Enhanced Lithium- and Sodium-Ion Storage in an Interconnected

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Enhanced Lithium- and Sodium-Ion Storage in an Interconnected Carbon Network Comprising Electronegative Fluorine Seok-Min Hong,†,‡ Vinodkumar Etacheri,§,∥ Chulgi Nathan Hong,§,⊥ Seung Wan Choi,† Ki Bong Lee,*,† and Vilas G. Pol*,§ †

Department of Chemical and Biological Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea Nuclear Fuel Cycle Process Development Division, Korea Atomic Energy Research Institute, 989-111 Daedeok-daero, Yuseong-gu, Daejeon 34057, Republic of Korea § Davidson School of Chemical Engineering, Purdue University, 480 Stadium Mall Drive, West Lafayette, Indiana 47907-2100, United States ∥ IMDEA Materials Institute, C/Eric Kandel 2, Getafe, Madrid 28906, Spain ⊥ Battery R&D, LG Chem, Ltd., 104-1 Moonji-dong, Yuseong-gu, Daejeon 305-380, Republic of Korea ‡

S Supporting Information *

ABSTRACT: Fluorocarbon (CxFy) anode materials were developed for lithium- and sodium-ion batteries through a facile one-step carbonization of a single precursor, polyvinylidene fluoride (PVDF). Interconnected carbon network structures were produced with doped fluorine in hightemperature carbonization at 500−800 °C. The fluorocarbon anodes derived from the PVDF precursor showed higher reversible discharge capacities of 735 mAh g−1 and 269 mAh g−1 in lithium- and sodium-ion batteries, respectively, compared to the commercial graphitic carbon. After 100 charge/discharge cycles, the fluorocarbon showed retentions of 91.3% and 97.5% in lithium (at 1C) and sodium (at 200 mA g−1) intercalation systems, respectively. The effects of carbonization temperature on the electrochemical properties of alkali metal ion storage were thoroughly investigated and documented. The specific capacities in lithium- and sodium-ion batteries were dependent on the fluorine content, indicating that the highly electronegative fluorine facilitates the insertion/extraction of lithium and sodium ions in rechargeable batteries. KEYWORDS: lithium-ion batteries, sodium-ion batteries, interconnected carbon network, fluorocarbon, polyvinylidene fluoride

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INTRODUCTION Portable electric devices such as laptops, cell phones, and electric vehicles are widely used and have led to a commercial need for storage systems that are more compact and have greater energy densities. Among the available rechargeable battery technologies, the lithium-ion battery is the most attractive storage technology owing to its light weight, reversible intercalation, and large output voltage.1−3 However, rechargeable batteries based on sodium show promise for use in stationary storage applications (e.g., solar and wind) because of the low cost and natural abundance of sodium. Various carbon architectures such as carbon fiber, nanotube, nanosheet, and sphere are currently utilized as anodes in both lithium-ion4−8 and sodium-ion batteries.9−12 Graphite, the most common carbon material, delivers a limited theoretical capacity of 372 mAh g−1 in lithium-ion batteries, while it is unsuitable in sodium-ion batteries because of the slightly larger diameter of the sodium ions.13,14 Alternatively, amorphous carbons composed of disordered graphitic planes are considered for sodium-ion-battery anodes.15 Furthermore, the © XXXX American Chemical Society

electrochemical performances can be enhanced through the modification of carbonaceous materials by doping heteroatoms such as nitrogen, boron, phosphorus, and sulfur.16−19 The presence of heteroatoms in carbon materials can effectively improve the electronic conductivity in and wettability of electrodes, providing enhanced charge transfer and electrode/ electrolyte interactions and therefore ion storage capacities. Li et al. (2012) introduced nitrogen-doped carbon nanotube (CNT) for lithium-ion storage with enhanced specific capacity of 494 mAh g−1, compared to 260 mAh g−1 for the pristine CNT.20 Wang et al. (2013) developed nitrogen-doped carbon nanofibers that delivered a reversible capacity of 134 mAh g−1 after 200 cycles at current density of 200 mA g−1 in sodium-ion batteries.18 Ling and Mizuno (2014) performed density functional theory (DFT) calculations for boron-doped graphene as an anode in sodium-ion batteries and demonReceived: March 9, 2017 Accepted: May 10, 2017

