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TBAPF6 hierarchical nanofiber gel membrane for lithium ion capacitor with ultrahigh ion conductivity and excellent interfacial compatibility
Electrochimica Acta 318 (2019) 801e808
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
PVDF/TBAPF6 hierarchical nanofiber gel membrane for lithium ion capacitor with ultrahigh ion conductivity and excellent interfacial compatibility Xianlei Shen a, b, 1, Zongjie Li a, c, 1, Nanping Deng a, c, Weimin Kang a, b, Jie Fan a, b, *, Yong Liu a, b, ** a
State Key Laboratory of Separation Membranes and Membrane Processes/National Center for International Joint Research on Separation Membranes, Tianjin Polytechnic University, Tianjin 300387, PR China School of Textile Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, PR China c School of Material Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 April 2019 Received in revised form 4 June 2019 Accepted 15 June 2019 Available online 19 June 2019
An efficient, convenient and flexible polyvinylidene fluoride/tetrabutylammonium hexafluorophosphate hierarchical nanofiber membrane (PVDF/TBAPF6 HNM) was successfully fabricated by one-step electrospinning and used as the separator of Li-ion capacitor (LIC). The PVDF/TBAPF6 HNMs displayed high ion conductivity of 4.28 103 S/cm, high mechanical strength, porosity, wettability and excellent thermal performance. Furthermore, the separator of PVDF/TBAPF6 HNM possessed lower interfacial resistance (134U) and better electrochemical stability, which exhibits better capacity retention and coulombic efficiency, compared with the commercial celgard 2340 separator. We believe that the gel polymer PVDF/TBAPF6 HNMs separator will have great potential in the field of the separator of LIC. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Lithium ion capacitor PVDF hierarchical nanofiber Gel membrane Ion conductivity Interfacial compatibility
1. Introduction The enormous growth of advanced intellectual products and emergence of electric vehicles have attributed to a high demand for high-efficiency energy storage systems with excellent energy and power density as well as cycle life [1]. Over the past twenty years, various energy storage elements with excellent performance have been proposed. Among these energy storage devices, Li-ion batteries (LIBs) and electric double-layer capacitors (EDLCs) are currently considered to be the most promising [2]. Compare with EDLCs providing higher power density (2-5 KW.Kg1) and long cycle life [3], LIBs possess higher energy densities (150200 Wh.Kg1) [4]. However, the power density and cycle life of LIBs
* Corresponding author. School of Textile Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, PR China. ** Corresponding author. School of Textile Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, PR China. E-mail addresses: [email protected] (J. Fan), [email protected] (Y. Liu). 1 These authors contributed equally to this work and should be considered cofirst authors. https://doi.org/10.1016/j.electacta.2019.06.095 0013-4686/© 2019 Elsevier Ltd. All rights reserved.
are relatively low. As a results, lithium ion capacitor (LIC), as a new capacitor for connecting LIBs and EDLCS, has attracted widespread attention from researchers because of different storage mechanisms at both ends [5]. LIC is composed of four parts including cathode, anode, separator and electrolyte, wherein EDLCs-type cathodes establish fast charge-discharge conditions, LIB-type anodes enable large capacitances [6]. Separator plays a key role in preventing physical contact of the electrodes meanwhile establishing free ionic transport and isolating electronic flow, which has significant influence on the safety performance and cycle capability of LIC [7e9]. At present, polyolefin microporous membranes have become the dominating separators composed of polyethylene (PE) and polypropylene (PP) owing to excellent mechanical property, glorious electrochemical stability and low cost, as well as particular performance that shuts the ion channel further preventing electrochemical reactions when a defective capacitor heats up arouse electrical short [10]. However, polyolefin separators are incompatible with the electrolyte due to their inherent limitations, such as low porosity, low electrolyte wettability, which directly affects the LIC performance [11e13]. Current researches largely focused on improving the absorption
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capacity between electrolytes and separators and suggested a large amount of strategies, including exposure to plasmas, electron beams, g-rays, physically grafting with hydrophilic molecules, phase inversion method and electrospinning technique [12,14,15]. Among these methods, electrospinning is an efficient technology which can produce long continuous fibers and control the structure of the fibers [16]. Additionally, the electrospun nanofiber membranes possesses high porosity and large specific surface area [17], which have a great potential in the application of separator to improve the property of LIC. Among the past few years, various electrospun polymers with excellent performances have been fabricated as separators of EDLCs or LIBs, including polyvinylidene fluoride (PVDF) [18], poly(vinylidene fluoride co-hexafluoropropylene) (PVDF-HFP) [19], polyacrylonitrile (PAN) [20]and polyethylene terephthalate (PET) [21]. It is well known that PVDF owns some appealing properties such as high polarity high dielectric constant (ε z 8.4), low degree of crystallinity and strongly electron affinity functional groups (eCeFe) [22]. Meanwhile, PVDF as a gel polymer electrolyte performs excellent ion conductivity [23]. Choi et al. [24] fabricated PVDF electrospun nanofiber membranes as polymer electrolyte with high ion conductivity(1.6e2.0 103 S/cm). These electrospun PVDF membranes had certain capacity to act as LIC separators, however, the porosity, electrolyte uptake and corresponding ionic conductivity of separator still needed to be further improved. There exit many emerging methods and strategies to improve the performance, including blending modification, compound modification and adding inorganic nanoparticles, etc. [25]. For example, Li et al. [26] blended PVDF with polydimethylsiloxane (PDMS) polymer to fabricate LIB, and found that the porosity and the electrolyte uptake increased significantly compared with those of pure elecrtospun PVDF membrane. Moreover, Lu et al. [27] prepared a poly(phthalazinone ether sulfone ketone) (PPESK)/PVDF/ PPESK tri-layer compound membrane via electrospinning and obtained good wettability in liquid electrolyte and higher ionic conductivity. Zhang et al. [28]coated the electrospun PVDF