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Hollow Glassy Carbon Nanofibers as Free-standing Li-ion Battery Anode
Electrochimica Acta 176 (2015) 1266–1271
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Enhanced Electrochemical Performance of Electrospun Ag/Hollow Glassy Carbon Nanofibers as Free-standing Li-ion Battery Anode Shilpa, Ashutosh Sharma* Department of Chemical Engineering, Indian Institute of Technology Kanpur, 208016, India
A R T I C L E I N F O
A B S T R A C T
Article history: Received 19 February 2015 Received in revised form 10 July 2015 Accepted 17 July 2015 Available online 26 July 2015
Silver with a high theoretical capacity for lithium storage is an attractive alloy based anode for Li-ion batteries, but large volume changes associated with AgLix alloy formation leads to electrode cracking, pulverization and rapid capacity fading. A buffer matrix, like the electrospun hollow carbon nanofibers, can reduce this problem to a great extent. Herein, we demonstrate the facile synthesis of a free-standing, binder free Ag-C hybrid electrode through co-axial electrospinning, where well dispersed Ag nanoparticles are embedded in hollow carbon nanofibers. Using this approach, the long cycle life of carbon is complemented with the high lithium storage capacity of Ag, resulting in a high performance anode. The Ag-C composite electrode delivers a capacity of 739 mAh g 1 (>conventional graphite anodes) at 50 mA g 1, with 85% capacity retention after 100 cycles. In addition, the Ag-C composite nanofibers are highly porous and exhibit a large accessible surface area (726.9 m2 g 1) with an average pore diameter of 6.07 nm. The encapsulation of Ag in the hollow interiors not only provides additional lithium storage sites but also enhances the electronic conductivity, which combined with the reduced lithium diffusion path lengths in the nanofibers result in faster charge-discharge kinetics and hence a high rate performance. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Lithium ion battery Silver nanoparticles Electrospinning Free-standing anode
1. Introduction Lithium ion batteries owing to their high energy densities are at present the most successful commercial energy storage systems dominating the portable electronics market. Also, being environment friendly, these batteries are gaining increasing research attention in the automobile industry for the development of clean electric/hybrid vehicles. However, the commercial graphite anode has a limited capacity for lithium storage (372 mA g 1) and is thereby inefficient in meeting the demands of higher energy/ power [1,2]. Over the past few years, lithium based alloys have gained considerable research attention as promising substitutes for the conventional graphite anodes. Tin-based composites as well as various transition metal oxides (MO) have been extensively studied in this context [3–5]. However, these systems react with lithium through the formation of Li2O followed by the formation of Li-M alloys. As the conversion reaction is electrochemically irreversible, it results in a large amount of irreversible capacity. To avert this problem, efforts are being focused to study the
* Corresponding author. Tel.: +91 512 259 7026. E-mail address: [email protected] (A. Sharma). http://dx.doi.org/10.1016/j.electacta.2015.07.093 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
alloying behavior of lithium with various elements like Bi, Ge, Mg, Sb, Si, Al, Pb, etc [6–16]. Silver is an attractive choice as it can accommodate a relatively high stoichiometric ratio of lithium through formation of various AgLix alloys (upto AgLi12) in a relatively low voltage range 0.25-0 V with respect to lithium [17]. Also, compared to Si and Sn, it has a high electrical conductivity and good diffusivity for lithium resulting in the faster electron transfer at high current rates. In most of the previous studies, silver has been used as a decorating material for enhancing the electrical conductivity of the composite [18–21]. As a lithium storage material, it has been used in the form of thin films in microbatteries, obtained through thermal evaporation, DC magnetron sputtering, rf sputtering or spray pyrolysis on metal substrates [17,22,23]. Similar to tin and silicon, silver also undergoes huge volume changes during charge/discharge reactions leading to electrode cracking and pulverization. Different approaches have been employed to improve the cycling stability of metal based electrodes. These include reducing the dimensions in the forms of nanofibers, nanoparticles, nanotubes, microporous and mesoprorous structures which can alleviate the volumetric stresses generated during cycling [24–26]. Other approaches like designing of silver based composites with carbon [27–29], conducting polymers [30], or by alloying with other metals have also been considered [31]. As regards the Ag/carbon composites explored
Shilpa, A. Sharma / Electrochimica Acta 176 (2015) 1266–1271
