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Electrochemical properties of carbon nanocoils and hollow graphite fibers as anodes for rechargeable lithium ion batteries
Accepted Manuscript Title: Electrochemical properties of carbon nanocoils and hollow graphite fibers as anodes for rechargeable lithium ion batteries Author: Liyong Wang Zhanjun Liu Quangui Guo Guizhen Wang Jinhua Yang Peng Li Xianglei Wang Lang Liu PII: DOI: Reference:
S0013-4686(16)30734-4 http://dx.doi.org/doi:10.1016/j.electacta.2016.03.160 EA 26990
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
28-1-2016 21-3-2016 26-3-2016
Please cite this article as: Liyong Wang, Zhanjun Liu, Quangui Guo, Guizhen Wang, Jinhua Yang, Peng Li, Xianglei Wang, Lang Liu, Electrochemical properties of carbon nanocoils and hollow graphite fibers as anodes for rechargeable lithium ion batteries, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.03.160 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Electrochemical properties of carbon nanocoils and hollow graphite fibers as anodes for rechargeable lithium ion batteries Liyong Wang a, b, Zhanjun Liua,*, Quangui Guoa, Guizhen Wang c, Jinhua Yang d, Peng Li e, Xianglei Wang f, Lang Liu a a
Key laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy
of Sciences, Taiyuan 030001, China b
Hebei Normal University for Nationalities, Chengde 067000, China
c
College of Materials and Chemical Engineering，Hainan University, Hainan 570228,
China d
e
Avic composite corporation Ltd., Beijing 101300, China
Department of Materials Engineering, Taiyuan Institute of Technology, Taiyuan
030008, China f
Ordos Qidi Incubator Service Center, Ordos 017010, China
* Corresponding author: Fax: +86 351 4084 106. E-mail address: [email protected] (Z. Liu).
1
ABSTRACT Carbon nanocoils (CNCs) have been used as anode materials for preparation of lithium ion batteries. As pure carbon material without any chemical modification, the graphitized CNCs anode exhibited larger capacities with good Coulombic efficiency, a higher rate capability, and better reversibility than the hollow graphite fibers (HGFs) anode. The excellent performance of the CNCs was possibly ascribed to the special structure and the high degree of graphitization. As a result, the CNCs anode exhibited high reversible capacity of 385.5 mA h g-1 at 50 mA g-1, 104.7% reversible capacity retention after 105 cycles, and superior reversible capability of 177.4 mA h g-1 at 1A g-1 after 100 cycles. This result indicated that CNCs could be an attractive choice as anode material for high-energy density and high-power lithium-ion batteries. Key words Carbon nanocoils, Hollow graphite fibers, Anode materials, Lithium ion batteries, High rate performance
2
1. Introduction The great demand for high-energy density and high-power lithium-ion batteries (LIBs) has spurred extensive research on electrode materials. [1-3]. The application of LIBs is indispensable in portable electronics, electric vehicles, and large-scale energy storage in the future [4, 5]. Up to now, silicon and carbon material are the two topical anode materials for LIBs. Silicon has been proved to have highest known theoretical lithium storage capacity (about 4200 mA h g-1) and the low working potential (about 0.5 V vs Li/Li+) among anode material for LIBs [6]. However, it had an unavoidable volume change of nearly 300% during the lithiation/delithiation process, which resulted in fast battery failure [7]. In order to deal with this problem, many researchers developed a lot of new types of silicon based composites to improve cycle life and reversible capacity [8-15]. For instance, silicon was designed and fabricated into silicon based nanowires, which maintained a discharge capacity close to 75% of the theoretical capacity, with little fading in cycling [16]. Silicon nanowires fabric could achieve a capacity of more than 800 mA h g-1 without the addition of conductive carbon or binder [17]. Though the silicon based composites obtained high capacity, the volume changes still existed, which was not produced by commercial company as anode for LIBs. Compared with pure silicon anodes, carbon material had little volume change and long cycle life with good safety during the cycling, which still held a dominant market share in LIBs. To further apply the lithium ion storage of carbon materials, many methods have been tried [18-22]. For example, the natural graphite spheres coated by 3