separator with SiO2 and obtained superior ionic conductivities and low electrode interfacial resistance of the separator. Additionally, tetrabutylammonium hexafluorophosphate(TBAPF6) was used as supporting electrolyte for increasing the conductivity of system and improving the electrochemical stability of electrolyte [29,30]. For example, Kiefer et al. [31] doped tetrabutylammonium hexafluorophosphate(TBAPF6) on the Polypyrrole (PPy) films by electropolymerization and the PPy/TBAPF6 film exhibited excellent conductivity. However, TBAPF6 has rarely been reported in the application of improving the battery separator properties. Herein, a novel PVDF/TBAPF6 LIC separator with high ion conductivity and low internal resistance was successfully fabricated by adding certain amount of TBAPF6 into the PVDF solution via electrospinning. A multiscale hierarchical nanofiber structure could be fabricated by the introduction of TBAPF6. The effect of the addition of TBAPF6 on the electrolyte uptake, liquid retention ability, ion conductivity and interfacial resistance of the separator were investigated, as well as the mechanical performance, thermal stability and LIC cycle performance. 2. Experimental 2.1. Materials PVDF(Mw ¼ 520,000) was purchased from Shanghai 3 F New Materials Co. Ltd, China. N,N-dimethylformamide (DMF) and acetone were supplied by Tianjin Kermel Chemical Reagent Co. Ltd, China. Tetrabutylammonium hexafluorophosphate (TBAPF6) (MW ¼ 387.41) was provided by damas-beta, China. The liquid
electrolyte of 1 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DC)/ethyl methyl carbonate (EMC) (1/1/1 wt/wt/wt) was supplied by New Materials of Guo Tai Hua Rong (Zhangjiagang, China). All of the chemical reagents were commercially purchased, without any treatment before used. 2.2. Synthesis TLNMs Firstly, PVDF was dissolved in the mixture of DMF and acetone with a volume ration of 3:1 to obtain the PVDF solution with a concentration of 16 wt%, then different contents of TBAPF6 were added into solution, stirring for 1 h to obtain different rations of PVDF/TBAPF6 solution. A commercially available electrospinning device (Shanghai Oriental Flying Nanotechnology Co., Ltd) was used for the fabrication of PVDF/TBAPF6 HNMs, as described previously. Briefly, the electrospinning conditions were optimized as follows: flow rate of 1 mL h1, tip-to-collector distance of 15 cm, and applied voltage of 30 kV. The obtained nanofiber membranes were dried at 60 C for 12 h in vacuum condition to remove solvent before being further used. The three samples were named as M1, M2 and M3, under the corresponding content of TBAPF6 of 0.025 mol/L,0.05 mol/L,0.075 mol/L, respectively. Pure PVDF nanofiber membrane was also prepared as control sample. The processes of manufacturing PVDF/TBAPF6 HNMs were shown in Fig. 1(a) and LIC schematic was illustrated in Fig. 1(c). 2.3. Electrode preparation and cell assembly The electrochemical performance of the prepared separator was tested by assembled the CR2032 coin type cells. Fig. 1(b) showed the components of the LIC, in which the electrode was made of the mixture of activated carbon (AC), conductive carbon (Super-P) and polyvinylidene fluoride (PVDF) as binder at ratio of 7:2:1 by weight. The mixture was smeared on the copper foils and then placed into vacuum oven for 24 h at 45 C. The anode needed to be prelithiated through contacting electrode and Li-metal foils under the pressure of 0.2 MPa and then immersed in electrolyte for 1.5 h before assembled into the LIC. 2.4. Characterization 2.4.1. Morphology The surface morphology of the PVDF/TBAPF6 HNMs was obtained by a field emission scanning electron microscope (FE-SEM, Gemini SEM500, Germany). 2.4.2. XRD The nanofiber crystallinity was characterized by D/MAX-2500 X-Ray Diffraction (Rigaku) with 2 h values in the 2q range between 10 and 55 . 2.4.3. FTIR Infrared absorption spectra were collected in the 6001500 cm1 range on a Fourier transform infrared spectrometer (FTIR, TENSPR37, Bruker). 2.4.4. Raman spectroscopy Raman spectroscopy was performed using a Renishaw in Via Raman microscope with an excitation wavelength of 532 nm and a power of 1 mw. 2.4.5. Mechanical properties Mechanical properties were measured using YG005 Electronic Single Fiber Strength Tester (Fangyuan Instrument Co., Ltd.,China) under the speed of testing at 10 mm min1 and the grip length of
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Fig.1. (a) The schematic illustration of the preparation of electrospun separator and battery assembling, (b) the components of the LIC, (c) the schematic of LIC.
20 mm. The examined sample dimension was 5 mm 50 mm. The tensile strength, T, of the membrane was calculated via eq (1).
T¼
X A*B
(1)
where X is the maximum load of membrane, A and B are the width and thickness of the membrane. The thickness of membrane was measured with a 0-10-3Q thickness gauge (Shanghai chuanlu measuring tool Co., Ltd.).
2.4.7. Electrolyte uptakes All the samples were dried for 24 h under vacuum condition at 60 C and then soaked into the select electrolyte for 1 h. The remaining electrolyte on the sample surface was removed by filter paper before measurement. The electrolyte uptake was calculated eq. (2) below.
Uptakeð%Þ ¼
Ww Wd 100% Wd
(2)
where Wd and Ww are the masses of dried and wet samples, which immersed into the electrolyte for 1 h respectively. 2.4.6. Thermal stability The thermal shrinkage property of the membrane was characterized by analyzing the dimensional changes of the circular samples after thermal treatment at 80 C, 100 C, 120 C, 140 C and 150 C for 1.5 h, respectively.
2.4.8. Porosity The porosity of the membrane was measured by immersing into n-butanol (Yingda Chemical Co., Ltd.) for 2 h and then the residual n-butanol adhering on the surface of sample was removed by wiped papers. The membrane porosity was calculated by eq. (3) as
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follows.
Porosityð%Þ ¼
Ww Wd 100% rb *Vd
(3)
where Wd and Ww are the weight of the membrane before (dry) and after soaking (wet) in n-butanol for 2 h, rb is the density of nbutanol (0.8098 g cm3), and Vd is the volume of the dry nanofiber membrane. 2.4.9. Contact angel measurement The contact angel (CA) of the specimen was measured by Contact Angle Meter (JC2000D, Powereach Co., Ltd., Shanghai, China) at 26 C. 2.4.10. Electrochemical characterize The ionic conductivity of the membrane was measured with stainless-steel (SS)/separator/SS blocking cells by electrochemical impedance spectroscopy (EIS, SI1260, Solatron) (CHI660D, Beijing Chinese science days Technology Co., Ltd.) with a frequency range from 0.01 Hz to 100 KHz. The ionic conductivity of the four samples were obtained by the following eq (4).
s¼
d Rs *A
(4)
where s is ionic conductivity, d is the thickness of separators. Rs and A are the bulk resistance of separators and the contact area between separators and SS, respectively. The interfacial resistance was measured on the CHI660D electrochemical workstation by AC impedance with Li/separator/Li cells at a scan rate of 1 mV s1 and a frequency range of 0.01 Hz ~ 100 KHz. Electrochemical stability window was measured in the voltage range of 2e7 V at a scan rate of 1 mV s1 by the linear sweep voltammetry (LSV) of Li/separator/SS cells using CHI660D electrochemistry workstation. The separator cycle stability was characterized by the capacity retention and coulombic efficiency of LIC. The assembled cell was measured after placing in argon atmosphere for 24 h using a LAND CT2001A battery-testing system (Lanhe Co., Ltd., Wuhan, China) in range of 0e3 V at 0.5C. 3. Results and discussion Fig. 2(a)-(d) are SEM images of pure PVDF and PVDF/TBAPF6 HNMs with different contents of TBAPF6. It was clear that hierarchical nanofibers with multi-scale diameter could be observed after adding TBAPF6. With the increase of TBAPF6 concentration, the number of fine nanofibers increased. According to our previous research [32], this was because that organic branched salts TBAPF6 could increase the conductivity of the precursor solution, which intensified the splitting of the jet in electrospinning. The fiber diameter frequencies were shown in Fig. 2(e)-(h). It was found that the average nanofiber diameters (AFDs, less than 100 nm) of PVDF/ TBAPF6 HNMs were smaller than that of pure PVDF nanofibers, in which their morphologies was homogenous and uniform with AFD of 132 nm. Additionally, the AFD of PVDF/TBAPF6 HNMs decreased with the increase of TBAPF6 concentration. Fig. 2 (i)-(j) showed the photos of pure PVDF and M2 separator after immersing in the liquid electrolyte and their corresponding SEM images. It could be seen that the gelatinizing phenomenon occurred when the two samples were soaked in the electrolyte and the gelation phenomena of M2 was much better than that of pure PVDF membrane. This phenomenon can be explained by the fact that the large specific surface
area caused by the hierarchical structure. Meanwhile, PVDF is a semi-crystalline polymer [33], which can be proved by the XRD spectra, as shown in Fig. 2(k). The intensities of peaks assigned to PVDF dramatically decreased after the addition of TBAPF6 and the amorphous domain of the membrane increased, which could facilitate the absorption and swelling of the liquid electrolyte in the membrane [34]. As a result, the electrolyte retention rate and ion conductivity of HNMs can be improved. Fig. 2(lem) showed the FTIR spectra and the Raman spectra of pristine PVDF and M2 membrane. The vibrational bands at 795,1149 cm1 correspond to the a-phase, and the peaks at 840 cm1,1402 cm1 correspond to the b-phase [35e38]. It is observed that b-phase content ratio increases with the introduction of TBAPF6. The Raman spectrum of the pristine electrospun PVDF membrane indicated the presence of major vibration bans observed at 800,877 cm1, it corresponds to the a phase of PVDF [39,40]. The Raman bands at 840 cm1 indicated the presence of b phase in PVDF membrane. As a result, both the FTIR spectra and Raman spectra confirmed the enhanced b phase in the PVDF/TBAPF6 HNMs, which is benefited to improve the morphologies of the elecrtrospun fibers [41] and enhancement of membrane dielectric constant and ac conductivity [42]. Therefore, the PVDF/TBAPF6 HNM has potential applied for LIC separator to promote ion transport performance. Generally, the mechanical property of electrospun nanofiber membrane is important for battery separator. Typically stressstrain curves of pure PVDF nanofiber membrane and PVDF/ TBAPF6 HNMs with different TBAPF6 contents were shown in Fig. 3(a). It was clear that the tensile breaking strength of all HNMs were higher than that of pure PVDF nanofiber membrane. When the concentration of TBAPF6 increased from 0 to 0.05 mol/L, the tensile breaking strength increased from 3.68 MPa to 5.54 MPa. It could be explained that this hierarchical structure can improve the mechanical properties of the membrane. This was because that the thick fibers acted as skeleton supports and the thin fibers interacted strongly with thick fibers through the bonding points and the entanglement, enhancing the mechanical property of membrane [43].Further increased the TBAPF6 concentration to 0.075 mol/L, the tensile breaking strength decreased to 4.97 MPa. This may be attributed to the reduced forces between PVDF molecules arising from the addition of excessive TBAPF6. These results indicated that the PVDF/TBAPF6 HNMs exhibited preferable mechanical strength comparing with pure PVDF nanofiber membrane that had a promotion for the safety and cycle performance for the LIC. The thermal stability and shrinkage resistance have great effects on the safety of LIC. Fig. 3(b) showed the thermal stability and shrinkage resistance of the Celgard 2340, pure PVDF, M1, M2 and M3 after the hot oven treatment from 80 C to 150 C, respectively. The results revealed that with the increasing of temperature, the commercial separator shrank at 120 C, and the color changed from white to transparent at 140 C, while no significant shrinkage was observed for PVDF and PVDF/TBAPF6 HNMs, demonstrating that these PVDF membranes had better shrinkage stability. The excellent thermal stability and shrinkage resistance is due to superior thermal resistance of PVDF itself compared with polyolefins. The large porosity and polyfluoropolymer could promote the electrolyte wettability of separator [44]. The electrolyte contact angles of pure PVDF and PVDF/TBAPF6 HNMs were shown in Fig. 4(a). It could be seen that the contact angel of the pure PVDF membrane (19 ) was smaller than that of the commercial separator (39.50 ), indicating that the electrospun porous PVDF separator had an excellent wettability compared to commercial separator. Meanwhile, the HNMs had smaller contact angle compared with pure PVDF membrane, performing excellent lyophilic properties. It can be mainly ascribed to the larger specific surface area [45] and low crystallinity of the HNMs, which was in accordance with the
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Fig. 2. SEM images of (a)pure PVDF and (b) M1, (c) M2; (d) M3, the nanofiber diameter distribution of (e) pure PVDF, (f) M1, (g) M2, (h) M3, morphology of pure PVDF (left) and M2(right) before(i) and after (j) immersing in electrolyte, (k) XRD patterns PVDF and M2, (l)FTIR spectra of PVDF and M2, (m) Raman spectra of PVDF and M2.
Fig. 3. (a)strain stress curve of pure PVDF and PVDF/TBAPF6 HNMs (b) the hot treatment pictures of Celgard 2340, pure PVDF and PVDF/TBAPF6 HNMs.
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Fig. 4. (a) The contact angles of commercial separator, pure PVDF and PVDF/TBAPF6 HNMs, (b) liquid retention rate and porosity of commercial separator, pure PVDF and PVDF/ TBAPF6 HNMs.
above SEM image and XRD patterns. The liquid retention rate and porosity of membranes were presented in Fig. 4(b). It could be found that the porosity and liquid retention rate of electrospun nanofiber membranes were significantly higher than that of commercial Celgard 2340. With the increase of TBAPF6 concentration, the porosity and liquid retention rate increased gradually. This was because the hierarchical structure could greatly increase the porosity and specific surface area of membranes and then increased the surface contacted area between fibers and electrolyte, leading to excellent electrolyte uptake [46]. Meanwhile, the dense hierarchical nanofiber structures can cause high electrolyte retention, which is benefited to the sustaining ion transport in the gel-type polymer separator. Evaluation of the ion transport capacity of the PVDF/TBAPF6 HNMs separator was conducted by electrochemical impedance spectroscopy (EIS). Cell composed of steel current collectors (SS), modified separator and electrolyte was subjected to an AC impedance to test the bulk resistance (Rs) and ionic conductivity at room temperature, the results were listed in Table 1. The Nyquist curves were shown in Fig.5(a). It could be seen that all the electrospun PVDF separators exhibited higher ion conductivity compared with commercial separator (Celgard2340). This may be caused by the fact that the low surface energy of C-F was beneficial to the increase of the electrolyte wettability and gelation polymer of PVDF facilitated fast ion transport [47,48]. Meanwhile, the HNMs exhibited higher ion conductivity than pure PVDF nanofiber membrane due to that the introduction of TBAPF6 can lower the resistance of the membrane. Additionally, the hierarchical structures gave rise to the increase of porosity and lager specific surface area and then enhanced the degree of separator gelation, which were beneficial to ionic transport between electrodes. However, it was worth nothing that the ion conductivity of M3 begun to reduce. This may be because that the ion migration channel became longer due to the excessive thin fibers, which decreased the ion transport efficiency in the electrolyte. In addition, it could be found that with the increase of TBAPF6 concentration, the bulk resistance of M1(0.86U), M2 (0.64U), M3 (0.52U) reduced due to the cationic characteristic of TBAPF6 [49]. The lithium ion transport and electrical resistance of PVDF/
TBAPF6 HNMs separator can be verified by the interfacial resistance analysis. Cell was assembled with separator/Li electrodes and the Nyquist curves were illustrated in Fig.5(b). It could be clearly seen that all electrospun PVDF separator exhibited lower semicircle than commercial separator owing to that electrospun PVDF nanofiber membrane owned higher porosity and better electrolyte wettability. Further, the PVDF/TBAPF6 HNMs separator possessed lower interfacial resistance than pure PVDF separator due to the remarkable improvement of porosity and electrolyte uptake with the addition of TBAPF6. Moreover, the trend of interfacial resistance from M1 to M3 was similar to ion resistance that listed in Table 1, and the M2 possessed the lowest interfacial resistance. The lower interfacial resistance can improve the cell electrochemical stability through enhancing the lithium ion migration efficiency and restraining the formation of solid electrolyte interphase (SEI) membrane [50]. The electrochemical stability was measured by linear sweep voltammetry (LSV) performed on the Li/separator/SS system cells. Generally, the working potential of practical lithium ion rechargeable batteries was ranging from 1.8 V to 3.5 V versus Liþ/Li [51]. Fig.5(c) shown The LSV curves of different separators were shown in Fig.5(c). It could be seen that no decomposition were observed below 3.6 V vs Liþ/Li for all PVDF nanofiber membrane and all the PVDF nanofiber membrane exhibited higher electrochemical
Table 1 The electrochemical performance of Commercial, PVDF, M1, M2 and M3. Samples
R S ( U)
Ơ (103 S/cm)
Ri (U)
Es (V)
Commercial PVDF M1 M2 M3
1.97 1.63 0.87 0.64 0.52
0.96 2.63 3.69 4.28 3.36
389 335 182 134 160
3.78 3.60 3.82 3.91 3.65
Fig. 5. (a)EIS results of the commercial separator, pure PVDF and different concentration of PVDF/TBAPF6 HNMs; (b) electrochemical impendence curves of the commercial separator, pure PVDF and different concentration of PVDF/TBAPF6 HNMs; (c) linear sweep voltammetry of the commercial separator, pure PVDF and different concentration of PVDF/TBAPF6 HNMs.
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Fig.6. (a) the rate capabilities of commercial separator and M2, (b) coulombic efficiency of commercial separator, M1, M2 and M3, (c) cycling performance and coulombic efficiency of commercial separator and M2.
stability window than commercial separator. Meanwhile, PVDF/ TBAPF6 HNMs possessed higher decomposition voltage (Es) than pure PVDF membrane, which demonstrated that the PVDF/TBAPF6 HNMs performed better electrochemical stability than pure PVDF membrane. These results indicated that PVDF/TBAPF6 HNMs were compatible with anode and could be applied to Li ion capacitor and the M2 was rather promising for enhancing LIC performance. Generally, the cell capacities are mainly determined by the type and structure of the electrodes. However, the physical structure and chemical composition of the separators also affect the ion transportation between the two electrodes, which is critically important in regulating the cell kinetics [52]. In order to verify the applicability of PVDF/TBAPF6 HNMs separator for LIC, the positive, negative electrode and M2 were constructed to accept cycle test with the voltage between 0 V and 3 V at 0.5C. As a control, the commercial separator was tested under the same condition. According to the Fig.6(a) it could be seen that the capacity retention of M2 was higher than that of commercial separator. The result can be explained that the high ionic conductivity of the gel membrane was favorable for the electrolyte uptake and ions transport. Therefore, the PVDF/TBAPF6 HNMs possessed more excellent ion transport kinetics. Furthermore, the capacities could almost return to the initial values when the current density reverted to 0.5C. Meanwhile, the coulombic efficiency of M1, M2, M3 were more stable than those of commercial separator after 10000 cycles and the introduction of TBAPF6 can effectively improve the coulombic efficiency. Additionally, it was worth nothing that M2 exhibited the most stable cycle performance, which can be attributed to the higher porosity, electrolyte wettability and excellent electrochemical performance. Moreover, the capacity retention rate of M2 was higher than that of commercial separator after 10000 cycles, as illustrated in Fig.6(b). The excellent electrochemical performance could be ascribed to the introduction of TBAPF6, which decreased the crystallization of PVDF and the internal resistance of
membrane. These results suggested that the PVDF/TBAPF6 HNMs separator exhibited excellent rate capability and cycle performance. 4. Conclusion A novel PVDF/TBAPF6 HNM as high-performance gel-type polymer electrolytes had been fabricated via one-step electrospinning. The introduction of TBAPF6 could efficiently increase the ion conductivity and decrease the internal resistance of separator. Furthermore, the hierarchical structures improved the mechanical properties, the electrolyte wettability, and the porosity of membranes. More importantly, The PVDF/TBAPF6 HNMs separator exhibited excellent rate capability and cycle performance. Therefore, PVDF/TBAPF6 HNMs were demonstrated to be a potential gel polymer electrolyte to substantially maintain the capability applying for high-performance LIC. Conflicts of interest The authors declare that they have no conflict of interest. Acknowledgments We are grateful for financial support from National Natural Science Foundation of China (No.51573133), China Postdoctoral Science Foundation Grant (2018M630276) and Natural Science Foundation of Ningbo (No. 2018A610104). References [1] K. Leng, F. Zhang, L. Zhang, T. Zhang, Y. Wu, Y. Lu, Y. Huang, Y. Chen, Graphene-based Li-ion hybrid supercapacitors with ultrahigh performance, Nano Research 6 (2013) 581e592. [2] N.S. Choi, Z. Chen, S.A. Freunberger, X. Ji, Y.K. Sun, K. Amine, G. Yushin, L.F. Nazar, J. Cho, P.G.J.A.C.I.E, Bruce, Challenges facing lithium batteries and
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
PVDF/TBAPF6 hierarchical nanofiber gel membrane for lithium ion capacitor with ultrahigh ion conductivity and excellent interfacial compatibility Xianlei Shen a, b, 1, Zongjie Li a, c, 1, Nanping Deng a, c, Weimin Kang a, b, Jie Fan a, b, *, Yong Liu a, b, ** a
State Key Laboratory of Separation Membranes and Membrane Processes/National Center for International Joint Research on Separation Membranes, Tianjin Polytechnic University, Tianjin 300387, PR China School of Textile Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, PR China c School of Material Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 April 2019 Received in revised form 4 June 2019 Accepted 15 June 2019 Available online 19 June 2019
An efficient, convenient and flexible polyvinylidene fluoride/tetrabutylammonium hexafluorophosphate hierarchical nanofiber membrane (PVDF/TBAPF6 HNM) was successfully fabricated by one-step electrospinning and used as the separator of Li-ion capacitor (LIC). The PVDF/TBAPF6 HNMs displayed high ion conductivity of 4.28 103 S/cm, high mechanical strength, porosity, wettability and excellent thermal performance. Furthermore, the separator of PVDF/TBAPF6 HNM possessed lower interfacial resistance (134U) and better electrochemical stability, which exhibits better capacity retention and coulombic efficiency, compared with the commercial celgard 2340 separator. We believe that the gel polymer PVDF/TBAPF6 HNMs separator will have great potential in the field of the separator of LIC. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Lithium ion capacitor PVDF hierarchical nanofiber Gel membrane Ion conductivity Interfacial compatibility
1. Introduction The enormous growth of advanced intellectual products and emergence of electric vehicles have attributed to a high demand for high-efficiency energy storage systems with excellent energy and power density as well as cycle life [1]. Over the past twenty years, various energy storage elements with excellent performance have been proposed. Among these energy storage devices, Li-ion batteries (LIBs) and electric double-layer capacitors (EDLCs) are currently considered to be the most promising [2]. Compare with EDLCs providing higher power density (2-5 KW.Kg1) and long cycle life [3], LIBs possess higher energy densities (150200 Wh.Kg1) [4]. However, the power density and cycle life of LIBs
* Corresponding author. School of Textile Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, PR China. ** Corresponding author. School of Textile Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, PR China. E-mail addresses: [email protected] (J. Fan), [email protected] (Y. Liu). 1 These authors contributed equally to this work and should be considered cofirst authors. https://doi.org/10.1016/j.electacta.2019.06.095 0013-4686/© 2019 Elsevier Ltd. All rights reserved.
are relatively low. As a results, lithium ion capacitor (LIC), as a new capacitor for connecting LIBs and EDLCS, has attracted widespread attention from researchers because of different storage mechanisms at both ends [5]. LIC is composed of four parts including cathode, anode, separator and electrolyte, wherein EDLCs-type cathodes establish fast charge-discharge conditions, LIB-type anodes enable large capacitances [6]. Separator plays a key role in preventing physical contact of the electrodes meanwhile establishing free ionic transport and isolating electronic flow, which has significant influence on the safety performance and cycle capability of LIC [7e9]. At present, polyolefin microporous membranes have become the dominating separators composed of polyethylene (PE) and polypropylene (PP) owing to excellent mechanical property, glorious electrochemical stability and low cost, as well as particular performance that shuts the ion channel further preventing electrochemical reactions when a defective capacitor heats up arouse electrical short [10]. However, polyolefin separators are incompatible with the electrolyte due to their inherent limitations, such as low porosity, low electrolyte wettability, which directly affects the LIC performance [11e13]. Current researches largely focused on improving the absorption
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capacity between electrolytes and separators and suggested a large amount of strategies, including exposure to plasmas, electron beams, g-rays, physically grafting with hydrophilic molecules, phase inversion method and electrospinning technique [12,14,15]. Among these methods, electrospinning is an efficient technology which can produce long continuous fibers and control the structure of the fibers [16]. Additionally, the electrospun nanofiber membranes possesses high porosity and large specific surface area [17], which have a great potential in the application of separator to improve the property of LIC. Among the past few years, various electrospun polymers with excellent performances have been fabricated as separators of EDLCs or LIBs, including polyvinylidene fluoride (PVDF) [18], poly(vinylidene fluoride co-hexafluoropropylene) (PVDF-HFP) [19], polyacrylonitrile (PAN) [20]and polyethylene terephthalate (PET) [21]. It is well known that PVDF owns some appealing properties such as high polarity high dielectric constant (ε z 8.4), low degree of crystallinity and strongly electron affinity functional groups (eCeFe) [22]. Meanwhile, PVDF as a gel polymer electrolyte performs excellent ion conductivity [23]. Choi et al. [24] fabricated PVDF electrospun nanofiber membranes as polymer electrolyte with high ion conductivity(1.6e2.0 103 S/cm). These electrospun PVDF membranes had certain capacity to act as LIC separators, however, the porosity, electrolyte uptake and corresponding ionic conductivity of separator still needed to be further improved. There exit many emerging methods and strategies to improve the performance, including blending modification, compound modification and adding inorganic nanoparticles, etc. [25]. For example, Li et al. [26] blended PVDF with polydimethylsiloxane (PDMS) polymer to fabricate LIB, and found that the porosity and the electrolyte uptake increased significantly compared with those of pure elecrtospun PVDF membrane. Moreover, Lu et al. [27] prepared a poly(phthalazinone ether sulfone ketone) (PPESK)/PVDF/ PPESK tri-layer compound membrane via electrospinning and obtained good wettability in liquid electrolyte and higher ionic conductivity. Zhang et al. [28]coated the electrospun PVDF separator with SiO2 and obtained superior ionic conductivities and low electrode interfacial resistance of the separator. Additionally, tetrabutylammonium hexafluorophosphate(TBAPF6) was used as supporting electrolyte for increasing the conductivity of system and improving the electrochemical stability of electrolyte [29,30]. For example, Kiefer et al. [31] doped tetrabutylammonium hexafluorophosphate(TBAPF6) on the Polypyrrole (PPy) films by electropolymerization and the PPy/TBAPF6 film exhibited excellent conductivity. However, TBAPF6 has rarely been reported in the application of improving the battery separator properties. Herein, a novel PVDF/TBAPF6 LIC separator with high ion conductivity and low internal resistance was successfully fabricated by adding certain amount of TBAPF6 into the PVDF solution via electrospinning. A multiscale hierarchical nanofiber structure could be fabricated by the introduction of TBAPF6. The effect of the addition of TBAPF6 on the electrolyte uptake, liquid retention ability, ion conductivity and interfacial resistance of the separator were investigated, as well as the mechanical performance, thermal stability and LIC cycle performance. 2. Experimental 2.1. Materials PVDF(Mw ¼ 520,000) was purchased from Shanghai 3 F New Materials Co. Ltd, China. N,N-dimethylformamide (DMF) and acetone were supplied by Tianjin Kermel Chemical Reagent Co. Ltd, China. Tetrabutylammonium hexafluorophosphate (TBAPF6) (MW ¼ 387.41) was provided by damas-beta, China. The liquid
electrolyte of 1 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DC)/ethyl methyl carbonate (EMC) (1/1/1 wt/wt/wt) was supplied by New Materials of Guo Tai Hua Rong (Zhangjiagang, China). All of the chemical reagents were commercially purchased, without any treatment before used. 2.2. Synthesis TLNMs Firstly, PVDF was dissolved in the mixture of DMF and acetone with a volume ration of 3:1 to obtain the PVDF solution with a concentration of 16 wt%, then different contents of TBAPF6 were added into solution, stirring for 1 h to obtain different rations of PVDF/TBAPF6 solution. A commercially available electrospinning device (Shanghai Oriental Flying Nanotechnology Co., Ltd) was used for the fabrication of PVDF/TBAPF6 HNMs, as described previously. Briefly, the electrospinning conditions were optimized as follows: flow rate of 1 mL h1, tip-to-collector distance of 15 cm, and applied voltage of 30 kV. The obtained nanofiber membranes were dried at 60 C for 12 h in vacuum condition to remove solvent before being further used. The three samples were named as M1, M2 and M3, under the corresponding content of TBAPF6 of 0.025 mol/L,0.05 mol/L,0.075 mol/L, respectively. Pure PVDF nanofiber membrane was also prepared as control sample. The processes of manufacturing PVDF/TBAPF6 HNMs were shown in Fig. 1(a) and LIC schematic was illustrated in Fig. 1(c). 2.3. Electrode preparation and cell assembly The electrochemical performance of the prepared separator was tested by assembled the CR2032 coin type cells. Fig. 1(b) showed the components of the LIC, in which the electrode was made of the mixture of activated carbon (AC), conductive carbon (Super-P) and polyvinylidene fluoride (PVDF) as binder at ratio of 7:2:1 by weight. The mixture was smeared on the copper foils and then placed into vacuum oven for 24 h at 45 C. The anode needed to be prelithiated through contacting electrode and Li-metal foils under the pressure of 0.2 MPa and then immersed in electrolyte for 1.5 h before assembled into the LIC. 2.4. Characterization 2.4.1. Morphology The surface morphology of the PVDF/TBAPF6 HNMs was obtained by a field emission scanning electron microscope (FE-SEM, Gemini SEM500, Germany). 2.4.2. XRD The nanofiber crystallinity was characterized by D/MAX-2500 X-Ray Diffraction (Rigaku) with 2 h values in the 2q range between 10 and 55 . 2.4.3. FTIR Infrared absorption spectra were collected in the 6001500 cm1 range on a Fourier transform infrared spectrometer (FTIR, TENSPR37, Bruker). 2.4.4. Raman spectroscopy Raman spectroscopy was performed using a Renishaw in Via Raman microscope with an excitation wavelength of 532 nm and a power of 1 mw. 2.4.5. Mechanical properties Mechanical properties were measured using YG005 Electronic Single Fiber Strength Tester (Fangyuan Instrument Co., Ltd.,China) under the speed of testing at 10 mm min1 and the grip length of
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Fig.1. (a) The schematic illustration of the preparation of electrospun separator and battery assembling, (b) the components of the LIC, (c) the schematic of LIC.
20 mm. The examined sample dimension was 5 mm 50 mm. The tensile strength, T, of the membrane was calculated via eq (1).
T¼
X A*B
(1)
where X is the maximum load of membrane, A and B are the width and thickness of the membrane. The thickness of membrane was measured with a 0-10-3Q thickness gauge (Shanghai chuanlu measuring tool Co., Ltd.).
2.4.7. Electrolyte uptakes All the samples were dried for 24 h under vacuum condition at 60 C and then soaked into the select electrolyte for 1 h. The remaining electrolyte on the sample surface was removed by filter paper before measurement. The electrolyte uptake was calculated eq. (2) below.
Uptakeð%Þ ¼
Ww Wd 100% Wd
(2)
where Wd and Ww are the masses of dried and wet samples, which immersed into the electrolyte for 1 h respectively. 2.4.6. Thermal stability The thermal shrinkage property of the membrane was characterized by analyzing the dimensional changes of the circular samples after thermal treatment at 80 C, 100 C, 120 C, 140 C and 150 C for 1.5 h, respectively.
2.4.8. Porosity The porosity of the membrane was measured by immersing into n-butanol (Yingda Chemical Co., Ltd.) for 2 h and then the residual n-butanol adhering on the surface of sample was removed by wiped papers. The membrane porosity was calculated by eq. (3) as
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follows.
Porosityð%Þ ¼
Ww Wd 100% rb *Vd
(3)
where Wd and Ww are the weight of the membrane before (dry) and after soaking (wet) in n-butanol for 2 h, rb is the density of nbutanol (0.8098 g cm3), and Vd is the volume of the dry nanofiber membrane. 2.4.9. Contact angel measurement The contact angel (CA) of the specimen was measured by Contact Angle Meter (JC2000D, Powereach Co., Ltd., Shanghai, China) at 26 C. 2.4.10. Electrochemical characterize The ionic conductivity of the membrane was measured with stainless-steel (SS)/separator/SS blocking cells by electrochemical impedance spectroscopy (EIS, SI1260, Solatron) (CHI660D, Beijing Chinese science days Technology Co., Ltd.) with a frequency range from 0.01 Hz to 100 KHz. The ionic conductivity of the four samples were obtained by the following eq (4).
s¼
d Rs *A
(4)
where s is ionic conductivity, d is the thickness of separators. Rs and A are the bulk resistance of separators and the contact area between separators and SS, respectively. The interfacial resistance was measured on the CHI660D electrochemical workstation by AC impedance with Li/separator/Li cells at a scan rate of 1 mV s1 and a frequency range of 0.01 Hz ~ 100 KHz. Electrochemical stability window was measured in the voltage range of 2e7 V at a scan rate of 1 mV s1 by the linear sweep voltammetry (LSV) of Li/separator/SS cells using CHI660D electrochemistry workstation. The separator cycle stability was characterized by the capacity retention and coulombic efficiency of LIC. The assembled cell was measured after placing in argon atmosphere for 24 h using a LAND CT2001A battery-testing system (Lanhe Co., Ltd., Wuhan, China) in range of 0e3 V at 0.5C. 3. Results and discussion Fig. 2(a)-(d) are SEM images of pure PVDF and PVDF/TBAPF6 HNMs with different contents of TBAPF6. It was clear that hierarchical nanofibers with multi-scale diameter could be observed after adding TBAPF6. With the increase of TBAPF6 concentration, the number of fine nanofibers increased. According to our previous research [32], this was because that organic branched salts TBAPF6 could increase the conductivity of the precursor solution, which intensified the splitting of the jet in electrospinning. The fiber diameter frequencies were shown in Fig. 2(e)-(h). It was found that the average nanofiber diameters (AFDs, less than 100 nm) of PVDF/ TBAPF6 HNMs were smaller than that of pure PVDF nanofibers, in which their morphologies was homogenous and uniform with AFD of 132 nm. Additionally, the AFD of PVDF/TBAPF6 HNMs decreased with the increase of TBAPF6 concentration. Fig. 2 (i)-(j) showed the photos of pure PVDF and M2 separator after immersing in the liquid electrolyte and their corresponding SEM images. It could be seen that the gelatinizing phenomenon occurred when the two samples were soaked in the electrolyte and the gelation phenomena of M2 was much better than that of pure PVDF membrane. This phenomenon can be explained by the fact that the large specific surface
area caused by the hierarchical structure. Meanwhile, PVDF is a semi-crystalline polymer [33], which can be proved by the XRD spectra, as shown in Fig. 2(k). The intensities of peaks assigned to PVDF dramatically decreased after the addition of TBAPF6 and the amorphous domain of the membrane increased, which could facilitate the absorption and swelling of the liquid electrolyte in the membrane [34]. As a result, the electrolyte retention rate and ion conductivity of HNMs can be improved. Fig. 2(lem) showed the FTIR spectra and the Raman spectra of pristine PVDF and M2 membrane. The vibrational bands at 795,1149 cm1 correspond to the a-phase, and the peaks at 840 cm1,1402 cm1 correspond to the b-phase [35e38]. It is observed that b-phase content ratio increases with the introduction of TBAPF6. The Raman spectrum of the pristine electrospun PVDF membrane indicated the presence of major vibration bans observed at 800,877 cm1, it corresponds to the a phase of PVDF [39,40]. The Raman bands at 840 cm1 indicated the presence of b phase in PVDF membrane. As a result, both the FTIR spectra and Raman spectra confirmed the enhanced b phase in the PVDF/TBAPF6 HNMs, which is benefited to improve the morphologies of the elecrtrospun fibers [41] and enhancement of membrane dielectric constant and ac conductivity [42]. Therefore, the PVDF/TBAPF6 HNM has potential applied for LIC separator to promote ion transport performance. Generally, the mechanical property of electrospun nanofiber membrane is important for battery separator. Typically stressstrain curves of pure PVDF nanofiber membrane and PVDF/ TBAPF6 HNMs with different TBAPF6 contents were shown in Fig. 3(a). It was clear that the tensile breaking strength of all HNMs were higher than that of pure PVDF nanofiber membrane. When the concentration of TBAPF6 increased from 0 to 0.05 mol/L, the tensile breaking strength increased from 3.68 MPa to 5.54 MPa. It could be explained that this hierarchical structure can improve the mechanical properties of the membrane. This was because that the thick fibers acted as skeleton supports and the thin fibers interacted strongly with thick fibers through the bonding points and the entanglement, enhancing the mechanical property of membrane [43].Further increased the TBAPF6 concentration to 0.075 mol/L, the tensile breaking strength decreased to 4.97 MPa. This may be attributed to the reduced forces between PVDF molecules arising from the addition of excessive TBAPF6. These results indicated that the PVDF/TBAPF6 HNMs exhibited preferable mechanical strength comparing with pure PVDF nanofiber membrane that had a promotion for the safety and cycle performance for the LIC. The thermal stability and shrinkage resistance have great effects on the safety of LIC. Fig. 3(b) showed the thermal stability and shrinkage resistance of the Celgard 2340, pure PVDF, M1, M2 and M3 after the hot oven treatment from 80 C to 150 C, respectively. The results revealed that with the increasing of temperature, the commercial separator shrank at 120 C, and the color changed from white to transparent at 140 C, while no significant shrinkage was observed for PVDF and PVDF/TBAPF6 HNMs, demonstrating that these PVDF membranes had better shrinkage stability. The excellent thermal stability and shrinkage resistance is due to superior thermal resistance of PVDF itself compared with polyolefins. The large porosity and polyfluoropolymer could promote the electrolyte wettability of separator [44]. The electrolyte contact angles of pure PVDF and PVDF/TBAPF6 HNMs were shown in Fig. 4(a). It could be seen that the contact angel of the pure PVDF membrane (19 ) was smaller than that of the commercial separator (39.50 ), indicating that the electrospun porous PVDF separator had an excellent wettability compared to commercial separator. Meanwhile, the HNMs had smaller contact angle compared with pure PVDF membrane, performing excellent lyophilic properties. It can be mainly ascribed to the larger specific surface area [45] and low crystallinity of the HNMs, which was in accordance with the
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Fig. 2. SEM images of (a)pure PVDF and (b) M1, (c) M2; (d) M3, the nanofiber diameter distribution of (e) pure PVDF, (f) M1, (g) M2, (h) M3, morphology of pure PVDF (left) and M2(right) before(i) and after (j) immersing in electrolyte, (k) XRD patterns PVDF and M2, (l)FTIR spectra of PVDF and M2, (m) Raman spectra of PVDF and M2.
Fig. 3. (a)strain stress curve of pure PVDF and PVDF/TBAPF6 HNMs (b) the hot treatment pictures of Celgard 2340, pure PVDF and PVDF/TBAPF6 HNMs.
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Fig. 4. (a) The contact angles of commercial separator, pure PVDF and PVDF/TBAPF6 HNMs, (b) liquid retention rate and porosity of commercial separator, pure PVDF and PVDF/ TBAPF6 HNMs.