earlier, Fu et al. employed a hydrothermal method for the fabrication of Ag/C core-shell rods, Hsieh et al. used Ag nanorods inserted in graphene supports and Dai et al. followed the flowdirected method for fabricating self-binding Ag/Graphene composites [27–29]. In this work, a free-standing hybrid electrode composed of silver nanoparticles embedded in hollow carbon nanofibers (diameter 300 nm) has been fabricated through a facile and up-scalable coaxial electrsopinning method followed by stabilization and carbonization steps. Our electrode is in the form of a thin (60 mm) nanofiber mat, which being mechanically stable and electrically conducting, has been used directly as an anode without requiring any inerts such as binder, conductive additive or a current collector, thus offering possibilities of delivering higher energy density. The embedding of silver nanoparticles in hollow nanofibers serves a dual role, by enhancing electronic conductivity along the fibers, and by forming alloys with lithium itself, thus increasing the overall capacity. The carbon in the nanofibers on the other hand helps to buffer the volume fluctuations associated with silver alloying-dealloying process producing a more stable cycling and rate performance. 2. Experimental 2.1. Materials Polyacrylonitrile (PAN, Mw: 150000), Poly(methyl methacrylate) (PMMA, Mw:120 000), N,N-dimethylformamide (DMF), and silver nitrate (AgNO3) used were obtained from Sigma Aldrich and were used directly as received. 2.2. Composite fabrication Co-axial electrospinning is employed followed by stabilization and carbonization steps for fabricating carbon nanofibers embedded with silver nanoparticles. Details of a typical co-axial electrsopinning setup are described elsewhere [32,33]. Briefly, two different polymer solutions were simultaneously pumped through the inner and outer co-axial needles (26 and 18 gauges respectively). The inner needle contained 5 wt% AgNO3 homogenously mixed in a 24 wt% PMMA solution, whereas the outer coaxial needle had a 9 wt % PAN solution in DMF. Electrospinning was performed in an environment with 30–40 % relative humidity. The flow rates of outer and inner fluids were kept constant at 4 and 7 ml min 1, respectively with the help of two separate syringe pumps. The chosen concentrations of Ag precursor, polymer, flow rates and applied voltage were optimized to obtain defect free uniform fibers without beads. On application of a high voltage, the polymer drop at the tip of the co-axial spinneret underwent stretching under the action of electrostatic forces producing co-axial nanofibers which were collected in an aligned manner on a rotating drum electrode. The distance between the tip of the spinneret and the rotating drum collector was fixed at 8 cm and the applied voltage was kept constant at 15 kV. A nanofiber mat approximately 60 mm was obtained after 8 hours of electrospinning. The as-spun nanofiber mat was subjected to stabilization in air atmosphere by heating at a rate of 1 C min 1 and holding the temperature constant at 250 C for 6 hours. During stabilization the nanofibers undergo cross-linking which is crucial for maintaining the structural integrity of the fibers [34]. After stabilization, the nanofiber mat was carbonized in nitrogen environment (flow rate 150 ml min 1) by heating at rate of 2 C min 1 upto 800 C with a holding time of 1 hour. During heating process, AgNO3 in the nanofibers completely decomposes 450 C yielding Ag nanoparticles.
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2.3. Structural and Electrochemical Characterization The morphology and the structure of the carbonized Ag-C composite nanofibers was studied using a field emission scanning electron microscope (FE-SEM, Quanta 200, Zeiss, Germany) and Transmission electron microscope (TEM, JEM-2100F JEOL, Japan). The elemental composition analysis of nanofibers was done using Energy dispersive X-ray spectroscopy (EDX) integrated with FESEM. The crystallinity of the composite was characterized through XRD analysis (X’Pert PRO, PANanalytical, Netherlands) using Cu Ka (l = 1.5416 Å) radiation. The Nitrogen adsorption–desorption isotherms were recorded using an Autosorb-1C machine (Quantachrome, USA) at 77K. The specific surface area and the pore size distribution of the composite fibers were estimated using the Brunauer–Emmett–Teller (BET) Barrett–Joyner–Halenda (BJH) and Density functional theory (DFT) methods. To conduct electrochemical measurements, the CNF mat was cut into a circular shape appropriate for a 2032 coin cell. The half cell was assembled inside a Glove Box having inert argon atmosphere. Lithium foil 750 mm (Sigma–Aldrich) thick was used as the counter electrode with polypropylene membrane (Celgard 2320) as separator and a 1 M LiPF6 solution in a solvent mixture of ethylene carbonate and diethyl carbonate (EC/DEC, 1:1 (v/v), Merck) as the electrolyte. After allowing 12 hours of ageing time, the cell was galvanostatically cycled between 0.01 to 2.5 V at current densities 50, 100, 200, 300 and 500 mA g 1 using a battery analyzer (MTI Corporation, USA). Cyclic voltammetry was carried out at a slow scan rate of 0.1 mV s 1 whereas impedance measurements were performed in the frequency range 100 kHz to 10 mHz with a perturbation a.c. amplitude of 10 mV using a PGSTAT 302N electrochemical workstation (Metrohm Autolab, Netherlands). 