Cellulosic Biomass-Reinforced Polyvinylidene Fluoride Separators ...

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Cellulosic Biomass-Reinforced Polyvinylidene Fluoride Separators with Enhanced Dielectric Properties and Thermal Tolerance Lei Li, Miao Yu, Chao Jia, Jianxin Liu, Yanyan Lv, Yanhua Liu, Yi Zhou, Chuanting Liu, and Ziqiang Shao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 31 May 2017 Downloaded from http://pubs.acs.org on June 2, 2017

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ACS Applied Materials & Interfaces

Cellulosic Biomass-Reinforced Polyvinylidene Fluoride Separators with Enhanced Dielectric Properties and Thermal Tolerance Lei Li, †,‡ Miao Yu, †,‡ Chao Jia, †,‡ Jianxin Liu, †,‡ Yanyan Lv, †,‡ Yanhua Liu, †,‡ Yi Zhou, †,‡ Chuanting Liu, †,‡ and Ziqiang Shao,*, †,‡ †

School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081,

China ‡

Beijing Engineering Research Centre of Cellulose and Its Derivatives, Beijing 100081, China

KEYWORDS: Cellulosic biomass material, Dielectric properties, Thermal tolerance, Separator, Lithium-ion battery.

ABSTRACT: Safety issues are critical barriers to large-scale energy storage applications for lithium-ion batteries. Using an ameliorated, thermally stable, shutdown separator is an effective method to overcome safety issues. Herein, we demonstrate a novel, cellulosic biomass materialblended polyvinylidene fluoride separator that was prepared using a simple non-solvent-induced phase separation technique. The process formed a microporous composite separator with reduced crystallinity, uniform pore size distribution, superior thermal tolerance, and enhanced electrolyte wettability and dielectric and mechanical properties. In addition, the separator has a superior

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capacity retention and better rate capability compared to the commercialized polypropylene microporous membrane. This fascinating membrane was fabricated via a relatively eco-friendly and cost-effective method and is an alternative, promising separator for high-power lithium-ion batteries.

1. INTRODUCTION Lithium-ion batteries (LIBs) possess advantages, such as a high energy density,1 a long cycle life,2 no memory effect, a low self-discharge rate and a low environmental impact, and they have been applied to areas in portable electronics.3 LIBs are believed to be promising for larger energy density power system, including electric/hybrid-electric electronic products and renewable energy plants in the future.4 Nevertheless, a major concern for large-scale, commercialized applications of LIBs is a potential security issue due to the use of flammable carbonate electrolytes and the existence of a series of potential thermal runaway reactions.5-7 Thus, developing advanced LIB devices is crucial and urgent.8-11 As an integral part of LIBs, the separators in LIBs must have excellent integrated features, such as long-term mechanical and dimensional stability, uniform thickness, shutdown functionality at high temperatures (120 ～ 180 °C), resistance to electrolyte impurities, and sufficient electrolyte wettability.12 The material options for separators mainly include polymers and polymer composites, such as poly(propylene) (PP), poly(ethylene) (PE), poly(vinylidene fluoride) (PVDF), poly(acrylonitrile) (PAN), and poly(ethylene oxide) (PEO).13 Among the polymers, the PVDF-based separators have been extensively applied in LIB devices due to their excellent aging tolerance, superior chemical resistance, high thermal stability and membrane forming properties.14 More importantly, pristine PVDF possesses a relatively high


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dielectric constant of 5 at a frequency of 1 kHz at room temperature.15 As noted by some references, polymer separators with high dielectric constants may prevent electrolyte exudation upon storage, increase the charge carrier concentration and enhance the ionic conductivity compared to separators with lower dielectric constants.16 However, the relatively high dielectric losses of PVDF may lead to dielectric heating and thermal breakdown,17-19 and the high crystallinity degree might restrict the conduction of lithium ions because the conduction mainly occurs in the amorphous region.20 Previous work has explored new approaches to conquer these drawbacks, including the introduction of low crystallinity or amorphous polymers,21 an electrode/separator assembly,16 surface coatings on the separators,22 composite membranes with organic or inorganic materials,2 and polymer structure modifications.23 However, some of these methods have negative effects, such as impurities from cross-linking or grafting, non-cost-effective preparation processes, and decreased mechanical properties.13 Alternative biomass materials have recently attracted substantial attention as an innovative solution for unparalleled advances in multiple energy storage devices.3, 24-26 Among the various eco-friendly, natural, resource-based materials reported to date, cellulose and its derivatives have been highlighted as potential building blocks because cellulose is naturally inexhaustible, low cost, lightweight, physicochemically steady, biocompatible, and recyclable, and it has a low thermal expansion rate.27-29 Extensive research has been devoted to the application of cellulosic materials to a wide variety of rechargeable power sources, such as supercapacitors and LIBs,30-35 with a concentration on separator membranes,36 flexible electrode substrates,33 electrode binders,37 electrolyte adhesives,38 mechanical buffers for metallic anodes,29 and porous collectors. Highsubstitution degree cyanoethyl cellulose (CEC), a type of cellulose synthesized from alkalified cellulose and acrylonitrile via Michael addition, has a high dielectric constant and low dielectric


