Three-Dimensional Nanoporous Polyethylene-Reinforced PVDF

Subscriber access provided by - Access paid by the | UCSB Libraries

Letter

Three dimensional (3D) nanoporous PE-reinforced PVDFHFP separator enabled by dual-solvent hierarchical gas liberation for ultrahigh rate lithium ion batteries Ripeng Luo, Ce Wang, Zuoxiang Zhang, Weiqiang Lv, Zhaohuan Wei, Yanning Zhang, Xiaoyan Luo, and Weidong He ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00091 • Publication Date (Web): 21 Feb 2018 Downloaded from http://pubs.acs.org on February 22, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Energy Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Energy Materials

Three

dimensional

(3D)

nanoporous

PE-reinforced

PVDF-HFP

separator enabled by dual-solvent hierarchical gas liberation for ultrahigh rate lithium ion batteries Ripeng Luo†, Ce Wang†, Zuoxiang Zhang†, Weiqiang Lv, Zhaohuan Wei, Yanning Zhang, Xiaoyan Luo and Weidong He* School of Energy Science and Engineering, University of Electronic Science and Technology

of

China,

Chengdu

610054,

PR

China.

[email protected]. †These authors contributed equally to this work.

1

ACS Paragon Plus Environment

E-mail:

ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 26

ABSTRACT: High-power lithium-ion batteries (LIBs) have extensive applications ranging from electronic devices to electric vehicles. The composition and structure of separators largely impact the rate performances of LIBs. Here, a three-dimensional (3D) nanoporous poly(vinylidenefluoride-hexafluoropropylene) (PVDF-HFP)- polyethylene (PE) composite separator is obtained through solvent liberation. The composite separator owns a high ionic conductivity of 1.01 mS·cm-1 at room temperature due to the high porosity up to 95.6% and the uniform 3D pore distribution. LiFePO4/Li half cells with the composite separator deliver record rate-capacities of 97 mAh·g-1 at 10 C and 57 mAh·g-1 at 20 C. PE in the composite separator significantly enhances the mechanical strength and thermal stability of the separator. Theoretical calculations show that the difference in the absorption energy between acetone and NMP solvent on PVDF-HFP is the major driving force for the formation of the inter-island structure, which provides massive Li+ transport channels during high rate battery cycling.

Keywords:

Poly(vinylidenefluoride-hexafluoropropylene);

lithium ion battery; Separator; Solvent liberation

2


Polyethylene;

High-rate

Page 3 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


High-rate lithium ion batteries (LIBs) are in great demand for applications in consumer electronics and electrical vehicles.1-5 The key to constructing the high-rate LIBs is to improve the Li+ conduction between electrodes and electron transfer in electrodes.6 Achieving efficient transport of Li+ ions within separators remains a challenge for high rate batteries. In particular, the rate performance of a battery is correlated closely with the porosity, pore size and pore distribution within the porous separator matrix. As ionic transport media, a porous polymer separator is commonly saturated with organic electrolyte containing lithium salt.7 Increasing electrolyte uptake typically enhances the Li+ conduction. Conventional polyolefin separators are lengthened and oriented in the same direction due to the anisotropic stretching route to form porosity.8 The low porosity and relatively-low wettability of such commercial separators lead to a quite limited electrolyte uptake, giving rise to poor high-rate performances of LIBs.9 High-porosity and wettability PVDF-HFP separators are commonly constructed with the incorporation of metal-oxide particles, including SiO2,10-11 TiO2,12 and Al2O3.13 In addition, electrospinning14-18 and phase inversion19-22 are frequently adopted to form porous polymer matrix. However, the aforementioned methods own intrinsic challenges. For instance, electrospun separators suffer from poor mechanical strength. Metal oxide doped separators suffer from poor pore distribution and phase inversion separators usually results in non-uniform pore size. Liu et al.23 developed a PVDF-HFP/PE composite gel 3



electrolyte separator by using a single tetrahydrofuran (THF) solvent with a high PE content of over 23 wt.%, well above 1.8-5.3 wt.% used in our work. As compared with the presented hierarchical solvent liberation method, the single solvent method tends to form dense microstructure of the separator because single solvent fails to result in the difference in evaporation rate and polymer affinity for fabricating high-porosity separators. Therefore, the single solvent method tends to end up with poor ionic conductivity and poor mechanical strength. In this work, PE-reinforced PVDF-HFP separators with 3D nanoporous structure were prepared through hierarchical solvent liberation. The uniform pore-distribution and interaction enable high absorption of electrolyte within separator and allow for high ionic conductivity and high-efficiency Li+ transport. By using the separator, the LiFePO4/Li half cell delivers rate-capacities of 97 mAh·g-1 at 10 C and 57 mAh·g-1 at 20 C. After adding PE, the separator owns no obvious shrinkage at 150 oC and enhanced mechanical strength up to 16 MPa, exhibiting remarkable thermal and mechanical robustness.