3. Results and Discussion Fig. 1(a) shows the FE-SEM image of the Ag-C composite indicating a well interconnected nanofiber web which can be cut into suitable dimensions and used directly as free-standing electrode in Li-ion battery. It can be observed that the nanofibers have a uniform average diameter 300 nm. The TEM image, Fig. 1(b) reveals that the nanofibers have hollow interiors impregnated with silver nanoparticles. The surface of the nanofibers is 30 nm thick with some silver particles also embedded in it. Energy dispersive X-ray spectra collected from different locations on Ag-C coaxial nanofibers reveal that it is composed of 25 wt% silver and 74 wt% carbon (Fig. 2). The crystallographic structure of the Ag-C composite nanofibers was characterized by XRD study (Fig. 3). The diffraction peaks at 2Q = 38.05, 44.30, 64.46, 77.44 correspond to the (111), (2 0 0), (2 2 0) and (3 11) reflection planes of the fcc Ag crystal confirming the formation of Ag nanoparticles on reduction during calcination and carbonization steps. The peak observed at 2Q = 26.01 can be assigned to the (0 0 2) carbon plane. The interlayer spacing (d002) as calculated using Bragg's equation is 0.342 nm which is close to that of graphitic carbon (0.335 nm). The Ag nanoparticles are well dispersed inside the hollow carbon nanofibers and exhibit polycrystalline feature. Fig. 4 depicts the nitrogen adsorption–desorption isotherm as well as the pore size distribution of the Ag-C composite nanofibers. The composite exhibits a high porosity with specific surface area of 726.9 m2 g 1. The total estimated pore volume is 1.104 cc g 1, of which meso and micro-pores contribute 0.3402 and 0.2956 cc g 1, respectively. The average pore diameter is found to be 6.07 nm. The adsorption/desorption curve represents a type IV isotherm typical for mesoporous materials. The hysteresis loop observed in certain portion of external pressure indicates capillary condensation in the
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Fig. 1. (a) FE-SEM micrograph of Ag-C composite nanofiber (inset showing the image of the free-standing nanofiber mat) (b) TEM image of the Ag-C composite showing silver nanoparticles embedded in hollow carbon nanofibers.
mesopores [35]. The high specific surface area of the composite enhances the interfacial contact between the Li ions and the electrode resulting in an improved Li storage capacity due to the presence of an increased number of accessible Li active sites. It also shortens of lithium diffusion and migration paths resulting in an improved rate performance. Fig. 5 displays the cyclic voltammogram of as-fabricated halfcell with free-standing Ag-C electrode as the anode, conducted at a slow scan rate of 0.1 mV s 1 between 0.01 to 3 V. The electrochemical process in the Ag-C electrode occurs through several steps including the formation of a solid electrolyte interface (SEI) layer in the voltage range 1.2–0.8 V. The predominant cathodic peak observed at around 0.2–0.35 V can be ascribed to the lithium insertion process. A broad anodic peak is observed at 0.35 V which corresponds to the extraction of lithium from the Ag-C composite electrode. The peak, however is not clearly resolved under the conditions used as extraction occurs in stages with voltage plateaus in the range 0.1–0.5 V. As the carbon in Ag-C composite is obtained by the pyrolysis of polyacrylonitrile polymer, it undergoes Li-insertion/extraction process in the voltage range below 1V as is typical for hard carbons [36]. As lithium insertion/ extraction occurs at potentials below 1V for both carbon and Ag, the redox peaks due to lithium insertion into carbon and that of Ag are not distinctly discernible in the cyclic voltammogram displayed. Fig. 6 shows the galvanostatic charge/discharge profiles of the Ag-C electrode for 1st, 2nd and 100th cycles at a current density of 50 mA g 1. The initial discharge/charge capacities of the composite (based on the total electrode mass) are 1035.25 and 739.65 mAh g 1 respectively, with an irreversible capacity loss of 295.6 mAh g 1 showing an initial columbic efficiency of 71.45%. Fig. 7 compares the cycling performance of the Ag-C composite electrode with a hollow carbon nanofiber (CNF) electrode and a solid CNF electrode for the first 100 cycles. After the first discharge cycle which leads to the SEI formation, the electrochemical reactions stabilize and the composite (Ag-C) delivers a reversible capacity of 600.15 mAh g 1 at the end of 100 cycles showing only a small capacity fade (18%). More remarkable is almost no loss of capacity after the initial 20 cycles indicating a robust stabilized
Fig. 2. EDX spectra of Ag-C coaxial composite nanofibers taken at three different regions on the composite reveal an average weight percentage of 25% in the composite.
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Fig. 5. Cyclic voltammogram of Ag-C electrode performed at a slow scan rate of 0.1 mV s 1. Fig. 3. X-ray diffraction pattern of Ag-C composite confirming fcc cubic structure.