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DOI: 10.1021/acsami.7b03456 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Scheme 1. Illustration of Fluorocarbon Production from PVDF via Carbonization for Anodes of Lithium- and Sodium-Ion Batteries

Material Characterization. The surface morphologies of PVDFderived fluorocarbon samples were characterized using scanning electron microscopy (SEM, Nova 200 NanoLab, FEI) and highresolution transmission electron microscopy (HRTEM, G2 T20, Tecnai). Energy dispersive X-ray spectroscopy coupled with SEM analysis was utilized to determine the elemental distribution in samples. X-ray photoelectron spectroscopy (XPS, ULVAC-PHI PHI5000 Versa Probe system) was used to identify the surface electronic states with Al Kα radiation (1486.6 eV). To obtain the textural properties of samples, N2 adsorption−desorption isotherms were measured at 77 K using a volumetric adsorption analyzer (ASAP2020, Micromeritics). Prior to the adsorption measurement, samples were degassed at 150 °C under vacuum (10 μmHg) for 12 h to remove any adsorbate and residual moisture. The specific surface area was estimated using the Brunauer−Emmett−Teller (BET) equation, and the total pore volume was determined from the adsorption amount obtained at the relative pressure of ∼0.99. The pore size distribution was estimated by nonlocalized density functional theory (NLDFT) calculation using the N2 adsorption data. A thermogravimetric analyzer (TGA, Q50, TA Instruments) was used to measure the weight loss of pristine PVDF powder at a heating rate of 3 °C min−1 under an argon gas flow. X-ray diffraction patterns were obtained with an X-ray diffractometer (XRD, Rigaku Smartlab diffractometer) using Cu−Kα X-ray source with a wavelength of λ = 0.154 nm and a 2θ range of 10−70°. A Raman spectrometer (DXR, Thermo scientific) using a He−Ne laser with an excitation wavelength of 632 nm was used in the 1000−2000 cm−1 wavenumber range to confirm possible defect formation. Electrochemical Characterization. To prepare electrodes for lithium- and sodium-ion batteries, PVDF-derived fluorocarbon, a conducting additive (carbon black), and a PVDF binder in a weight ratio of 70:20:10 were blended with a N-methylpyrrolidone (NMP) solvent. The slurry was coated on a copper foil using a doctor blade and dried at 80 °C in a vacuum oven. For the lithium-ion battery, electrodes assembled into coin 2032 cells using a Celgard 2500 polypropylene separator and Li-foil as a counter electrode. The electrolyte consists of 1 M LiPF6 in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) with a volumetric ratio of 1:1:1. For the sodium intercalation system, a sodium metal foil counter electrode, a glass fiber separator, and an electrolyte consisting of 1 M NaClO4 in propylene carbonate (PC) solvent were assembled into coin cells. For the sodium-ion battery, a fluoroethylene carbonate (FEC) additive was used in the cells to achieve sodium insertion/ deinsertion with good cyclability.27 The electrochemical cells were prepared in an argon-filled glovebox. Galvanostatic charge−discharge tests at various current densities were performed in the potential range 0−3 V at ambient temperature using a Neware battery tester. Cyclic voltammetry (CV) was conducted at 0.1 mV s−1 and electrochemical

strated that the capacity of boron-doped graphene reaches 2.52 times of that of hard carbon.19 Given fluorine is the most electronegative element, enhanced electrochemical performance is anticipated on use of fluorocarbons. Fluorination could provide maximum charge polarization improving the electrochemical activity of energyrelated reactions.21 Many fluorine-doped carbon materials are generally prepared by thermal annealing with fluorinecontaining gases such as CF4, F2, ClF3, NF3, and XeF2.22−25 However, such gases are very toxic, corrosive, and hazardous and can cause the ignition of organic material on contact. Therefore, much caution is required during their handling, and the thermal annealing method is not recommended in general laboratory conditions. As an alternative approach to reduce toxicity and complexity, fluorocarbon can be prepared from fluorine-containing solid carbon precursors.26 In this study, we introduced a facile and safe one-pot method for the preparation of interconnected carbon network comprising fluorine as an anode for lithium- and sodium-ion storing rechargeable batteries. Polyvinylidene fluoride (PVDF) was used as a fluorine-containing solid carbon precursor. The PVDF-derived disordered carbon can contain fluorine, and therefore it is considered a promising anode material for both lithium- and sodium-ion batteries. In addition, the porous structure with moderate surface area of fluorocarbon can improve ionic diffusion due to its better electrode/electrolyte interface, eventually increasing the overall electrochemical performance. To the best of our knowledge, this was the first instance that utilized PVDF-derived fluorocarbon in both lithium- and sodium-ion storage. We thoroughly investigated the effects of fluorine content on the specific capacities at various current densities in lithium- and sodium-ion storage systems and compared them with the commercial graphitic carbon electrode.

