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Construction of core-shell nanofiber membrane with enhanced interface compatibility for lithium-metal battery
Solid State Ionics 347 (2020) 115266
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Construction of core-shell nanofiber membrane with enhanced interface compatibility for lithium-metal battery ⁎
Xianli Songa,b,c, Wen Qid, Haitao Zhangb,c, , Gongying Wanga,b,
T
⁎
a
Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, China National Engineering Laboratory for VOCs Pollution Control Material & Technology, University of Chinese Academy of Sciences, Beijing 101408, China c Beijing Key Laboratory of Ionic Liquids Clean Process, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, China d Materials Center, Beijing Institute of Collaborative Innovation, Beijing 100081, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Nanocomposite membrane Coaxial electrospinning Interface compatibility Lithium-metal batteries Ionic liquid
Improving interface compatibility is critical to the solid-state lithium metal batteries. Thus, a novel type of coreshell nanocomposite polymer fiber membrane was prepared by a coaxial electrospinning technique. The coreshell nanofiber membranes contains poly (propylene carbonate) (PPC) in the shell, and poly(vinylidene fluorideco-hexafluoropropylene) (PVDF-HFP) that containing in-situ generated silica in the core. The structure, topography and compositions of the sample were investigated by scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS). The nanocomposite membranes exhibit three-dimensional framework structure with well-dispersed fibers, and could be transformed readily into gel polymer electrolytes (GPEs) by being soaked in an ionic liquid solution. Our study showed that the core-shell PVDF-HFP-SiO2@PPC polymer electrolyte exhibited a relatively high electrolyte uptake of 460%. The core-shell electrolyte was found to play a significant role in increasing the interface compatibility. The optimized core-shell PVDF-HFP-SiO2@PPC electrolyte exhibited an enhanced ionic conductivity (1.05 mS cm−1) in comparison with the blend PVDF-HFP-SiO2PPC electrolyte (0.5 mS cm−1) at 25 °C. This study demonstrates that the optimization of composition and microstructure is efficient in the fabrication of high-performance membranes for lithium-metal batteries.
1. Introduction Currently, the rechargeable lithium metal batteries have regained intensive attentions owing to the increasing demand in high energy density energy storage devices. Li metal exhibits high reactivity during repeated Li plating-stripping cycles, leading to lower efficiency and poor cycling stability, which could possibly trigger safety issues [1–3]. Over the last two decades, solid electrolytes have been used as promising ones to improve the safety performances, which can be treated as alternative of liquid electrolytes owing to the distinct advantages such as excellent interface stability, leakage proof, easy fabrication and outstanding mechanical properties [4–7]. Gel polymer electrolytes (GPEs), can be considered to exist in a state between liquid and solid, and are usually achieved by adding inorganic salt, polymer host, and low molecular weight plasticizing agents to common solvent [8–10]. The low molecular weight plasticizers are inadequate in the preparation of electrolyte due to the corrosive, volatile, and flammable characteristics. Ionic liquids, are generally utilized as green solvents, and substitute for low molecular weight plasticizers in the fabrication of highly
⁎
safe GPEs. The addition of ionic liquids (ILs) into polymer electrolytes to obtain GPEs is beneficial to chemical stability and electrochemical stability window [11]. Generally, ILs were added into the polymer electrolytes to fabricate high-performance GPEs. Beside the ionic liquids, the selection of suitable host polymer is critical to high-quality electrolytes. Poly(propylene carbonate) (PPC) is an environment friendly macromolecule with amorphous alternating copolymer. PPC exhibits excellent compatibility with lithium salts and has good interfacial contact with electrodes, which is a promising material for polymer electrolyte. The previous study revealed that PPC could be used as a buffer layer in the sandwich-like composite solid electrolyte, and it was beneficial to the interfacial compatibility during cycling [16]. Yang et al. also presented a strategy to improve the interfacial compatibility by introducing the PPC as adaptive buffer layer [17]. Coaxial electrospinning has been proved to be an effective technique to prepare multifunctional core-shell nanofibers synchronously. The selection of appropriate materials for core and shell substrates is extremely significant to the performances of core-shell nanofiber membranes [12–15]. PVDF-HFP exhibits relatively high ionic
Corresponding authors. E-mail addresses: [email protected] (H. Zhang), [email protected] (G. Wang).
https://doi.org/10.1016/j.ssi.2020.115266 Received 10 December 2019; Received in revised form 14 February 2020; Accepted 16 February 2020 0167-2738/ © 2020 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic illustration for the fabrication of core-shell PVDF-HFP-SiO2@PPC nanofiber membrane.
continuous nanofibers with core-shell structured. For a comparison, the as-prepared solution containing PVDF-HFP in-situ silica mixed with PPC was placed in a plastic syringe equipped with a needle-diameter of 0.5 mm under similar condition. The membrane was designated as PVDF-HFP-SiO2-PPC. The thickness of the membrane was about 150 μm. Before being stored in the glove box, the prepared nanofibrous mats were dried under vacuum at 50 °C overnight. The GPEs were fabricated by stacking the nanofibrous membranes in the solution of LiTFSI in EMIMBF4.
conductivity and high mechanical strength with semi-crystalline characteristic, which is considered to be a promising polymer electrolyte material [18]. The high dielectric constant of PVDF-HFP can be beneficial to dissociate lithium salt and enhance the carrier concentration [19–22]. Meanwhile, it exhibited a high stability during the reductive/ oxidative electrochemical process [18]. The addition of inert fillers, such as silica (SiO2), zirconia (ZrO2), and molecule sieves, could enhance the mechanical properties and ionic conductivity, which can facilitate the transport of Li+ [23–26]. Furthermore, the addition of inert fillers via in-situ synthesis of inorganic backbone compared to mechanical mixing would be conducive to uniform dispersion and better interfacial contact [27,28]. In this work, the PVDF-HFP-SiO2@PPC core-shell nanofibers were devised and fabricated by a coaxial electrospinning technique (illustrated in Fig. 1). PPC was selected as the shell material to improve the compatibility with commonly used electrodes. PVDF-HFP and in-situ generated silica were designed as the core matrix to increase the mechanical stability and the stability in the reductive/oxidative electrochemical environment. The obtained results indicate that this core-shell PVDF-HFP-SiO2@PPC composite electrolyte is very attractive for highperformance lithium metal batteries.