Ag-C structure and composition. Although, the hollow and solid CNFs exhibit comparable capacity retention, they display a lower lithium storage capacity of 387.16 mAh g 1 and 228.18 mAh g 1 respectively, at the end of 100 cycles. The increased capacity of the Ag-C composite can be ascribed to the incorporation of silver nanoparticles in its 1-dimensional nanofibers. The void space surrounding the silver nanoparticles as well as the carbon in nanofibers sufficiently relax the volumetric stress generated on expansion/contraction of silver on alloying/dealloying producing a stable cycling performance. Fig. 8 shows the FE-SEM images of the nanofiber mat electrode (Ag-C) at the end of 100 cycles at different magnifications. It can seen in images that the electrode has sufficient mechanical stability owing to its intertwined web of nanofibers and thus is able to retain its structure and the fiber morphology even after 100 cycles. The EDX analysis conducted on the Ag-C electrode at the end of 100 cycles reveal that Ag dispersion of 22 wt% is retained in the electrode after 100 cycles (Fig. 9). The elements P, F and O detected in the EDX spectra are due the electrolyte and products of electrolyte decomposition in the SEI. (Li is not observed in the obtained EDX spectra as EDX detector can not detect elements below atomic number 5).
Fig. 6. Galvanostatic charge–discharge profile of the Ag-C electrode for the 1st, 2nd and 100th cycles at 50 mA g 1 current density.
Fig. 4. N2 adsorption/desorption isotherm and pore-size distribution of the Ag-C composite.
Fig. 7. Comparison of the specific capacity of Ag-C composite electrode with hollow and solid CNF electrodes.
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Fig. 8. (a), (b), (c) and (d): FE-SEM images of the Ag-C composite electrode after 100 cycles at different magnifications.
of 15 minutes before commencing the measurement. Fig. 12 displays impedance spectra of the Ag-C composite electrode measured in the as-fabricated state and at the end of 5th, 40th and 60th discharge cycles. In each spectrum, two semicircles can be easily identified in the high and intermediate frequency zone followed by a low frequency beeline. In the high frequency zone, the intercept on the real axis denotes the combined resistance due to the electrolyte, electrical contacts and the separator (Rs). The semicircle in the high frequency region represents the lithium ion migration impedance through the solid electrolyte interface layer (RSEI). The mid-frequency semicircle is associated with the alloying process that the composite metal/carbon anode undergoes with lithium and the charge transfer resistance encountered at the electrolyte-anode interface (RCT). The sloping beeline in the low Fig. 9. EDX spectra of the Ag-C electrode taken at the end of 100 charge/discharge cycles.
Fig. 10 shows the ex situ XRD pattern obtained for the Ag-C electrode after 100 cycles (charged to 3 V). Diffraction peaks are observed at 2Q = 38.12, 44.25, 64.43 and 77.44 corresponding to the (111), (2 0 0), (2 2 0) and (3 11) planes of the fcc Ag crystal. Fig. 11 displays the rate performance of the Ag-C electrode cycled continually at different current rates. The composite delivers a capacity of 521.9, 464.1, 437.8 and 399.1 mAh g 1 at 100, 200, 300 and 500 mA g 1, respectively. Again, when the current is lowered back to 50 mA g 1, the composite is able to resume a capacity of 608.3 mAh g 1 demonstrating excellent rate capability. This excellent rate performance indicates good electronic conductivity in the composite which enables faster charge transport. In order to further understand the electrochemical behavior of the Ag-C composite electrode, electrochemical impedance spectroscopy (EIS) measurements were performed in the frequency range 100 kHz–10 mHz with an a.c. voltage amplitude of 5 mV. For each EIS measurement, the cell was first discharged at a constant current (50 mA g 1) upto 0.05 V and was allowed a relaxation time
Fig. 10. Ex situ XRD pattern of the Ag-C composite electrode at the end of 100th cycle (charged to 3 V).
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alloys but also enhances the electronic conductivity resulting in faster charge-discharge kinetics. The presence of a mechanically strong and flexible carbon matrix in the nanofibers combined with the void spaces around the silver particles plays a shielding role absorbing the volumetric stresses developed during chargedischarge process. The superior electrochemical performance suggests that the electrode may be a potential candidate for future LIBs in applications requiring high energy/power densities. Acknowledgement The support received from Department of Science and Technology (DST), New Delhi to Nanoscience Center at IITK Kanpur is gratefully acknowledged. References
Fig. 11. Rate performance of the Ag-C electrode at different current densities.
Fig. 12. Nyquist plot of the Ag-C composite electrode in the frequency range 100 kHz–10 mHz: as fabricated, and at the end of 5th, 40th and 60th discharge cycles.