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loss tangent and is considered to have the potential to compensate for the defects of PVDF-based separators.39 Meanwhile, its excellent mechanical and electrochemical properties, resistance to microbial attack and acid erosion, excellent thermal stability, and low moisture regain can also contribute to the safety and commercialization of separators. Moreover, another cellulosic material, cellulose nanofibers (CNF), were deemed to be an effective additive for enhancing the mechanical properties and reducing the overall crystallinity of a material due to its high elastic modulus, hydrophilicity, large aspect ratio, low density and small thermal expansion coefficient.40 Advanced separator membranes with improved dielectric properties and thermal resistance are being considered to manufacture next-generation, high-safety and high-performance cells because of their excellent insulating properties and high-energy storage density. In particular, the theoretical understanding and interpretation of the comparative phenomenon for the design and synthesis of high-dielectric functional separators along with concurrent endeavors to promote ion transport kinetics and avoid interior short-circuit failures have not been reported. Herein, inspired by the facile functionalization of CEC and CNF, we propose a green material strategy for the development of novel ternary-composited separators. A non-solvent-induced phase separation (NIPS) method was adopted because the process can be easily controlled to avoid perforation, obtain a small and evenly distributed micro-pore size, and achieve a high tensile strength biaxial and puncture strength compared to other methods such as the dry-spinning/hot-drawing process.41 This work will shed light on an ideal polyolefin substitute as an environmental energy storage material due to its specific functionality, easy processability and reliability. 2. EXPERIMENTAL SECTION 2.1. Materials and Chemicals


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PVDF (Mw=3.6×105, HSV 900) was purchased from Aladdin with 97% hydrolysis and a molecular weight of 72 000 g mol-1. Acrylonitrile and N, N’-Dimethylformamide (DMF) was provided by Sinopharm Chemical Reagent. CEC was prepared in our laboratory using cotton (M30) and acrylonitrile.15, 39 The substitution degree (DS) was 2.46 (see FTIR spectrum in Figure S1 and calculation method in eq S1, Supporting Information). CNF were prepared via oxalic acid hydrolysis and high-pressure homogenization of bleached eucalyptus pulp (see TEM images and FTIR spectrum in Figure S1-S2, Supporting Information).42 Lithium (Li), lithium cobalt oxide (LiCoO2) and steel slices (SS) were purchased from Beijing Nonferrous Metals Research Institute (battery grade). The electrolyte used in the half-cell consisted of LiPF6 in a solvent mixture of ethylene carbonate (EC, AR), diethyl carbonate (DEC, AR), and ethyl methyl carbonate (EMC, AR) (LiPF6/EC/DEC/EMC, 1:1:1:1, v:v:v:v) obtained from Dong Guan Shanshan Battery Materials Co. Ltd. 2.2. Preparation of the Composite Membranes A non-solvent-induced phase separation (NIPS) wet-process method (Figure 1a) was used to fabricate the composite membranes.41 The different precursor PVDF/CEC/CNF mixed solutions were designed with suitable compositions (Table S1). The total solid content of the different precursor solutions was 15 wt %, and CNF were added in the form of a homogeneous suspension of DMF. First, PVDF, CEC and CNF were mixed and stirred in a suspension of DMF for 180 min in a 40 °C bath at 500 rpm, and the homogeneous mixtures were cooled to room temperature and centrifuged at 10 000 rpm to completely remove the internal bubbles. Then, the suspensions were evenly cast on a glass plate surface at room temperature using a doctor blade with a 2.5 cm·s-1 velocity and 150 mm thickness. The plates were immediately immersed in a coagulation bath of deionized water at room temperature for 24 h. Finally, the resultant separators with a thickness of