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EXPERIMENTAL SECTION

Synthesis of PVDF-Derived Fluorocarbon. PVDF (Kynar 761, Elf AtoChem) was used as a precursor for fluorocarbon preparation. PVDF was loaded in a cylindrical tube furnace, and the furnace temperature was increased to the target temperature of 500−800 °C at a heating rate of 3 °C min−1 under an argon gas flow at 200 mL min−1. After the target temperature was reached, heating at a constant temperature was maintained for 1 h for carbonization of PVDF. Here, the PVDF-derived fluorocarbon is denoted as CPx, where x represents the carbonization temperature. B

DOI: 10.1021/acsami.7b03456 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. SEM images of (a) pristine PVDF and (b and c) CP500. (d) Layered SEM image of CP500, corresponding EDX maps of (e) carbon and (f) fluorine, and (g) EDX patterns. (h and i) TEM images of CP500. impedance spectroscopy (EIS) tests were conducted using a Gamry Reference-600 electrochemical workstation. The electrochemical performance of the PVDF-derived carbon was also compared with that of Conocophillips CGP-A12 commercial graphitic electrodes.

TEM images in Figure 1h,i show short-range order of carbon atoms mimicking worm-like structures. Figure 2a shows the high-resolution F 1s XPS spectra of fluorocarbons prepared at different temperatures, and the peak at ∼687.2 cm−1 corresponds to the C−F semi ionic bond.28 The fluorine contents in the fluorocarbons are summarized in Table 1. About 7.95% fluorine was measured in CP500, while the amount of fluorine gradually decreased with increasing carbonization temperature and fluorine was fully removed in CP800. The decomposition of fluorine was further investigated by TGA analysis. During the carbonization of PVDF, a significant weight decrease was recorded at ∼420 °C (Figure S1 of the Supporting Information, SI). Therefore, PVDFderived fluorocarbons were prepared at carbonization temperatures above 500 °C. The N2 adsorption−desorption isotherms of fluorocarbons measured at 77 K are shown in Figure 3. Similar isotherm shapes were obtained for all the prepared fluorocarbons and they correspond to the Type I isotherm of IUPAC classification, indicating microporous structure. The fluorocarbons dominantly have pore diameters of ∼0.62 nm from the pore size distributions derived from NLDFT. The textural properties of PVDF-derived fluorocarbon are summarized in

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RESULTS AND DISCUSSION Characteristics of PVDF-Derived Fluorocarbon. The one-pot, solid state, scalable production of fluorocarbon from the PVDF precursor by carbonization at a high temperature is shown in Scheme 1. The fluorine content in the fluorocarbon was controlled by adjusting carbonization temperature at 500− 800 °C in an argon gas flow. The obtained fluorocarbons were used as an anode electrode to store lithium and sodium ions in secondary batteries. The morphologies of pristine PVDF and PVDF-derived fluorocarbon were investigated by SEM and TEM (Figure 1). The pristine PVDF had a spherical particle morphology with a diameter of ∼200 nm (Figure 1a). However, it lost its spherical shape and converted into an interconnected carbon network at 500 °C (CP500) as shown in Figure 1b. The high-resolution SEM image of CP500 in Figure 1c depicts nanometer-scale rough surfaces. EDX mapping (Figure 1d−g) confirmed that the fluorine was well distributed in the obtained carbon. The C

DOI: 10.1021/acsami.7b03456 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. (a) Wide-scan XPS spectra and (b−e) high-resolution F 1s XPS spectra. The result demonstrates complete loss of F at 800 °C.

Figure 3. (a) N2 adsorption−desorption isotherms of fluorocarbons at 77 K and (b) pore size distribution of fluorocarbons prepared at 500− 800 °C.