frequency zone denotes the diffusion of lithium into the bulk of the electrode. Based on the Nyquist plot, an equivalent circuit model is constructed (shown in the inset of Fig. 12), consisting of two resistors and constant phase elements (CPE1 and CPE2) in parallel with a Warburg impedance element. 4. Conclusion A simple co-axial electrospinning technique with the potential for upscaling is used to fabricate a free standing, Ag-C composite electrode in which silver nanoparticles are embedded in hollow carbon nanofibers. The electrode displays remarkably enhanced capacity (739.65 mAh g 1) with stable cycling (<20% capacity fade after 100 cycles and no significant loss after 20 cycles) and rate performance (399.15 mAh g 1 at 500 mA g 1), compared to hollow CNFs (387.16 mAh g 1) and solid CNFs (228.18 mAh g 1) without encapsulated silver. The incorporation of silver in the nanofibers not only provides additional lithium storage sites by forming AgLix
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Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Enhanced Electrochemical Performance of Electrospun Ag/Hollow Glassy Carbon Nanofibers as Free-standing Li-ion Battery Anode Shilpa, Ashutosh Sharma* Department of Chemical Engineering, Indian Institute of Technology Kanpur, 208016, India
A R T I C L E I N F O
A B S T R A C T
Article history: Received 19 February 2015 Received in revised form 10 July 2015 Accepted 17 July 2015 Available online 26 July 2015
Silver with a high theoretical capacity for lithium storage is an attractive alloy based anode for Li-ion batteries, but large volume changes associated with AgLix alloy formation leads to electrode cracking, pulverization and rapid capacity fading. A buffer matrix, like the electrospun hollow carbon nanofibers, can reduce this problem to a great extent. Herein, we demonstrate the facile synthesis of a free-standing, binder free Ag-C hybrid electrode through co-axial electrospinning, where well dispersed Ag nanoparticles are embedded in hollow carbon nanofibers. Using this approach, the long cycle life of carbon is complemented with the high lithium storage capacity of Ag, resulting in a high performance anode. The Ag-C composite electrode delivers a capacity of 739 mAh g 1 (>conventional graphite anodes) at 50 mA g 1, with 85% capacity retention after 100 cycles. In addition, the Ag-C composite nanofibers are highly porous and exhibit a large accessible surface area (726.9 m2 g 1) with an average pore diameter of 6.07 nm. The encapsulation of Ag in the hollow interiors not only provides additional lithium storage sites but also enhances the electronic conductivity, which combined with the reduced lithium diffusion path lengths in the nanofibers result in faster charge-discharge kinetics and hence a high rate performance. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Lithium ion battery Silver nanoparticles Electrospinning Free-standing anode
1. Introduction Lithium ion batteries owing to their high energy densities are at present the most successful commercial energy storage systems dominating the portable electronics market. Also, being environment friendly, these batteries are gaining increasing research attention in the automobile industry for the development of clean electric/hybrid vehicles. However, the commercial graphite anode has a limited capacity for lithium storage (372 mA g 1) and is thereby inefficient in meeting the demands of higher energy/ power [1,2]. Over the past few years, lithium based alloys have gained considerable research attention as promising substitutes for the conventional graphite anodes. Tin-based composites as well as various transition metal oxides (MO) have been extensively studied in this context [3–5]. However, these systems react with lithium through the formation of Li2O followed by the formation of Li-M alloys. As the conversion reaction is electrochemically irreversible, it results in a large amount of irreversible capacity. To avert this problem, efforts are being focused to study the
* Corresponding author. Tel.: +91 512 259 7026. E-mail address: [email protected] (A. Sharma). http://dx.doi.org/10.1016/j.electacta.2015.07.093 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
alloying behavior of lithium with various elements like Bi, Ge, Mg, Sb, Si, Al, Pb, etc [6–16]. Silver is an attractive choice as it can accommodate a relatively high stoichiometric ratio of lithium through formation of various AgLix alloys (upto AgLi12) in a relatively low voltage range 0.25-0 V with respect to lithium [17]. Also, compared to Si and Sn, it has a high electrical conductivity and good diffusivity for lithium resulting in the faster electron transfer at high current rates. In most of the previous studies, silver has been used as a decorating material for enhancing the electrical conductivity of the composite [18–21]. As a lithium storage material, it has been used in the form of thin films in microbatteries, obtained through thermal evaporation, DC magnetron sputtering, rf sputtering or spray pyrolysis on metal substrates [17,22,23]. Similar to tin and silicon, silver also undergoes huge volume changes during charge/discharge reactions leading to electrode cracking and pulverization. Different approaches have been employed to improve the cycling stability of metal based electrodes. These include reducing the dimensions in the forms of nanofibers, nanoparticles, nanotubes, microporous and mesoprorous structures which can alleviate the volumetric stresses generated during cycling [24–26]. Other approaches like designing of silver based composites with carbon [27–29], conducting polymers [30], or by alloying with other metals have also been considered [31]. As regards the Ag/carbon composites explored
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earlier, Fu et al. employed a hydrothermal method for the fabrication of Ag/C core-shell rods, Hsieh et al. used Ag nanorods inserted in graphene supports and Dai et al. followed the flowdirected method for fabricating self-binding Ag/Graphene composites [27–29]. In this work, a free-standing hybrid electrode composed of silver nanoparticles embedded in hollow carbon nanofibers (diameter 300 nm) has been fabricated through a facile and up-scalable coaxial electrsopinning method followed by stabilization and carbonization steps. Our electrode is in the form of a thin (60 mm) nanofiber mat, which being mechanically stable and electrically conducting, has been used directly as an anode without requiring any inerts such as binder, conductive additive or a current collector, thus offering possibilities of delivering higher energy density. The embedding of silver nanoparticles in hollow nanofibers serves a dual role, by enhancing electronic conductivity along the fibers, and by forming alloys with lithium itself, thus increasing the overall capacity. The carbon in the nanofibers on the other hand helps to buffer the volume fluctuations associated with silver alloying-dealloying process producing a more stable cycling and rate performance. 2. Experimental 2.1. Materials Polyacrylonitrile (PAN, Mw: 150000), Poly(methyl methacrylate) (PMMA, Mw:120 000), N,N-dimethylformamide (DMF), and silver nitrate (AgNO3) used were obtained from Sigma Aldrich and were used directly as received. 2.2. Composite fabrication Co-axial electrospinning is employed followed by stabilization and carbonization steps for fabricating carbon nanofibers embedded with silver nanoparticles. Details of a typical co-axial electrsopinning setup are described elsewhere [32,33]. Briefly, two different polymer solutions were simultaneously pumped through the inner and outer co-axial needles (26 and 18 gauges respectively). The inner needle contained 5 wt% AgNO3 homogenously mixed in a 24 wt% PMMA solution, whereas the outer coaxial needle had a 9 wt % PAN solution in DMF. Electrospinning was performed in an environment with 30–40 % relative humidity. The flow rates of outer and inner fluids were kept constant at 4 and 7 ml min 1, respectively with the help of two separate syringe pumps. The chosen concentrations of Ag precursor, polymer, flow rates and applied voltage were optimized to obtain defect free uniform fibers without beads. On application of a high voltage, the polymer drop at the tip of the co-axial spinneret underwent stretching under the action of electrostatic forces producing co-axial nanofibers which were collected in an aligned manner on a rotating drum electrode. The distance between the tip of the spinneret and the rotating drum collector was fixed at 8 cm and the applied voltage was kept constant at 15 kV. A nanofiber mat approximately 60 mm was obtained after 8 hours of electrospinning. The as-spun nanofiber mat was subjected to stabilization in air atmosphere by heating at a rate of 1 C min 1 and holding the temperature constant at 250 C for 6 hours. During stabilization the nanofibers undergo cross-linking which is crucial for maintaining the structural integrity of the fibers [34]. After stabilization, the nanofiber mat was carbonized in nitrogen environment (flow rate 150 ml min 1) by heating at rate of 2 C min 1 upto 800 C with a holding time of 1 hour. During heating process, AgNO3 in the nanofibers completely decomposes 450 C yielding Ag nanoparticles.
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2.3. Structural and Electrochemical Characterization The morphology and the structure of the carbonized Ag-C composite nanofibers was studied using a field emission scanning electron microscope (FE-SEM, Quanta 200, Zeiss, Germany) and Transmission electron microscope (TEM, JEM-2100F JEOL, Japan). The elemental composition analysis of nanofibers was done using Energy dispersive X-ray spectroscopy (EDX) integrated with FESEM. The crystallinity of the composite was characterized through XRD analysis (X’Pert PRO, PANanalytical, Netherlands) using Cu Ka (l = 1.5416 Å) radiation. The Nitrogen adsorption–desorption isotherms were recorded using an Autosorb-1C machine (Quantachrome, USA) at 77K. The specific surface area and the pore size distribution of the composite fibers were estimated using the Brunauer–Emmett–Teller (BET) Barrett–Joyner–Halenda (BJH) and Density functional theory (DFT) methods. To conduct electrochemical measurements, the CNF mat was cut into a circular shape appropriate for a 2032 coin cell. The half cell was assembled inside a Glove Box having inert argon atmosphere. Lithium foil 750 mm (Sigma–Aldrich) thick was used as the counter electrode with polypropylene membrane (Celgard 2320) as separator and a 1 M LiPF6 solution in a solvent mixture of ethylene carbonate and diethyl carbonate (EC/DEC, 1:1 (v/v), Merck) as the electrolyte. After allowing 12 hours of ageing time, the cell was galvanostatically cycled between 0.01 to 2.5 V at current densities 50, 100, 200, 300 and 500 mA g 1 using a battery analyzer (MTI Corporation, USA). Cyclic voltammetry was carried out at a slow scan rate of 0.1 mV s 1 whereas impedance measurements were performed in the frequency range 100 kHz to 10 mHz with a perturbation a.c. amplitude of 10 mV using a PGSTAT 302N electrochemical workstation (Metrohm Autolab, Netherlands). 3. Results and Discussion Fig. 1(a) shows the FE-SEM image of the Ag-C composite indicating a well interconnected nanofiber web which can be cut into suitable dimensions and used directly as free-standing electrode in Li-ion battery. It can be observed that the nanofibers have a uniform average diameter 300 nm. The TEM image, Fig. 1(b) reveals that the nanofibers have hollow interiors impregnated with silver nanoparticles. The surface of the nanofibers is 30 nm thick with some silver particles also embedded in it. Energy dispersive X-ray spectra collected from different locations on Ag-C coaxial nanofibers reveal that it is composed of 25 wt% silver and 74 wt% carbon (Fig. 2). The crystallographic structure of the Ag-C composite nanofibers was characterized by XRD study (Fig. 3). The diffraction peaks at 2Q = 38.05, 44.30, 64.46, 77.44 correspond to the (111), (2 0 0), (2 2 0) and (3 11) reflection planes of the fcc Ag crystal confirming the formation of Ag nanoparticles on reduction during calcination and carbonization steps. The peak observed at 2Q = 26.01 can be assigned to the (0 0 2) carbon plane. The interlayer spacing (d002) as calculated using Bragg's equation is 0.342 nm which is close to that of graphitic carbon (0.335 nm). The Ag nanoparticles are well dispersed inside the hollow carbon nanofibers and exhibit polycrystalline feature. Fig. 4 depicts the nitrogen adsorption–desorption isotherm as well as the pore size distribution of the Ag-C composite nanofibers. The composite exhibits a high porosity with specific surface area of 726.9 m2 g 1. The total estimated pore volume is 1.104 cc g 1, of which meso and micro-pores contribute 0.3402 and 0.2956 cc g 1, respectively. The average pore diameter is found to be 6.07 nm. The adsorption/desorption curve represents a type IV isotherm typical for mesoporous materials. The hysteresis loop observed in certain portion of external pressure indicates capillary condensation in the
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Fig. 1. (a) FE-SEM micrograph of Ag-C composite nanofiber (inset showing the image of the free-standing nanofiber mat) (b) TEM image of the Ag-C composite showing silver nanoparticles embedded in hollow carbon nanofibers.
mesopores [35]. The high specific surface area of the composite enhances the interfacial contact between the Li ions and the electrode resulting in an improved Li storage capacity due to the presence of an increased number of accessible Li active sites. It also shortens of lithium diffusion and migration paths resulting in an improved rate performance. Fig. 5 displays the cyclic voltammogram of as-fabricated halfcell with free-standing Ag-C electrode as the anode, conducted at a slow scan rate of 0.1 mV s 1 between 0.01 to 3 V. The electrochemical process in the Ag-C electrode occurs through several steps including the formation of a solid electrolyte interface (SEI) layer in the voltage range 1.2–0.8 V. The predominant cathodic peak observed at around 0.2–0.35 V can be ascribed to the lithium insertion process. A broad anodic peak is observed at 0.35 V which corresponds to the extraction of lithium from the Ag-C composite electrode. The peak, however is not clearly resolved under the conditions used as extraction occurs in stages with voltage plateaus in the range 0.1–0.5 V. As the carbon in Ag-C composite is obtained by the pyrolysis of polyacrylonitrile polymer, it undergoes Li-insertion/extraction process in the voltage range below 1V as is typical for hard carbons [36]. As lithium insertion/ extraction occurs at potentials below 1V for both carbon and Ag, the redox peaks due to lithium insertion into carbon and that of Ag are not distinctly discernible in the cyclic voltammogram displayed. Fig. 6 shows the galvanostatic charge/discharge profiles of the Ag-C electrode for 1st, 2nd and 100th cycles at a current density of 50 mA g 1. The initial discharge/charge capacities of the composite (based on the total electrode mass) are 1035.25 and 739.65 mAh g 1 respectively, with an irreversible capacity loss of 295.6 mAh g 1 showing an initial columbic efficiency of 71.45%. Fig. 7 compares the cycling performance of the Ag-C composite electrode with a hollow carbon nanofiber (CNF) electrode and a solid CNF electrode for the first 100 cycles. After the first discharge cycle which leads to the SEI formation, the electrochemical reactions stabilize and the composite (Ag-C) delivers a reversible capacity of 600.15 mAh g 1 at the end of 100 cycles showing only a small capacity fade (18%). More remarkable is almost no loss of capacity after the initial 20 cycles indicating a robust stabilized
Fig. 2. EDX spectra of Ag-C coaxial composite nanofibers taken at three different regions on the composite reveal an average weight percentage of 25% in the composite.
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Fig. 5. Cyclic voltammogram of Ag-C electrode performed at a slow scan rate of 0.1 mV s 1. Fig. 3. X-ray diffraction pattern of Ag-C composite confirming fcc cubic structure.
Ag-C structure and composition. Although, the hollow and solid CNFs exhibit comparable capacity retention, they display a lower lithium storage capacity of 387.16 mAh g 1 and 228.18 mAh g 1 respectively, at the end of 100 cycles. The increased capacity of the Ag-C composite can be ascribed to the incorporation of silver nanoparticles in its 1-dimensional nanofibers. The void space surrounding the silver nanoparticles as well as the carbon in nanofibers sufficiently relax the volumetric stress generated on expansion/contraction of silver on alloying/dealloying producing a stable cycling performance. Fig. 8 shows the FE-SEM images of the nanofiber mat electrode (Ag-C) at the end of 100 cycles at different magnifications. It can seen in images that the electrode has sufficient mechanical stability owing to its intertwined web of nanofibers and thus is able to retain its structure and the fiber morphology even after 100 cycles. The EDX analysis conducted on the Ag-C electrode at the end of 100 cycles reveal that Ag dispersion of 22 wt% is retained in the electrode after 100 cycles (Fig. 9). The elements P, F and O detected in the EDX spectra are due the electrolyte and products of electrolyte decomposition in the SEI. (Li is not observed in the obtained EDX spectra as EDX detector can not detect elements below atomic number 5).
Fig. 6. Galvanostatic charge–discharge profile of the Ag-C electrode for the 1st, 2nd and 100th cycles at 50 mA g 1 current density.
Fig. 4. N2 adsorption/desorption isotherm and pore-size distribution of the Ag-C composite.
Fig. 7. Comparison of the specific capacity of Ag-C composite electrode with hollow and solid CNF electrodes.
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Fig. 8. (a), (b), (c) and (d): FE-SEM images of the Ag-C composite electrode after 100 cycles at different magnifications.
of 15 minutes before commencing the measurement. Fig. 12 displays impedance spectra of the Ag-C composite electrode measured in the as-fabricated state and at the end of 5th, 40th and 60th discharge cycles. In each spectrum, two semicircles can be easily identified in the high and intermediate frequency zone followed by a low frequency beeline. In the high frequency zone, the intercept on the real axis denotes the combined resistance due to the electrolyte, electrical contacts and the separator (Rs). The semicircle in the high frequency region represents the lithium ion migration impedance through the solid electrolyte interface layer (RSEI). The mid-frequency semicircle is associated with the alloying process that the composite metal/carbon anode undergoes with lithium and the charge transfer resistance encountered at the electrolyte-anode interface (RCT). The sloping beeline in the low Fig. 9. EDX spectra of the Ag-C electrode taken at the end of 100 charge/discharge cycles.
Fig. 10 shows the ex situ XRD pattern obtained for the Ag-C electrode after 100 cycles (charged to 3 V). Diffraction peaks are observed at 2Q = 38.12, 44.25, 64.43 and 77.44 corresponding to the (111), (2 0 0), (2 2 0) and (3 11) planes of the fcc Ag crystal. Fig. 11 displays the rate performance of the Ag-C electrode cycled continually at different current rates. The composite delivers a capacity of 521.9, 464.1, 437.8 and 399.1 mAh g 1 at 100, 200, 300 and 500 mA g 1, respectively. Again, when the current is lowered back to 50 mA g 1, the composite is able to resume a capacity of 608.3 mAh g 1 demonstrating excellent rate capability. This excellent rate performance indicates good electronic conductivity in the composite which enables faster charge transport. In order to further understand the electrochemical behavior of the Ag-C composite electrode, electrochemical impedance spectroscopy (EIS) measurements were performed in the frequency range 100 kHz–10 mHz with an a.c. voltage amplitude of 5 mV. For each EIS measurement, the cell was first discharged at a constant current (50 mA g 1) upto 0.05 V and was allowed a relaxation time
Fig. 10. Ex situ XRD pattern of the Ag-C composite electrode at the end of 100th cycle (charged to 3 V).
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alloys but also enhances the electronic conductivity resulting in faster charge-discharge kinetics. The presence of a mechanically strong and flexible carbon matrix in the nanofibers combined with the void spaces around the silver particles plays a shielding role absorbing the volumetric stresses developed during chargedischarge process. The superior electrochemical performance suggests that the electrode may be a potential candidate for future LIBs in applications requiring high energy/power densities. Acknowledgement The support received from Department of Science and Technology (DST), New Delhi to Nanoscience Center at IITK Kanpur is gratefully acknowledged. References
Fig. 11. Rate performance of the Ag-C electrode at different current densities.
Fig. 12. Nyquist plot of the Ag-C composite electrode in the frequency range 100 kHz–10 mHz: as fabricated, and at the end of 5th, 40th and 60th discharge cycles.
frequency zone denotes the diffusion of lithium into the bulk of the electrode. Based on the Nyquist plot, an equivalent circuit model is constructed (shown in the inset of Fig. 12), consisting of two resistors and constant phase elements (CPE1 and CPE2) in parallel with a Warburg impedance element. 4. Conclusion A simple co-axial electrospinning technique with the potential for upscaling is used to fabricate a free standing, Ag-C composite electrode in which silver nanoparticles are embedded in hollow carbon nanofibers. The electrode displays remarkably enhanced capacity (739.65 mAh g 1) with stable cycling (<20% capacity fade after 100 cycles and no significant loss after 20 cycles) and rate performance (399.15 mAh g 1 at 500 mA g 1), compared to hollow CNFs (387.16 mAh g 1) and solid CNFs (228.18 mAh g 1) without encapsulated silver. The incorporation of silver in the nanofibers not only provides additional lithium storage sites by forming AgLix
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