2.3. LiFePO4 cathode preparation The positive electrode was composed of LiFePO4, PVDF (poly(vinylidene fluoride), and super P. Initially, LiFePO4 (80%), super P (10%) were mixed in a mortar, and then 10 wt% PVDF was added to the above mixture. Then, the mixture was dissolved in N-methyl-2-pyrrolidone until a homogenous slurry was obtained. After coating the above slurry on the aluminum foil, the electrodes were dried in the vacuum oven at 110 °C for 24 h. 2.4. Electrochemical characterization of GPEs
2. Experimental
All batteries assembly was packaged in 2025 coin cells in an argonatmosphere glove box (O2 and H2O were below 0.01 ppm). The ionic conductivity (σ) of the electrolyte was calculated from the electrolyte bulk resistance (R) measured with stainless steel by AC impedance technique by the following formula:
2.1. Materials PPC (Mw = 120,000 from Empower Materials Inc., USA) and PVDFHFP (Aldrich, Mw = 450,000) were dried at 60 °C before utilization. Solvent N-methyl-2-pyrrolidone, acetone, and N, N-Dimethylforamide (DMF) were purchased from Aladdin. Tetraethoxysilane (TEOS) was purchased from Sigma-Aldrich.
σ=
d RS
where S is the surface area of the stainless steel. The cyclic voltammetry (CV, 0.1 mV s−1) using LiFePO4 cathode as the working electrode and Li as the reference electrode was obtained with CHI600E electrochemical workstation. The interfacial stability and battery cycling performance were carried out on LAND battery testing systems (LAND CT2001A).
2.2. Preparation of core-shell Nanofibrous composite polymer electrolyte Two types of electrospun nanofibers, consisting of core-shell nanofibrous membrane PVDF-HFP-SiO2@PPC and blend nanofibrous membrane PVDF-HFP-SiO2-PPC was prepared. The PPC solutions was prepared by dissolving blended solvent of DMF/acetone (1:1, w/w) and then magnetically stirred at concentration of 19 wt%. Another solution of 17% of PVDF-HFP in the same mixed solvent was prepared under magnetic stirring, followed by adding the required quantity of TEOS for generating in-situ silica. The core-shell nanofibrous membranes consisting of nanofibers with core (PPC)-shell (PVDF-HFP, in-situ silica) were fabricated by coaxial electrospinning. A 18 kV positive voltage was applied during the electrospinning process. The distance between the spinneret-to-collector was kept at 15 cm. The flow rate of the core solution was kept at 0.6 mL h−1, and the shell flow rate was kept at 0.72 mL h−1. The core flow rate was slower than the shell flow rate, which can benefit forming
2.5. Materials characterizations The morphology of as-prepared materials was investigated by a scanning electron microscopy (SEM) (JSM-7001F) with accelerating voltage of 5.0 kV. EDS was used to determine the composition and distribution of the single nanofiber. Fourier transform infrared (FT-IR) spectra were recorded by Thermo Nicolet 380 spectrometer. The electrochemical characterizations were tested using CR2025 coin-type cells. The contact angle was conducted on the Drop Shape Analyzer 100. The solution of LiTFSI in EMIMBF4 droplets (3 μL) was spread onto the surface of the membranes and then the images were captured in a continuous model. 2
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Fig. 2. FESEM images of electrospun core-shell PPC-PVDF-HFP-SiO2 and PPC@PVDF-HFP-SiO2 membrane (a, c), and corresponding fiber diameter distributions diagram (b, d), respectively.
3. Results and discussion
vibrations respectively, which are strong features of PVDF-HFP copolymer. The band at 783 and 1229 cm−1 can be assigned to the stretching vibration of the CeO bond, the stretching vibration of the C]O is about 1745 cm−1 [30]. In the spectrum of the nanofiber membrane with in-situ SiO2, the bands of SiO2 overlapped with groups of PVDF-HFP. Thus, EDS spectra were used to confirm the presence of SiO2 in the internal of the composite nanofiber membrane [31]. The presence of all characteristic peaks of PVDF-HFP, PPC, and SiO2 in the spectrum of the composite membrane indicates that the core-shell composite membrane was fabricated successfully [32]. Additionally, EDS was used to investigate the composition and distribution of core-shell PVDF-HFP-SiO2@PPC fibers [33]. From the elemental analysis by a line scan across a single fiber, element F was attributed to PVDF-HFP, and element Si was attributed to SiO2 in Fig. 4b. The amount of element F and Si increases in the middle of single nanofiber, indirect verification of the successful construction of the coaxial nature of the core-shell nanofibers. Meanwhile, the SEM image in Fig. S1 was performed to further prove the core-shell nanofibers.
3.1. Morphology of membranes The morphology of the core-shell structure of PVDF-HFP-SiO2-PPC and PVDF-HFP-SiO2@PPC nanofiber mats were investigated by SEM in Fig. 2. From the SEM image, a three-dimensional framework structure is formed by interconnecting well-dispersed fibers. This unique porous structure facilitates efficient capture and retention of the electrolyte, and contributes to the smooth diffusion of the electrolyte into the battery assembly. The diameters of the PVDF-HFP-SiO2@PPC core-shell membrane and PVDF-HFP-SiO2-PPC blend membranes were estimated to be 250–300 nm and 350–400 nm respectively in Fig. 2(b, d). The increased viscosity of the blend of precursor solution results in the broader distribution of fiber diameter [29]. The surface wettability of the composite electrolyte was generally explored by contact angle measurement. This property has significant impact on the ionic conductivity, and thus affects the capacity and cycle performance of the battery. A semi-quantitative measurement of the differences in the wettability of membranes can be investigated by comparing the contact angles of the membranes. As shown in Fig. 3, the evolution of contact angles gradually decreased over time. The contact angle of the PVDF-HFP-SiO2@PPC membranes was initially 27.7° at the initial time, and then decreased to 16.3° after 5 s in Fig. 3c and d, which is relatively lower than the PVDF-HFP-SiO2-PPC. The smaller contact angle was obtained on core-shell PVDF-HFP-SiO2@PPC fibers than PVDF-HFP-SiO2-PPC, indicating that the core-shell fibers play an effect on enhancing the wettability. This result indicates that the core-shell fibers facilitate the affinity between the different polymer components and allows rapid penetration of the ionic liquid electrolyte. The improvement in wettability of the core-shell PVDF-HFP-SiO2@PPC nanofibers fabricated via coaxial electrospinning technology is highly attractive for solid electrolyte application of Li metal batteries. FTIR spectra of the blending PVDF-HFP-SiO2-PPC and core-shell PVDF-HFP-SiO2@PPC fibers are shown in Fig. 4a. The absorption peaks at 1403 cm−1, 980 cm−1, and 882 cm−1 can be assigned to CeF stretching vibration, CeF stretching, CeC symmetric stretching
3.2. Electrochemical stability The number of cation transports is more critical for battery applications, because the low lithium transference number results in concentration polarization, which is detrimental to the performance of the battery. The lithium transference number was calculated according to the Bruce-Vincent formula [34–36]:
t+Li =
IS (∆V − I0 R 0) I0 (∆V − IS RS)
where I0 is the current at the initial state, Is is the steady-state current. The ΔV is the potential applied across the battery, R0 is the resistance at the initial state, Rs is the impedance after polarization. The current-time curves and the corresponding electrochemical impedance spectra (Fig. 5) before and after polarization were collected to calculate the Li+ ion transference number. Li+ transference number and electrolytes uptake amount for the PVDF-HFP-SiO2-PPC and PVDF3
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Fig. 3. Contact angle measurements of (a, b) PPC-PVDF-HFP-SiO2 membrane, and (c, d) PPC@PVDF-HFP-SiO2 core-shell membrane.