Table 1. Textural Properties of PVDF-Derived Carbons Obtained from N2 Adsorption Isotherm at 77 K and Fluorine Content Determined from XPS Analysis sample

SBET (m2 g−1)

VTa (cm3 g−1)

VMb (cm3 g−1)

F content (at%)

CP400 CP500 CP600 CP800

878 979 1009 1024

0.372 0.408 0.445 0.453

0.358 0.397 0.408 0.414

7.95 5.02 2.08 0

they are higher than the theoretical value of 0.34 nm,33 indicating that a high sodium-ion storage capacity is possible in the prepared fluorocarbons. The Raman spectra of fluorocarbons synthesized at various carbonization temperatures are exhibited in Figure 4b. Two distinct phonon peaks at ∼1340 (D band) and ∼1580 cm−1 (G band) are observed. The former is associated with the disorder with the mode of A1g symmetry relevant to the sp3-hybridized carbon, while the latter corresponds to the E2g mode related to the sp2-hybridized carbon vibration. The intensity ratio of D band to G band increased steadily from 0.85 to 0.93 with increasing carbonization temperature from 500 to 800 °C, and it is attributed to the increased defects from the pore development in PVDF caused by fluorine removal.26 Electrochemical Performance of PVDF-Derived Fluorocarbon. The lithium- and sodium-ion storage behaviors in the fluorocarbon were investigated by recording cyclic voltammograms (CV) between 0 and 3 V at a scan rate of 1 mV s−1 (Figure 5). The irreversible cathodic peaks at 0.39 V versus Li/Li+ (Figure 5a) and 0.13 V versus Na/Na+ (Figure 5b) at the first discharge process reflect the decomposition of electrolyte and the known formation of solid electrolyte interphase (SEI) on the surface of the fluorocarbon.34,35 There was no anodic peak in the fluorocarbon compared to the commercial graphitic electrodes (Figure S2). In the subsequent cycles, the peaks disappeared as the dense SEI layer formed in the first discharge which can prevent the direct contact of

Total pore volume at p/po ≈ 0.99. bMicropore volume determined from the Dubinin−Radushkevich equation.

a

Table 1. With increasing carbonization temperature, the specific BET surface area and total pore volume were increased from 878 m2 g−1 and 0.372 cm3 g−1 for CP500 to 1024 m2 g−1 and 0.453 cm3 g−1 for CP800, respectively. From the results of XPS and textural properties, it can be suggested that increasing carbonization temperature led to decomposition of fluorinecontaining functional groups and development of porous structure.29 The high surface area and porous structure in the fluorocarbons may contribute to the improvement of electrode/electrolyte contact interface and ion diffusion paths, respectively, and eventually enhance the ion/charge transfer.30,31 The XRD patterns of fluorocarbons (Figure 4a) show two broad peaks at 2θ of around 21° and 44°, which correspond to the reflection of (002) and (001) planes in the graphitic carbon, respectively, representing an amorphous carbon structure.32 The interlayer spacing (d002) values obtained from (002) plane of the fluorocarbons are calculated to be 0.42−0.43 nm, and D

DOI: 10.1021/acsami.7b03456 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 5. Cyclic voltammograms of CP500 at a scan rate of 1 mV s−1 in (a) lithium- and (b) sodium-ion batteries.

Figure 4. (a) X-ray diffraction patterns and (b) Raman spectra of fluorocarbons prepared at 500−800 °C.

sodium-ion batteries, respectively. Although the fully carbonized PVDF (CP800) showed similar lithium storage capacity to the commercial graphitic carbon in the slow rate of C/10 and C/5, significant capacity fading was obtained in commercial graphite anodes at the high current density of C/2 and 1C. In the sodium-ion battery, the commercial graphite anode showed very poor capacity which is inappropriate for usage; however, a high storage capacity could be delivered by the fluorocarbons even at high current densities. Moreover, when the current rate was returned to its initial current density of C/10 (for the lithium-ion battery) and 500 mA g−1 (for the sodium-ion battery) after varied current cycling, the high rate capabilities were recovered in both lithium- and sodium-ion batteries, indicating a good reversibility. In the use of secondary battery, long-term cyclic stability with high Coulombic efficiencies is as essential as high capacities. Figure 7c,d shows the cyclic tests of CP500 in lithium- and sodium-ion storage, respectively. To reduce the effects of initial irreversible performance on the cycling test, three cycles were conducted at a slow rate: current densities of C/10 in lithiumion batteries and 50 mA g−1 in sodium-ion batteries. For lithium-ion batteries, high specific capacities of 587 and 363 mAh g−1 could be obtained after 100 cycles at current densities of C/10 and 1C, respectively, and the stable Coulombic efficiency of 99.6% with cyclic stability of 91.3% was shown at the high discharge−charge rate of 1C. Besides, the sodium ions delivered specific capacity of 129 and 101 mAh g−1 after 100 cycles for CP500 at the rates of 50 and 200 mAh g−1, respectively, giving Coulombic efficiency of nearly 100% with cyclic stability of 97.5%. During the subsequent cycles, no

