Cobalt oxide-porous carbon composite derived from CO2 for the enhanced performance of lithium-ion battery

Journal of CO₂ Utilization 30 (2019) 28–37

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Cobalt oxide-porous carbon composite derived from CO2 for the enhanced performance of lithium-ion battery Won Yeong Choia, Dong Kyu Leea, Hee-Tak Kima, Jang Wook Choib, Jae W. Leea,

T

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a Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon, 34141, Republic of Korea b School of Chemical and Biological Engineering and Institute of Chemical Process (ICP) Seoul National University, 1 Gwanak-ro, Gwanak-gu, 08826, Seoul, Republic of Korea

A R T I C LE I N FO

A B S T R A C T

Keywords: Cobalt oxide Porous carbon Carbon dioxide Lithium-ion battery anode

As the necessity of eco-friendly energy storage devices grows, lithium-ion batteries (LIBs) have emerged as a promising alternative. While metal oxides, due to their high theoretical capacity, have been investigated as anode materials for LIBs, shortcomings such as poor electrical conductivity and segregation of metal oxide are obstacles in actual practice. To solve these problems, this study synthesized cobalt oxide/porous carbon (Co/ CPC) composites through a facile one-step thermal process under gaseous carbon dioxide (CO2) atmosphere. Asprepared composites showed cobalt oxide nanoparticles well-integrated with the CO2-derived porous carbon (CPC) and exhibited superior electrochemical performance via a synergistic effect of the cobalt oxide and the CPC. The composites demonstrated a reversible capacity of 1179 mA h·g−1 at a current density of 1000 mA·g−1 and stably retained this capacity over 300 cycles. Therefore, Co/CPC composites prepared from CO2 can be economical and eco-friendly anode materials due to their green synthesis route and outstanding electrochemical performance.

1. Introduction As a promising energy carrier, lithium-ion batteries (LIBs) are showing a dramatic growth in practice due to their outstanding strengths of high energy density and reduced environmental harmfulness compared to previously used batteries (e.g. nickel-cadmium batteries and lead-acid batteries) [1]. While LIBs have already been applied in many commercial electrical devices including smart phones, continual progress in technology calls for enhanced energy storage systems for electrical vehicles. As an anode material of lithium-ion battery, graphite with a theoretical capacity of 372 mA h·g−1 has been widely used in industry. However, since the graphite has a limitation due to its low specific capacity, other anode materials have been developed for widespread commercialization. The representative alternative anode materials are transition metal oxides (TMOs). As TMOs store energy by reacting with a large quantity of lithium ions owing to its multiple oxidation states, it shows a high theoretical capacity (e.g. Co3O4 : 890 mA h·g−1; CoO : 716 mA h·g−1; Fe3O4 : 925 mA h·g−1; MnO : 756 mA h·g−1; CuO : 674 mA h·g−1) [2]. Despite this remarkable performance, some drawbacks of low electrical conductivity and unstable phase transition

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during lithiation and delithiation hinder the electrochemical ability of TMOs in actual applications [3]. To make up for the deficiencies of TMOs, many efforts have been made, such as forming nano-sized TMO particles or creating void spaces to protect the structure by alleviating the volume change arising from the energy storage process [4–6]. Among various attempts, the most promising method has been hybridization of TMOs with carbonaceous materials. Various carbon sources have been adopted as carbonaceous materials, including graphene, carbon nanosheet, carbon nanotube, carbon nanofiber, amorphous carbon, and so on [7–11]. However, the experimental procedures to synthesize such carbon materials and to hybridize them with TMOs are too complex, requiring complex process steps such as pre-treatment, hydrothermal reaction, and several heating processes under air, argon, or nitrogen gas. Thus, the complex experimental procedure must be simplified and synergistic performance must be obtained by combining TMO with a carbon material, leading to practical applications with improved economic feasibility. It was reported that porous carbon can be synthesized from carbon dioxide (CO2) gas at atmospheric experimental conditions [12]. Enhancement of the electrochemical performance due to high electrical conductivity was found to result when these materials were applied to

Corresponding author. E-mail address: [email protected] (J.W. Lee).

https://doi.org/10.1016/j.jcou.2019.01.001 Received 15 November 2018; Received in revised form 28 December 2018; Accepted 5 January 2019 2212-9820/ © 2019 Elsevier Ltd. All rights reserved.