HFP-SiO2@PPC at room temperature are listed in Table 1. The tLi+ of the polymer electrolyte of core-shell PVDF-HFP-SiO2@PPC increases in comparison with that of blend PVDF-HFP-SiO2-PPC. The improvement of tLi+ could diminish the concentration gradient during the charge and discharge processes, suggesting that core-shell favor the lithium iondominant conduction behavior. The temperature-dependent ionic conductivity was investigated by the electrochemical impedance spectroscopy using a symmetric configuration of the SS/GPEs/SS cell. Fig. 6a and b show the impedance plots of PVDF-HFP-SiO2-PPC and PVDF-HFP-SiO2@PPC electrolyte. The plot of ionic conductivity at various temperatures between 25 and 80 °C was shown in Fig. 6c. Since increasing the temperature not only improves the ionic mobility, but also enhances the mobility of the polymer chains, thereby promoting the ion migration. The ionic conductivity of electrolyte present the same trend-positively correlated with the temperature. Meanwhile, the plots of both electrolytes appear linear, which suggests that their ionic conducting behaviors obey the typical Arrhenius behavior. The ionic conductivity of core-shell electrolyte PVDF-HFP-SiO2@PPC (1.05 mS cm−1) is higher than that of blend electrolyte PVDF-HFPSiO2-PPC (0.5 mS cm−1) at 25 °C. These results indicate that the ether bond of the PPC polymer chain could promote the Li+ migration
through the interaction between the oxygen atoms, thus weakened the bond between the fluorine atom and Li+ [37]. The electrolyte uptake amount is shown in Table 1. The core-shell membrane PVDF-HFPSiO2@PPC show higher electrolyte uptake than the blend membrane, which is attributed to the high porosity and the easy infiltration of ionic liquids into the inner cavities through the interconnected pores of the electrolyte, and is beneficial to enhance the ionic conductivity. Therefore, the core-shell composite electrolyte has a positive utility on both lithium-ion transference numbers and ionic conductivity. The cyclic voltammetry test of the cell in the voltage range of 2.9–4.1 V was operated at 0.1 mV s−1 scan rate to further investigated the electrochemical performance of the electrolyte (see Fig. 7a). The intercalation/deintercalation potential is noticed at about 3.77 V and 3.16 V, respectively [22]. To investigate the compatibility of the coreshell PVDF-HFP-SiO2@PPC gel polymer electrolyte against the lithium metal, depositing/striping of Li|GPE|Li symmetric cells was measured under a constant current density of 0.1 mA cm-2. It indicated that the PVDF-HFP-SiO2@PPC cell has lower overpotentials in comparison with the PVDF-HFP-SiO2-PPC cell, as shown in Fig. 7b. The overpotential for PVDF-HFP-SiO2@PPC and PVDF-HFP-SiO2-PPC cells showed a stable potential of 170 mV and 200 mV before 150 h, respectively. The increased overpotential of PVDF-HFP-SiO2-PPC cell after 150 h of charge-
Fig. 4. (a) FTIR spectra of PVDF-HFP-SiO2@PPC core-shell membrane nanofiber and PVDF-HFP-SiO2-PPC blend membrane nanofiber, (b) EDS elemental analysis by line scanning mapping of single PVDF-HFP-SiO2@PPC core-shell membrane nanofiber. 4
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Fig. 5. DC polarization plot electrolyte of core-shell PVDF-HFP-SiO2@ PPC and PVDF-HFP-SiO2-PPC sandwiched between two symmetric Li electrodes with applied voltage 50 mV (a, c), and impedance spectra before and after polarization (b, d), respectively. The inset in (c) shows the equivalent circuit used for Nyquist plot fitting.