fluorocarbon with electrolyte. The redox peaks occurring at lower potential in both lithium- and sodium-ion batteries are ascribed to ions insertion/extraction. In addition, the second and following cyclic voltammograms remained almost unchanged, suggesting that the fluorocarbon electrodes are electrochemically stable and the process is reversible after the first cycle. The high specific discharge capacities of 1283 mAh g−1 and 689 mAh g−1 were obtained in CP500 for lithium-ion (Figure 6a) and sodium-ion (Figure 6b) batteries, respectively. The lower capacity for sodium-ion storage than lithium-ion storage is due to different sodiation/desodiation kinetics due to the larger ion radius of Na+ (102 nm) than that of Li+ (76 nm) and shared carbons.36 The Coulombic efficiencies of the initial cycles of CP500 in lithium- and sodium-ion batteries were 59.0 and 39.7%, respectively. The relatively low initial Coulombicefficiencies are mainly caused by the irreversible capacity losses, which are commonly found in anode materials during the first discharge process due to the inevitable formation of SEI film and decomposition of electrolyte, coinciding with the CV results. The similar results were also obtained from the fluorocarbons prepared at different temperatures (Figure S3). As exhibited in Figure 6c,d, the second cycle discharge capacities of CP500 were 735 mAh g−1 for the lithium-ion battery and 269 mAh g−1 for the sodium-ion battery with Coulombic efficiencies of 95.4 and 85.9%, respectively, providing an expectation of good cyclic performance. Figure 7a,b compares the rate performances for the fluorocarbons and commercial graphite in lithium- and E

DOI: 10.1021/acsami.7b03456 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 6. First cycle discharge and charge of CP500 in (a) lithium- and (b) sodium-ion batteries at a constant current density of C/10 and 50 mA g−1, respectively. Second cycle voltage profiles of CP500 at various current densities in (c) lithium- and (d) sodium-ion batteries.

Figure 7. Electrochemical rate performances of fluorocarbons and commercial graphite in (a) lithium- and (b) sodium-ion batteries after the first cycle. Galvanostatic cycling performance of CP500 at a constant current in (c) lithium- and (d) sodium-ion batteries.

F

DOI: 10.1021/acsami.7b03456 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces obvious change was exhibited in voltage profiles, indicating highly stable reactions in the fluorocarbon electrodes (Figure S4). From these results, it is confirmed that the lithium- or sodium-ions and electrons are rapidly transported through electrodes and electrolyte with reversible lithiation/delithiation or sodiation/desodiation processes, respectively. To comprehend the effect of fluorine in the fluorocarbons on the electrochemical performances, the relation between the fluorine content in fluorocarbon and the obtained specific capacity in the second cycle at different current densities was investigated (Figure 8a,b). It is noteworthy that the specific

Figure 9. Electrochemical impedance spectra of fluorocarbons prepared at 500 and 800 °C and commercial graphite in (a) lithiumand (b) sodium-ion batteries.

tion.39 Further investigation of the application of these novel anodes in a full-cell configuration, cell balancing, and adjustment of pore size in the PVDF-derived fluorocarbon are in progress.

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CONCLUSIONS A facile and safe method for preparing fluorocarbon electrode materials from controlled carbonization of PVDF was reported along with a methodical study of the effect of the carbonization temperature on the characteristics of the prepared materials. Upon PVDF carbonization, a highly porous interconnected carbon network was obtained, and increasing carbonization temperature from 500 to 800 °C led to gradual removal of the fluorine content. The fluorocarbon synthesized at 500 °C (CP500) showed outstanding electrochemical capacities of 735 mAh g−1 in lithium-ion batteries and 269 mAh g−1 in sodiumion batteries at the current density of C/10 and 50 mAh g−1, respectively, which are higher than those of commercial graphite. Moreover, the fluorocarbon also showed excellent cyclic stability preserving 91.3 and 97.5% after 100 cycles in lithium- and sodium-ion batteries, respectively. The obtained specific capacity was highly dependent on the fluorine content in the PVDF-derived fluorocarbon in both lithium- and sodium-ion storage, suggesting that the highly electronegative fluorine facilitates insertion/extraction of lithium and sodium ions in the carbon. This study demonstrates that PVDF-derived fluorocarbon is expected as a promising anode for both rechargeable lithium- and sodium-ion batteries.