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approximately 50 μm were dried at 60 °C for 24 h to remove any residual solvent. All prefabricated membranes were stored in a glove box filled with argon gas (Lab 2000, O2 and H2O < 0.1 ppm) prior to use. Meanwhile, a commercialized PP separator was examined for comparison. In the following, the commercialized PP separator, pristine PVDF membrane, and the PVDF separators composited with CNF-1%, CNF-1% and CEC-2%, CNF-1% and CEC-4%, CNF-1% and CEC-6% are referred as PP, PVDF, PVDF-CNF, PVDF-2, PVDF-4, and PVDF-6, respectively. 2.3. Characterizations The morphology of the different membranes was measured using a scanning electron microscope (SEM, Hitachi S-4800) together with the EDS mapping. The contact angles of a 2 μL water droplet on the samples were determined using a dynamic contact angle analyzer (KRUSS DSA100). The pore distribution was evaluated through Brunauer-Emmett-Teller (BET, Quantachrome NOVA-4000e) nitrogen sorption-desorption measurements. The crystalline structure of the different membranes was verified using a wide-angle X-ray diffractometer (XRD, D8 ADVANCE). The thermal shrinkage behavior of the separators (diameter of 1.8 cm) was determined by storing them in an oven at every 20 °C from 120 °C to 240 °C for 1.0 h.36 The porosity of the membranes was analyzed by immersing them in n-butanol for 12 h.43 The thickness of the membranes was measured using a thickness gauge (CHY.C2). The mechanical properties of the separators were characterized using a universal testing instrument (Instron 3369) at room temperature. The electrolyte uptake measurements for the separators were investigated by soaking the separators in an electrolyte solution of 1 M LiPF6/EC/DEC/EMC (1:1:1:1, v:v:v:v) in a glove box.44 The bulk impedance and ionic conductivity of a prepared separator were recorded using electrochemical impedance spectroscopy (EIS) (CHI660E electrochemical workstation) at different temperatures from 0 °C to 80 °C. Lithium-ion transference numbers were determined via


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AC impedance measurements in combination with a DC polarization step according to the method suggested by Bruce et al.45 The dielectric properties of the blend separators were investigated using an impedance analyzer (Agilent 4294A).39 The interfacial stabilities of the electrolyte-soaked membranes and the lithium metal electrode were characterized using the interfacial impedance.46 The chemical compositions of the membrane surface were examined using an XPS analyzer (Amicus Budget) and FTIR (Spectrum Two). The melting points of the membranes were determined by DSC (NETZSCH STA 449F3). The open-circuit voltages (OCV) at different temperatures were measured using an electrochemical workstation with a thermometer (YC727UD) by attaching a thermocouple to the battery surface.47 Nail penetration tests were conducted by penetrating a 2.5 mm steel nail through a cell with a thickness of 4.2 mm, a width of 61 mm and a length of 88 mm (Model-426188), and the temperature and voltage of the cells were recorded using data recorders (HIOKI 8430-21).48 (All details are shown in the Supporting Information.) 2.4. Battery Assembly and Electrochemical Measurements The button-type half-cells (LIR2025) were assembled by sandwiching the blend membranes between the LiCoO2 anodes and Li metal cathodes after immersing them in a liquid electrolyte. All the steps are executed in a glove box filled with argon gas. The Shenzhen Neware battery testing equipment (CT-3008W) was used to evaluate the cycling stability and discharge rate performance of the cells. The cells were cycled at a constant charge/discharge current density of 0.2 C/0.2 C to determine the cycling performance. The discharge rates were varied from 0.2 to 8.0 C at a charge current of 0.2 C and at a voltage range of 2.8 ~ 4.2 V to measure the discharge rate performance.49 3 RESULTS AND DISCUSSION


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The morphology and pore size distribution of the pristine PVDF, PVDF/CNF, and PVDF/CEC/CNF separators were first investigated via SEM and BET (Figure 2a, see cross-section images in Figure S3, Supporting Information). The SEM observation confirmed that the surfaces of the separators were highly homogeneous without obvious defects, and the hollow, 3D structure with linked micropores (see the model of the cross-section in Figure 1b) was eventually formed through the NIPS process. Furthermore, all the membranes possessed an approximate thickness of 50 μm and an average pore size of < 0.5 μm, which increased slightly with the increase in the CEC content. The PVDF-CNF membrane showed a smoother surface because the gaps in the PVDF crystal area were effectively connected by the nanoscale fibers, and a distinct surface layer assembled. However, for the CEC-composited membranes, a significant larger pore size and pore volume were observed in SEM and BET. Meanwhile, the cortex of the membranes was gradually penetrated. This can be explained by the CEC blending increasing the viscosity of the scraping solution in the NIPS process (Figure 1a), which leads to a reduced speed for the phase separation process and expansion of the pore structure.41 In addition, the relatively slow phase separation process can reduce the formation of crystal nuclei and cause the crystals to grow more completely, which leads to the construction of larger pores.16 To compare the dielectric properties of different separators, we characterized the dielectric constants (Figure 3a), dielectric loss tangent (Figure 3b) and AC conductivity (Figure S4, Supporting Information) of the separators. The frequency dependence of the dielectric permittivity at room temperature showed that the pristine PVDF membrane had a dielectric constant of 3.50 at 100 Hz, which was much lower than that of conventional PVDF membranes owing to the microporous structure.15 The composition of CEC significantly enhanced the dielectric constants of the PVDF/CEC/CNF composite membranes, and this may be partly attributed to the fact that