pyrolytic carbon displayed excellent cyclability, with capacity above 320 mA h g-1 at 15th cycle, which was higher than that of the natural graphite spheres. This method decreased the specific surface area, inhibited the solid electrolyte interface (SEI) film formation around the internal cracks, and inhibited formation of the thin and compact SEI film on the outer surface of the coated natural graphite spheres [23]. The spherical graphite coated by propylene carbonate and ethylene carbonate as anode materials was also studied. They had a good reversible capacity as high as around 360 mA h g-1, which was close to the theoretical value of 372 mA h g-1. During the cycling the Coulombic efficiencies of the above anode materials were over 90%, which was proper for application as anode material in LIBs [24]. One-dimensional carbon nanofibers grown on the two-dimensional graphene sheets as anode material showed high reversible capacity of 667 mA h g-1, which was superior to those of pure graphene, natural graphite, and carbon nanotubes [25]. Carbon nanocoils (CNCs) were one of representative chiral materials with nanostructure. So far, researchers studying CNCs have found many outstanding properties, such as light weight, high electrical and thermal conductivity, large surface area and unique superelasticity due to their three-dimensional helical nature [26-29]. These properties may do favor for CNCs to obtain a promising electrochemical performance as anode material for LIBs. 2. Experimental 2.1 Raw materials The CNCs could be synthesized by atomic layer deposition or chemical vapor 4
deposition [30-32]. The CNCs used in this work were synthesized by chemical vapor deposition using acetylene as a carbon source and copper nanoparticles as catalysts at 523 K followed by a heat treatment at 1173 K for 1 h. The hollow graphite fibers (HGFs) were made by melt spinning in our laboratory. The softening point of isotropic pitch was 553 K. The isotropic pitch was separated into toluene soluble (TS, 31.08 wt. %) and insoluble (TI) fractions. The fraction of TI was separated into quinoline soluble (QS, 46.03 wt. %) and insoluble (QI, 22.89 wt. %). The isotropic pitch was firstly spun into fibers using the spinnerette under a nitrogen atmosphere. As-spun fibers were thermo-oxidatively stabilized at 553 K for 0.5 h in oxidation oven. The stabilized fibers were carbonized at 1273 K for 0.5 h under nitrogen atmosphere in a tube furnace. The CNCs and hollow carbon fibers were finally graphitized at 3073 K under argon atmosphere. 2.2 Characterizations and Electrochemical Evaluation The crystallite structure of materials were characterized by X-ray diffraction system (XRD; D8 advance, Brucker.) equipped with Cu Ka radiation between 10-90o at a scan rate 5o/min. The values of interplanar spacing (d002) were characterized. Field-emission scanning electron microscopy (SEM, JSM-7001F) and field-emission transmission electron microscopy (TEM, JEM-2100F) were used to observe the microstructure of the samples. The working electrodes consisted of active material, acetylene black and polyvinylidene fluoride (PVDF) at a weight ratio of 70:20:10. CR 2016-type coin 5
cells were assembled in a glovebox with Ar atmosphere. Li foil was used as the counter electrode and reference electrode, with microporous polypropylene film (Celgard 2400) as the separator. The electrolyte was composed of 1 M LiPF6 in ethylene carbonate-dimethyl carbonate (1:1 by volume) and 5 vol. % vinylene carbonate. The cells were galvanostatically discharged and charged using a battery test system (LAND CT 2001A model, Wuhan Jinnuo Electronics Ltd.) in the voltage range of 0.01-2 V at room temperature. 3. Results and Discussion The quasi-one-dimensional chiral CNCs could be clearly observed by SEM, as shown in Fig. 1a. A lot of CNCs were randomly stacked to form three-dimensional porous and interpenetrating network. The CNCs had a diameter of approximately 100 nm. They were tightly coiled and had very uniform coil pitches and coil diameters. In order to have a convenient comparison, HGFs were prepared to be control specimens as shown in Fig. 1b. The external diameter of GHFs was approximately
19.9 0.6 m , and their wall thickness was approximately 5.5 0.4 m . Fig. 2a displayed a representative TEM image of the CNCs, which kept regular helical forms. In Fig. 2b the graphite stripes of the CNCs folded or overlapped each other, which showed diffraction rings taken from the entire area. The graphite stripes of the HGFs highlighted diffraction spot taken from the entire area, which indicated a highly graphitic structure (in Fig. 2c). The ribbons of graphite grain of CNCs and HGFs could be observed obviously during the Fig. 2 (b, c), which indicated that they both could have good extent of graphitization and good electric conductivity. 6
XRD was carried out to further investigate the structure of the samples (Fig. 2d). In order to avoid the shifting and broadening of the diffraction profiles, the internal silicon standard was used in XRD analysis. The interplanar spacing of the CNCs was measured to be 0.3411 nm, which was indicative of the slightly turbostratic structure. The interplanar spacing of the HGFs was 0.3375 nm, which was lower than that of the CNCs. The cycle performance of the CNCs and the HGFs was carried out at a current density of 50 mA g-1. The charge-discharge curves of the CNCs and HGFs were shown in Fig. 3(a, b). The HGFs electrode delivered its initial discharge and charge capacity of 443.7 and 375.1 mA h g-1, respectively, with a Coulombic efficiency of 84.5 % (Fig. 3a). After 50 cycle its reversible capacity gradually decayed to 338.9 mA h g-1 with steady Coulombic efficiency (Fig. 3c). In the case of the CNCs, it exhibited a first discharge capacity of 378.5 mA h g-1 and a first charge capacity of 368.2 mA h g-1, respectively, which had a Coulombic efficiency of 97.3 % (Fig. 3b). Then the reversible capacity began to increase, and kept a high capacity over 368 mA h g-1 in the next 25 cycles. The CNCs electrode maintained a discharge capacity of 385.3 mA h g-1 and a charge capacity of 385.5 mA h g-1 after 105 cycles (Fig. 3d). The charge-discharge curves of the CNCs overlapped quite well, which also indicated good cycling performance (Fig. 3b). The above results revealed that CNCs anode had a better cyclability with high reversible capacity than that of the HGFs. The higher capacity surpassing the theoretical value of graphite might be ascribed to the micro-pores in fibers, which could store the lithium. Therefore, the capacity could be 7
higher than the theoretical value of graphite [33]. Rate capability was an important factor for the use of LIBs in power application. We investigated the corresponding rate capability with the various rates stepwise increased from 50 mA g-1 to 800 mA g-1 and then switched back. The HGFs delivered the reversible capacity of 350.4, 300.3, and 232.7 mA h g-1 after 5, 10, and 15 cycles. With the increase of the current densities, the reversible capacity started to decrease. At the current densities of 400 and 800 mA g-1, the HGFs obtained the reversible capacity of 137.9 and 49.7 mA h g-1, respectively (Fig. 3e). After the high rate measurements, the HGFs electrode tested at 50 mA g-1 reached a reversible capacity of 345.2 mA h g-1, which was in accordance with the value of the first 5th cycle. The reversible capacity of the CNCs electrode were 364.1, 289.8, and 230.1 mA h g-1 under the current densities of 50, 100, and 200 mA g-1, which had no obvious difference with that of the HGFs. But at the current density of 400 mA g-1 the CNCs obtained the reversible capacity of 184.8 mA h g-1 (as shown in Fig. 3f), which was higher than that of the HGFs (137.9 mA h g-1). Surprisingly at the current density of 800 mA g-1 the reversible capacity was 145.2 mA h g-1, which was two times higher than that of the HGFs (49.7 mA h g-1). This should be attributed to the conductive three-dimensional interpenetrating network of the CNCs, which could enhance the structural integrity of the electrode. Notably, after measured under the high current densities, the reversible capacities of CNCs electrode cycled at 50 mA g-1 were able to recover to the values tested under the initial constant current density of 50 mA g-1. Compared with the HGFs, the CNCs had a better rate capability at the high current 8
density, which offered a promising potential utilization in high-energy-density lithium-ion cells. To further study the cycle performance of the CNCs, higher current densities of 0.1 and 1 A g-1 were imposed on. The initial discharge and charge capacity were 327.2 and 315.8 mA h g-1, respectively, with a Coulombic efficiency of 96.5 % at the current density of 0.1 A g-1. After 400 cycles, the reversible capacity of 335.6 mA h g-1 was remained, which was rarely reported for CNCs anodes (as shown in Fig. 4a). At the same time, the higher current density of 1 A g-1 was imposed on the CNCs anode. After the first several cycles, the reversible capacity increased steadily as shown in Fig. 4b. The CNCs reached the highest reversible capacity of 182.1 mA h g-1, at the 97th cycle. The CNCs reached the lithium storage capacity of 182.4 mA h g-1 with the reversible capacity of 177.4 mA h g-1, with a Coulombic efficiency of 97.3 % at the 100th cycle. It was amazing that the CNCs showed a highly robust electrochemical performance, which could obtain a quite high reversible capacity under the rather high current density after 100 cycles. A typical cyclic voltammetry (CV) measurement of the CNCs and the HGFs electrodes in the voltage range of 0.01 - 2.0 V at a sweep rate of 1 mV/s was shown in Fig. 5 (a, b). In the case of the CNCs electrode, during its first cathodic half-cycle, the discharge current was clearly observed at starting potential of 1.45 V as shown in Fig. 5a. Then the discharge current became large gradually, which could be attributed to the formation of SEI film that resulted in an initial irreversible capacity [34, 35]. In the first anodic scan, there was an obvious anodic peak appearing at 0.52 V. During 9
the following anodic scan, the corresponding anodic peaks were observed at 0.48 and 0.49 V, respectively. Fig. 5b showed the CV curves of the HGFs. In the first cathodic half-cycle, the discharge current became large suddenly at starting potential of 1.14 V as shown by the inset, which resulted in the formation of the SEI film [34, 35]. In the first anodic scan, the anodic peak appeared at 0.55 V. In addition, the anodic current peaks of the following scan presented a slightly enhanced intensity compared with the first scan. Judging from the second and third cathodic-anodic scan curves of the two electrodes, they overlapped quite well, suggesting their good cycling performance. The electrical conductivity of the CNCs and the HGFs was verified by electrochemical impedance spectroscopy (EIS) measurements as shown in Fig. 5c. The EIS spectra consisted of two semicircles and a straight slopping line. The first semicircle at the high frequency region might result from the formation of the SEI. The second semicircle at the medium frequency region was probably because of the interfacial charge transfer impedance. The straight slopping line at low frequency mainly was related to the lithium diffusion impedance. In contrast, the CNCs had larger contact impedance than that of the HGFs, possibly attributed to their large specific surface area mainly due to an external surface. But the charge transfer impedance of the CNCs was less than the HGFs’ one. The straight diffusion tail suggested that the CNCs’ conductive network facilitated the diffusion and transport of lithium ions between the electrode and the electrolyte, thus reducing the lithium ion diffusion resistance.
10
4. Conclusions
In conclusion, the CNCs were employed as active anode materials for lithium ion batteries. At the current density of 50 mA g-1, the CNCs anode exhibited a reversible capacity of 385.5 mA h g-1 after 105 cycles, which was higher than 338.9 mA h g-1 of the HGFs anode for 50 cycles. The CNCs showed a reversible capacity of 335.6 mA h g-1 at 0.1 A g-1 after 400 cycles, which indicated longer cycle life and higher reversible capacity in contrast to HGFs. Cycled under the higher current density of 1A g-1, the CNCs still obtained a reversible capacity of 177.4 mA h g-1 after 100 cycles, which exhibited promising cycling performance. When cycled at the higher current density of 800 mA g-1, the reversible capacity of the CNCs reached a value of 145.2 mA h g-1, which was two times higher than that of the HGFs (49.7 mA h g-1). This revealed that the CNCs had better rate capability in contrast to the HGFs. The graphitized CNCs as a pure carbon anode material showed a highly robust electrochemical performance, which would be a promising choice as anode material for the LIBs. Acknowledgement
This work was supported by the National Natural Science Foundation of China (51362010,
11564011),
Project
of
Scientific
Technological
Research
and
Development Plan of Chengde (20155006), Natural Science Foundation of Hainan Province (514207, 514212), the Shanxi Province Foundation for Youths (201505027).
11
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Figures
Fig. 1 SEM images: (a) the CNCs, (b) the HGFs.
Fig. 2 TEM images: (a, b) the CNCs (inset shows the electron diffraction pattern). (c) the HGFs (inset shows the electron diffraction pattern). (d) X-ray diffraction pattern of the CNCs and the HGFs.
17
Fig. 3 The charge-discharge curves: (a) the HGFs, (b) the CNCs. Cycle performance: (c) the HGFs, (d) the CNCs. Rate capability: (e) the HGFs, (f) the CNCs.
18
Fig. 4 Cycle performance of the CNCs: (a) at the current density of the 0.1 A g-1, (b) at the current density of the 1 A g-1.
Fig. 5 (a) Cyclic voltammetry curves of the CNCs. (b) Cyclic voltammetry curves of the HGFs (inset shows the cathodic half-cycle curves at about 1.0 V). (c) Nyquist plots of the electrodes of the CNCs and the HGFs.
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S0013-4686(16)30734-4 http://dx.doi.org/doi:10.1016/j.electacta.2016.03.160 EA 26990
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
28-1-2016 21-3-2016 26-3-2016
Please cite this article as: Liyong Wang, Zhanjun Liu, Quangui Guo, Guizhen Wang, Jinhua Yang, Peng Li, Xianglei Wang, Lang Liu, Electrochemical properties of carbon nanocoils and hollow graphite fibers as anodes for rechargeable lithium ion batteries, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.03.160 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Electrochemical properties of carbon nanocoils and hollow graphite fibers as anodes for rechargeable lithium ion batteries Liyong Wang a, b, Zhanjun Liua,*, Quangui Guoa, Guizhen Wang c, Jinhua Yang d, Peng Li e, Xianglei Wang f, Lang Liu a a
Key laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy
of Sciences, Taiyuan 030001, China b
Hebei Normal University for Nationalities, Chengde 067000, China
c
College of Materials and Chemical Engineering，Hainan University, Hainan 570228,
China d
e
Avic composite corporation Ltd., Beijing 101300, China
Department of Materials Engineering, Taiyuan Institute of Technology, Taiyuan
030008, China f
Ordos Qidi Incubator Service Center, Ordos 017010, China
* Corresponding author: Fax: +86 351 4084 106. E-mail address: [email protected] (Z. Liu).
1
ABSTRACT Carbon nanocoils (CNCs) have been used as anode materials for preparation of lithium ion batteries. As pure carbon material without any chemical modification, the graphitized CNCs anode exhibited larger capacities with good Coulombic efficiency, a higher rate capability, and better reversibility than the hollow graphite fibers (HGFs) anode. The excellent performance of the CNCs was possibly ascribed to the special structure and the high degree of graphitization. As a result, the CNCs anode exhibited high reversible capacity of 385.5 mA h g-1 at 50 mA g-1, 104.7% reversible capacity retention after 105 cycles, and superior reversible capability of 177.4 mA h g-1 at 1A g-1 after 100 cycles. This result indicated that CNCs could be an attractive choice as anode material for high-energy density and high-power lithium-ion batteries. Key words Carbon nanocoils, Hollow graphite fibers, Anode materials, Lithium ion batteries, High rate performance
2
1. Introduction The great demand for high-energy density and high-power lithium-ion batteries (LIBs) has spurred extensive research on electrode materials. [1-3]. The application of LIBs is indispensable in portable electronics, electric vehicles, and large-scale energy storage in the future [4, 5]. Up to now, silicon and carbon material are the two topical anode materials for LIBs. Silicon has been proved to have highest known theoretical lithium storage capacity (about 4200 mA h g-1) and the low working potential (about 0.5 V vs Li/Li+) among anode material for LIBs [6]. However, it had an unavoidable volume change of nearly 300% during the lithiation/delithiation process, which resulted in fast battery failure [7]. In order to deal with this problem, many researchers developed a lot of new types of silicon based composites to improve cycle life and reversible capacity [8-15]. For instance, silicon was designed and fabricated into silicon based nanowires, which maintained a discharge capacity close to 75% of the theoretical capacity, with little fading in cycling [16]. Silicon nanowires fabric could achieve a capacity of more than 800 mA h g-1 without the addition of conductive carbon or binder [17]. Though the silicon based composites obtained high capacity, the volume changes still existed, which was not produced by commercial company as anode for LIBs. Compared with pure silicon anodes, carbon material had little volume change and long cycle life with good safety during the cycling, which still held a dominant market share in LIBs. To further apply the lithium ion storage of carbon materials, many methods have been tried [18-22]. For example, the natural graphite spheres coated by 3
pyrolytic carbon displayed excellent cyclability, with capacity above 320 mA h g-1 at 15th cycle, which was higher than that of the natural graphite spheres. This method decreased the specific surface area, inhibited the solid electrolyte interface (SEI) film formation around the internal cracks, and inhibited formation of the thin and compact SEI film on the outer surface of the coated natural graphite spheres [23]. The spherical graphite coated by propylene carbonate and ethylene carbonate as anode materials was also studied. They had a good reversible capacity as high as around 360 mA h g-1, which was close to the theoretical value of 372 mA h g-1. During the cycling the Coulombic efficiencies of the above anode materials were over 90%, which was proper for application as anode material in LIBs [24]. One-dimensional carbon nanofibers grown on the two-dimensional graphene sheets as anode material showed high reversible capacity of 667 mA h g-1, which was superior to those of pure graphene, natural graphite, and carbon nanotubes [25]. Carbon nanocoils (CNCs) were one of representative chiral materials with nanostructure. So far, researchers studying CNCs have found many outstanding properties, such as light weight, high electrical and thermal conductivity, large surface area and unique superelasticity due to their three-dimensional helical nature [26-29]. These properties may do favor for CNCs to obtain a promising electrochemical performance as anode material for LIBs. 2. Experimental 2.1 Raw materials The CNCs could be synthesized by atomic layer deposition or chemical vapor 4
deposition [30-32]. The CNCs used in this work were synthesized by chemical vapor deposition using acetylene as a carbon source and copper nanoparticles as catalysts at 523 K followed by a heat treatment at 1173 K for 1 h. The hollow graphite fibers (HGFs) were made by melt spinning in our laboratory. The softening point of isotropic pitch was 553 K. The isotropic pitch was separated into toluene soluble (TS, 31.08 wt. %) and insoluble (TI) fractions. The fraction of TI was separated into quinoline soluble (QS, 46.03 wt. %) and insoluble (QI, 22.89 wt. %). The isotropic pitch was firstly spun into fibers using the spinnerette under a nitrogen atmosphere. As-spun fibers were thermo-oxidatively stabilized at 553 K for 0.5 h in oxidation oven. The stabilized fibers were carbonized at 1273 K for 0.5 h under nitrogen atmosphere in a tube furnace. The CNCs and hollow carbon fibers were finally graphitized at 3073 K under argon atmosphere. 2.2 Characterizations and Electrochemical Evaluation The crystallite structure of materials were characterized by X-ray diffraction system (XRD; D8 advance, Brucker.) equipped with Cu Ka radiation between 10-90o at a scan rate 5o/min. The values of interplanar spacing (d002) were characterized. Field-emission scanning electron microscopy (SEM, JSM-7001F) and field-emission transmission electron microscopy (TEM, JEM-2100F) were used to observe the microstructure of the samples. The working electrodes consisted of active material, acetylene black and polyvinylidene fluoride (PVDF) at a weight ratio of 70:20:10. CR 2016-type coin 5
cells were assembled in a glovebox with Ar atmosphere. Li foil was used as the counter electrode and reference electrode, with microporous polypropylene film (Celgard 2400) as the separator. The electrolyte was composed of 1 M LiPF6 in ethylene carbonate-dimethyl carbonate (1:1 by volume) and 5 vol. % vinylene carbonate. The cells were galvanostatically discharged and charged using a battery test system (LAND CT 2001A model, Wuhan Jinnuo Electronics Ltd.) in the voltage range of 0.01-2 V at room temperature. 3. Results and Discussion The quasi-one-dimensional chiral CNCs could be clearly observed by SEM, as shown in Fig. 1a. A lot of CNCs were randomly stacked to form three-dimensional porous and interpenetrating network. The CNCs had a diameter of approximately 100 nm. They were tightly coiled and had very uniform coil pitches and coil diameters. In order to have a convenient comparison, HGFs were prepared to be control specimens as shown in Fig. 1b. The external diameter of GHFs was approximately
19.9 0.6 m , and their wall thickness was approximately 5.5 0.4 m . Fig. 2a displayed a representative TEM image of the CNCs, which kept regular helical forms. In Fig. 2b the graphite stripes of the CNCs folded or overlapped each other, which showed diffraction rings taken from the entire area. The graphite stripes of the HGFs highlighted diffraction spot taken from the entire area, which indicated a highly graphitic structure (in Fig. 2c). The ribbons of graphite grain of CNCs and HGFs could be observed obviously during the Fig. 2 (b, c), which indicated that they both could have good extent of graphitization and good electric conductivity. 6
XRD was carried out to further investigate the structure of the samples (Fig. 2d). In order to avoid the shifting and broadening of the diffraction profiles, the internal silicon standard was used in XRD analysis. The interplanar spacing of the CNCs was measured to be 0.3411 nm, which was indicative of the slightly turbostratic structure. The interplanar spacing of the HGFs was 0.3375 nm, which was lower than that of the CNCs. The cycle performance of the CNCs and the HGFs was carried out at a current density of 50 mA g-1. The charge-discharge curves of the CNCs and HGFs were shown in Fig. 3(a, b). The HGFs electrode delivered its initial discharge and charge capacity of 443.7 and 375.1 mA h g-1, respectively, with a Coulombic efficiency of 84.5 % (Fig. 3a). After 50 cycle its reversible capacity gradually decayed to 338.9 mA h g-1 with steady Coulombic efficiency (Fig. 3c). In the case of the CNCs, it exhibited a first discharge capacity of 378.5 mA h g-1 and a first charge capacity of 368.2 mA h g-1, respectively, which had a Coulombic efficiency of 97.3 % (Fig. 3b). Then the reversible capacity began to increase, and kept a high capacity over 368 mA h g-1 in the next 25 cycles. The CNCs electrode maintained a discharge capacity of 385.3 mA h g-1 and a charge capacity of 385.5 mA h g-1 after 105 cycles (Fig. 3d). The charge-discharge curves of the CNCs overlapped quite well, which also indicated good cycling performance (Fig. 3b). The above results revealed that CNCs anode had a better cyclability with high reversible capacity than that of the HGFs. The higher capacity surpassing the theoretical value of graphite might be ascribed to the micro-pores in fibers, which could store the lithium. Therefore, the capacity could be 7
higher than the theoretical value of graphite [33]. Rate capability was an important factor for the use of LIBs in power application. We investigated the corresponding rate capability with the various rates stepwise increased from 50 mA g-1 to 800 mA g-1 and then switched back. The HGFs delivered the reversible capacity of 350.4, 300.3, and 232.7 mA h g-1 after 5, 10, and 15 cycles. With the increase of the current densities, the reversible capacity started to decrease. At the current densities of 400 and 800 mA g-1, the HGFs obtained the reversible capacity of 137.9 and 49.7 mA h g-1, respectively (Fig. 3e). After the high rate measurements, the HGFs electrode tested at 50 mA g-1 reached a reversible capacity of 345.2 mA h g-1, which was in accordance with the value of the first 5th cycle. The reversible capacity of the CNCs electrode were 364.1, 289.8, and 230.1 mA h g-1 under the current densities of 50, 100, and 200 mA g-1, which had no obvious difference with that of the HGFs. But at the current density of 400 mA g-1 the CNCs obtained the reversible capacity of 184.8 mA h g-1 (as shown in Fig. 3f), which was higher than that of the HGFs (137.9 mA h g-1). Surprisingly at the current density of 800 mA g-1 the reversible capacity was 145.2 mA h g-1, which was two times higher than that of the HGFs (49.7 mA h g-1). This should be attributed to the conductive three-dimensional interpenetrating network of the CNCs, which could enhance the structural integrity of the electrode. Notably, after measured under the high current densities, the reversible capacities of CNCs electrode cycled at 50 mA g-1 were able to recover to the values tested under the initial constant current density of 50 mA g-1. Compared with the HGFs, the CNCs had a better rate capability at the high current 8
density, which offered a promising potential utilization in high-energy-density lithium-ion cells. To further study the cycle performance of the CNCs, higher current densities of 0.1 and 1 A g-1 were imposed on. The initial discharge and charge capacity were 327.2 and 315.8 mA h g-1, respectively, with a Coulombic efficiency of 96.5 % at the current density of 0.1 A g-1. After 400 cycles, the reversible capacity of 335.6 mA h g-1 was remained, which was rarely reported for CNCs anodes (as shown in Fig. 4a). At the same time, the higher current density of 1 A g-1 was imposed on the CNCs anode. After the first several cycles, the reversible capacity increased steadily as shown in Fig. 4b. The CNCs reached the highest reversible capacity of 182.1 mA h g-1, at the 97th cycle. The CNCs reached the lithium storage capacity of 182.4 mA h g-1 with the reversible capacity of 177.4 mA h g-1, with a Coulombic efficiency of 97.3 % at the 100th cycle. It was amazing that the CNCs showed a highly robust electrochemical performance, which could obtain a quite high reversible capacity under the rather high current density after 100 cycles. A typical cyclic voltammetry (CV) measurement of the CNCs and the HGFs electrodes in the voltage range of 0.01 - 2.0 V at a sweep rate of 1 mV/s was shown in Fig. 5 (a, b). In the case of the CNCs electrode, during its first cathodic half-cycle, the discharge current was clearly observed at starting potential of 1.45 V as shown in Fig. 5a. Then the discharge current became large gradually, which could be attributed to the formation of SEI film that resulted in an initial irreversible capacity [34, 35]. In the first anodic scan, there was an obvious anodic peak appearing at 0.52 V. During 9
the following anodic scan, the corresponding anodic peaks were observed at 0.48 and 0.49 V, respectively. Fig. 5b showed the CV curves of the HGFs. In the first cathodic half-cycle, the discharge current became large suddenly at starting potential of 1.14 V as shown by the inset, which resulted in the formation of the SEI film [34, 35]. In the first anodic scan, the anodic peak appeared at 0.55 V. In addition, the anodic current peaks of the following scan presented a slightly enhanced intensity compared with the first scan. Judging from the second and third cathodic-anodic scan curves of the two electrodes, they overlapped quite well, suggesting their good cycling performance. The electrical conductivity of the CNCs and the HGFs was verified by electrochemical impedance spectroscopy (EIS) measurements as shown in Fig. 5c. The EIS spectra consisted of two semicircles and a straight slopping line. The first semicircle at the high frequency region might result from the formation of the SEI. The second semicircle at the medium frequency region was probably because of the interfacial charge transfer impedance. The straight slopping line at low frequency mainly was related to the lithium diffusion impedance. In contrast, the CNCs had larger contact impedance than that of the HGFs, possibly attributed to their large specific surface area mainly due to an external surface. But the charge transfer impedance of the CNCs was less than the HGFs’ one. The straight diffusion tail suggested that the CNCs’ conductive network facilitated the diffusion and transport of lithium ions between the electrode and the electrolyte, thus reducing the lithium ion diffusion resistance.
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4. Conclusions
In conclusion, the CNCs were employed as active anode materials for lithium ion batteries. At the current density of 50 mA g-1, the CNCs anode exhibited a reversible capacity of 385.5 mA h g-1 after 105 cycles, which was higher than 338.9 mA h g-1 of the HGFs anode for 50 cycles. The CNCs showed a reversible capacity of 335.6 mA h g-1 at 0.1 A g-1 after 400 cycles, which indicated longer cycle life and higher reversible capacity in contrast to HGFs. Cycled under the higher current density of 1A g-1, the CNCs still obtained a reversible capacity of 177.4 mA h g-1 after 100 cycles, which exhibited promising cycling performance. When cycled at the higher current density of 800 mA g-1, the reversible capacity of the CNCs reached a value of 145.2 mA h g-1, which was two times higher than that of the HGFs (49.7 mA h g-1). This revealed that the CNCs had better rate capability in contrast to the HGFs. The graphitized CNCs as a pure carbon anode material showed a highly robust electrochemical performance, which would be a promising choice as anode material for the LIBs. Acknowledgement
This work was supported by the National Natural Science Foundation of China (51362010,
11564011),
Project
of
Scientific
Technological
Research
and
Development Plan of Chengde (20155006), Natural Science Foundation of Hainan Province (514207, 514212), the Shanxi Province Foundation for Youths (201505027).
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Figures
Fig. 1 SEM images: (a) the CNCs, (b) the HGFs.
Fig. 2 TEM images: (a, b) the CNCs (inset shows the electron diffraction pattern). (c) the HGFs (inset shows the electron diffraction pattern). (d) X-ray diffraction pattern of the CNCs and the HGFs.
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Fig. 3 The charge-discharge curves: (a) the HGFs, (b) the CNCs. Cycle performance: (c) the HGFs, (d) the CNCs. Rate capability: (e) the HGFs, (f) the CNCs.
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Fig. 4 Cycle performance of the CNCs: (a) at the current density of the 0.1 A g-1, (b) at the current density of the 1 A g-1.
Fig. 5 (a) Cyclic voltammetry curves of the CNCs. (b) Cyclic voltammetry curves of the HGFs (inset shows the cathodic half-cycle curves at about 1.0 V). (c) Nyquist plots of the electrodes of the CNCs and the HGFs.
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