Results and discussion Figure 1a shows the scanning electron microscopic (SEM) images of the commercial tri-layer Celgard 2325 separator.24 One-dimensional oriented and lengthened pores produced with a dry method are observed. Figure 1b shows the SEM image of the 4


Page 4 of 26

Page 5 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


separator based on acetone solvent in our previous research,25 labeled as PVDF-HFP@acetone. The separator has increased porosity with larger pores size as compared with Celgard 2325. However, the compact structure with 2D pore distribution impedes electrolyte uptake for Li+ transport. Figures 1c and 1d present the spatial structure of composite separator synthesized by dual-solvent acetone and NMP. The composite separator shows inter-island structure with abundant pores. Acetone and NMP own boiling points of 56.5 °C and 203 °C, corresponding to great difference in evaporation rate. During evaporation, acetone forms in-situ pores within the polymer matrix. Simultaneously, PVDF-HFP dissolves in NMP and forms dense and strong organic framework. The nanoporous inter-island structure gives rise to 3D ionic transport channels for high-efficiency Li+ transport. The X-ray diffraction (XRD) pattern of the composite separator is shown in Figure 2a. Typical α-phase PVDF-HFP gives rise to strong characteristic diffraction peaks at 18.6°, 20.8° and 26.6°, corresponding to (020), (100), (110) planes of the semi-crystalline structure. To confirm the incorporation of polyethylene within the PVDF-HFP matrix, PE grains, pure PVDF-HFP separator and composite separator prepared by PVDF-HFP and PE, are analyzed by Fourier transform infrared spectroscopy (FTIR). As shown in Figure 2b, the characteristic peaks of bare PE occur at 2930 cm-1, 2850 cm-1 and 1476 cm-1, corresponding to antisymmetric stretching, symmetric stretching and bending vibrations of C-H in –CH2- group, respectively. The 5



aforementioned bands are detected in the FTIR plot of the PVDF-HFP/PE separator. Moreover, the characteristic peak of C-H at 3600 cm-1-3800 cm-1 in the composite separator confirms the existence of PE. The results demonstrate that PE is successful introduced into the PVDF-HFP separator. Figure 2c shows thermogravimetry (TG) curves of pure PVDF-HFP separator and the composite separator. A portion of PE in composite separator begins to melt at 92 °C, in accordance with the melting point of PE. Significant weight loss occurs at over 460 °C for both separators. The mechanical property of the separator is critical for the safety performance of the lithium ion batteries, especially at high temperatures. The tensile strength and strain of the PVDF-HFP based separators are compared, as shown in Figure 2d. Composite separators were prepared with different weights of PE, labeled as PVDF-HFP/PE-0.5 (1.8 wt% of PE), PVDF-HFP/PE-1.0 (3.6 wt% of PE), and PVDF-HFP/PE-1.5 (5.3 wt% of PE). The PVDF-HFP@NMP separator was prepared by adopting the hierarchical solvent liberation method without PE. The PVDF-HFP/PE-1.0 separator owns a tensile strength of 16.1 MPa, which is four times larger than that of the PVDF-HFP@acetone separator. The composite separator sustains its integration with more than 31% strain while PVDF-HFP@NMP only endures a small strain of 12.7%. The adoption of PE in the separator efficiently promotes the mechanical stretching ratio and avoids fracture. Infrared thermography images of the separators are shown in Figure 2e. Composite separator 6