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fuel cell catalysts [13–19], or supercapacitor electrode materials [20–24]. However, the favorable nature of the hierarchical pore structure and high electrical conductivity in CO2-derived carbon with a facile synthetic pathway has never explored for a LIB system. To this end, this study aims to synthesize a composite of CO2-derived porous carbon and transition metal oxide for a lithium-ion battery anode material because a single synthesis step of CO2 conversion to the composite of hierarchical porous carbon and metal oxide is possible. To simplify the experimental procedure, carbon dioxide is utilized as a mild oxidant in the formation of the metal oxide through the oxidation process of the metal oxide precursor [25]. This work introduces a sandwich-like hybridized cobalt oxide/ porous carbon (Co/CPC) composite, in which cobalt oxide particles are cohesively embedded between multiple carbon flakes. It will be demonstrated that the as-synthesized novel composite exhibits superior reversible capacity for a long cycle without fading, and tolerates even high current densities, providing excellent stability and rate capability when applied as an anode material of a lithium-ion battery. Employing various microscopic and spectroscopic analyses, we will show that this performance can be achieved through a synergistic effect of cobalt oxide having a high theoretical capacity, and conductive carbon matrix with a three-dimensional structure. Furthermore, in view of the synthesis method, the Co/CPC composite is a suitable electrode material for the next generation of lithium-ion batteries because the synthesis process has not only a simple one-step heat treatment at atmospheric pressure and moderate temperature, but also utilizes carbon dioxide as both carbon source and mild oxidant for the entire synthesis process. The Co/CPC composite has good potentials as an alternative electrode material for enhancing applications of high-value added energy storage devices because of the high electrochemical performance, competitive synthesis route and greenhouse gas utilization.

while the total mass was fixed at 2.0 g. The synthesized products were denoted as Co/CPC-0.5 and Co/CPC-1, respectively, according to the ratio of cobalt precursor to NaBH4. For comparison, the carbon material (CO2-derived Porous Carbon, CPC) was prepared from gaseous CO2 and NaBH4 while the Co control group was synthesized using CO2 and cobalt(Ⅱ) acetate tetrahydrate, through the same procedure used for the Co/CPC composite synthesis.

2. Materials and methods

2.3. Electrochemical measurements

2.1. Synthesis of cobalt oxide/porous carbon (Co/CPC) composite from CO2 and precursors

The electrochemical properties were analyzed in the form of a CR2032 type coin cell. The cell was assembled in an Ar-filled glove box. Each of the synthesized samples (CPC, Co/CPC-0.5, Co/CPC-1, and Co control group) was homogeneously mixed with polyvinylidene fluoride (PVDF) and carbon black at a mass ratio of 6: 2: 2 by 1500 rpm stirring in N-methyl-2-pyrrolidone (NMP) solvent for 24 h. The as-prepared mixture was spread on a copper foil and then dried in an 80 °C oven for a day. The mass loading of the sample was 0.1∼0.4 mg·cm−2; this sample was used as a working electrode after punching it with a hole of diameter 10 mm. Lithium metal was used as the counter electrode and porous polypropylene (Celgard 2400) was used as a separator. 1 M LiPF6 in ethylene carbonate/diethyl carbonate (EC/DEC = 1:1 v/v) was employed as an electrolyte. To determine the electrochemical performance, galvanostatic charge-discharge (GCD) cycle tests at a current density of 1000 mA·g−1 and cyclic voltammetry (CV) measurements at a scan rate of 0.2 mV·s−1 were carried out on the WBCS3000 L battery cycler system in a voltage range of 0.01 to 3.0 V vs. Li/Li+. The resistance was obtained from the electrochemical impedance spectroscopy (EIS) measurement using the VSP model of a Biologic potentiometer at frequencies from 100 kHz to 0.05 Hz, with a sinusoidal signal of 10 mV.