19 mAh g−1 at 0.1 C, 0.5 C, 1 C, 2 C and 3 C, respectively. The coreshell PVDF-HFP-SiO2@PPC electrolyte exhibited better rate performance compared to the blend electrolyte. The capacity retained 80 mAh g−1 at a rate of 3.0 C. When the current density went back to 0.1 C, the capacity of PVDF-HFP-SiO2@PPC returned to its original value, indicating a highly reversible rate capability of the core-shell electrolyte. The enhanced rate performance can be attributed to the relatively higher electrolyte uptake, Li+ transference number, and ion conductivity, which stem from the special characteristic of the coreshell composites electrolyte [42,43]. The electrochemical properties of the batteries with PVDF-HFP-SiO2@PPC and PVDF-HFP-SiO2-PPC were further investigated. The charge/discharge curves of LiFePO4/Li batteries at a rate of 0.5 C are displayed in Fig. 8b. The charge/discharge curve displays a flat plateau at about 3.5 V and 3.4 V, which are corresponded to the potential of LiFePO4 cathode [44–46]. The battery with the PVDF-HFP-SiO2@PPC owns a high discharge capacity and small polarization. The small ΔV indicates a low interfacial impedance and high stability of the interphases in Li half batteries. The rate performance is shown in Fig. 8c. Fig. 8c shows the voltage profiles of LiFePO4|Li cell tested with various voltage ranging from 2.9 V to 4.1 V at 50 °C. The specific capacity decreased as the current density was
discharge might be caused by the growth of non-uniform lithium depositions. This cell failed after 220 h, which might be led by the dendrite-induced short [38–40]. This phenomenon might be attributed to an unstable interface between PVDF-HFP-SiO2-PPC and Li electrode, which would lead to high interfacial resistance and serious polarization [38,39,41]. For the PVDF-HFP-SiO2@PPC, the voltage did not exhibit unstable fluctuation, indicating a uniform lithium deposition in 300 h. The improvement implies that the core-shell membrane PVDF-HFPSiO2@PPC may facilitate uniform Li electrodeposition, which may be ascribed to the improved ionic conductivity and enhanced Li-ion transference number [43]. Electrochemical impedance spectra of the symmetric Li batteries were performed at different temperatures (see Fig. 7c, d). The resistances of the electrolyte declined with increasing temperature. The batteries assembled with the PVDF-HFP-SiO2@PPC exhibited lower interface resistance than PVDF-HFP-SiO2-PPC, suggesting that the PVDF-HFP-SiO2@PPC could exhibit good compatibility with lithium metal. Fig. 8a showed the rate performance of batteries with PVDF-HFPSiO2@PPC and PVDF-HFP-SiO2-PPC. When the current density increased, the batteries with PVDF-HFP-SiO2-PPC show discharge capacities of 167 mAh g−1, 158 mAh g−1, 138 mAh g−1, 51 mAh g−1 and
Fig. 6. Impedance plots of (a) PVDF-HFP-SiO2-PPC and (b) PVDF-HFP-SiO2@PPC at different temperature, (c) the Arrhenius plots of PVDF-HFP-SiO2-PPC and PVDFHFP-SiO2@PPC. 5
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Table 1 Fitted impedance parameters of electrolyte and the corresponding lithium ion transference numbers (tLi+), electrolyte uptake amount. Sample
R0/ohm
Rs/ohm
I0/μA
Is/μA
tLi+
Electrolytes uptake amount (wt%)
PVDF-HFP-SiO2-PPC PVDF-HFP-SiO2@PPC
307.54 128.62
360.55 182.25
11.1 38.3
7.8 29.4
0.69 0.77
300 460
ionic conductivity, and electrolyte wettability compared to the blend nanofibrous PVDF-HFP-SiO2-PPC. Besides, benefiting from the synergistic effects of the PVDF-HFP and in-situ SiO2 as core material and PPC as shell material, the batteries using core-shell electrolyte exhibit stable charge/discharge voltage platform and cycle performance. The results endow core-shell PVDF-HFP-SiO2@PPC electrolytes a high potential for lithium metal batteries in practical application fields. Therefore, this class of electrolyte with a well-designed core-shell nanofiber would be a promising electrolyte for high performance and safe lithium metal batteries.
enhanced from 0.1 C to 3 C, which is attributed to the increase of overpotential [47,48]. It is also found that the charge plateau rises while the discharge plateau declines with the current rate, which should originate from the polarization effect [42]. Fig. 8d shows the charge and discharge properties of devices assembled with PVDF-HFP-SiO2@PPC, PVDF-HFP-SiO2-PPC in the applied potential range from 2.9 V to 4.1 V. The capacity retention is almost 94% for the PVDF-HFP-SiO2@PPC, in comparison with the PVDF-HFP-SiO2-PPC of 78% at 1 C after 100 cycles. The core-shell PVDF-HFP-SiO2@PPC electrolyte showed a higher capacity and capacity retention compare to the cells with PVDF-HFP-SiO2-PPC. It is also observed that core-shell PVDF-HFP-SiO2@PPC cell show relatively stable performance with good capacity retention at room temperature (Fig. S2). The performance improvement of core-shell PVDF-HFPSiO2@PPC electrolyte is attributed to synchronously prepared multifunctional core-shell nanofiber membrane via choosing appropriate materials for core and shell substrates [49].The improved performances can be attributed to combining the merits of the shell PPC and the core matrix of PVDF-HFP, containing in-situ generated SiO2, in which the former contributes to the improve the wettability and ionic conductivity, and latter enhances the mechanical and electrochemical stability. This result indicates the suitability of electrolyte based on the electrospun core-shell PVDF-HFP-SiO2@PPC electrolyte for the practical application of Li metal battery.
CRediT authorship contribution statement Xianli Song: Investigation, Data curation, Writing - original draft. Wen Qi: Visualization. Haitao Zhang: Supervision, Writing review & editing. Gongying Wang: Writing - review & editing. Acknowledgment This work was financially supported by the National Key Research and Development Program of China (2016YFB0100303), the National Natural Science Foundation of China (No. 21878308), the Key Research Program of Frontier Sciences (QYZDY-SSW-JSC011), the Beijing Natural Science Foundation (Grant No. 2184134), and the K.C.Wong Education Foundation.
4. Conclusion Declaration of competing interests The core-shell membrane via coaxial electrospinning technology was successfully fabricated. The GPEs based on the core-shell PVDFHFP-SiO2@PPC exhibits prominent improvement in electrolyte uptake,
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to Fig. 7. (a) CV curves of LiFePO4| PVDF-HFPSiO2@ PPC | Li cells with a scan rate of 0.1 mV s−1 at room temperature, (b) Cycling stability of symmetric lithium cells of PVDFHFP-SiO2-PPC and PVDF-HFP-SiO2@PPC at a current density of 0.1 mA cm−2 at room temperature, Nyquist plots of symmetric lithium cells of (c) PVDF-HFP-SiO2-PPC and (d) PVDFHFP-SiO2@PPC at different temperatures.