Figure 8. Correlation between the specific capacity at various current densities and fluorine content of fluorocarbon in (a) lithium- and (b) sodium-ion batteries.

capacity of the fluorocarbon was strongly dependent on the fluorine content for both lithium- and sodium-ion batteries. It is thought that the highly electronegative fluorine in the material enhances the electrochemically active sites and electronic conductivity, and hence increases the ion storage capacity. Electrochemical impedance spectroscopy (EIS) measurements were carried out to get insights into the difference in electrochemical performance and electrode kinetics of CP500, CP800, and commercial graphite anode. As shown in Figure 9, the semicircles across the high-medium frequency region followed by a linear slope at the low frequency are typical for carbon-based electrodes.37,38 The semicircles are attributed to the charge transfer resistance in the electrode/electrolyte interface, while the linear slope is due to Warburg impedance from solid-state metal ion diffusion. The smaller semicircle of CP500 compared to that of CP800 and commercial graphitic carbon indicates that CP500 possesses lower charge transfer resistance impedance, and thus fast electron transport can be expected in both lithium- and sodium-ion insertion/extracG

DOI: 10.1021/acsami.7b03456 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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(10) Zhou, X. S.; Dai, Z. H.; Bao, J. C.; Guo, Y. G. Wet Milled Synthesis of an Sb/MWCNT Nanocomposite for Improved Sodium Storage. J. Mater. Chem. A 2013, 1, 13727−13731. (11) Ding, J.; Wang, H.; Li, Z.; Kohandehghan, A.; Cui, K.; Xu, Z.; Zahiri, B.; Tan, X.; Lotfabad, E. M.; Olsen, B. C.; Mitlin, D. Carbon Nanosheet Frameworks Derived from Peat Moss as High Performance Sodium Ion Battery Anodes. ACS Nano 2013, 7, 11004−11015. (12) Alcántara, R.; Lavela, P.; Ortiz, G. F.; Tirado, J. L. Carbon Microspheres Obtained from Resorcinol-Formaldehyde as HighCapacity Electrodes for Sodium-Ion Batteries. Electrochem. Solid-State Lett. 2005, 8, A222−A225. (13) Wen, Y.; He, K.; Zhu, Y. J.; Han, F. D.; Xu, Y. H.; Matsuda, I.; Ishii, Y.; Cumings, J.; Wang, C. S. Expanded Graphite as Superior Anode for Sodium-Ion Batteries. Nat. Commun. 2014, 5, 4033−4042. (14) Nishi, Y. Lithium Ion Secondary Batteries; Past 10 Years and the Future. J. Power Sources 2001, 100, 101−106. (15) Palomares, V.; Serras, P.; Villaluenga, I.; Hueso, K. B.; Carretero-Gonzalez, J.; Rojo, T. Na-Ion Batteries, Recent Advances and Present Challenges to Become Low Cost Energy Storage Systems. Energy Environ. Sci. 2012, 5, 5884−5901. (16) Bulusheva, L. G.; Okotrub, A. V.; Kurenya, A. G.; Zhang, H.; Zhang, H.; Chen, X.; Song, H. Electrochemical Properties of NitrogenDoped Carbon Nanotube Anode in Li-Ion Batteries. Carbon 2011, 49, 4013−4023. (17) Paraknowitsch, J. P.; Thomas, A. Doping Carbons Beyond Nitrogen: An Overview of Advanced Heteroatom Doped Carbons with Boron, Sulphur and Phosphorus for Energy Applications. Energy Environ. Sci. 2013, 6, 2839−2855. (18) Wang, Z.; Qie, L.; Yuan, L.; Zhang, W.; Hu, X.; Huang, Y. Functionalized N-Doped Interconnected Carbon Nanofibers as an Anode Material for Sodium-Ion Storage with Excellent Performance. Carbon 2013, 55, 328−334. (19) Ling, C.; Mizuno, F. Boron-Doped Graphene as a Promising Anode for