2.2. Characterization High resolution powder X-ray diffractometry (XRD) was conducted on a SmartLab (Rigaku) with Cu Kα (λ = 1.5406 Å) radiation for a 2θ range of 15° to 70°, a scan rate of 2°·min−1, and a step size of 0.02°. The tube current and voltage were set to 200 mA and 45 kV, respectively. The morphology of the samples was characterized using scanning electron microscopy (SEM, SU8230, Hitachi) and field-emission transmission electron microscopy (300 kV FE-TEM, Tecnai G² F30 S-Twin, FEI). The samples for TEM were prepared by dropping a diluted suspension onto a copper grid. To analyze surface chemical compositions, X-ray photoelectron spectroscopy (XPS) survey was performed using a Sigma Probe (Thermo VG Scientific) system equipped with an Al Kα radiation (hν = 1486 eV) under a chamber pressure of 5 × 10−10 mbar. Thermogravimetric analysis (TGA) was carried out on a LABSYS Evo (SETARAM) from room temperature to 1000 °C with a ramping rate of 10 °C min−1 under air condition. The Brunauer-Emmett-Teller (BET) specific surface area and pore size distribution (PSD) were estimated by nitrogen adsorption-desorption at 77 K with three-flex surface characterization (Micromeritics). Raman spectra were acquired using a LabRAM HR Evolution Visible_NIR (HORIBA) with an Ar ion laser radiation of 514.5 nm in a range of 200 to 4000 cm−1.

The composite of Co/CPC was synthesized by simple one-step heating of both cobalt oxide precursor and CO2 reduction agent under a CO2 atmosphere. First, cobalt(Ⅱ) acetate tetrahydrate (Co (CH3COO)2·4H2O, Junsei Chemical) was dissolved in 10 ml DI water for 30 min sonication and then dried in an 100 °C oven for 24 h to remove water from the hydrated cobalt precursor. The as-prepared cobalt precursor was ground and mechanically mixed by mortar and pestle with a CO2 reduction agent of sodium borohydride (NaBH4, purity > 98%, Sigma Aldrich). The mixture was placed in an alumina crucible and then heated inside an alumina tube furnace (GSL1100X, MTI). The furnace was heated from room temperature to 400 °C with a ramping rate of 2 °C min−1 under argon atmosphere (Ar, 50 ml·min−1) and then further heated to 500 °C with a ramping rate of 5 °C min−1 under CO2 flow of 76 ml·min−1. The temperature was maintained at 500 °C for 2 h under the same CO2 atmosphere. After the reaction, the product was washed with DI water and ethanol through the stirring and filtration method and then dried in an 80 °C oven for a day. The mass ratios of cobalt(Ⅱ) acetate tetrahydrate to NaBH4 were adjusted to 0.5 and 1,

Fig. 1. Schematic illustration of the synthesis of Co/CPC composites by one-step heating process from sodium borohydride (NaBH4) and cobalt(Ⅱ) acetate (Co(Ac)2) under carbon dioxide (CO2) atmosphere. 29