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the CEC in the composite separators can be considered a microcapacitor network.39 In contrast, the dielectric loss tangent slightly declined with the increase in the CEC content, which can be explained by the lower dielectric loss tangent of CEC compared to PVDF. Meanwhile, the formation of conductive paths and leakage current by CNF could connect the structural elements of PVDF. The relatively low dielectric losses of membranes should reduce dielectric heating, prevent thermal breakdown and increase the safety of LIBs.17-19 In addition, the AC conductivity of the composite membranes increased with both the content of the CEC and the frequency, and this increase can be ascribed to the decreased distance between the carriers at a higher CEC mass fraction and charge flowing via hopping.15 The wettability of the separators was characterized by the contact angle measurements, and the results confirmed the beneficial effect of the cellulosic materials (see Table 1 and Figure S5). The water contact angle on the pristine PVDF separator was 143°, and it decreased to 99° on the CNF-blended PVDF separator and 45° on the PVDF-6 separator. The obvious decrease in the contact angle can be ascribed to the introduction of the hydrophilic surface of cellulosic materials.36 The surface was expected to improve capacity retention in hydrophilic electrolyte solutions and heighten the ionic conductivity.43 To further discuss the effect of the membrane structure on ion conductivity, the porosity of the different membranes was measured in detail and correlated with the electrolyte intake (see Table 1, Figure S6 and Figure S7). With the rise in the porosity, the electrolyte intake and ion conductivity of the separators gradually increased. The reasons for the increase can be explained by the following, first, the electrolyte is usually present as a liquid in the pore structure and a gel in the amorphous region, and the migration rate of the charge carriers is much faster in the liquid than in the gel phase. Thus, improving the porosity may effectively facilitate the conductivity of


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membranes.50 Second, the –CN group introduced by CEC can increase the conductivity owing to the number of lone pair electrons provided by the electronegative atom.16 The suppressed penetration resulting from the PVDF-CNF surface layer may limit the migration of the carrier, which cannot be ignored, but this effect may be masked by the superior electrolyte wettability of the layer.51 The crystalline region in the porous membrane can provide the necessary mechanical strength and thermal stability, while the amorphous region can effectively absorb the electrolyte and form the gel phase. Therefore, adjusting the crystallinity of the membrane to achieve synergistic improvement in the mechanical properties and ability to absorb the electrolyte is critical.16 The XRD patterns of the pristine PVDF, CEC and blending separators with different compositions are shown in Figure 3c, and the crystallinity of the PVDF in different separators is displayed in Table 1. For the pristine CEC, three diffraction peaks can be found at approximately 10.4°, 18.0° and 21.0°, and these peaks can be ascribed to the regularity of the polymer chains.15 The pristine PVDF membrane has two diffraction peaks at 2 θ=18 .3° and 19.9°, which correspond to the (100) and (110) crystal planes of the α-phase, respectively. Meanwhile, the γ-phase is present at the superposition point of the peaks located at 18.5° and 20.0°, which are associated with the (020) and (110) crystal planes, respectively. The peak at 2 θ=20.4°, corresponding to the (110) and (200) reflections of the β-phase, is more distinct in the blend separators.52 This result clearly indicated that the higher CEC contents are beneficial for the formation of the β-phase. In the same peak, the full-width at half max of the blending separators declined with the increase in the cellulosic material content, which indicated that the CNF and CEC compositions may effectively inhibit the growth of the PVDF crystalline grains. These results can be explained by (i) the addition of nanofibers, which can effectively destroy the ordered crystal structure of the PVDF and CEC, and