Na-Ion Batteries. Phys. Chem. Chem. Phys. 2014, 16, 10419−10424. (20) Li, X.; Liu, J.; Zhang, Y.; Li, Y.; Liu, H.; Meng, X.; Yang, J.; Geng, D.; Wang, D.; Li, R.; Sun, X. High Concentration Nitrogen Doped Carbon Nanotube Anodes with Superior Li+ Storage Performance for Lithium Rechargeable Battery Application. J. Power Sources 2012, 197, 238−245. (21) Jeon, I. Y.; Ju, M. J.; Xu, J. T.; Choi, H. J.; Seo, J. M.; Kim, M. J.; Choi, I. T.; Kim, H. M.; Kim, J. C.; Lee, J. J.; Liu, H. K.; Kim, H. K.; Dou, S. X.; Dai, L. M.; Baek, J. B. Edge-Fluorinated Graphene Nanoplatelets as High Performance Electrodes for Dye-Sensitized Solar Cells and Lithium Ion Batteries. Adv. Funct. Mater. 2015, 25, 1170−1179. (22) Nakajima, T. Surface Modification of Carbon Anodes for Secondary Lithium Battery by Fluorination. J. Fluorine Chem. 2007, 128, 277−284. (23) Robinson, J. T.; Burgess, J. S.; Junkermeier, C. E.; Badescu, S. C.; Reinecke, T. L.; Perkins, F. K.; Zalalutdniov, M. K.; Baldwin, J. W.; Culbertson, J. C.; Sheehan, P. E.; Snow, E. S. Properties of Fluorinated Graphene Films. Nano Lett. 2010, 10, 3001−3005. (24) Nakajima, T.; Shibata, S.; Naga, K.; Ohzawa, Y.; Tressaud, A.; Durand, E.; Groult, H.; Warmont, F. Surface Structure and Electrochemical Characteristics of Plasma-Fluorinated Petroleum Cokes for Lithium Ion Battery. J. Power Sources 2007, 168, 265−271. (25) Jayasinghe, R.; Thapa, A. K.; Dharmasena, R. R.; Nguyen, T. Q.; Pradhan, B. K.; Paudel, H. S.; Jasinski, J. B.; Sherehiy, A.; Yoshio, M.; Sumanasekera, G. U. Optimization of Multi-Walled Carbon Nanotube based CFx Electrodes for Improved Primary and Secondary Battery Performances. J. Power Sources 2014, 253, 404−411. (26) Hong, S.-M.; Lim, G.; Kim, S. H.; Kim, J. H.; Lee, K. B.; Ham, H. C. Preparation of Porous Carbons Based on Polyvinylidene Fluoride for CO2 Adsorption: A Combined Experimental and Computational Study. Microporous Mesoporous Mater. 2016, 219, 59−65. (27) Komaba, S.; Ishikawa, T.; Yabuuchi, N.; Murata, W.; Ito, A.; Ohsawa, Y. Fluorinated Ethylene Carbonate as Electrolyte Additive for

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b03456. Additional TGA data of PVDF, cyclic voltammograms for graphite, first cycle discharge and charge voltage profiles of fluorocarbons, and 5th and 100th cycle discharge and charge voltage profiles (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +82 2 3290 4851. Fax: +82 2 926 6102. E-mail: [email protected] (K.B.L.). *Tel.: +01 765 494 0044. Fax: +01 765 494 0805. E-mail: [email protected] (V.G.P.). ORCID

Ki Bong Lee: 0000-0001-9020-8646 Vilas G. Pol: 0000-0002-4866-117X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a National Research Foundation (NRF) grant funded by the Korean government’s Ministry of Science, ICT and Future Planning, through the Basic Science Research Program (2015R1A1A1A05001363), and the R&D Center for Reduction of Non-CO2 Greenhouse Gases (2013001690013) funded by the Korean Ministry of Environment (MOE) as the Global Top Environment R&D Program. VP would like to thank the US Office of Naval Research’s NEPTUNE Program for funding provided under grant number N00014-15-1-2833 for this work.

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REFERENCES

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DOI: 10.1021/acsami.7b03456 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX