as a high-performance anode material for sodium-ion batteries. Adv. Energy Mater. 2014, 4, 1301584. (6) Menachem, C.; Peled, E.; Burstein, L.; Rosenberg, Y. Characterization of modified NG7 graphite as an improved anode for lithium-ion batteries. J. Power Sources 1997, 68, 277-282.


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TOC Graphic


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Figure 1. Structure of 3-D graphene-like ordered microporous carbon. (a) SEM image of the Y zeolitetemplated carbon. (b) TEM image (main picture) and Fourier diffractogram (inset) of the carbon. (c) 3-D electron-density map of carbon framework formed within pore channels of FAU-type zeolite. The electrondensity map reproduced with permission from ref. 21. The iso-surface level of the electron density is set to 0.25 electrons per Å3 (green) and 0.35 electrons per Å3 (magenta). Light gray areas represent crosssectional cuts 84x89mm (300 x 300 DPI)


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Figure 2. LIB anodic performances of the Y zeolite-templated carbon. (a) Galvanostatic charge/discharge profiles measured at a current density of 0.1 A g-1. (b) Cycling performance measured at a current density of 0.1 A g-1. (c) Rate performance measured at various current densities increasing from 0.1 to 5.0 A g-1. (d) Cycling performance measured at a current density of 5.0 A g-1. 84x75mm (300 x 300 DPI)



Figure 3. STEM-EELS elemental analysis of the Y zeolite-templated carbon electrode. (a) STEM image of the carbon sample obtained after the first full lithiation. (b,c) EELS mapping images for C (b) and Li (c) of the lithiated sample. (d) STEM image of the carbon sample obtained after the subsequent full delithiation. (e,f) EELS mapping images for C (e) and Li (f) of the delithiated sample. 82x49mm (300 x 300 DPI)


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Figure 4. STEM-EDS elemental analysis of the Y zeolite-templated carbon electrode. (a) STEM image of the carbon obtained after the first lithiation/delithiation cycle. (b) EDS line scan profiles of the C, O, F, and P Kedge signal measured along the red line shown on the left side image. 84x41mm (300 x 300 DPI)



Figure 5. Solid-state NMR characterization of the Y zeolite-templated carbon electrode. (a) 7Li and (b) 13C solid-state MAS NMR spectra of the Y zeolite-templated carbon electrode obtained after the first full lithiation and subsequent delithiation. The unlithiated sample was additionally compared in (b). Asterisks in the 7Li and 13C NMR spectra indicate spinning sidebands. The inverted triangles in the 13C NMR spectra of the lithiated and delithiated samples mark the peaks of alkyl and carbonate groups of the SEI layer components.48 82x52mm (300 x 300 DPI)


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Anomalously High Lithium Storage in Three ... - ACS Publications

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