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3. Results and discussion

XRD analysis was performed (Fig. S1). Thus, the cobalt oxide content in Co/CPC-0.5 and Co/CPC-1 is approximately 46.3 and 61.6 wt%, respectively. XRD patterns of the synthesized Co/CPC composites and of the Co control group were investigated to identify what types of cobalt oxides were formed in the synthesis process. In the case of the carbon material, CPC, a broad peak of 2θ = 20–30° representing an amorphous carbon was observed [12,13,15]. As shown in Fig. 2b, all of the samples contained clear diffraction peaks centered at 2θ = 19.0, 31.3, 36.9, 38.5, 44.8, 55.7, 59.4, and 65.2°, belonging to the (111), (220), (311), (222), (400), (422), (511), and (440) planes and matched well to the cubic Co3O4 phase (Space group: Fd-3 m, JCPDF: 00-042-1467), indicating that Co3O4 is the main component. While the Co control group showed clear peaks originating solely from Co3O4, the Co/CPC composites showed other small peaks of CoO in addition to that of Co3O4. Meanwhile, the peak intensity varied after CPC was incorporated into the cobalt oxides. The broadened peak in the Co/CPC composites suggests that the carbon material prevented the cobalt oxides from aggregation, leading to a decrease in the crystallite size of the cobalt oxides [28,29]. Due to its higher carbon content, Co/CPC-0.5 has smaller cobalt oxide particles than Co/CPC-1, in view of the peak width of XRD patterns, supporting the idea of the prevention of Co3O4 aggregation by CO2-derived carbon flakes. To examine the morphology of the samples, SEM analyses were conducted. First, CPC in Fig. 3a has the appearance of multiple carbon plates that are stacked and linked to each other in various directions. The void spaces between sandwiched carbon plates are believed to be sufficient for cobalt oxides to form. In the case of the Co control group shown in Fig. 3b, small Co3O4 particles, which were 100 nm in size, are agglomerated to form larger particle chunks. Meanwhile, the structures of the Co/CPC composites in Fig. 3c–d are comprised of nano-sized cobalt oxide particles that were well anchored to the plate-like carbon material supports. Distributed growth of cobalt oxide particles rather than the bulk particle development was established because insertion of cobalt oxide particles took place in void spaces formed by the threedimensional carbon matrix, inhibiting aggregation. For further understanding of the morphology of the composites, TEM images were obtained. In Fig. 4a and d, evenly dispersed nanosized cobalt oxide particles on the carbon plates are observed. When comparing between Co/CPC-0.5 and Co/CPC-1, the average size of the cobalt oxide particles and their distance are smaller and longer in Co/ CPC-0.5 than those in Co/CPC-1. This agrees to the XRD peak width tendency shown in Fig. 2b. High-resolution transmission electron microscopy (HRTEM) characterizations in Fig. 4b and e show the integrated architecture, in which the cobalt oxide particles are wrapped inside the carbon material. Lattice fringes measured at 2.3, 2.4, 2.8, and 4.6 Å are consistent with (222), (311), (220), and (111) planes of cubic Co3O4 phase. Furthermore, the selected-area electron diffraction (SAED) patterns in Fig. 4c and f indicate the formation of crystalline Co3O4, as was observed in the XRD analysis. The elemental mapping images of Co/CPC-0.5 in Fig. 4k–m also clearly support the existence of cobalt and oxygen atoms at the same positions, indicating the formation of cobalt oxide particles. This cage-like interconnected structure of nano-sized cobalt oxide and the porous carbon is expected to be advantageous in the electrochemical test because the small sizes of cobalt oxide particles minimize the volume change occurring from the lithiation-delithiation process and enable efficient interaction within the composites. Also, the enclosed architecture will protect the cobalt oxide particles from segregating from the carbon matrix even in repeated charge-discharge processes. The surface chemical compositions of the composites were elucidated by XPS. Survey spectra verified the presence of Co, O, C, and a trace of B atoms in the composites (Fig. S2). Co2p spectra of both Co/ CPC-0.5 and Co/CPC-1 have two distinct peaks of 2p1/2 and 2p3/2 at 796.8 eV and 780.9 eV. The spin-energy separation of ˜16 eV indicates that the Co2+ oxidation states are dominant at the surface of the composites [30,31]. Thus, the less oxidized form of cobalt oxide, the

Fig. 1 illustrates the experimental scheme of the synthesis route of cobalt oxide/porous carbon (Co/CPC) composites. The composites of cobalt oxide and CPC were produced via oxidation-reduction reactions between CO2 and the mixture of cobalt precursor and NaBH4. At temperatures higher than 400 °C, CO2 is deposited as a solid carbon on NaBH4, which is a strong reducing agent [12,13]. Thus, during the reaction, carbon atoms in gaseous carbon dioxide are combined with oxygen, boron, or other carbon atoms, and then became solid carbon materials with a porous structure [15,18]. Simultaneously, because CO2 acts as an mild oxidant for the metal oxide precursor, oxygen atoms in carbon dioxide participate in the oxidation reaction of the cobalt precursor to form cobalt oxides such as Co3O4 and CoO [25]. Because the formation of both porous carbon material and cobalt oxides occurred concurrently, cobalt oxide particles could be either distributed or enclosed by carbon materials evenly. Therefore, the Co/CPC composites with this beneficial structure are expected to facilitate interaction between cobalt oxide and the carbon material. Contents of carbon and cobalt oxides in the Co/CPC composites were determined by thermogravimetric analyses (TGA). There are three noticeable weight losses in Fig. 2a. The first weight loss occurring below 100 °C is attributed to the evaporation of residual water, and then the burning of the amorphous carbon follows. The decarbonation of amorphous carbon begins gradually from low temperatures, peaking at around 400 °C [26,27]. The last weight loss after 900 °C is due to the decomposition of by-products containing B, O, and Co atoms. At temperatures above 1000 °C, only the Co3O4 phase was detected when the

