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graphite fluoride composites cathode for high power and energy densities lithium primary batteries
Synthetic Metals 220 (2016) 560–566
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Polythiophene/graphite fluoride composites cathode for high power and energy densities lithium primary batteries Xiaodong Yina , Yu Lia,b,c,d, Yiyu Fenga,b,c,d , Wei Fenga,b,c,d,* a
School of Materials Science and Engineering, Tianjin University, Tianjin 300072, PR China Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, PR China Key Laboratory of Advanced Ceramics and Machining Technology, Ministry of Education, Tianjin 300072, PR China d Tianjin Key Laboratory of Composite and Functional Materials, Tianjin 300072, PR China b c
A R T I C L E I N F O
Article history: Received 16 March 2016 Received in revised form 23 July 2016 Accepted 27 July 2016 Available online xxx Keywords: Lithium primary battery Graphite fluorides Polythiophene Cathode materials Rate capability
A B S T R A C T
A series of polythiophene/graphite fluoride (PTh/CFx) composites have been synthesized by in situ polymerization of thiophene monomers on the surface of CFx. Transmission electron microscopy (TEM) demonstrates that 22.94 wt.% PTh/CFx is the suitable ratio for the composite due to the uniform and complete PTh coating on the surface of CFx particles. Electrochemical impedance spectra (EIS) measurements confirm that the coating of PTh decreases the charge transfer resistance of CFx cathode significantly. Conductive PTh serves as the conducting additive as well as the porous adsorbing agent, so the rate capability of PTh/CFx composites improves remarkably compared to pure CFx cathode. The amount of PTh coating affects the electrochemical performances of PTh/CFx composites, and the one containing 22.94 wt.% PTh can be discharged at high rate up to 4C delivering a maximum power density of 4997 W kg1, associated with a high 1707 Wh kg1 energy density. ã 2016 Published by Elsevier B.V.
1. Introduction The use of graphite fluoride (CFx) as cathode of lithium primary batteries firstly studied by Watanabe [1], which has been commercially developed in Japan in 1970 by the Matsushita Co. As one of cathode materials in lithium primary batteries, CFx presents many advantages such as high energy density (up to 1500 Wh kg1), high and flat discharge potential (about 2.2–2.4 V), and broad temperature range of use in the practical batteries (from 40 C to 170 C) [2]. CFx is synthesized by the chemical reaction of fluorine with carbon at high temperature, and the value of x can vary from 0 to 1.3, depending on the fluorination temperature [3]. However, because of the covalence of the C F bond, CFx shows a very low electrical conductivity, resulting in a low rate capability and an initial potential delay, which hampers its application in high-power devices. It was an effective method to solve this problem that CFx with lower fluorination content improved its rate capability [4–6]. For example, sub-fluorinated carbon nanofibres could sustain high discharge rate up to 6C due to the presence of unfluorinated path
* Corresponding author at: School of Materials Science and Engineering, Tianjin University, Tianjin 300072, PR China. E-mail address: [email protected] (W. Feng). http://dx.doi.org/10.1016/j.synthmet.2016.07.032 0379-6779/ã 2016 Published by Elsevier B.V.
within the CFx [5]. The mixture of CFx with other cathode materials such as silver vanadium oxide or manganese dioxide to develop a hybrid structure was also utilized to modify the rate capability [7–9]. The addition of some amount of nanostructured conductive additives, such as carbon nanotubes [9] or graphene [10], into the CFx cathode can significantly improve its rate capability due to the formation of conductive paths for the charges transport. Recently, it was found that substituting nanostructured carbon materials for conventional graphite as the starting materials of CFx was an effective approach to improve the rate capability thanks to the special nanostructures [5,11]. Besides the methods mentioned above, surface coating was considered as an easy and effective way to enhance the electronic conductivity of CFx [12,13]. Conducting polymers have received numerous attentions because they can play different roles in improving the cathodes as a result of their good electrochemical stabilities and favorable morphologies [14–16]. For example, the electrodeposition of polypyrrole on CFx cathode improved its rate capability and the delivered power density achieved to 5235 W kg1 at 4C rate [13]. Among the various conducting polymers, because polythiophene (PTh) can participate in lithiation and delithiation reactions, contributing to the specific capacity of the composites [17,18], it has been served as the conducting layers in various cathode materials to improve their electrochemical performances [19–21]. In this work, a series of PTh/CFx composites
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were prepared by in situ polymerization. The influence of the different PTh ratios on the electrochemical performances was studied. PTh particles as a conductive additive were coated on the surface of CFx powder. The conductivity of CFx cathode was significantly enhanced by the PTh coating, which decreased the polarization of discharge and hence improves the rate capability. 2. Experimental 2.1. Material preparation The PTh/CFx composites were synthesized by in situ polymerization. At first, 1 g CFx powders (x = 1.0, Alfa Aesar, A Johnson Matthey Company) and some amount of anhydrous FeCl3 (Tianjin Guangfu Fine Chemical Research Institute) were dispersed in 100 mL CHCl3 solution to form a homogenous suspension with the help of ultrasonication. Before the polymerization, the suspension was bubbled with argon gas for 30 min to remove the oxygen. While the suspension was magnetically agitated, PTh monomers (0.1, 0.25 and 0.5 g; J&K Scientific Ltd.) dissolved in 50 mL CHCl3 was slowly added into the suspension in 1 h, and the molar ratio of thiophene to FeCl3 was kept at 1:4. The polymerization reaction was carried out in ice-bath under a continuous argon flow for 10 h so that thiophene monomers tended to link by a-a position, which could enhance the electrical conductivity of PTh effectively [21]. After the polymerization, 100 mL methanol was added to the mixture to dissolve the remaining FeCl3. The mixture was then filtered to remove the iron ions as well as the remained monomers, and the insoluble solid was washed by methanol and deionized water for several times. After this, the solid was poured into the solution of 1 M hydrochloric acid and the mixture was stirred for 2 h at room temperature. Then the solid product was collected by filtration and washed using deionized water until the filtrate being neutral. Finally, the brown product was dried at 80 C for 24 h under vacuum to obtain the PTh/CFx composite. The proposed synthesis route of PTh/CFx composite is illustrated in Scheme 1. 2.2. Materials characterization The thermal behavior of the samples was examined by thermogravimetry analysis (TGA, Shimadzu DTG-60) from room temperature up to 800 C at a heating rate of 5 C min1 in a dynamic
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atmosphere of nitrogen (35 mL min1). The morphology of CFx and PTh/CFx composites was observed by field-emission scanning electron microscopy (SEM, hitachi S-4800) and field-emission transmission electron microscopy (TEM, Philips Tecnai G2 F20). Fourier Transform infrared (FTIR) spectroscopy characterized the structure of the synthesized sample. The FTIR spectra were recorded with Perkin–Elmer Spectrum One FTIR Spectrophotometer which used KBr pellets in the region 4000–500 cm1. X-ray diffraction (XRD) measurements were performed on a Philips diffractometer, which was composed of a quartz monochromator, a Cu Ka radiation source at a scan rate of 10 min1 and a goniometric plate. Fourprobe testing instrument (SX1934, Jiangsu) was used to measure the conductivity, and made it on a disc-shaped pellet by four-point direct current method at room temperature. 2.3. Electrochemical measurement The electrodes were composed of cathode materials (bare CFx or CFx/PTh composites), conductive additive (Super P carbon) and binder (polyvinylidene difluoride, PVDF) with a weight ratio of 80:10:10. In order to obtain the homogenous and slightly viscous slurry, N-methyl pyrrolidone (NMP) was added. The electrode film was obtained by the means of the slurry coating on an Al foil, and the film was dried at 120 C for 12 h. The electrochemical performances of CFx/PTh composites were investigated by assembling 2032 coin type batteries, so the diameter of punched electrode disks was about 18 mm, which were further dried at 80 C for 8 h in a vacuum. The mass loading for each cathode was 2– 3 mg cm2. And then, the shaped disks were put into a glove-box filled with argon (Mikrouna Co., Advanced 2440/750) for cell assembly. A metallic lithium disc, a microporous polypropylene/ polyethylene/polypropylene film and the solution of 1 M LiPF6 in ethylene carbonate: dimethyl carbonate (EC:DMC, 1:1, vol%) were used as the anode, separator and electrolyte, respectively. The discharged test of coin cells was operated under constant current densities (Land CT2001A, Wu Han Jin Nuo Electronics Co., China) at room temperature, and the termination potential was 1.5 V (vs Li/ Li+). A three-electrode electrochemical cell included a Li flake as the counter electrode and an extra Li wire as the reference electrode, and the EIS was measured using Advanced Electrochemical System Parstat 2263 over the frequency range from 0.01 to 10 kHz.
Scheme 1. Schematic illustration of the proposed synthesis route of PTh/CFx composites.
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3. Results and discussion 3.1. Composition of PTh/CFx composites Fig. 1 shows the comparison of the mass losses of CFx, PTh and PTh/CFx composites upon heating in a nitrogen atmosphere. The obvious weight loss of CFx between 450 and 650 C is derived from the self-decomposition of CFx [22]. In the case of PTh, it decomposes with a two-step weight loss behavior. The first weight loss step occurs between 130 and 360 C, attributed to the loss of dopant anions bound to the PTh chain, and the second step weight loss occurs between 360 and 740 C, assigned to the degradation of the skeletal PTh chain structure [23,24]. There is no significant change in the CFx decomposition temperature when the initia mass ratio of PTh to CFx below to 1:4, although it slightly shifts to the lower value with the increase of PTh amount due to the interaction between main chains of PTh and CFx surface. When the initial mass ratio of PTh to CFx is 1:2, the composite shows the weight loss behavior similar to that of PTh, indicating the agglomeration of PTh particles [25]. On the basis of the total mass loss up to 800 C, the mass ratio of PTh in the PTh/CFx composite can be determined as 8.66, 22.94 and 33.80 wt.%, respectively, which is close to the initial reactant ratio. 3.2. Microstructures of PTh/CFx composites The SEM images of CFx and PTh/CFx composites are shown in Fig. 2. The pristine CFx particles (Fig. 2a) have a smoother surface with sharper edges and the particle size ranges from 2 to 6 mm. According to the synthesis route, the absorbed Fe3+ on the outer surface of CFx particles initiated the polymerization of thiophene monomers, and hence an electrically conductive PTh layer was formed on the surface of CFx particles. The PTh coated CFx powders tend to aggregate due to the interaction of static electricity of PTh layers on the surface of CFx [12], and PTh/CFx composites develop a porous structure. From high magnification images, it can be found that the surface of PTh/CFx composites is in the flakelike morphology with highly porous structures and the PTh coating layer becomes less porous with the increasing content of PTh in the composite. In order to observe the PTh coating on the surface of CFx, TEM observation was carried out and the TEM images are shown in Fig. 3a–c. When the PTh is in a low level (Fig. 3a), CFx particle can only be coated by some granulated PTh, with large area of bare surface. When the PTh amount reaches 22.94 wt.% (Fig. 3b), CFx particle surface is coated by a thin PTh layer uniformly and
completely. When the PTh content continues to increase up to 33.80 wt.% (Fig. 3c), PTh coating on CFx particle surface gets thickening and some individual PTh particles appear in the composite. An extra HRTEM image of 22.94 wt% CFx/PTh composite is shown in Fig. 3d. Based on the proposed synthesis route, it was clearly observed that the translucent PTh layer (the lighter area) coated on the surface of CFx particles (the dark region), which is similar to the previous reports [19–21]. The PTh coating plays an important role to enhance the electrochemical performances of composites because it not only enhances the electronic conductivity of the composite by reducing the particle-to-particle contact resistance but also increases the contact area between the cathode and the electrolyte [20,21]. Therefore, the reasonable amount PTh can form an uniform and complete coating, and the insufficient and excessive amount of PTh may cause uncoated or too thickness coating. 3.3. Structural characterization of PTh/CFx composites Fig. 4(a) shows the FTIR spectra of CFx, PTh and PTh/CFx composites. The spectral data of the PTh was consistent with those reported previously [26–28]. The strong absorption band at 789 cm1 is assigned to the C H out-of-plane stretching vibration of the 2,5-disubstituted thiophene ring. The band at 1433 cm1 is attributed to the stretching peak of C¼C and the bands at 1658 and 1324 cm1 are indicative of the C C asymmetric and symmetric stretching vibrations, respectively. The two bands attributed to the C S asymmetric and symmetric stretching vibrations are found at 1036 and 694 cm1. The appearance of bands at 694, 789, and 1036 cm1 confirms that the thiophene monomers are mainly polymerized through the a-a conjunction shown in the synthesis route, leading to an excellent electronic conductivity of PTh, which is crucial for the improvements of electrochemical performances [29,30]. The band at 3427 cm1 is attributed to the O H stretching vibration from crystal or absorbed water [24]. On the other hand, the strong absorption band at 1217 cm1 in the CFx sample is assigned to the characteristic of the C F covalent bonds and the bands at 2924 cm1 are indicative of the CH stretching vibration [31]. The weak polymer signals of PTh/CFx in which the mass ratio of PTh is below 22.94 wt.% is caused by the relatively low fraction of PTh, which is consistent with TGA analysis. The absence of new bands in PTh/CFx composites and the similarity of PTh and PTh/CFx composites spectra indicate that no chemical reaction occurs between CFx and PTh, and PTh serves as a conducting additive in the composites. To confirm the structure of PTh/CFx further, the XRD patterns are given in Fig. 4(b). The XRD pattern of PTh shows a typical amorphous structure with a broad peak centered at 2u = 20 . The XRD patterns of the CFx display three distinct peaks at 12.5, 26, and 41, which is in agreement with the previous reports [32–34]. The peak at 12.5 is the (001) reflection attributed to a high-level fluorinated hexagonal system, and the broad peak at 26 , assigned to (002) reflection, is generated by the poor regularity along the stacking direction. The peak around 41 represents the (100) reflection and is ascribed to the C C in-plane length in the reticular system. The XRD spectra of the PTh/CFx composites don’t change a lot compared with the pattern of CFx except for the appearance of slight broad PTh peaks centered at 2u = 20 , demonstrating that the crystal structure of CFx remains original and no new phase forms during the in-situ polymerization process [35]. 3.4. Electrochemical analysis of PTh/CFx composites
Fig. 1. TGA analysis of PTh, CFx and PTh/CFx composites.
Fig. 5 shows the galvanostatic discharge curves of PTh/CFx composites. The pure CFx cathode is also discharged at the same
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Fig. 2. SEM images of CFx (a), 8.66 wt.% PTh/CFx (b), 22.94 wt.% PTh/CFx (c) and 33.80 wt.% PTh/CFx (d). The insets show the SEM images at high magnifications of the corresponding samples.
Fig. 3. TEM images of 8.66 wt.% PTh/CFx (a), 22.94 wt.% PTh/CFx (b), 33.80 wt.% PTh/CFx (c) and the HRTEM image of 22.94 wt.% PTh/CFx (d).
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Fig. 4. FT-IR spectra (a) and XRD patterns (b) of PTh, CFx and PTh/CFx composites.
Fig. 5. Galvanostatic discharge curves at different rates of CFx (a), 8.66 wt.% PTh/CFx (b), 22.94 wt.% PTh/CFx (c), and 33.8 wt.% PTh/CFx (d).
rates for the comparison. The theoretical specific capacities of purchased CFx (x = 1.0) and PTh are 865 mAh g1 and 82 mAh g1, respectively [36]. The discharge current at different rates is calculated by the theoretical value of individual cathode. The electrode components, the theoretical specific capacities and electrochemical performances of pure CFx and CFx/PTh composites cathodes are listed in Table 1. And these values are calculated for a potential cut-off at 1.5 V vs. Li/Li+. The specific capacity of CFx is 784 mAh g1 at 0.05C, and it hardly delivers any capacity at 1C, like previous reports [5,9,10,12]. The introduction of PTh coating on CFx particles improves the rate capability of CFx and reduces the voltage delay after discharge effectively. The increased polarization with the increase of discharge rates is attributed to the difference
in the potential of the solvated lithium ion [37,38], which indicates the lithium ion diffusion controlled reaction. The PTh layer reduces the particle-to-particle contact resistance, which enhances the electrical conductivity of the composite. From Table 2, the electronic conductivity of 8.66 wt.% PTh/CFx composite is 5.28 105 S cm1, while CFx is a kind of insulators. When the content of PTh reaches 22.94 wt.%, the electronic conductivity of the composite is 3.42 103 S cm1, and the further increase of PTh content in the composite enhances little of electronic conductivity. In addition, from SEM images, the surface of CFx/PTh composites shows the flakelike morphology with highly porous structures. The porous structure of PTh layer allows the electrolyte to distribute throughout the electrode, which enhances the lithium ion
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Table 1 Electrochemical performances of CFx and PTh/CFx. samples
Electrode components
Theoretical Capacity (mAh g1)
Maximum Crate
Maximum Capacity (mAh g1)
Maximum W (Wh kg1)
Maximum P (W kg1)
CFx
80 wt.% CFx 10 wt.% Super P 10 wt.% PVDF 6.93 wt.% PTh 73.07 wt.% CFx 10 wt.% Super P 10 wt.% PVDF 18.35 wt.% PTh 61.65 wt.% CFx 10 wt.% Super P 10 wt.% PVDF 27.04 wt.% PTh 52.96 wt.% CFx 10 wt.% Super P 10 wt.% PVDF
865
1C
784.4
1806
1252
797.19
2C
802.6
1871
2693
685.38
4C
715.0
1707
4997
600.35
4C
603.1
1428
5460
8.66 wt.% PTh/CFx
22.94 wt.% PTh/ CFx
33.80 wt.% PTh/ CFx
Table 2 The electronic conductivities of PTh/CFx and PTh. Samples
Electronic conductivity (S cm1)
8.66 wt.% PTh/CFx 22.94 wt.% PTh/CFx 33.8 wt.% PTh/CFx PTh
5.28 105 3.42 103 3.75 103 0.34
diffusion coefficient subsequently. Therefore, the rate capability of CFx/PTh composite cathode is better than pure CFx cathode and the potential of CFx/PTh composite cathode is higher than that of pure CFx cathode at high rates, which becomes more significant with the increase of PTh content in the composite. However, the excessive PTh content in the composites reduces the specific capacity of composites contrarily due to the low specific capacity of PTh [39]. Therefore, the electrochemical performances of PTh/CFx composites can be tuned by the content of PTh, and 22.94 wt.% PTh/CFx composite shows the best electrochemical performances. The specific capacity of this composite is 715 mAh g1 at 0.05C rate, close to the theoretical capacity, and the specific capacity is about 270 mAh g1 at 4C rate. Fig. 6 presents the Ragone plots, which gives the variation of the energy density vs. the power density for CFx and PTh/CFx. The higher energy density and the lower power density values are observed at low discharge rates (0.1C). The increase of the
Fig. 6. Ragone plots of CFx and PTh/CFx composites.
discharge rate leads to an obvious decrease in energy density due to the drop of the discharge voltage as well as the specific capacity. The addition of PTh can effectively enhance the rate capability of the composites, leading to the high power densities. Both 22.94 wt. % and 33.80 wt.% PTh/CFx composites can be discharged at 4C rate, and the delivered power density is 4997 and 5460 W kg1, respectively. However, the energy density of 33.80 wt.% PTh/CFx at low discharge rate is significantly lower than that of other composites due to the excessive PTh amount and the agglomeration of PTh particles. Therefore, it can be further demonstrated that 22.94 wt.% PTh/CFx is the proper composite, exhibiting an excellent rate capability without the loss of energy density at low rate. In order to understand more details of the PTh coating effects, the impedance analysis of PTh/CFx, discharged at 0.1C rate, at 10% depth of discharge (DOD) states is performed, and Fig. 7 shows the Nyquist plots. According to the previous report [6], Fig. 7 inset shows the equivalent circuit used for the interpretation of the impedance spectra. The bulk ohmic resistance (Rb), which is contributed to the combination of current collector, electrode, separator and electrolyte, is low for all the cathode materials due to the batteries were prepared by reaction in the same electrolyte and adding the same conductive additive. On the interface between the discharge product shell and the liquid electrode, it can be characterized by Rct and Qct at high frequency that the charge transfer through the electric double layer. On the surface of
Fig. 7. The impedance spectra of CFx and PTh/CFx composites measured from a three-electrode electrochemical cell at 10% levels of DOD. The inset is the equivalent circuit for analysis of the lithium primary batteries.
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cathode, the discharge product shell can be characterized by R1 (resistance to lithium ion diffusion on the discharge product shell) and Q1 (a constant phase element) at high frequency. Due to the lack of lithium ion diffusion in active materials, another constant phase element Qint is applied. And Rct is the synergetic effects of the contact resistance between product shell resistance, chargetransfer resistance and conductive particles, which corresponds to the depressed semicircle in the impedance plot. The introduction of PTh coating brings no new semicircles in the impedance spectra, indicating that the electrolyte can easily access the coated CFx because of the porous structure of PTh. The conductive PTh coating also provides electron and ion channels for electrochemical reactions and enhance the electronic conductivity of CFx cathode significantly. The value of Rct decreases with the increase of PTh amount, but the difference of Rct between 22.94 wt.% and 33.80 wt.% PTh/CFx is not significant. Meanwhile, the frequencies in the onsets between the semicircle and the slopping straight line represent the reaction kinetics, which are also marked in Fig. 7, and the higher frequency indicates the faster kinetics [40]. The reaction kinetics becomes faster with the increase of PTh amount in the PTh/CFx composites until that up to 22.94 wt.%. Moreover, the porous structure of PTh coating can also avoid the cracking of cathode as well as the blocking of lithium ions due to the swelled LiF crystals formed during the electrochemical reaction [41]. Therefore, the porous PTh coating on CFx surface by in situ polymerization is an effective method to improve the electrochemical performance of CFx cathode. 4. Conclusions PTh/CFx composite materials have been synthesized by in situ polymerization for the first time. The introduction of PTh coating can improve the electronic conductivity of CFx, and hence accelerates the charge transfer reaction, which was confirmed by EIS measurements. The PTh coating also acts as the electrolyte absorbent due to the porous structure, facilitating the access of electrolyte to CFx and fulfilling the utilization of cathode completely. TEM images demonstrate that an uniform and complete coating can be formed when the PTh content reaches 22.94 wt.%. The 22.94 wt.% PTh/CFx maintains the high energy density of CFx at low discharge rate and exhibits the excellent rate capability, which delivers 4997 W kg1 power density at 4C rate, while the maximum power density of CFx is only 1252 W kg1. The PTh/CFx composites containing 22.94 wt.% PTh is a promising cathode material for lithium primary battery. Acknowledgements This work was financially supported by National Natural Science Funds for Distinguished Young Scholars (51425306), the National Basic Research Program of China (Grant No. 2016YFA0202302), National Natural Science Foundation of China
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Polythiophene/graphite fluoride composites cathode for high power and energy densities lithium primary batteries Xiaodong Yina , Yu Lia,b,c,d, Yiyu Fenga,b,c,d , Wei Fenga,b,c,d,* a
School of Materials Science and Engineering, Tianjin University, Tianjin 300072, PR China Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, PR China Key Laboratory of Advanced Ceramics and Machining Technology, Ministry of Education, Tianjin 300072, PR China d Tianjin Key Laboratory of Composite and Functional Materials, Tianjin 300072, PR China b c
A R T I C L E I N F O
Article history: Received 16 March 2016 Received in revised form 23 July 2016 Accepted 27 July 2016 Available online xxx Keywords: Lithium primary battery Graphite fluorides Polythiophene Cathode materials Rate capability
A B S T R A C T
A series of polythiophene/graphite fluoride (PTh/CFx) composites have been synthesized by in situ polymerization of thiophene monomers on the surface of CFx. Transmission electron microscopy (TEM) demonstrates that 22.94 wt.% PTh/CFx is the suitable ratio for the composite due to the uniform and complete PTh coating on the surface of CFx particles. Electrochemical impedance spectra (EIS) measurements confirm that the coating of PTh decreases the charge transfer resistance of CFx cathode significantly. Conductive PTh serves as the conducting additive as well as the porous adsorbing agent, so the rate capability of PTh/CFx composites improves remarkably compared to pure CFx cathode. The amount of PTh coating affects the electrochemical performances of PTh/CFx composites, and the one containing 22.94 wt.% PTh can be discharged at high rate up to 4C delivering a maximum power density of 4997 W kg1, associated with a high 1707 Wh kg1 energy density. ã 2016 Published by Elsevier B.V.
1. Introduction The use of graphite fluoride (CFx) as cathode of lithium primary batteries firstly studied by Watanabe [1], which has been commercially developed in Japan in 1970 by the Matsushita Co. As one of cathode materials in lithium primary batteries, CFx presents many advantages such as high energy density (up to 1500 Wh kg1), high and flat discharge potential (about 2.2–2.4 V), and broad temperature range of use in the practical batteries (from 40 C to 170 C) [2]. CFx is synthesized by the chemical reaction of fluorine with carbon at high temperature, and the value of x can vary from 0 to 1.3, depending on the fluorination temperature [3]. However, because of the covalence of the C F bond, CFx shows a very low electrical conductivity, resulting in a low rate capability and an initial potential delay, which hampers its application in high-power devices. It was an effective method to solve this problem that CFx with lower fluorination content improved its rate capability [4–6]. For example, sub-fluorinated carbon nanofibres could sustain high discharge rate up to 6C due to the presence of unfluorinated path
* Corresponding author at: School of Materials Science and Engineering, Tianjin University, Tianjin 300072, PR China. E-mail address: [email protected] (W. Feng). http://dx.doi.org/10.1016/j.synthmet.2016.07.032 0379-6779/ã 2016 Published by Elsevier B.V.
within the CFx [5]. The mixture of CFx with other cathode materials such as silver vanadium oxide or manganese dioxide to develop a hybrid structure was also utilized to modify the rate capability [7–9]. The addition of some amount of nanostructured conductive additives, such as carbon nanotubes [9] or graphene [10], into the CFx cathode can significantly improve its rate capability due to the formation of conductive paths for the charges transport. Recently, it was found that substituting nanostructured carbon materials for conventional graphite as the starting materials of CFx was an effective approach to improve the rate capability thanks to the special nanostructures [5,11]. Besides the methods mentioned above, surface coating was considered as an easy and effective way to enhance the electronic conductivity of CFx [12,13]. Conducting polymers have received numerous attentions because they can play different roles in improving the cathodes as a result of their good electrochemical stabilities and favorable morphologies [14–16]. For example, the electrodeposition of polypyrrole on CFx cathode improved its rate capability and the delivered power density achieved to 5235 W kg1 at 4C rate [13]. Among the various conducting polymers, because polythiophene (PTh) can participate in lithiation and delithiation reactions, contributing to the specific capacity of the composites [17,18], it has been served as the conducting layers in various cathode materials to improve their electrochemical performances [19–21]. In this work, a series of PTh/CFx composites
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were prepared by in situ polymerization. The influence of the different PTh ratios on the electrochemical performances was studied. PTh particles as a conductive additive were coated on the surface of CFx powder. The conductivity of CFx cathode was significantly enhanced by the PTh coating, which decreased the polarization of discharge and hence improves the rate capability. 2. Experimental 2.1. Material preparation The PTh/CFx composites were synthesized by in situ polymerization. At first, 1 g CFx powders (x = 1.0, Alfa Aesar, A Johnson Matthey Company) and some amount of anhydrous FeCl3 (Tianjin Guangfu Fine Chemical Research Institute) were dispersed in 100 mL CHCl3 solution to form a homogenous suspension with the help of ultrasonication. Before the polymerization, the suspension was bubbled with argon gas for 30 min to remove the oxygen. While the suspension was magnetically agitated, PTh monomers (0.1, 0.25 and 0.5 g; J&K Scientific Ltd.) dissolved in 50 mL CHCl3 was slowly added into the suspension in 1 h, and the molar ratio of thiophene to FeCl3 was kept at 1:4. The polymerization reaction was carried out in ice-bath under a continuous argon flow for 10 h so that thiophene monomers tended to link by a-a position, which could enhance the electrical conductivity of PTh effectively [21]. After the polymerization, 100 mL methanol was added to the mixture to dissolve the remaining FeCl3. The mixture was then filtered to remove the iron ions as well as the remained monomers, and the insoluble solid was washed by methanol and deionized water for several times. After this, the solid was poured into the solution of 1 M hydrochloric acid and the mixture was stirred for 2 h at room temperature. Then the solid product was collected by filtration and washed using deionized water until the filtrate being neutral. Finally, the brown product was dried at 80 C for 24 h under vacuum to obtain the PTh/CFx composite. The proposed synthesis route of PTh/CFx composite is illustrated in Scheme 1. 2.2. Materials characterization The thermal behavior of the samples was examined by thermogravimetry analysis (TGA, Shimadzu DTG-60) from room temperature up to 800 C at a heating rate of 5 C min1 in a dynamic
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atmosphere of nitrogen (35 mL min1). The morphology of CFx and PTh/CFx composites was observed by field-emission scanning electron microscopy (SEM, hitachi S-4800) and field-emission transmission electron microscopy (TEM, Philips Tecnai G2 F20). Fourier Transform infrared (FTIR) spectroscopy characterized the structure of the synthesized sample. The FTIR spectra were recorded with Perkin–Elmer Spectrum One FTIR Spectrophotometer which used KBr pellets in the region 4000–500 cm1. X-ray diffraction (XRD) measurements were performed on a Philips diffractometer, which was composed of a quartz monochromator, a Cu Ka radiation source at a scan rate of 10 min1 and a goniometric plate. Fourprobe testing instrument (SX1934, Jiangsu) was used to measure the conductivity, and made it on a disc-shaped pellet by four-point direct current method at room temperature. 2.3. Electrochemical measurement The electrodes were composed of cathode materials (bare CFx or CFx/PTh composites), conductive additive (Super P carbon) and binder (polyvinylidene difluoride, PVDF) with a weight ratio of 80:10:10. In order to obtain the homogenous and slightly viscous slurry, N-methyl pyrrolidone (NMP) was added. The electrode film was obtained by the means of the slurry coating on an Al foil, and the film was dried at 120 C for 12 h. The electrochemical performances of CFx/PTh composites were investigated by assembling 2032 coin type batteries, so the diameter of punched electrode disks was about 18 mm, which were further dried at 80 C for 8 h in a vacuum. The mass loading for each cathode was 2– 3 mg cm2. And then, the shaped disks were put into a glove-box filled with argon (Mikrouna Co., Advanced 2440/750) for cell assembly. A metallic lithium disc, a microporous polypropylene/ polyethylene/polypropylene film and the solution of 1 M LiPF6 in ethylene carbonate: dimethyl carbonate (EC:DMC, 1:1, vol%) were used as the anode, separator and electrolyte, respectively. The discharged test of coin cells was operated under constant current densities (Land CT2001A, Wu Han Jin Nuo Electronics Co., China) at room temperature, and the termination potential was 1.5 V (vs Li/ Li+). A three-electrode electrochemical cell included a Li flake as the counter electrode and an extra Li wire as the reference electrode, and the EIS was measured using Advanced Electrochemical System Parstat 2263 over the frequency range from 0.01 to 10 kHz.
Scheme 1. Schematic illustration of the proposed synthesis route of PTh/CFx composites.
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3. Results and discussion 3.1. Composition of PTh/CFx composites Fig. 1 shows the comparison of the mass losses of CFx, PTh and PTh/CFx composites upon heating in a nitrogen atmosphere. The obvious weight loss of CFx between 450 and 650 C is derived from the self-decomposition of CFx [22]. In the case of PTh, it decomposes with a two-step weight loss behavior. The first weight loss step occurs between 130 and 360 C, attributed to the loss of dopant anions bound to the PTh chain, and the second step weight loss occurs between 360 and 740 C, assigned to the degradation of the skeletal PTh chain structure [23,24]. There is no significant change in the CFx decomposition temperature when the initia mass ratio of PTh to CFx below to 1:4, although it slightly shifts to the lower value with the increase of PTh amount due to the interaction between main chains of PTh and CFx surface. When the initial mass ratio of PTh to CFx is 1:2, the composite shows the weight loss behavior similar to that of PTh, indicating the agglomeration of PTh particles [25]. On the basis of the total mass loss up to 800 C, the mass ratio of PTh in the PTh/CFx composite can be determined as 8.66, 22.94 and 33.80 wt.%, respectively, which is close to the initial reactant ratio. 3.2. Microstructures of PTh/CFx composites The SEM images of CFx and PTh/CFx composites are shown in Fig. 2. The pristine CFx particles (Fig. 2a) have a smoother surface with sharper edges and the particle size ranges from 2 to 6 mm. According to the synthesis route, the absorbed Fe3+ on the outer surface of CFx particles initiated the polymerization of thiophene monomers, and hence an electrically conductive PTh layer was formed on the surface of CFx particles. The PTh coated CFx powders tend to aggregate due to the interaction of static electricity of PTh layers on the surface of CFx [12], and PTh/CFx composites develop a porous structure. From high magnification images, it can be found that the surface of PTh/CFx composites is in the flakelike morphology with highly porous structures and the PTh coating layer becomes less porous with the increasing content of PTh in the composite. In order to observe the PTh coating on the surface of CFx, TEM observation was carried out and the TEM images are shown in Fig. 3a–c. When the PTh is in a low level (Fig. 3a), CFx particle can only be coated by some granulated PTh, with large area of bare surface. When the PTh amount reaches 22.94 wt.% (Fig. 3b), CFx particle surface is coated by a thin PTh layer uniformly and
completely. When the PTh content continues to increase up to 33.80 wt.% (Fig. 3c), PTh coating on CFx particle surface gets thickening and some individual PTh particles appear in the composite. An extra HRTEM image of 22.94 wt% CFx/PTh composite is shown in Fig. 3d. Based on the proposed synthesis route, it was clearly observed that the translucent PTh layer (the lighter area) coated on the surface of CFx particles (the dark region), which is similar to the previous reports [19–21]. The PTh coating plays an important role to enhance the electrochemical performances of composites because it not only enhances the electronic conductivity of the composite by reducing the particle-to-particle contact resistance but also increases the contact area between the cathode and the electrolyte [20,21]. Therefore, the reasonable amount PTh can form an uniform and complete coating, and the insufficient and excessive amount of PTh may cause uncoated or too thickness coating. 3.3. Structural characterization of PTh/CFx composites Fig. 4(a) shows the FTIR spectra of CFx, PTh and PTh/CFx composites. The spectral data of the PTh was consistent with those reported previously [26–28]. The strong absorption band at 789 cm1 is assigned to the C H out-of-plane stretching vibration of the 2,5-disubstituted thiophene ring. The band at 1433 cm1 is attributed to the stretching peak of C¼C and the bands at 1658 and 1324 cm1 are indicative of the C C asymmetric and symmetric stretching vibrations, respectively. The two bands attributed to the C S asymmetric and symmetric stretching vibrations are found at 1036 and 694 cm1. The appearance of bands at 694, 789, and 1036 cm1 confirms that the thiophene monomers are mainly polymerized through the a-a conjunction shown in the synthesis route, leading to an excellent electronic conductivity of PTh, which is crucial for the improvements of electrochemical performances [29,30]. The band at 3427 cm1 is attributed to the O H stretching vibration from crystal or absorbed water [24]. On the other hand, the strong absorption band at 1217 cm1 in the CFx sample is assigned to the characteristic of the C F covalent bonds and the bands at 2924 cm1 are indicative of the CH stretching vibration [31]. The weak polymer signals of PTh/CFx in which the mass ratio of PTh is below 22.94 wt.% is caused by the relatively low fraction of PTh, which is consistent with TGA analysis. The absence of new bands in PTh/CFx composites and the similarity of PTh and PTh/CFx composites spectra indicate that no chemical reaction occurs between CFx and PTh, and PTh serves as a conducting additive in the composites. To confirm the structure of PTh/CFx further, the XRD patterns are given in Fig. 4(b). The XRD pattern of PTh shows a typical amorphous structure with a broad peak centered at 2u = 20 . The XRD patterns of the CFx display three distinct peaks at 12.5, 26, and 41, which is in agreement with the previous reports [32–34]. The peak at 12.5 is the (001) reflection attributed to a high-level fluorinated hexagonal system, and the broad peak at 26 , assigned to (002) reflection, is generated by the poor regularity along the stacking direction. The peak around 41 represents the (100) reflection and is ascribed to the C C in-plane length in the reticular system. The XRD spectra of the PTh/CFx composites don’t change a lot compared with the pattern of CFx except for the appearance of slight broad PTh peaks centered at 2u = 20 , demonstrating that the crystal structure of CFx remains original and no new phase forms during the in-situ polymerization process [35]. 3.4. Electrochemical analysis of PTh/CFx composites
Fig. 1. TGA analysis of PTh, CFx and PTh/CFx composites.
Fig. 5 shows the galvanostatic discharge curves of PTh/CFx composites. The pure CFx cathode is also discharged at the same
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Fig. 2. SEM images of CFx (a), 8.66 wt.% PTh/CFx (b), 22.94 wt.% PTh/CFx (c) and 33.80 wt.% PTh/CFx (d). The insets show the SEM images at high magnifications of the corresponding samples.
Fig. 3. TEM images of 8.66 wt.% PTh/CFx (a), 22.94 wt.% PTh/CFx (b), 33.80 wt.% PTh/CFx (c) and the HRTEM image of 22.94 wt.% PTh/CFx (d).
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Fig. 4. FT-IR spectra (a) and XRD patterns (b) of PTh, CFx and PTh/CFx composites.
Fig. 5. Galvanostatic discharge curves at different rates of CFx (a), 8.66 wt.% PTh/CFx (b), 22.94 wt.% PTh/CFx (c), and 33.8 wt.% PTh/CFx (d).
rates for the comparison. The theoretical specific capacities of purchased CFx (x = 1.0) and PTh are 865 mAh g1 and 82 mAh g1, respectively [36]. The discharge current at different rates is calculated by the theoretical value of individual cathode. The electrode components, the theoretical specific capacities and electrochemical performances of pure CFx and CFx/PTh composites cathodes are listed in Table 1. And these values are calculated for a potential cut-off at 1.5 V vs. Li/Li+. The specific capacity of CFx is 784 mAh g1 at 0.05C, and it hardly delivers any capacity at 1C, like previous reports [5,9,10,12]. The introduction of PTh coating on CFx particles improves the rate capability of CFx and reduces the voltage delay after discharge effectively. The increased polarization with the increase of discharge rates is attributed to the difference
in the potential of the solvated lithium ion [37,38], which indicates the lithium ion diffusion controlled reaction. The PTh layer reduces the particle-to-particle contact resistance, which enhances the electrical conductivity of the composite. From Table 2, the electronic conductivity of 8.66 wt.% PTh/CFx composite is 5.28 105 S cm1, while CFx is a kind of insulators. When the content of PTh reaches 22.94 wt.%, the electronic conductivity of the composite is 3.42 103 S cm1, and the further increase of PTh content in the composite enhances little of electronic conductivity. In addition, from SEM images, the surface of CFx/PTh composites shows the flakelike morphology with highly porous structures. The porous structure of PTh layer allows the electrolyte to distribute throughout the electrode, which enhances the lithium ion
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Table 1 Electrochemical performances of CFx and PTh/CFx. samples
Electrode components
Theoretical Capacity (mAh g1)
Maximum Crate
Maximum Capacity (mAh g1)
Maximum W (Wh kg1)
Maximum P (W kg1)
CFx
80 wt.% CFx 10 wt.% Super P 10 wt.% PVDF 6.93 wt.% PTh 73.07 wt.% CFx 10 wt.% Super P 10 wt.% PVDF 18.35 wt.% PTh 61.65 wt.% CFx 10 wt.% Super P 10 wt.% PVDF 27.04 wt.% PTh 52.96 wt.% CFx 10 wt.% Super P 10 wt.% PVDF
865
1C
784.4
1806
1252
797.19
2C
802.6
1871
2693
685.38
4C
715.0
1707
4997
600.35
4C
603.1
1428
5460
8.66 wt.% PTh/CFx
22.94 wt.% PTh/ CFx
33.80 wt.% PTh/ CFx
Table 2 The electronic conductivities of PTh/CFx and PTh. Samples
Electronic conductivity (S cm1)
8.66 wt.% PTh/CFx 22.94 wt.% PTh/CFx 33.8 wt.% PTh/CFx PTh
5.28 105 3.42 103 3.75 103 0.34
diffusion coefficient subsequently. Therefore, the rate capability of CFx/PTh composite cathode is better than pure CFx cathode and the potential of CFx/PTh composite cathode is higher than that of pure CFx cathode at high rates, which becomes more significant with the increase of PTh content in the composite. However, the excessive PTh content in the composites reduces the specific capacity of composites contrarily due to the low specific capacity of PTh [39]. Therefore, the electrochemical performances of PTh/CFx composites can be tuned by the content of PTh, and 22.94 wt.% PTh/CFx composite shows the best electrochemical performances. The specific capacity of this composite is 715 mAh g1 at 0.05C rate, close to the theoretical capacity, and the specific capacity is about 270 mAh g1 at 4C rate. Fig. 6 presents the Ragone plots, which gives the variation of the energy density vs. the power density for CFx and PTh/CFx. The higher energy density and the lower power density values are observed at low discharge rates (0.1C). The increase of the
Fig. 6. Ragone plots of CFx and PTh/CFx composites.
discharge rate leads to an obvious decrease in energy density due to the drop of the discharge voltage as well as the specific capacity. The addition of PTh can effectively enhance the rate capability of the composites, leading to the high power densities. Both 22.94 wt. % and 33.80 wt.% PTh/CFx composites can be discharged at 4C rate, and the delivered power density is 4997 and 5460 W kg1, respectively. However, the energy density of 33.80 wt.% PTh/CFx at low discharge rate is significantly lower than that of other composites due to the excessive PTh amount and the agglomeration of PTh particles. Therefore, it can be further demonstrated that 22.94 wt.% PTh/CFx is the proper composite, exhibiting an excellent rate capability without the loss of energy density at low rate. In order to understand more details of the PTh coating effects, the impedance analysis of PTh/CFx, discharged at 0.1C rate, at 10% depth of discharge (DOD) states is performed, and Fig. 7 shows the Nyquist plots. According to the previous report [6], Fig. 7 inset shows the equivalent circuit used for the interpretation of the impedance spectra. The bulk ohmic resistance (Rb), which is contributed to the combination of current collector, electrode, separator and electrolyte, is low for all the cathode materials due to the batteries were prepared by reaction in the same electrolyte and adding the same conductive additive. On the interface between the discharge product shell and the liquid electrode, it can be characterized by Rct and Qct at high frequency that the charge transfer through the electric double layer. On the surface of
Fig. 7. The impedance spectra of CFx and PTh/CFx composites measured from a three-electrode electrochemical cell at 10% levels of DOD. The inset is the equivalent circuit for analysis of the lithium primary batteries.
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cathode, the discharge product shell can be characterized by R1 (resistance to lithium ion diffusion on the discharge product shell) and Q1 (a constant phase element) at high frequency. Due to the lack of lithium ion diffusion in active materials, another constant phase element Qint is applied. And Rct is the synergetic effects of the contact resistance between product shell resistance, chargetransfer resistance and conductive particles, which corresponds to the depressed semicircle in the impedance plot. The introduction of PTh coating brings no new semicircles in the impedance spectra, indicating that the electrolyte can easily access the coated CFx because of the porous structure of PTh. The conductive PTh coating also provides electron and ion channels for electrochemical reactions and enhance the electronic conductivity of CFx cathode significantly. The value of Rct decreases with the increase of PTh amount, but the difference of Rct between 22.94 wt.% and 33.80 wt.% PTh/CFx is not significant. Meanwhile, the frequencies in the onsets between the semicircle and the slopping straight line represent the reaction kinetics, which are also marked in Fig. 7, and the higher frequency indicates the faster kinetics [40]. The reaction kinetics becomes faster with the increase of PTh amount in the PTh/CFx composites until that up to 22.94 wt.%. Moreover, the porous structure of PTh coating can also avoid the cracking of cathode as well as the blocking of lithium ions due to the swelled LiF crystals formed during the electrochemical reaction [41]. Therefore, the porous PTh coating on CFx surface by in situ polymerization is an effective method to improve the electrochemical performance of CFx cathode. 4. Conclusions PTh/CFx composite materials have been synthesized by in situ polymerization for the first time. The introduction of PTh coating can improve the electronic conductivity of CFx, and hence accelerates the charge transfer reaction, which was confirmed by EIS measurements. The PTh coating also acts as the electrolyte absorbent due to the porous structure, facilitating the access of electrolyte to CFx and fulfilling the utilization of cathode completely. TEM images demonstrate that an uniform and complete coating can be formed when the PTh content reaches 22.94 wt.%. The 22.94 wt.% PTh/CFx maintains the high energy density of CFx at low discharge rate and exhibits the excellent rate capability, which delivers 4997 W kg1 power density at 4C rate, while the maximum power density of CFx is only 1252 W kg1. The PTh/CFx composites containing 22.94 wt.% PTh is a promising cathode material for lithium primary battery. Acknowledgements This work was financially supported by National Natural Science Funds for Distinguished Young Scholars (51425306), the National Basic Research Program of China (Grant No. 2016YFA0202302), National Natural Science Foundation of China
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