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Fig. 8. (a) Comparison of the rate capabilities of PVDF-HFP-SiO2@PPC, PVDFHFP-SiO2-PPC at 50 °C, (b) Comparison of the charge-discharge curves of PVDF-HFPSiO2@PPC, PVDF-HFP-SiO2-PPC at 50 °C and the rate of 0.5 C, (C) Voltage profiles of LiFePO4|PVDF-HFP-SiO2@PPC | Li half cells at 50 °C and various rates, (d) Cycling performance of Li half cells assembled with PVDF-HFP-SiO2@PPC, PVDF-HFP-SiO2-PPC at 1 C at 50 °C.
influence the work reported in this paper. [13]
Appendix A. Supplementary data
[14]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ssi.2020.115266. [15]
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Contents lists available at ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Construction of core-shell nanofiber membrane with enhanced interface compatibility for lithium-metal battery ⁎
Xianli Songa,b,c, Wen Qid, Haitao Zhangb,c, , Gongying Wanga,b,
T
⁎
a
Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, China National Engineering Laboratory for VOCs Pollution Control Material & Technology, University of Chinese Academy of Sciences, Beijing 101408, China c Beijing Key Laboratory of Ionic Liquids Clean Process, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, China d Materials Center, Beijing Institute of Collaborative Innovation, Beijing 100081, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Nanocomposite membrane Coaxial electrospinning Interface compatibility Lithium-metal batteries Ionic liquid
Improving interface compatibility is critical to the solid-state lithium metal batteries. Thus, a novel type of coreshell nanocomposite polymer fiber membrane was prepared by a coaxial electrospinning technique. The coreshell nanofiber membranes contains poly (propylene carbonate) (PPC) in the shell, and poly(vinylidene fluorideco-hexafluoropropylene) (PVDF-HFP) that containing in-situ generated silica in the core. The structure, topography and compositions of the sample were investigated by scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS). The nanocomposite membranes exhibit three-dimensional framework structure with well-dispersed fibers, and could be transformed readily into gel polymer electrolytes (GPEs) by being soaked in an ionic liquid solution. Our study showed that the core-shell PVDF-HFP-SiO2@PPC polymer electrolyte exhibited a relatively high electrolyte uptake of 460%. The core-shell electrolyte was found to play a significant role in increasing the interface compatibility. The optimized core-shell PVDF-HFP-SiO2@PPC electrolyte exhibited an enhanced ionic conductivity (1.05 mS cm−1) in comparison with the blend PVDF-HFP-SiO2PPC electrolyte (0.5 mS cm−1) at 25 °C. This study demonstrates that the optimization of composition and microstructure is efficient in the fabrication of high-performance membranes for lithium-metal batteries.
1. Introduction Currently, the rechargeable lithium metal batteries have regained intensive attentions owing to the increasing demand in high energy density energy storage devices. Li metal exhibits high reactivity during repeated Li plating-stripping cycles, leading to lower efficiency and poor cycling stability, which could possibly trigger safety issues [1–3]. Over the last two decades, solid electrolytes have been used as promising ones to improve the safety performances, which can be treated as alternative of liquid electrolytes owing to the distinct advantages such as excellent interface stability, leakage proof, easy fabrication and outstanding mechanical properties [4–7]. Gel polymer electrolytes (GPEs), can be considered to exist in a state between liquid and solid, and are usually achieved by adding inorganic salt, polymer host, and low molecular weight plasticizing agents to common solvent [8–10]. The low molecular weight plasticizers are inadequate in the preparation of electrolyte due to the corrosive, volatile, and flammable characteristics. Ionic liquids, are generally utilized as green solvents, and substitute for low molecular weight plasticizers in the fabrication of highly
⁎
safe GPEs. The addition of ionic liquids (ILs) into polymer electrolytes to obtain GPEs is beneficial to chemical stability and electrochemical stability window [11]. Generally, ILs were added into the polymer electrolytes to fabricate high-performance GPEs. Beside the ionic liquids, the selection of suitable host polymer is critical to high-quality electrolytes. Poly(propylene carbonate) (PPC) is an environment friendly macromolecule with amorphous alternating copolymer. PPC exhibits excellent compatibility with lithium salts and has good interfacial contact with electrodes, which is a promising material for polymer electrolyte. The previous study revealed that PPC could be used as a buffer layer in the sandwich-like composite solid electrolyte, and it was beneficial to the interfacial compatibility during cycling [16]. Yang et al. also presented a strategy to improve the interfacial compatibility by introducing the PPC as adaptive buffer layer [17]. Coaxial electrospinning has been proved to be an effective technique to prepare multifunctional core-shell nanofibers synchronously. The selection of appropriate materials for core and shell substrates is extremely significant to the performances of core-shell nanofiber membranes [12–15]. PVDF-HFP exhibits relatively high ionic
Corresponding authors. E-mail addresses: [email protected] (H. Zhang), [email protected] (G. Wang).
https://doi.org/10.1016/j.ssi.2020.115266 Received 10 December 2019; Received in revised form 14 February 2020; Accepted 16 February 2020 0167-2738/ © 2020 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic illustration for the fabrication of core-shell PVDF-HFP-SiO2@PPC nanofiber membrane.
continuous nanofibers with core-shell structured. For a comparison, the as-prepared solution containing PVDF-HFP in-situ silica mixed with PPC was placed in a plastic syringe equipped with a needle-diameter of 0.5 mm under similar condition. The membrane was designated as PVDF-HFP-SiO2-PPC. The thickness of the membrane was about 150 μm. Before being stored in the glove box, the prepared nanofibrous mats were dried under vacuum at 50 °C overnight. The GPEs were fabricated by stacking the nanofibrous membranes in the solution of LiTFSI in EMIMBF4.
conductivity and high mechanical strength with semi-crystalline characteristic, which is considered to be a promising polymer electrolyte material [18]. The high dielectric constant of PVDF-HFP can be beneficial to dissociate lithium salt and enhance the carrier concentration [19–22]. Meanwhile, it exhibited a high stability during the reductive/ oxidative electrochemical process [18]. The addition of inert fillers, such as silica (SiO2), zirconia (ZrO2), and molecule sieves, could enhance the mechanical properties and ionic conductivity, which can facilitate the transport of Li+ [23–26]. Furthermore, the addition of inert fillers via in-situ synthesis of inorganic backbone compared to mechanical mixing would be conducive to uniform dispersion and better interfacial contact [27,28]. In this work, the PVDF-HFP-SiO2@PPC core-shell nanofibers were devised and fabricated by a coaxial electrospinning technique (illustrated in Fig. 1). PPC was selected as the shell material to improve the compatibility with commonly used electrodes. PVDF-HFP and in-situ generated silica were designed as the core matrix to increase the mechanical stability and the stability in the reductive/oxidative electrochemical environment. The obtained results indicate that this core-shell PVDF-HFP-SiO2@PPC composite electrolyte is very attractive for highperformance lithium metal batteries.