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(ii) the interdiffusion between the molecular chains of CEC and PVDF, which may hinder their crystallization.15 The characteristic peaks of crystalline CEC almost completely disappeared in the composite film, which also confirmed this result. In the process of the electrolyte impregnating the separators, the electrolyte first fills the pores and then infiltrates into the amorphous zone. The crystalline region is usually not swollen by the electrolyte, thus, in these separators, the decrease in the crystallinity may facilitate an improvement of the ionic conductivity.52-53 In the following discussion, we examined which parameter (porosity or crystallinity) had a larger influence on ion conductivity to provide insight into how to improve the separator design. The Arrhenius model (see eq S6, Supporting Information) was used to analyze the experimental data and illuminate the kinetic mechanism of the ion conductivity process. The variation in ionic conductivity (σ) with temperature (T) is depicted in Figure 3d. Logσ and 1/T exhibited a good linear relationship (Table S2, Supporting Information) in the range from 0 to 80 °C, which indicated that the Arrhenius equation was suitable for describing the ion conductivity behavior in the system. Hence, the migration of charge carriers mainly occurred in the electrolyte-soaked pores instead of in the amorphous area. Accordingly, the activation energy values in all the samples were between 16.03 and 20.76 kJ mol-1 (Table 1, see Supporting Information for details), which confirmed that the difference in the porosity and the average pore size did not significantly affect the apparent activation energy of the ion transport. The activation energies of the all-solid polymer electrolyte and the electrolyte were approximately 86 and 4 kJ mol-1, respectively.45 The activation energy in these systems was between those energies and closer to the latter energy, which further indicated that the carrier transfer mainly occurred in the pores. Indeed, the lithium-ion transference number (Table 1 and Figure S8, see Supporting Information for details) was 0.32 for the pristine PVDF separator and was ameliorated with the increase in the cellulosic material content, which


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indicated that the surface of all of the separators can interact with the solvent molecules and ions in the electrolyte.54 A LIB separator should be selected by mechanically robust and thermally stable so as to bear the high tension after frequent collisions and avoid short-circuits caused by the debris, irregular electrode surface and growth of lithium dendrites.53, 55-57 We tested the stress-strain curves and thermal shrinkage of different composited separators (see Figure 3e-f) to evaluate their properties. As shown in the curves, the tensile strength of PVDF, PVDF-CNF, PVDF-2, PVDF-4 and PVDF6 reached 4.76, 7.74, 11.44, 12.66 and 14.30 MPa, respectively, which demonstrated that certain amounts of CNF have positive effects on the strength of the PVDF-based separators owing to the large aspect ratio and high elastic modulus of CNF.58 In addition, the flexible long chains of CEC and the abundant polar groups of —CN on the surface can highly orient molecular chains, which is also conducive to enhancing the intermolecular force.15 The thermal shrinkage of the separators was characterized by measuring their dimensional changes when they suffered thermal treatment at every 20 °C from 120 °C to 240 °C for 1.0 h (Figure 3f, see photographic images in Figure S9, Supporting Information). The difference in heat shrinkage rate between the PVDF-based separators and the PP separator was more distinct as the temperature increased, and the shrinkage of the pristine PP and PVDF separators was over 95% at 240 °C because their melting temperatures are approximately 200 °C and 170 °C, respectively.20, 59 In comparison, the dimensional changes in the composite separators were not obvious over a wide range of temperatures, and the shrinkage of the PVDF-CNF, CEC-2, CEC-4 and CEC-6 membranes was 56%, 50%, 44%, and 40%, respectively. The results can be explained by the negligible thermal shrinkage of the native cellulosic materials.5,

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Remarkably, long-term mechanical and dimensional stability can

effectively avoid internal short-circuits at elevated temperatures and high charge/discharge rates.36


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The ability to shutdown cells in the case of a short-circuit or overheating is another desirable safety characteristic of separators.47, 61-62 The DSC analysis (Figure 4a) indicated that the melting peaks of the pristine PVDF separator and the PVDF-6 separator were at approximately 170 °C, whereas CEC and CNF exhibited superior thermal stability up to 280 °C. These results suggested that no significant changes occurred in the crystal form of the PVDF during the membrane formation and that PVDF-6 possessed great potential for shutdown functionality near 170 °C. The AC impedance spectra (Figure 4b) showed shutdown occurred at approximately 170 °C owing to the melting of PVDF, which agreed with the DSC curves. The SEM image (the insert of Figure 4b) after heat treatment at 170 °C and 180 °C for 1 h showed that the PVDF completely melted and blocked the pores of the PVDF-6 membrane. The open-circuit voltages (OCV) of the batteries in the range of 0 to 200 °C (Figure 4b) was also considered to further study the cell performance with PVDF-6 separators. The OCV was essentially unchanged as the temperature increased to 180 °C, which indicated that the melting of the PVDF cannot directly cause an internal shortcircuit. Nevertheless, an obvious fluctuation was observed between 180 °C and 195 °C, and the value swiftly fell to zero at approximately 200 °C. A possible explanation for the results could be that the high temperature caused a chemical reaction between the active electrode materials and the liquid electrolyte or pinholes grew in the composite separator.47 These results supported the shutdown function of PVDF-6, and in these separators, the PVDF melted close to the pores upon reaching the thermal runaway temperature. The cellulosic material provided a high mechanical strength from the negligible thermal-shrinkage behavior.36 Electrochemical impedance spectra (EIS) were used to investigate the electrochemical performance of the as-prepared composite separators in the following.63 Figure 5a depicts the Nyquist plots obtained from the liquid electrolyte-soaked separators with a lithium electrode. As