Page 6 of 26

Page 7 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


maintains its integration after 200 s at 150 °C, whereas obvious dimensional shrinkage occurs in primitive phase inversion separators.19 At 150 °C Celgard 2325 begins to deform at 100 s and shrinks severely after 200 s. Electrolyte uptake reflects the efficiency of ionic conduction of separators. Figure 3a presents the comparison on electrolyte absorption between composite separator and Celgard 2325. The composite separator delivers an electrolyte absorption ratio of 216%, which is five times as high as that of commercial Celgard 2325 separator (43%). Electrochemical Impedance Spectroscopy (EIS) is employed to measure the temperature dependence of ionic conductivity. Figure 3b displays the correlation between ionic conductivity and temperature from 25-75 °C. The insert of Figure 3b depicts the Nyquist plot of the composite separator. At room temperature, the PVDF-HFP/PE-1.0 separator delivers a larger ionic conductivity of 1.01 mS·cm-1 as compared to 0.21 mS·cm-1 of Celgard 2325. The high ionic conductivity of the composite separator is attributed to the high electrolyte absorption and homogenous pore distribution, which allows for high-efficiency Li+ transport. The activation energy of the PVDF-HFP/PE-1.0 separator is smaller as compared with that of the commercial separator, which suggests a lower energy for Li+ diffusion due to the large interior space within the porous composite separator. Ionic conductivities of the PVDF-HFP/PE-1.0 and other reported PVDF based separators are compared in Table 1. 7



Page 8 of 26

Charge-discharge tests were performed to evaluate the electrochemical performance of PVDF-HFP/PE separators by using LiFePO4 as electrode and Li-metal as counter electrode. Figure 4a presents the initial charge-discharge curves of the batteries with pure PVDF-HFP and PVDF-HFP/PE separators at 0.1 C. The PVDF-HFP/PE-1.0 separator delivers the highest discharge capacity of 164.7 mAh·g-1, a value close to the theoretical capacity of LiFePO4 with a Columbic efficiency of 99.2%. The PVDF-HFP/PE-1.0 battery owns a higher initial discharge capacity and Columbic efficiency as compared with the PVDF-HFP@NMP, indicating improved

electrochemical

reversibility. The rate

performance of the PVDF-HFP/PE separators is compared with those of commercial Celgard 2325 and PVDF-HFP@acetone. As shown in Figure 4b, the battery with PVDF-HFP/PE-1.0 separator owns a higher capacity of 154 mAh·g-1 at 0.5 C, as compared with that of Celgard 2325 of 130 mAh·g-1. The PVDF-HFP/PE-1.0 separator has nearly zero capacity fading after 60 cycles at 0.5-1-0.5 C. As comparison, the PVDF-HFP@acetone separator undergoes 11.6% capacity decay, from 129 mAh·g-1 to 114 mAh·g-1. The worse cycle stability is mainly attributed to the morphological decay in the PVDF@acetone separator.25 The porosity of the PVDF-HFP@acetone is largely limited due to the interaction of electrolyte. The composite separator with PE is capable of stabilizing the morphology and maintaining the electrolyte absorption, giving rise to the remarkable cycle stability of the batteries. As shown in Figure 4c, the high-rate 8


Page 9 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


performance of PVDF-HFP/PE-1.0 is analyzed at 10 C to 25 C for 100 cycles. At different rates, the initial discharge capacity values are 97.4 mAh·g-1, 70.8 mAh·g-1, 56.6 mAh·g-1 and 41.3 mAh·g-1 at 10 C, 15 C, 20 C and 25 C, respectively. Cycling at 10 C, the half-cell assembled with the composite separator owns an initial capacity of 97.4 mAh·g-1 and 92.5 mAh·g-1 after 100 cycles with a capacity retention of 94.9%. As comparison, Celgard 2325 delivers an initial capacity of 63.5 mAh·g-1 and 54.5 mAh·g-1 after 100 cycles at the same condition. The gel-polymer separator presents an extraordinary high rate capacity and high stability, exhibiting great potential for high rate applications. Figure 4d demonstrates the cycle performance of the battery with the composite separator at 3 C as compared with Celgard 2325. The composite separator battery allows for 1000 cycles without capacity decay. Moreover, the average capacity achieves a value of 126 mAh·g-1. As comparison, the battery with the Celgard 2325 separator delivers an initial capacity of 99 mAh·g-1 and a severe decay of 47.4% after 1000 cycles. The crucial properties and electrochemical performances between the commercial and PVDF-HFP/PE-1.0 separators are compared in Table 2. Figure S1 presents the Nyquist plots of the Li|LiFePO4 half-cells with Celgard 2325 and PVDF-HFP/PE-1.0 separators. The impedance parameters were analyzed by ZSimpWin Software. The PVDF-HFP/PE-1.0 battery presents a small charge transfer resistance (Rct) of 166.8 Ω, well below that (261.7 Ω) of the battery based on Celgard 9