2.3. LiFePO4 cathode preparation The positive electrode was composed of LiFePO4, PVDF (poly(vinylidene fluoride), and super P. Initially, LiFePO4 (80%), super P (10%) were mixed in a mortar, and then 10 wt% PVDF was added to the above mixture. Then, the mixture was dissolved in N-methyl-2-pyrrolidone until a homogenous slurry was obtained. After coating the above slurry on the aluminum foil, the electrodes were dried in the vacuum oven at 110 °C for 24 h. 2.4. Electrochemical characterization of GPEs
2. Experimental
All batteries assembly was packaged in 2025 coin cells in an argonatmosphere glove box (O2 and H2O were below 0.01 ppm). The ionic conductivity (σ) of the electrolyte was calculated from the electrolyte bulk resistance (R) measured with stainless steel by AC impedance technique by the following formula:
2.1. Materials PPC (Mw = 120,000 from Empower Materials Inc., USA) and PVDFHFP (Aldrich, Mw = 450,000) were dried at 60 °C before utilization. Solvent N-methyl-2-pyrrolidone, acetone, and N, N-Dimethylforamide (DMF) were purchased from Aladdin. Tetraethoxysilane (TEOS) was purchased from Sigma-Aldrich.
σ=
d RS
where S is the surface area of the stainless steel. The cyclic voltammetry (CV, 0.1 mV s−1) using LiFePO4 cathode as the working electrode and Li as the reference electrode was obtained with CHI600E electrochemical workstation. The interfacial stability and battery cycling performance were carried out on LAND battery testing systems (LAND CT2001A).
2.2. Preparation of core-shell Nanofibrous composite polymer electrolyte Two types of electrospun nanofibers, consisting of core-shell nanofibrous membrane PVDF-HFP-SiO2@PPC and blend nanofibrous membrane PVDF-HFP-SiO2-PPC was prepared. The PPC solutions was prepared by dissolving blended solvent of DMF/acetone (1:1, w/w) and then magnetically stirred at concentration of 19 wt%. Another solution of 17% of PVDF-HFP in the same mixed solvent was prepared under magnetic stirring, followed by adding the required quantity of TEOS for generating in-situ silica. The core-shell nanofibrous membranes consisting of nanofibers with core (PPC)-shell (PVDF-HFP, in-situ silica) were fabricated by coaxial electrospinning. A 18 kV positive voltage was applied during the electrospinning process. The distance between the spinneret-to-collector was kept at 15 cm. The flow rate of the core solution was kept at 0.6 mL h−1, and the shell flow rate was kept at 0.72 mL h−1. The core flow rate was slower than the shell flow rate, which can benefit forming
2.5. Materials characterizations The morphology of as-prepared materials was investigated by a scanning electron microscopy (SEM) (JSM-7001F) with accelerating voltage of 5.0 kV. EDS was used to determine the composition and distribution of the single nanofiber. Fourier transform infrared (FT-IR) spectra were recorded by Thermo Nicolet 380 spectrometer. The electrochemical characterizations were tested using CR2025 coin-type cells. The contact angle was conducted on the Drop Shape Analyzer 100. The solution of LiTFSI in EMIMBF4 droplets (3 μL) was spread onto the surface of the membranes and then the images were captured in a continuous model. 2
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Fig. 2. FESEM images of electrospun core-shell PPC-PVDF-HFP-SiO2 and PPC@PVDF-HFP-SiO2 membrane (a, c), and corresponding fiber diameter distributions diagram (b, d), respectively.
3. Results and discussion
vibrations respectively, which are strong features of PVDF-HFP copolymer. The band at 783 and 1229 cm−1 can be assigned to the stretching vibration of the CeO bond, the stretching vibration of the C]O is about 1745 cm−1 [30]. In the spectrum of the nanofiber membrane with in-situ SiO2, the bands of SiO2 overlapped with groups of PVDF-HFP. Thus, EDS spectra were used to confirm the presence of SiO2 in the internal of the composite nanofiber membrane [31]. The presence of all characteristic peaks of PVDF-HFP, PPC, and SiO2 in the spectrum of the composite membrane indicates that the core-shell composite membrane was fabricated successfully [32]. Additionally, EDS was used to investigate the composition and distribution of core-shell PVDF-HFP-SiO2@PPC fibers [33]. From the elemental analysis by a line scan across a single fiber, element F was attributed to PVDF-HFP, and element Si was attributed to SiO2 in Fig. 4b. The amount of element F and Si increases in the middle of single nanofiber, indirect verification of the successful construction of the coaxial nature of the core-shell nanofibers. Meanwhile, the SEM image in Fig. S1 was performed to further prove the core-shell nanofibers.
3.1. Morphology of membranes The morphology of the core-shell structure of PVDF-HFP-SiO2-PPC and PVDF-HFP-SiO2@PPC nanofiber mats were investigated by SEM in Fig. 2. From the SEM image, a three-dimensional framework structure is formed by interconnecting well-dispersed fibers. This unique porous structure facilitates efficient capture and retention of the electrolyte, and contributes to the smooth diffusion of the electrolyte into the battery assembly. The diameters of the PVDF-HFP-SiO2@PPC core-shell membrane and PVDF-HFP-SiO2-PPC blend membranes were estimated to be 250–300 nm and 350–400 nm respectively in Fig. 2(b, d). The increased viscosity of the blend of precursor solution results in the broader distribution of fiber diameter [29]. The surface wettability of the composite electrolyte was generally explored by contact angle measurement. This property has significant impact on the ionic conductivity, and thus affects the capacity and cycle performance of the battery. A semi-quantitative measurement of the differences in the wettability of membranes can be investigated by comparing the contact angles of the membranes. As shown in Fig. 3, the evolution of contact angles gradually decreased over time. The contact angle of the PVDF-HFP-SiO2@PPC membranes was initially 27.7° at the initial time, and then decreased to 16.3° after 5 s in Fig. 3c and d, which is relatively lower than the PVDF-HFP-SiO2-PPC. The smaller contact angle was obtained on core-shell PVDF-HFP-SiO2@PPC fibers than PVDF-HFP-SiO2-PPC, indicating that the core-shell fibers play an effect on enhancing the wettability. This result indicates that the core-shell fibers facilitate the affinity between the different polymer components and allows rapid penetration of the ionic liquid electrolyte. The improvement in wettability of the core-shell PVDF-HFP-SiO2@PPC nanofibers fabricated via coaxial electrospinning technology is highly attractive for solid electrolyte application of Li metal batteries. FTIR spectra of the blending PVDF-HFP-SiO2-PPC and core-shell PVDF-HFP-SiO2@PPC fibers are shown in Fig. 4a. The absorption peaks at 1403 cm−1, 980 cm−1, and 882 cm−1 can be assigned to CeF stretching vibration, CeF stretching, CeC symmetric stretching
3.2. Electrochemical stability The number of cation transports is more critical for battery applications, because the low lithium transference number results in concentration polarization, which is detrimental to the performance of the battery. The lithium transference number was calculated according to the Bruce-Vincent formula [34–36]:
t+Li =
IS (∆V − I0 R 0) I0 (∆V − IS RS)
where I0 is the current at the initial state, Is is the steady-state current. The ΔV is the potential applied across the battery, R0 is the resistance at the initial state, Rs is the impedance after polarization. The current-time curves and the corresponding electrochemical impedance spectra (Fig. 5) before and after polarization were collected to calculate the Li+ ion transference number. Li+ transference number and electrolytes uptake amount for the PVDF-HFP-SiO2-PPC and PVDF3
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Fig. 3. Contact angle measurements of (a, b) PPC-PVDF-HFP-SiO2 membrane, and (c, d) PPC@PVDF-HFP-SiO2 core-shell membrane.