Fig. 2. (a) TGA analysis of Co/CPC-0.5 and Co/CPC-1 up to 1000 ℃ with a ramping rate of 10 ℃·min−1 under an air condition. (b) XRD patterns of Co control group and the Co/CPC composites. 30

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Fig. 3. SEM images of (a) CPC; (b) Co control group; (c) Co/CPC-0.5; (d) Co/CPC-1.

the existence of micropores and mesopores in the structure. These abundant pores of a wide range of sizes are formed by combination of CPC and cobalt oxides, and will bring positive effects in the electrochemical performance because large accessibility between the composites and electrolyte not only accelerates the reactivity of lithiationdelithiation of cobalt oxides but also increases the number of lithiumions stored in the pores [38,39]. The electrochemical performance of the samples as an anode material in a lithium-ion battery was evaluated in the voltage range of 0.01 to 3.0 V vs. Li/Li+. Fig. 6 shows CV curves of CPC, Co control group, and the Co/CPC composites acquired during the charge-discharge process. In the first cycle, CPC has one cathodic peak at 0.70 V, while the other samples including the Co control group show an additional peak near 0.90 V. The peak located in common at 0.70 V is associated with the irreversible solid electrolyte interphase (SEI) layer formation, and the peak near 0.90 V indicates the reduction of Co3O4 to Co. From the second cycle, the Co control group retains one cathodic and one anodic peak at 1.05 V and 2.15 V, respectively. It testifies that only Co3O4 is involved in the lithium-ion storage process of the Co control group through the reaction depicted in eq. 1 below. Meanwhile, the Co/ CPC composites have two cathodic and anodic peaks in subsequent cycles. The additional cathodic peak at 1.25 V is attributed to the reduction of CoO to Co, and the two anodic peaks are ascribed to the series oxidation of Co to CoO and Co3O4 phases [40–42]. Some of the Co and Li2O generated from the reduction of Co3O4 in the first discharge process underwent two reverse reactions of eq. 1 and eq. 2 during the charge process, forming both Co3O4 and CoO [43–45]. Therefore, in the charge-discharge process in the Co/CPC composites, both Co3O4 and CoO participate in the lithium-ion storage process. In the charge and discharge curves of the Co/CPC composites (Fig. S3), plateaus appeared around the voltages at which the peaks were observed in the CV curve. Two plateaus at 1.3 V and 1.0 V in the discharge curve indicate the reduction of cobalt oxide to cobalt metal, and two plateaus at 1.1 V and 2.0 V in the charge curve mean the oxidation of cobalt metal to cobalt oxide phases.