2325. After cycling 5 cycles at 10 C, PVDF-HFP/PE-1.0 battery owns a Rct of 209.1 Ω while Celgard 2325 battery owns a significantly increased Rct of 323.7 Ω, which could be attributed to surface layer formation. Comparison of the electrochemical impedance is presented in Table 3. The PVDF-HFP/PE-1.0 separator owns a smaller kinetic impedance of the charge transfer at the electrolyte-electrode interface, an important factor for the rate performance of the LIBs. A significantly small charge transfer resistance of the PVDF-HFP/PE-1.0 battery is responsible for the remarkable high-rate performance. The high electrolyte affinity and high porosity give rise to the low Rct, which contributes to excellent high-rate performances. To investigate the formation of the 3D inter-island structure of the composite separator, the interaction energies between PVDF-HFP and acetone/NMP were calculated. The calculations were performed by subtracting the self-consistent field calculation (SCF) energies of both geometry optimized functional groups and solvent molecules from the total energy of the optimized configuration when the solvent molecules were placed near the functional groups of PVDF-HFP. Four different functional groups were chosen, including CH2F2, C2H4F2, C3F8 and C3H2F6, to illustrate the interaction between solvent molecules and different sites in PVDF-HFP chains. CH2F2 and C2H4F2 represent the site of main PVDF-HFP chains, whereas C3F8 and C3H2F6 represent the site on HFP branch chains. The result in Figure 5 shows that the interactions between acetone and various sites in 10


Page 10 of 26

Page 11 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PVDF-HFP are much lower than the interactions between NMP and PVDF-HFP, especially for the interactions with the HFP branch chains. For instance, the adsorbed interaction between acetone and C3F8/C2H2F6 is -0.02 eV, whereas the adsorption interactions between NMP and C3F8/C2H2F6 are -0.15 eV and -0.22 eV. The results indicate that NMP has stronger adsorption energy and higher affinity than that of acetone. The higher affinity of PVDF-HFP to NMP indicates that more PVDF-HFP is integrated into NMP rather than acetone. During the natural drying of the solvents, the much-faster extraction of acetone generates great number of pores. Due to the three-dimensional hierarchical pores, the separator is created with uniform polymeric framework with abundant ion diffusion channels, which gives rise to the ultrahigh rate performances of the separator.

Conclusion In this work, a 3D nanoporous PE reinforced PVDF-HFP composite separator with an inter-island structure is prepared for ultrahigh rate LIBs. Based on hierarchical solvent liberation, the preparation adopts a bi-solvent system. The acetone-NMP bi-solvent gives rise to the difference in evaporation rate and polymer affinity, which is the key to forming the high porosity of 95.6% and the 3D hierarchical pore distribution. The high-efficiency ionic transport channels of the separator with the hierarchical structure give rise to record rate-capacities of 97 mAh·g-1 at 10 C and 57 mAh·g-1 at 20 C. With PE, the separator presents only a subtle shrinkage as temperature increases to 150 °C and an enhanced 11



tensile strength of 16 MPa, which is attributed to the reinforcement effect of PE within the polymeric matrix.

Experimental Section Preparation of PVDF-HFP/PE composite separator. The PVDF-HFP powder was purchased from Solvay Co. Ltd. (Mw = 100,000 g·mol-1, 20 wt% HFP). At 50 oC, 0.6 g PVDF-HFP powder was added into 8.0 mL acetone under constant stirring, until the milky mixture changed into transparent gel-like colloid. Simultaneously, 0.1 g undissolved PVDF-HFP was dispersed in 1.0 mL acetone at room temperature under stirring, which formed rich space to hold residual electrolyte.25 PE was dissolved in NMP at 50 oC to prepare saturated solution with solubility of 0.0276 g·mL-1. The volume of PE-NMP solution ranged from 0.5 mL to 1.5 mL in this experiment. The aforementioned NMP-based and acetone-based solutions were mixed together with stirring at room temperature until the homogeneous slurry sample was obtained. The as-prepared homogeneous slurry was cast on the substrate and naturally dried at room temperature. The thickness of the resulted separator ranged from 20 µm to 24 µm. By using different amounts of PE-treated NMP (0.5, 1.0, 1.5 ml), different separators were synthesized, labeled as PVDF-HFP/PE-0.5 (1.8 wt% of PE), PVDF-HFP/PE-1.0 (3.6 wt% of PE) and PVDF-HFP/PE-1.5 (5.3 wt% of PE), respectively. The polymer mass ratio of the prepared separators was determined and chosen based on the optimal mechanical strength after the 12


Page 12 of 26

Page 13 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


synthesis.