HFP-SiO2@PPC at room temperature are listed in Table 1. The tLi+ of the polymer electrolyte of core-shell PVDF-HFP-SiO2@PPC increases in comparison with that of blend PVDF-HFP-SiO2-PPC. The improvement of tLi+ could diminish the concentration gradient during the charge and discharge processes, suggesting that core-shell favor the lithium iondominant conduction behavior. The temperature-dependent ionic conductivity was investigated by the electrochemical impedance spectroscopy using a symmetric configuration of the SS/GPEs/SS cell. Fig. 6a and b show the impedance plots of PVDF-HFP-SiO2-PPC and PVDF-HFP-SiO2@PPC electrolyte. The plot of ionic conductivity at various temperatures between 25 and 80 °C was shown in Fig. 6c. Since increasing the temperature not only improves the ionic mobility, but also enhances the mobility of the polymer chains, thereby promoting the ion migration. The ionic conductivity of electrolyte present the same trend-positively correlated with the temperature. Meanwhile, the plots of both electrolytes appear linear, which suggests that their ionic conducting behaviors obey the typical Arrhenius behavior. The ionic conductivity of core-shell electrolyte PVDF-HFP-SiO2@PPC (1.05 mS cm−1) is higher than that of blend electrolyte PVDF-HFPSiO2-PPC (0.5 mS cm−1) at 25 °C. These results indicate that the ether bond of the PPC polymer chain could promote the Li+ migration
through the interaction between the oxygen atoms, thus weakened the bond between the fluorine atom and Li+ [37]. The electrolyte uptake amount is shown in Table 1. The core-shell membrane PVDF-HFPSiO2@PPC show higher electrolyte uptake than the blend membrane, which is attributed to the high porosity and the easy infiltration of ionic liquids into the inner cavities through the interconnected pores of the electrolyte, and is beneficial to enhance the ionic conductivity. Therefore, the core-shell composite electrolyte has a positive utility on both lithium-ion transference numbers and ionic conductivity. The cyclic voltammetry test of the cell in the voltage range of 2.9–4.1 V was operated at 0.1 mV s−1 scan rate to further investigated the electrochemical performance of the electrolyte (see Fig. 7a). The intercalation/deintercalation potential is noticed at about 3.77 V and 3.16 V, respectively [22]. To investigate the compatibility of the coreshell PVDF-HFP-SiO2@PPC gel polymer electrolyte against the lithium metal, depositing/striping of Li|GPE|Li symmetric cells was measured under a constant current density of 0.1 mA cm-2. It indicated that the PVDF-HFP-SiO2@PPC cell has lower overpotentials in comparison with the PVDF-HFP-SiO2-PPC cell, as shown in Fig. 7b. The overpotential for PVDF-HFP-SiO2@PPC and PVDF-HFP-SiO2-PPC cells showed a stable potential of 170 mV and 200 mV before 150 h, respectively. The increased overpotential of PVDF-HFP-SiO2-PPC cell after 150 h of charge-
Fig. 4. (a) FTIR spectra of PVDF-HFP-SiO2@PPC core-shell membrane nanofiber and PVDF-HFP-SiO2-PPC blend membrane nanofiber, (b) EDS elemental analysis by line scanning mapping of single PVDF-HFP-SiO2@PPC core-shell membrane nanofiber. 4
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Fig. 5. DC polarization plot electrolyte of core-shell PVDF-HFP-SiO2@ PPC and PVDF-HFP-SiO2-PPC sandwiched between two symmetric Li electrodes with applied voltage 50 mV (a, c), and impedance spectra before and after polarization (b, d), respectively. The inset in (c) shows the equivalent circuit used for Nyquist plot fitting.