CoO phase, was synthesized near the surface. In other words, most of the main product, the Co3O4 phase, is located inside the carbon material rather than at the exposed surface. Detailed analyses of the Co2p spectrum showed four peaks that are attributed to 2p1/2 of Co2+ (796.6 eV) and Co3+ (794.8 eV), and 2p3/2 of Co2+ (780.8 eV) and Co3+ (779.6 eV) [10,32,33]. Moreover, two satellite peaks of 2p1/2 and 2p3/2 are also observed, which are common characteristics of Co3O4 as previously reported [34]. The O1s spectrum features three peaks at 533.0 eV, 531.6 eV, and 530.0 eV, attributed to CeO, C]O, and CoeO groups, respectively. The Raman spectra of the CPC and Co/CPC composites are represented in Fig. 5a. All the samples display two peaks of the D band (1350 cm−1) and G band (1590 cm−1) related to the presence of disorders due to the sp3 carbon configuration and the in-plane bondstretching mode of the sp2 carbon configuration, respectively [35]. The relative peak intensity values of the D band and G band (ID/IG) tend to decrease in the order of CPC (0.927), Co/CPC-0.5 (0.825), and Co/CPC1 (0.808). Since CPC is mainly composed of amorphous carbon, the decrease of ID/IG means an increase in defects or disorder [17,35–37]. The carbon material in the Co/CPC composites becomes more disordered because an incorporation of cobalt precursor in the synthesis process causes the carbon to deviate from ordered growth. Since we do not have any annealing process for the composite above 700 °C, the additional graphitization does not contribute to the ordering of the carbon structure [18,20,23]. The porous architecture of the Co/CPC composites was verified by nitrogen (N2) adsorption-desorption measurements. The resultant isotherm and pore size distribution (PSD) estimated by non-local density functional theory (NLDFT) are given in Fig. 5c and d. When compared to that of the Co control group (Fig. 5b), Brunauer-Emmett-Teller (BET) surface area are greatly increased to 488.5 and 291.2 m2·g−1 for Co/ CPC-0.5 and Co/CPC-1 from 17.4 m2·g−1 for the Co control group. Pore volumes also increased more than ten times from 0.059 cm3·g−1 for the Co control group to 0.828 and 0.526 cm3·g−1 for Co/CPC-0.5 and Co/ CPC-1, respectively. While the Co control group has few pores in the structure, both of the Co/CPC composites show sharp pore size peaks near 2 nm and a wide range of peaks between 2 to 50 nm, ascertaining

Co3 O4 + 8Li+ + 8e− ↔ 3Co + 4Li2 O 31

(1)

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Fig. 4. Low and high magnification TEM images and selected-area electron diffraction (SAED) patterns of (a,b,c) Co/CPC-0.5; (d,e,f) Co/CPC-1; (g,h,i) Co control group. (j) STEM image of Co/CPC-0.5 and corresponding elemental mapping of (k) carbon; (l) oxygen; (m) cobalt.

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Fig. 5. (a) Raman spectra of the CPC and Co/CPC composites. N2 adsorption-desorption isotherm and related pore size distribution (inset) of (b) Co control group; (c) Co/CPC-0.5; (d) Co/CPC-1.

CoO + 2Li+ + 2e− ↔ Co + Li2 O

CPC. Under a current density of 1000 mA·g−1, Co/CPC-0.5 and Co/ CPC-1 exhibited discharge capacities of 1953 and 1857 mA h·g−1 at the first cycle, and 1125 mA h·g−1 and 928 mA h·g−1 at the 50th cycle. That level was maintained over 300 cycles without degradation, showing values of 1179 mA h·g−1 and 952 mA h·g−1 at the 300th cycle. Meanwhile, the capacities of both the Co/CPC composites gradually increased in the middle of the cycle. This is attributed to the activation process arising from the hierarchically porous structure of the Co/CPC composites. Because the Co/CPC composites have a wide range of pore sizes including micropores and mesopores, the internal surfaces on the small pores are difficult to access during the early cycles. However, as more lithium-ions are accessible to internal pores, more favorable diffusion pathways are established over a number of cycles, resulting in the capacity increase [40,47]. Such high specific capacity and superior cyclability were achieved because the unique architecture of the anchored cobalt oxide particles between sandwich-like porous carbon layers prevented cobalt oxides from aggregation and allowed utilization of many cobalt oxide particles in the lithium-ion storage process. Structural stability was also accomplished by nano-sized cobalt oxides and carbon support. Small sizes of cobalt oxide particles reduced the degree of volume change, and the CPC robustly maintained the structure of the composites against the rough reactions that accompany phase transitions. Moreover, the cage-like confined cobalt oxides on the conductive CPC network facilitated intimate electrical connection within the active material through compact contact, interrupting segregation of cobalt oxides from the electrode. Lastly, the large surface area of micro- and mesopores plays a role as both an efficient channel for charge carriers (e.g. electrons, ions) and a place for energy storage

(2)

Fig. 7a and b display the cycling stabilities and coulombic efficiencies of the Co control group and Co/CPC composites up to 300 cycles at a current density of 1000 mA·g−1. The coulombic efficiencies of Co/CPC-0.5 and Co/CPC-1 are initially 63.4 and 57.8%, and then steadily increase and reach 98.5 and 99.4% at the 300th cycle, respectively. The coulombic efficiency at the first cycle is generally lower than that at subsequent cycles owing to irreversible capacity arising from the SEI layer formation, unstable phase conversion, structural change, and so on [46]. However, the initial coulombic efficiencies (ICEs) of the Co/CPC composites were higher than that of the Co control group (51.6%). It implies that the irreversible processes can be suppressed when the Co3O4 particles are well distributed and confined in the interstitial space of the carbon flakes. Among the samples, the Co control group exhibited the lowest discharge capacity (Fig. 7a). As shown in the SEM and TEM images of the Co control group, cobalt oxide particles were agglomerated, such that the agglomerated particles could not all participate in the lithiumion storage process, resulting in low reversible capacity. Moreover, the capacity decreased gradually over the cycles. This decrease arose from the collapse of the structure in the Co control group due to significant volume changes during the phase transition in the lithiation-delithiation process. The unstable structural changes cause detachment of cobalt oxide grains from the electrode, reducing the amount of active material that can be involved in the energy storage process. By contrast, the Co/CPC composites retained an excellent reversible capacity for long cycles due to the synergistic effect of cobalt oxide and 33

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Fig. 6. First, second, third, and fifth CV graphs of (a) CPC; (b) Co control group (c) Co/CPC-0.5; (d) Co/CPC-1 at a scan rate of 0.2 mV·s−1.

lower current density of 500 mA·g−1 again, the capacity of Co/CPC composites returned to its original state. As many electrons and ions are readily accessible after hybridizing a carbon matrix with a large surface area, lithium-ions can be reliably stored with fast transfers through three-dimensional framework. When the spent composite was collected after the rate performance test, almost the same appearance was observed, even after the composite was subjected to various current densities for 100 cycles (Fig. S4). Such tremendous structural stability demonstrates the steady capacity retention in cycling tests and allows excellent capacity at high current densities. On the other hand, the superior electrochemical performance of the Co/CPC composites was possible due to not only evenly embedded cobalt oxide nanoparticles but also the improvement in the electrical conductivity of the composite. In the electrochemical impedance spectroscopy (EIS) measurements shown in Fig. 7d, the diameter of a semicircle signifies the charge-transfer resistance, which is greatly reduced in the Co/CPC composites compared to that in the Co control group [42]. The existence of carbon material supplemented the poor electrical conductivity of the cobalt oxides, facilitating transport of electrons and lithium ions through the composite [54]. Furthermore, as the charge and discharge cycles progressed, the diameter of the semicircles decreased and the length of the low-frequency line shortened from the 1 st cycle to the 100th cycle, indicating the improved chargetransfer and quickened solid-state lithium-ion diffusion (Fig. S5). This suggests that the composite is stabilized during cycling in the aspect of the lithium-ion storage at active sites [51,55]. Therefore, with the asmentioned structural properties and large surface area, the enhanced electrical conductivity due to the formation of the Co/CPC composites enables this material to retain fast reaction kinetics, excellent cycle

[47–50]. The high surface area of the Co/CPC composites promotes reactivity of composites and lithium-ions by providing large accessibility between them. At the interfaces of the porous Co/CPC composites, lithium ions can be adsorbed or intercalated through active sites such as surface atoms, pores, and defects. These active sites can be built more during repeated charge-discharge processes [40,41,51,52]. In addition, the grain boundaries of cobalt metal and Li2O generated from the lithiation process of cobalt oxides contribute to extra energy storage of the composites via the adsorptive mechanism of lithium-ions [53]. Because of these beneficial structural properties, the Co/CPC composites are able to utilize the highest number of active sites, yielding both excellent capacity and superior cycle stability. The electrochemical performances of the Co/CPC composites are superior to or competitive with those in other studies in this field. Some characteristics, and electrochemical test results for other cobalt oxide derivatives in previous studies are listed in Table 1 for comparison. Considering a current density and cycle number, the Co/CPC composites hold the superb electrochemical ability. As a high power density is regarded as a crucial factor in future lithium-ion battery applications, charge-discharge capacities at various current densities were investigated (Fig. 7c). The Co/CPC composites revealed outstanding rate capability under intense electrochemical conditions, delivering 1119, 961, 833, 658, and 525 mA h·g−1 for Co/ CPC-0.5, and 856, 730, 618, 478, and 351 mA h g−1 for Co/CPC-1 at current densities of 500, 1000, 2000, 5000, and 10,000 mA g−1, respectively. Even at a high current density of 10,000 mA g−1, Co/CPC0.5 preserved a reversible capacity of 525 mA h g−1, which is higher than the theoretical capacity of graphite. When a current density was switched abruptly from a higher current density of 10,000 mA·g−1 to a 34

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Fig. 7. (a) Charge and discharge capacity of Co/CPC-0.5, Co/CPC-1, and Co control group at a current density of 1000 mA·g−1 up to 300 cycles. (b) Coulombic efficiencies of the Co/CPC composites at a current density of 1000 mA·g−1 up to 300 cycles. (c) Charge and discharge capacity of the Co/CPC composites at various current densities from 500 to 10,000 mA·g−1. (d) EIS measurement of the Co/CPC composites and Co control group. Table 1 Representative information on cobalt oxide/carbon composites as anode materials of lithium-ion batteries. Composites

Cobalt oxide weight [%]

Surface area [m2·g−1]

Current density [mA·g−1]

Capacity [mAh·g−1]

Cycles

Ref.

Co/CPC-0.5 Co/CPC-1 Co3O4-NS/N-rGO Graphene-coated Co3O4 fibers Co3O4 NP/N-graphene Graphene- Co3O4 microsphere C-doped hollow Co3O4 NF CoO- Co3O4-RGO CoO/BPC Co3O4@Graphene core-shell Co3O4/NG hybrid Co3O4@CNT

46.3 61.6 90 78.3 66.5 . 94 . 72.2 76.2 72.5 70.2

488.5 291.2 99.7 43.2 95.3 79.2 25.8 150 348 66.2 80.1 293

1000

1179 952 859 1000 812 820 1121 994 1057 ∼700 1236 ∼700

300

This study

50 40 230 35 100 200 200 200 200 100

[10] [56] [57] [58] [59] [32] [60] [61] [62] [63]

1000 100 1000 100 200 100 200 1000 89 10

Deposition of CO2-derived porous carbon (CPC) on the cobalt oxides prevented cobalt oxide particle agglomeration and mitigated volume changes stemming from the lithium-ion storage process. Also, CPC improved lithium-ion transport by enlarging the surface area and the electrical conductivity. The synergistic effect of cobalt oxide and CPC yielded a high charge-discharge capacity with a long cycle life (1179 mA h·g−1 after 300 cycles @ 1000 mA·g−1). Especially, carbon dioxide (CO2), which is the most dominant greenhouse gas, was utilized in the synthesizing process of the composites, giving additional significance to this study in view of CO2 utilization. Considering all of these strengths, the Co/CPC composites are promising candidates as anode materials in lithium-ion batteries and have high potential for

stability, and outstanding rate capability. 4. Conclusions Novel composites of cobalt oxide and porous carbon (Co/CPC) have been prepared using cobalt precursor (Co(CH3COO)2·4H2O) and reducing agent (NaBH4) under carbon dioxide (CO2) atmosphere. The experimental procedure requires only a facile one-step heating process at mild conditions of 1 atm and 500 °C. The prepared Co/CPC composites showed sandwich-like well-anchored cobalt oxide nanoparticles (∼100 nm) on a carbon framework and exhibited superior electrochemical performance compared to that of bare cobalt oxide. 35

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further development. [23]

Acknowledgements

[24]

The authors acknowledge support from the Korea CCS R & D program funded by the Ministry of Science and ICT (NRF2014M1A8A1049297). This research was also supported in part by the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) (NRF2015K1A4A3047100).

[25]

[26]

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Appendix A. Supplementary data [28]

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jcou.2019.01.001.

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