Characterizations. The surface morphology of the composite separator was measured by the SU8020 scanning electron microscope (Hitachi, Japan) after gold sputtering. The crystallinity of the material was measured by X-ray diffraction

(MAX-2550-VB3, Rigaku) at a 2θ range of 10o-80o with Cu targeted Kα diffraction. The bonding and functional groups of the separators were analyzed by Fourier-transform Infrared Spectroscopy (Nicolet iS5, Thermo Fisher Scientific, USA) in the range of 4000 cm-1 to 1000 cm-1. The thermal analysis of the separators was performed by a thermogravimetric analyzer (NETZSCH, Germany) with a temperature increasing step at 10 °C·min-1 from room temperature to 800 °C at air atmosphere. Mechanical strength of the separators was evaluated by using the CMT6104 Universal Tester (MTS Systems Corporation, Shanghai). Electrolyte uptake was measured by weighing method. Propylene carbonate was used for absorption measurement. The absorption ratio was measured by following formula,

where m* is the mass of the separator with complete electrolyte uptake, and m0 is the mass of bare separator. Electrochemical measurements. In this study, LIR2032 coin cells were chosen for 13



electrochemical test. The separators were cut into small discs with a diameter of 18 mm to fit the button cell. Positive electrode selection was commercial carbon-coated lithium iron phosphate. Cathode slurry was made up of LiFePO4 active material, carbon black as conductive filler and PVDF as binder, with the mass ratio of 8:1:1. The active material loading of the electrode was 2 mg·cm-2~3 mg·cm-2. Separator was sandwiched by cathode and lithium metal after soaking into liquid electrolyte ( ZS002 1M LiPF6 2%VC in EC/DMC/EMC/DEC=30:15:35:20 in volume). The batteries were assembled in a LS800 glove box (Dellix, Chengdu) and placed inside the glove box for 12 h before the test. Electrochemical performances including the capacity and cyclic performances of the batteries, were evaluated by a Granville battery test system (Blue, Wuhan). The voltage ranged from 2.5 V to 4.2 V at room temperature. Ionic conductivity of the separator was measured by AC impedance analysis. The as-prepared and commercial separators were assembled with a stainless steel / separator / stainless steel configuration. The separators were immersed in the electrolyte for 12 h before conducting the impedance analysis. CHI660E electrochemical workstation (Chen Hua, Shanghai) was applied as test instrument. The frequency ranged from 0.1 Hz to 106 Hz with the amplitude of 10 mV at different temperatures (25 oC, 35 oC, 45 oC, 55 oC, 65 o

C and 75 oC). Ionic conductivity is calculated by following formula,

14


Page 14 of 26

Page 15 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


where L is the thickness, A is the area of the separator, and R is the real-part resistance obtained from horizontal axis of the Nyquist diagram. The half-cell impedance was measured by AC impedance analysis. The cells were deposited overnight after assembly. The primitive electrochemical impedance data was simulated by the ZSIMPWIN software. Computational Methods. The atomic configurations and interaction energies between PVDF-HFP and solvent molecules (acetone, NMP) were calculated by using density functional theory within Perdew–Berke–Ernzerh of generalized gradient approximation (GGA-PBE), as implemented in the Dmol3 package. The double numerical plus polarization (DNP) basis sets with effective core potential were employed to describe atomic potentials. Self-consistent field calculations (SCF) were carried out until the SCF tolerance was below 1×10-6.

ASSOCIATED CONTENT Supporting Information. Essential equations employed in this work for calculations of electrolyte uptake, porosity and ionic conductivity. Nyquist diagram used for impedance analysis of the Li|LiFePO4 half cells. Acknowledgments. The work was supported by the Fundamental Research Funds for the 15



Chinese Central Universities (Grant No. ZYGX2015Z003), the National Natural Science Foundation of China (Grant No. 21403031 and 51501030) and the Science & Technology Support Funds of Sichuan Province (Grant No. 2016GZ0151).

REFERENCES (1) Jansen, A. N.; Kahaian, A. J.; Kepler, K. D.; Nelson, P. A.; Amine, K.; Dees, D. W.; Vissers, D. R.; Thackeray, M. M. Development of a High-power Lithium-ion Battery. J. Power Sources 1999, 81, 902-905. (2) Lam, L. T.; Louey, R. Development of Ultra-battery for Hybrid-electric Vehicle Applications. J. Power Sources 2006, 158 , 1140-1148. (3) Kang, K. S.; Meng, Y. S.; Breger, J.; Grey, C. P.; Ceder, G. Electrodes with High Power and High Capacity for Rechargeable Lithium Batteries. Science 2006, 311, 977-980. (4) Pasta, M.; Wessells, C. D.; Huggins, R. A.; Cui, Y. A High-rate and Long Cycle Life Aqueous Electrolyte Battery for Grid-scale Energy Storage. Nat. Commun 2012, 3, 1149. (5) Arai, J.; Yamaki, T.; Yamauchi, S.; Yuasa, T.; Maeshima, T.; Sakai, T.; Koseki, M.; Horiba, T. Development of a High Power Lithium Secondary Battery for Hybrid Electric Vehicles. J. Power Sources 2005, 146 , 788-792. (6) Liu, H.; Strobridge, F. C.; Borkiewicz, O. J.; Wiaderek, K. M.; Chapman, K. W.; Chupas, P. J.; Grey, C. P. Capturing Metastable Structures During High-rate Cycling of LiFePO4 Nanoparticle Electrodes. Science 2014, 344. (7) Sousa, R. E.; Nunes-Pereira, J.; Costa, C. M.; Silva, M. M.; Lanceros-Méndez, S.; Hassoun, J.; Scrosati, B.; Appetecchi, G. B. Influence of the Porosity Degree of Poly(Vinylidene Fluoride-co-Hexafluoropropylene) Separators in the Performance of Li-ion batteries. J. Power Sources 2014, 263, 29-36. (8) Djian, D.; Alloin, F.; Martinet, S.; Lignier, H.; Sanchez, J. Y. Lithium-ion Batteries with High Charge Rate Capacity: Influence of the Porous Separator. J. Power Sources 2007, 172, 416-421. (9) Djian, D.; Alloin, F.; Martinet, S.; Lignier, H. Macroporous Poly(Vinylidene Fluoride) Membrane as a Separator for Lithium-ion Batteries with High Charge Rate Capacity. J. Power Sources 2009, 187, 575-580. (10) Jeong, H.-S.; Choi, E.-S.; Lee, S.-Y.; Kim, J. H. Evaporation-induced, Close-packed Silica Nanoparticle-embedded Nonwoven Composite Separator Membranes for 16


Page 16 of 26

Page 17 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


High-voltage/High-rate Lithium-ion Batteries: Advantageous Effect of Highly Percolated, Electrolyte-philic Microporous Architecture. J. Membrane. Sci 2012, 415–416, 513-519. (11) Xiao, W.; Li, X.-H.; Guo, H.-J.; Wang, Z.-X.; Zhang, Y.-H.; Zhang, X.-P. Preparation of Core–shell Structural Single Ionic Conductor SiO2@Li+ and Its Application in PVDF–HFP-based Composite Polymer Electrolyte. Electrochim. Acta 2012, 85, 612-621. (12) Kim, K. M.; Park, N.-G.; Ryu, K. S.; Chang, S. H. Characteristics of PVdF-HFP/TiO2 Composite Membrane Electrolytes Prepared by Phase Inversion and Conventional Casting Methods. Electrochim. Acta 2006, 51, 5636-5644. (13) Jeong, H.-S.; Kim, D.-W.; Jeong, Y. U.; Lee, S.-Y. Effect of Phase Inversion on Microporous Structure Development of Al2O3/Poly(Vinylidene Fluoride-Hexafluoropropylene)-based Ceramic Composite Separators for Lithium-ion Batteries. J. Power Sources 2010, 195 , 6116-6121. (14) Chen, W.-Y.; Liu, Y.-B.; Ma, Y.; Liu, J.-Z.; Liu, X.-R. Improved Performance of PVdF-HFP/PI Nanofiber Membrane for Lithium Ion Battery Separator Prepared by a Bicomponent Cross-electrospinning Method. Mater. Lett 2014, 133, 67-70. (15) Croce, F.; Focarete, M. L.; Hassoun, J.; Meschini, I.; Scrosati, B. A Safe, High-rate and High-energy Polymer Lithium-ion Battery Based on Gelled Membranes Prepared by Electrospinning. Energ Environ Sci 2011, 4 , 921-927. (16) Rao, M.-M.; Geng, X.-Y.; Liao, Y.-H.; Hu, S.-J.; Li, W.-S. Preparation and Performance of Gel Polymer Electrolyte Based on Electrospun Polymer Membrane and Ionic Liquid for Lithium Ion Battery. J. Membrane. Sci 2012, 399, 37-42. (17) Xiao, Q.-Z.; Li, Z.-H.; Gao, D.-S.; Zhang, H.-L. A Novel Sandwiched Membrane as Polymer Electrolyte for Application in Lithium-ion Battery. J. Membrane. Sci 2009, 326, 260-264. (18) Zhu, Y.-S.; Xiao, S.-Y.; Shi, Y.; Yang, Y.-Q.; Hou, Y.-Y.; Wu, Y.-P. A Composite Gel Polymer Electrolyte with High Performance Based on Poly(Vinylidene Fluoride) and Polyborate for Lithium Ion Batteries. Adv. Energy. Mater 2014, 4, 1300647-n/a. (19) Pu, W.-H.; He, X.-M.; Wang, L.; Jiang, C.-Y.; Wan, C.-R. Preparation of PVDF–HFP Microporous Membrane for Li-ion Batteries by Phase Inversion. J. Membrane. Sci 2006, 272, 11-14. (20) Li, D.; Shi, D.-Q.; Xia, Y.-G.; Qiao, L.; Li, X.-F.; Zhang, H.-M. Superior Thermally Stable and Nonflammable Porous Polybenzimidazole Membrane with High Wettability for High-Power Lithium-Ion Batteries. ACS App. Mater. Inter 2017, 9, 8742-8750. (21) Du Pasquier, A.; Warren, P. C.; Culver, D.; Gozdz, A. S.; Amatucci, G. G.; Tarascon, J. M. Plastic PVDF-HFP Electrolyte Laminates Prepared by a Phase-inversion Process. Solid State Ionics 2000, 135, 249-257. (22) Lee, Y.-M.; Kim, J.-W.; Choi, N.-S.; Lee, J.-A.; Seol, W.-H.; Park, J.-K. Novel Porous Separator Based on PVdF and PE Non-woven Matrix for Rechargeable Lithium 17



Batteries. J. Power Sources 2005, 139, 235-241. (23) Liu, X.-J.; Kusawake, H.; Kuwajima, S. Preparation of a PVdF-HFP/Polyethylene Composite Gel Electrolyte with Shutdown Function for Lithium-ion Secondary Battery. J. Power Sources 2001, 97, 661-663. (24) Arora, P.; Zhang, Z.-M. Battery Separators. Chem. Rev 2004, 104, 4419-4462. (25) Ye, L.-H.; Shi, X.-Y.; Zhang, Z.-X.; Liu, J.-N.; Jian, X.; Waqas, M.; He, W.-D. An Efficient Route to Polymeric Electrolyte Membranes with Interparticle Chain Microstructure Toward High-Temperature Lithium-Ion Batteries. Adv. Mater. Interfaces 2017, 4, 1601236-n/a. (26) Ulaganathan, M.; Mathew, C.M.; Rajendran, S. Highly Porous Lithium-ion Conducting Solvent-free Poly(Vinylidene Fluoride-co-Hexafluoropropylene)/Poly(Ethyl Methacrylate) Based Polymer Blend Electrolytes for Li Battery Applications. Electrochim. Acta 2013, 93, 230-235. (27) Ulaganathan, M.; Nithya, R.; Rajendran, S.; Raghu, S., Li-ion Conduction on Nanofiller Incorporated PVdF-co-HFP Based Composite Polymer Blend Electrolytes for Flexible Battery Applications. Solid State Ionics 2012, 218, 7-12. (28) Ulaganathan, M.; Lei, Y. -L.; Flora, X. H.; Yan, Q., Charge Transport, Mechanical and Storage Performances of Sepiolite Based Composite Polymer Electrolytes. ChemistrySelect 2016, 1, 5821-5827. (29) Hao, J.-L.; Xiao, Q. -Z.; Lei, G. -T.; Li, Z. -H.; Wu, L. -J. A Novel Polyvinylidene Fluoride/Microfiber Composite Gel Polymer Electrolyte with an Interpenetrating Network Structure for Lithium Ion Battery. Electrochim. Acta 2014, 125, 450-456. (30) Zhu, Y.-S.; Wang, F.-X.; Liu, L.-L.; Xiao, S.-Y.; Chang, Z.; Wu, Y.-P. Composite of a Nonwoven Fabric with Poly(Vinylidene Fluoride) as a Gel Membrane of High Safety for Lithium Ion Battery. Energ Environ. Sci. 2013, 6, 618-624.

18


Page 18 of 26

Page 19 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1 (a) Photo of as-reported composite separator. SEM images of (b) Celgard 2325,24 PVDF-HFP based separators with different synthetic methods: (c) PVDF-HFP@acetone, (d) and (e) composite separators at different magnifications.

19



Figure 2 (a) XRD and (b) FTIR patterns of PVDF-HFP/PE composite separator. (c) Stress-strain curves of phase-inversion, PVDF-HFP@acetone, PVDF-HFP@NMP and 20


Page 20 of 26

Page 21 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


composite separators. (d) Thermogravimetry (TG) curves of phase inversion and composite separators. (e) Thermography photographs through infrared radiations.

Figure 3 (a) The diagram of time-dependent electrolyte uptake ratio of composite separators and Celgard 2325. (b) EIS plots of ionic conductivity data of Celgard 2325, composite separators and Arrhenius plots of PVDF-HFP/PE-1.0 separator.

21



Figure 4 (a) Electrochemical performances of LiFePO4 tested by PVDF-HFP@acetone, PVDF-HFP@NMP, PVDF-HFP/PE-0.5, 1.0, 1.5 separators during initial cycle at the rate of 0.1 C. (b) Variable rate performance curves of LiFePO4 tested by Celgard 2325, PVDF-HFP@acetone, PVDF-HFP/PE-0.5, 1.0, 1.5 separators. (c) High rate testing of LiFePO4 with PVDF-HFP/PE-1.0 and Celgard 2325 separator at 10 C-25 C for 100 cycles. (d) Cycle performances of LiFePO4 with PVDF-HFP/PE-1.0 and Celgard 2325 separator at 3 C for ultra-long 1000 cycles.

22


Page 22 of 26

Page 23 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5 DFT optimized adsorption structures and calculated adsorption energy values between functional groups of PVDF-HFP and solvent molecules: (a) C2H2F2 and acetone, (b) C2H4F2 and acetone, (c) C3F8 and acetone, (d) C3H2F6 and acetone, (e) C2H2F2 and NMP, (f) C2H4F2 and NMP, (g) C3F8 and NMP, (h) C3H2F6 and NMP.

23



Table of Contents

24


Page 24 of 26

Page 25 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Comparison of ionic conductivity between the as-prepared separator and other reported PVDF based separators. Ionic conductivity, S·cm-1

Separator

Ref.

PVDF-HFP/PE-1.0

1.01×10-3

-

PVDF

1.2×10-3

Pu et al.19

PVDF-HFP/PEMA blends

0.918×10-3

PVDF-HFP/PVAc incorporated with BaTiO3

5.53×10-3

PVDF-LiTFSI with sepiolite

3.58×10-4

PVDF/microfiber

6.5×10-4

Hao et al.29

PVDF-HFP/PI

1.46×10-3

Chen et al.14

Nonwoven-supported PVDF-HFP

3.0×10-4

Zhu et al.30

Ulaganathan al.26-28

et

Table 2. Comparison of crucial properties and electrochemical performances between Celgard 2325 and PVDF-HFP/PE-1.0 separators.

Separator

Porosity

Ionic conductivity, mS·cm-1

Maximum Initial discharge capacity -1 temperature, at 10-15-20-25C, mAh·g o C

Celgard 2325

43%

0.21

63.5, 39.2, 27.8, 11.0

Three-Dimensional Nanoporous Polyethylene-Reinforced PVDF

Recommend Documents