19 mAh g−1 at 0.1 C, 0.5 C, 1 C, 2 C and 3 C, respectively. The coreshell PVDF-HFP-SiO2@PPC electrolyte exhibited better rate performance compared to the blend electrolyte. The capacity retained 80 mAh g−1 at a rate of 3.0 C. When the current density went back to 0.1 C, the capacity of PVDF-HFP-SiO2@PPC returned to its original value, indicating a highly reversible rate capability of the core-shell electrolyte. The enhanced rate performance can be attributed to the relatively higher electrolyte uptake, Li+ transference number, and ion conductivity, which stem from the special characteristic of the coreshell composites electrolyte [42,43]. The electrochemical properties of the batteries with PVDF-HFP-SiO2@PPC and PVDF-HFP-SiO2-PPC were further investigated. The charge/discharge curves of LiFePO4/Li batteries at a rate of 0.5 C are displayed in Fig. 8b. The charge/discharge curve displays a flat plateau at about 3.5 V and 3.4 V, which are corresponded to the potential of LiFePO4 cathode [44–46]. The battery with the PVDF-HFP-SiO2@PPC owns a high discharge capacity and small polarization. The small ΔV indicates a low interfacial impedance and high stability of the interphases in Li half batteries. The rate performance is shown in Fig. 8c. Fig. 8c shows the voltage profiles of LiFePO4|Li cell tested with various voltage ranging from 2.9 V to 4.1 V at 50 °C. The specific capacity decreased as the current density was
discharge might be caused by the growth of non-uniform lithium depositions. This cell failed after 220 h, which might be led by the dendrite-induced short [38–40]. This phenomenon might be attributed to an unstable interface between PVDF-HFP-SiO2-PPC and Li electrode, which would lead to high interfacial resistance and serious polarization [38,39,41]. For the PVDF-HFP-SiO2@PPC, the voltage did not exhibit unstable fluctuation, indicating a uniform lithium deposition in 300 h. The improvement implies that the core-shell membrane PVDF-HFPSiO2@PPC may facilitate uniform Li electrodeposition, which may be ascribed to the improved ionic conductivity and enhanced Li-ion transference number [43]. Electrochemical impedance spectra of the symmetric Li batteries were performed at different temperatures (see Fig. 7c, d). The resistances of the electrolyte declined with increasing temperature. The batteries assembled with the PVDF-HFP-SiO2@PPC exhibited lower interface resistance than PVDF-HFP-SiO2-PPC, suggesting that the PVDF-HFP-SiO2@PPC could exhibit good compatibility with lithium metal. Fig. 8a showed the rate performance of batteries with PVDF-HFPSiO2@PPC and PVDF-HFP-SiO2-PPC. When the current density increased, the batteries with PVDF-HFP-SiO2-PPC show discharge capacities of 167 mAh g−1, 158 mAh g−1, 138 mAh g−1, 51 mAh g−1 and
Fig. 6. Impedance plots of (a) PVDF-HFP-SiO2-PPC and (b) PVDF-HFP-SiO2@PPC at different temperature, (c) the Arrhenius plots of PVDF-HFP-SiO2-PPC and PVDFHFP-SiO2@PPC. 5
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Table 1 Fitted impedance parameters of electrolyte and the corresponding lithium ion transference numbers (tLi+), electrolyte uptake amount. Sample
R0/ohm
Rs/ohm
I0/μA
Is/μA
tLi+
Electrolytes uptake amount (wt%)
PVDF-HFP-SiO2-PPC PVDF-HFP-SiO2@PPC
307.54 128.62
360.55 182.25
11.1 38.3
7.8 29.4
0.69 0.77
300 460
ionic conductivity, and electrolyte wettability compared to the blend nanofibrous PVDF-HFP-SiO2-PPC. Besides, benefiting from the synergistic effects of the PVDF-HFP and in-situ SiO2 as core material and PPC as shell material, the batteries using core-shell electrolyte exhibit stable charge/discharge voltage platform and cycle performance. The results endow core-shell PVDF-HFP-SiO2@PPC electrolytes a high potential for lithium metal batteries in practical application fields. Therefore, this class of electrolyte with a well-designed core-shell nanofiber would be a promising electrolyte for high performance and safe lithium metal batteries.
enhanced from 0.1 C to 3 C, which is attributed to the increase of overpotential [47,48]. It is also found that the charge plateau rises while the discharge plateau declines with the current rate, which should originate from the polarization effect [42]. Fig. 8d shows the charge and discharge properties of devices assembled with PVDF-HFP-SiO2@PPC, PVDF-HFP-SiO2-PPC in the applied potential range from 2.9 V to 4.1 V. The capacity retention is almost 94% for the PVDF-HFP-SiO2@PPC, in comparison with the PVDF-HFP-SiO2-PPC of 78% at 1 C after 100 cycles. The core-shell PVDF-HFP-SiO2@PPC electrolyte showed a higher capacity and capacity retention compare to the cells with PVDF-HFP-SiO2-PPC. It is also observed that core-shell PVDF-HFP-SiO2@PPC cell show relatively stable performance with good capacity retention at room temperature (Fig. S2). The performance improvement of core-shell PVDF-HFPSiO2@PPC electrolyte is attributed to synchronously prepared multifunctional core-shell nanofiber membrane via choosing appropriate materials for core and shell substrates [49].The improved performances can be attributed to combining the merits of the shell PPC and the core matrix of PVDF-HFP, containing in-situ generated SiO2, in which the former contributes to the improve the wettability and ionic conductivity, and latter enhances the mechanical and electrochemical stability. This result indicates the suitability of electrolyte based on the electrospun core-shell PVDF-HFP-SiO2@PPC electrolyte for the practical application of Li metal battery.
CRediT authorship contribution statement Xianli Song: Investigation, Data curation, Writing - original draft. Wen Qi: Visualization. Haitao Zhang: Supervision, Writing review & editing. Gongying Wang: Writing - review & editing. Acknowledgment This work was financially supported by the National Key Research and Development Program of China (2016YFB0100303), the National Natural Science Foundation of China (No. 21878308), the Key Research Program of Frontier Sciences (QYZDY-SSW-JSC011), the Beijing Natural Science Foundation (Grant No. 2184134), and the K.C.Wong Education Foundation.
4. Conclusion Declaration of competing interests The core-shell membrane via coaxial electrospinning technology was successfully fabricated. The GPEs based on the core-shell PVDFHFP-SiO2@PPC exhibits prominent improvement in electrolyte uptake,
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to Fig. 7. (a) CV curves of LiFePO4| PVDF-HFPSiO2@ PPC | Li cells with a scan rate of 0.1 mV s−1 at room temperature, (b) Cycling stability of symmetric lithium cells of PVDFHFP-SiO2-PPC and PVDF-HFP-SiO2@PPC at a current density of 0.1 mA cm−2 at room temperature, Nyquist plots of symmetric lithium cells of (c) PVDF-HFP-SiO2-PPC and (d) PVDFHFP-SiO2@PPC at different temperatures.
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Fig. 8. (a) Comparison of the rate capabilities of PVDF-HFP-SiO2@PPC, PVDFHFP-SiO2-PPC at 50 °C, (b) Comparison of the charge-discharge curves of PVDF-HFPSiO2@PPC, PVDF-HFP-SiO2-PPC at 50 °C and the rate of 0.5 C, (C) Voltage profiles of LiFePO4|PVDF-HFP-SiO2@PPC | Li half cells at 50 °C and various rates, (d) Cycling performance of Li half cells assembled with PVDF-HFP-SiO2@PPC, PVDF-HFP-SiO2-PPC at 1 C at 50 °C.
influence the work reported in this paper. [13]
Appendix A. Supplementary data
[14]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ssi.2020.115266. [15]
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