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shown in the figure, the initial interfacial resistance values of the CNF-CEC-blended separators were conspicuously less than that of the pristine PVDF separator. This difference might be attributed to the intensified electrolyte wettability and the expanded microporous structure.40, 53, 6465

With the increase of the CEC content in the separators, the interfacial stability between the

membrane and the metal lithium electrode improved, and the resistance of the Li-ion batteries using the PVDF-6 separator was lower than that of the batteries with other separators. This is partially attributed to the fact that the PVDF-6 separator can hold many electrolytes and facilitate Li-ion migration.5, 21, 36, 55 The rate performances were utilized to prove the excellent power performance of the CNFCEC-blended separators, as shown in Figure 5b.66 The discharging capacities of the cells with the CNF-CEC-blended separators were much higher than the cells with the commercial PP separator and pristine PVDF separator at various rates, which demonstrated that these cells exhibited a better rate capability. For example, the PVDF-6 separator maintained a specific capacity of 119.0 mAh g-1 at 0.5 C, whereas the capacity of commercial polypropylene separator was 110.0 mAh g-1. The specific capacities of the cells using the PVDF-6 separator and PP separator at 8.0 C were 92.5 and 60.2 mAh g-1, respectively. Interestingly, the reversible capacities of the cells with the PVDF6 and PP separator were 123.4 and 116.0 mAh g-1 when the rate reverted to 0.2 C at last, and these values were very close to the initial capacity. The improved rate capability might be attributed to the enhanced ionic conductivity and decreased interfacial resistance of the composite separators.45 To further understand the electrochemical capabilities, the cycling capabilities of the various Li0|separator|LiCoO2 half-cells were tested to determine the practical application properties of the cellulosic material-composited membranes.67 Figure 5c shows that the discharge capacity increased from 123.0 to 124.5 mAh g-1 when the cell was assembled with the PVDF-6 separator


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instead of the PP separator. All of the cells showed a Coulombic efficiency of ca. 100%, and the cells with the PVDF-CNF-CEC separators had slightly higher values than those with the commercial PP and pristine PVDF separators. This difference may be partly attributed to the fact that the ternary-blended separators has a higher liquid electrolyte retention ability than the original PP and PVDF separators,36 which results in a higher ionic conductivity. Capacity fading occurred in all of the batteries in 50 cycles because of the structural instability of LiCoO2 and side reactions that occur during the charge/discharge;21 however, the fading of the cells assembled with the PVDF-6 separator was much slower than the fading in cells assembled with the PP and PVDF separators. The possible explanation for this could be that the PVDF-6 separator can reduce the irreversible loss of active components.51 In addition, the cell configured with the PVDF-6 separator still had a specific capacity of 123.2 mAh g-1 with a decay rate of only 0.01% per cycle after 50 cycles, but in the battery using the PP separator, the capacity declined to 110.0 mAh g-1 with a decay rate of 0.23% per cycle. The potential explanation for this result is that fewer lithium dendrites formed in the cell with the PVDF-6 separator, and based on this, a higher concentration of Li-ions can be obtained in the cells, resulting in the reduced electrode polarization, which is related to anion accumulation and the constrained anion concentration gradient to promote Li-ion transport through the interfacial layer.66 More evidence was found via the morphological analysis of the cycled lithium electrodes.23, 65-66 Figure 6 depicts the SEM images of the lithium metal anode disassembled from different LIBs after 50 cycles. According to the images, the surfaces of the lithium-metal electrodes disassembled from the cell with the PVDF-6 separator were smooth without substantial dendrites; in contrast, the electrodes of the pristine PVDF and PP separators showed the formation of dendrites with snowflake-shaped morphologies (see local enlarged image in Figure 6b). This morphological difference indicated that the dendrite formation during cycling