above SEM image and XRD patterns. The liquid retention rate and porosity of membranes were presented in Fig. 4(b). It could be found that the porosity and liquid retention rate of electrospun nanofiber membranes were significantly higher than that of commercial Celgard 2340. With the increase of TBAPF6 concentration, the porosity and liquid retention rate increased gradually. This was because the hierarchical structure could greatly increase the porosity and specific surface area of membranes and then increased the surface contacted area between fibers and electrolyte, leading to excellent electrolyte uptake [46]. Meanwhile, the dense hierarchical nanofiber structures can cause high electrolyte retention, which is benefited to the sustaining ion transport in the gel-type polymer separator. Evaluation of the ion transport capacity of the PVDF/TBAPF6 HNMs separator was conducted by electrochemical impedance spectroscopy (EIS). Cell composed of steel current collectors (SS), modified separator and electrolyte was subjected to an AC impedance to test the bulk resistance (Rs) and ionic conductivity at room temperature, the results were listed in Table 1. The Nyquist curves were shown in Fig.5(a). It could be seen that all the electrospun PVDF separators exhibited higher ion conductivity compared with commercial separator (Celgard2340). This may be caused by the fact that the low surface energy of C-F was beneficial to the increase of the electrolyte wettability and gelation polymer of PVDF facilitated fast ion transport [47,48]. Meanwhile, the HNMs exhibited higher ion conductivity than pure PVDF nanofiber membrane due to that the introduction of TBAPF6 can lower the resistance of the membrane. Additionally, the hierarchical structures gave rise to the increase of porosity and lager specific surface area and then enhanced the degree of separator gelation, which were beneficial to ionic transport between electrodes. However, it was worth nothing that the ion conductivity of M3 begun to reduce. This may be because that the ion migration channel became longer due to the excessive thin fibers, which decreased the ion transport efficiency in the electrolyte. In addition, it could be found that with the increase of TBAPF6 concentration, the bulk resistance of M1(0.86U), M2 (0.64U), M3 (0.52U) reduced due to the cationic characteristic of TBAPF6 [49]. The lithium ion transport and electrical resistance of PVDF/
TBAPF6 HNMs separator can be verified by the interfacial resistance analysis. Cell was assembled with separator/Li electrodes and the Nyquist curves were illustrated in Fig.5(b). It could be clearly seen that all electrospun PVDF separator exhibited lower semicircle than commercial separator owing to that electrospun PVDF nanofiber membrane owned higher porosity and better electrolyte wettability. Further, the PVDF/TBAPF6 HNMs separator possessed lower interfacial resistance than pure PVDF separator due to the remarkable improvement of porosity and electrolyte uptake with the addition of TBAPF6. Moreover, the trend of interfacial resistance from M1 to M3 was similar to ion resistance that listed in Table 1, and the M2 possessed the lowest interfacial resistance. The lower interfacial resistance can improve the cell electrochemical stability through enhancing the lithium ion migration efficiency and restraining the formation of solid electrolyte interphase (SEI) membrane [50]. The electrochemical stability was measured by linear sweep voltammetry (LSV) performed on the Li/separator/SS system cells. Generally, the working potential of practical lithium ion rechargeable batteries was ranging from 1.8 V to 3.5 V versus Liþ/Li [51]. Fig.5(c) shown The LSV curves of different separators were shown in Fig.5(c). It could be seen that no decomposition were observed below 3.6 V vs Liþ/Li for all PVDF nanofiber membrane and all the PVDF nanofiber membrane exhibited higher electrochemical
Table 1 The electrochemical performance of Commercial, PVDF, M1, M2 and M3. Samples
R S ( U)
Ơ (103 S/cm)
Ri (U)
Es (V)
Commercial PVDF M1 M2 M3
1.97 1.63 0.87 0.64 0.52
0.96 2.63 3.69 4.28 3.36
389 335 182 134 160
3.78 3.60 3.82 3.91 3.65
Fig. 5. (a)EIS results of the commercial separator, pure PVDF and different concentration of PVDF/TBAPF6 HNMs; (b) electrochemical impendence curves of the commercial separator, pure PVDF and different concentration of PVDF/TBAPF6 HNMs; (c) linear sweep voltammetry of the commercial separator, pure PVDF and different concentration of PVDF/TBAPF6 HNMs.
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Fig.6. (a) the rate capabilities of commercial separator and M2, (b) coulombic efficiency of commercial separator, M1, M2 and M3, (c) cycling performance and coulombic efficiency of commercial separator and M2.
stability window than commercial separator. Meanwhile, PVDF/ TBAPF6 HNMs possessed higher decomposition voltage (Es) than pure PVDF membrane, which demonstrated that the PVDF/TBAPF6 HNMs performed better electrochemical stability than pure PVDF membrane. These results indicated that PVDF/TBAPF6 HNMs were compatible with anode and could be applied to Li ion capacitor and the M2 was rather promising for enhancing LIC performance. Generally, the cell capacities are mainly determined by the type and structure of the electrodes. However, the physical structure and chemical composition of the separators also affect the ion transportation between the two electrodes, which is critically important in regulating the cell kinetics [52]. In order to verify the applicability of PVDF/TBAPF6 HNMs separator for LIC, the positive, negative electrode and M2 were constructed to accept cycle test with the voltage between 0 V and 3 V at 0.5C. As a control, the commercial separator was tested under the same condition. According to the Fig.6(a) it could be seen that the capacity retention of M2 was higher than that of commercial separator. The result can be explained that the high ionic conductivity of the gel membrane was favorable for the electrolyte uptake and ions transport. Therefore, the PVDF/TBAPF6 HNMs possessed more excellent ion transport kinetics. Furthermore, the capacities could almost return to the initial values when the current density reverted to 0.5C. Meanwhile, the coulombic efficiency of M1, M2, M3 were more stable than those of commercial separator after 10000 cycles and the introduction of TBAPF6 can effectively improve the coulombic efficiency. Additionally, it was worth nothing that M2 exhibited the most stable cycle performance, which can be attributed to the higher porosity, electrolyte wettability and excellent electrochemical performance. Moreover, the capacity retention rate of M2 was higher than that of commercial separator after 10000 cycles, as illustrated in Fig.6(b). The excellent electrochemical performance could be ascribed to the introduction of TBAPF6, which decreased the crystallization of PVDF and the internal resistance of
membrane. These results suggested that the PVDF/TBAPF6 HNMs separator exhibited excellent rate capability and cycle performance. 4. Conclusion A novel PVDF/TBAPF6 HNM as high-performance gel-type polymer electrolytes had been fabricated via one-step electrospinning. The introduction of TBAPF6 could efficiently increase the ion conductivity and decrease the internal resistance of separator. Furthermore, the hierarchical structures improved the mechanical properties, the electrolyte wettability, and the porosity of membranes. More importantly, The PVDF/TBAPF6 HNMs separator exhibited excellent rate capability and cycle performance. Therefore, PVDF/TBAPF6 HNMs were demonstrated to be a potential gel polymer electrolyte to substantially maintain the capability applying for high-performance LIC. Conflicts of interest The authors declare that they have no conflict of interest. Acknowledgments We are grateful for financial support from National Natural Science Foundation of China (No.51573133), China Postdoctoral Science Foundation Grant (2018M630276) and Natural Science Foundation of Ningbo (No. 2018A610104). References [1] K. Leng, F. Zhang, L. Zhang, T. Zhang, Y. Wu, Y. Lu, Y. Huang, Y. Chen, Graphene-based Li-ion hybrid supercapacitors with ultrahigh performance, Nano Research 6 (2013) 581e592. [2] N.S. Choi, Z. Chen, S.A. Freunberger, X. Ji, Y.K. Sun, K. Amine, G. Yushin, L.F. Nazar, J. Cho, P.G.J.A.C.I.E, Bruce, Challenges facing lithium batteries and
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