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could be effectively suppressed by the composition of the PVDF separator, which was partially attributed to the enhanced porosity and wettability of the PVDF-6 membrane.46, 65-66 Therefore, the better cycle performance of the battery with the PVDF-6 separator can account for most of the restricted dendrite growth. The structural stability of the separator during the cycle is also an important factor for maintaining cycle performance.23, 54, 59 To evaluate the structural stability of the separator, XPS and FTIR tests of the PVDF-6 separator were carried out before and after 50 cycles. As seen from Figure 7a, the PVDF-6 separator before and after cycling showed similar peaks at 289.8 eV, 401.3 eV, 531.6 eV, and 685.1 eV, which can be assigned to C 1s, N 1s, O 1s, and F 1s, respectively,23, 63, 68

and demonstrates that the separator exhibited good electrochemical inertness. After cycling,

a new peak at 55.1 eV appeared in the potential region of P 2p, which is residual electrolyte.69 In addition, no obvious peak at 55.5 eV for Li 1s was observed, which supported that the surface of the separator had negligible residual lithium dendrites. In the FTIR spectra (Figure 7b), the peaks at 1182 cm-1 and 1072 cm-1 can be attributed to —CF2 and C—F in the PVDF matrix,70 and the tiny peaks at 893 cm-1 and 2260 cm-1 can be assigned to the β-glyosidic linkages of the glucose ring and nitrile groups (C≡N) in cellulosic materials,15 respectively. The two FTIR spectra were almost identical, which further indicated that the PVDF-6 separator maintained its structure after 50 cycles. To investigate the changes in the morphology of the porous separator, the surface and cross section SEM images of the PVDF-6 separator after cycling were measured (Figure 7c-d). The surface and cross-section of the PVDF-6 separator were homogeneous without obvious lithium dendrites or defects, similar to Figure 1e and Figure S3e, which implied that this cycling process did not block the original pores of the PVDF-6 separator.23 The pore size distribution of the separator from the BET tests further confirmed this result; the separator maintained an average


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pore size of ca. 0.5 μm. From the elemental mapping in Figure 7e-f, the nitrogen and fluoride were homogeneously distributed on the separator after 50 cycles. It indicated that the PVDF-6 separator was sufficient to long-term usage. This can be explained by the inertness of the components in PVDF-6 separator.15, 39-40, 71 To determine the safety performance of the CNF-CEC-blended PVDF separator, we utilized a nail penetration test on the pre-assembled cells.48, 68 The results are presented in Figure 8a-b. As shown in the figure, the LIBs with a PVDF separator emitted a remarkable amount of smoke, burned fiercely, and expanded. In contrast, the cells with the PVDF-6 separator kept their initial shapes without significant expansion or rupture. The two types of LIBs exhibited vastly different thermal and voltage responses. The temperatures of the LIBs with the PVDF separator sharply increased to 380 °C, and the voltages suddenly declined to 0 V (Figure 8a), which can be ascribed to an enormous amounts of heat caused by short-circuiting.68 In contrast, the voltage of the LIBs with the PVDF-6 separator instantaneously fell to 4.0 V and was steady for a long time; the temperature rose from 25 to 109 °C and then dropped slowly (Figure 8b). Thus, CEC and CNF play a crucial role in maintaining the original dimensions of the PVDF-6 separator and avoid immediate contact between the cathode and the anode. The viscous CEC and CNF have good expansion properties, which filled the space around the pinhole.48 These results confirm that the PVDF-6 separator greatly reinforces the safety of cells and maintains the excellent cycling performance. Overall, the electrochemical performances and safety of the PVDF/CNF/CEC separators were impressive and superior to that of other separators. The reason for the improvement can be explained as follows, (i) the presence of a large number of sub-micron, interior holes causes the membrane to possess a high porosity and store a large amount of the electrolyte, improving the


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ionic conductivity;40, 53-56, 61, 65 (ii) the blending of CNF and CEC materials with a large number of polar groups on their surfaces facilitates the improvement in the compatibility between the membrane and the electrolyte and further expands the storage capacity of the electrolyte;37, 48, 50, 52, 72

(iii) the addition of CEC can reduce dielectric losses, thus, the dielectric heating was

suppressed;15, 17-19, 39 (iv) CEC and CNF may destroy the regularity of the PVDF molecular chains and increase the proportion of the amorphous regions;15 and (v) flexible macromolecular chains of cellulosic materials could create favorable conditions for ion migration, enhance the mechanical properties and improve the thermal stability.40, 53, 65 4 CONCLUSION Microporous polymer separators based on a PVDF/CNF/CEC blend with various compositions were prepared using the NIPS wet-process. The obtained composite membranes had reduced crystallinity, excellent porosity, enhanced dielectric properties and remarkable electrolyte wettability. The thermal stabilities and mechanical properties increased with the increase in the CEC content, and under the optimal formula condition, the tensile strength was 14.30 MPa and the ionic conductivity was 1.26 mS cm-1. CEC and CNF play a crucial role in increasing the Li+ content in LIBs and improving the safety of LIBs. The electrochemical characterization further confirmed that the lithium-ion battery assembled with the PVDF/CNF/CEC composited separators exhibited preferable comprehensive performances, such as the cycle ability and rate capability. Therefore, this novel and eco-friendly separator can be considered a high-performance separator and is a good candidate for high-performance LIBs.


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Figure 1. Schematic illustration for the experimental process (a) and structures of the PVDF separator before and after blending (b). The molecular chains of CEC were interspersed between flexible molecular chains of PVDF, resulting in a decrease in crystallinity, and CNF were uniformly filled in the composite system, which played a role in strengthening and toughening.

Figure 2. SEM image of pristine PVDF (a), PVDF-CNF (b), PVDF-CNF-2% CEC (c), PVDFCNF-4% CEC (d) and PVDF-CNF-6% CEC (e) membranes and their pore size distribution (f).


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Figure 3. Characterization of different membranes. Dependence of the dielectric permittivity (a), dielectric loss (b), and XRD patterns (c); temperature dependence of the ionic conductivity of the separator/electrolyte system (d), stretching curves (e), thermal shrinkage properties (f).

Figure 4. Results for the shutdown properties of the PVDF-CNF-6% CEC separator. DSC curve of the PVDF and PVDF-CNF-6% CEC (a) separators, and the AC impedance spectra and opencircuit voltages (OCV) of the batteries (b). The insert shows the surface and cross-section SEM images of the separator after thermal treatment at 170 °C and 180 °C.

Figure 5. Electrochemical performance of sandwich-like devices assembled with different membranes. The Nyquist plots of the separators in symmetric lithium coin cells (a), rate performance of the battery at a rate of 0.2, 0.5, 1, 2, 4, 6, 8 and 0.2 C in the voltage range of 2.84.2 V (b), cycle performance at 0.2 C (c).


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Figure 6. SEM images of lithium-metal electrodes disassembled from cells after the 50th cycle using PP (a), a local enlarged image (b), PVDF (c), and PVDF-CNF-6% CEC (d) separators.

Figure 7. Structural stability of the separator during the cycle. High-resolution XPS spectra of the PVDF-CNF-6% CEC separators (a), FTIR spectra (b) of the PVDF-CNF-6% CEC separator before and after the 50th cycle. Surface and cross-section SEM of the PVDF-CNF-6% CEC separator after the 50th cycle (c, d). The insert shows its pore size distribution. Elemental mapping of the PVDF-CNF-6% CEC separator after 50 cycles (e, f).


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Figure 8. Voltage-temperature profiles of LIBs with PVDF (a) and PVDF-CNF-6% CEC (b) separators. The insert shows digital images during the nail penetration tests.

Table 1. The performance of lithium-ion battery separators Sample

PP

a

b

c

d

e

Crystallinity degree (%)

-

49

47

40

35

33

Thickness (μm)

25

50

49

50

50

49

Water contact angle (°)

-

143

99

55

50

45

Porosity (%)

40

55.2

56.2

58.2

59.4

60.2

Electrolyte uptake (%)

110

280

300

330

360

370

Bulk resistance (Ω)

4.4

6.8

4.2

3.2

2.5

2.0

Conductivity (mS cm-1)

0.29 0.37

0.6

0.79

1.01

1.26

Activation energy of the ion transport (kJ mol-1) -

20.64 17.30 20.18 20.76 16.03

Lithium-ion transference number

0.36

-

0.37

0.36

0.40

0.42

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. The composition table of different precursor solutions; details of the experimental; the FTIR spectrum of CEC and CNF; the TEM image of CNF; cross-section SEM images of samples; AC conductivity curves of different separators; calculation process of Lithium-ion transference numbers; linear fitting


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process of ionic conductivity under different temperature; hydrophilic, electrolyte uptake and conductivity of the samples; separator images before and after heat-treatment. (PDF) AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This work was supported by the Municipal Science and Technology Projects of Beijing (grant numbers Z141103004414111). Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by the Municipal Science and Technology Projects of Beijing (grant numbers Z141103004414111) and Beijing Key Laboratory for Chemical Power Source and Green Catalysis. We thank the Instrumental Analysis Center of Beijing Institute of Technology and ACS Chemwox.

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