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Zeolitic imidazolate framework-cellulose nanofiber hybrid membrane as Li-Ion battery separator: Basic membrane property and battery performance
Journal of Power Sources 454 (2020) 227878
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Zeolitic imidazolate framework-cellulose nanofiber hybrid membrane as Li-Ion battery separator: Basic membrane property and battery performance Xiuxuan Sun a, Meichun Li a, Suxia Ren b, Tingzhou Lei b, Sang-Young Lee c, Sunyoung Lee d, Qinglin Wu a, * a
School of Renewable Natural Resources, Louisiana State University AgCenter, Baton Rouge, LA, 70803, United States Key Biomass Energy Laboratory of Henan Province, Zhengzhou, 45008, China c Department of Energy Engineering, School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan, 689-798, South Korea d Department of Forest Products, National Institute of Forest Science, Seoul, 02455, Republic of Korea b
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� ZIF8-CNF membrane synthesized and used as lithium-ion battery (LIB) separator. � More homogeneously distributed pores in the manufactured membrane. � Better thermal, mechanical and surface wetting property of the membrane. � LIB’s discharge retention stability, cycling performance and rate capacity improved. � ZIF8-CNF nanocomposite membrane as a future sustainable LIB separator material.
A R T I C L E I N F O
A B S T R A C T
Keywords: Cellulose nanofibers Zeolitic imidazolate framework Li-ion battery separator Membrane property Battery performance
Zeolitic imidazolate framework-8 (ZIF8) concept was introduced in the fabrication of li-ion battery (LIB) sepa rator for the first time. The ZIF8 crystals were synthesized on the surface of cellulose nanofibers (CNFs) by mixing 2-methylimidazole (Hmim) with zinc nitrate hexahydrate (Zn) in methanol. ZIF8- or Zn (mim)2-CNF composite membrane was fabricated and the effect of ZIF8 on the pore structure of CNF network was investigated. The pores of the ZIF8-CNF membrane distributed more homogeneously than that of the pure CNF membrane. The porosity increased from 42% (pure CNF membrane) to 55% (composite membrane). The successful synthesis of ZIF8 crystals on CNF surface was confirmed through elemental composition and crystalline structure analysis of the composite. Compared with polymer based separator, the ZIF8-2-CNF membrane exhibited better thermal stability, mechanical property (isotropic vs. anisotropic), thermal expansion behavior (15.53 vs. 178.90 ppm/k), and surface wettability (13.31� vs. 96.18� ). The LIB fabricated with ZIF8-CNF membrane had a comparable cycling stability and a better discharge retention stability (88.3% vs 80.2%) in comparison with these of a trilayer commercial polymer membrane. The environmentally friendly ZIF8-CNF nanocomposite membrane can be considered as a good alternative for manufacturing LIBs.
* Corresponding author. E-mail address: [email protected] (Q. Wu). https://doi.org/10.1016/j.jpowsour.2020.227878 Received 1 March 2019; Received in revised form 27 January 2020; Accepted 10 February 2020 Available online 22 February 2020 0378-7753/© 2020 Elsevier B.V. All rights reserved.
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1. Introduction
of ZIF8-CNF composite membranes in LIBs.
Transition from fossil energy to sustainable energy and wide-spread application of portable electronics, and electric vehicles promote a rapid growth of Li-ion battery (LIB) industry [1]. Currently, considerable effort is being made to improve electrode materials [2,3], separators [4, 5], and electrolyte systems [6,7] for the LIB to meet the increasing market demands of LIBs with large energy density and improved rate capabilities. LIB separators play a critical role in keeping anode and cathode isolation and good ion transmission rate. Currently, polymer separators such as polypropylene (PP), Polyethylene (PE), or the hybrids are extensively used in the LIB industry. The limitations of the polymer separators include poor surface wettability, high thermal expansion, and low thermal stability [8]. In order to solve these problems, alternative materials and coated polymers are considered. Cellulose is the most abundant renewable bio-polymer in the world [9]. Cellulose nanofibers (CNFs) are prepared from cellulose using me chanical defibrillation, acid hydrolysis, and/or TEMPO treatments [10, 11]. CNF membranes exhibit better thermal stability, good electrolyte wettability and excellent thermal expansion behavior in comparison with traditional polymer membrane [12]. Different types of cellulose-based separators have been fabricated, including microfibrillar cellulose/polysulfonamide separator through a paper making method [13], and polyborate coated cellulose nonwoven separator through the electrospinning combined with dip-coating method [14,15]. For example, the discharge capacity of LIB assembled with cellulose/poly vinylidene fluoride-hexafluoropropylene (PVDF-HFP) composite sepa rator was 115 mA h g 1 at 1.0 C, which is larger than 106 mA h g 1 from LIB with traditional PP separator [16]. The LIBs, using LiCoO as active material of cathode, graphite as anode, and cellulose/polysulfonamide composite membrane as separator, exhibited a capacity retention ratio of 85% after one hundred cycles and worked well with a normal cycling profile at high temperature [13]. Cellulose/polydopamine separator exhibited excellent mechanical strength and ion conductivity (0:95 � 10 3 S cm 1 ), which is favorable for the long-term reliable performance of LIBs [17]. However, major drawbacks about CNF-based membranes are heterogeneous distribution of pores. Large pores (>1 μm) often exist in some areas of the separator, while other areas of the separator are too densely packed due to strong hydrogen bonding among cellulose fibrils [10]. In order to prevent dendritic lithium penetration through sepa rator pores, some general requirements of pore size are less than 1 μm [4]. Chun et al. used different solvent mixture systems in the CNF membrane making process to improve the pore structure of CNF sepa rators, leading to a satisfactory ion conductivity (0.75 mS cm 1 ) [18]. Zeolitic imidazolate framework-8 (ZIF8) is a subclass of metalorganic framework (MOF), which was originally discovered and used in the hydrogen storage media [19]. The 2-methylimidazole (Hmim) zinc complex, designated as Zn (mim)2 or ZIF8 [20], exhibits excellent thermal and chemical stability. The material is widely used in gas sep aration [21], selective sensor [22], drug delivery [23], and catalytic applications [24]. The pore size of ZIF8-based composite (e.g., ZIF8-CNF complex) can be controlled through different solvent systems at different temperatures [25]. The tunable pore size of the ZIF8 complex provides a possible strategy to facilitate the Li-ion flow through the separator and improve the interior battery resistance and electrolyte filling time [4]. The objective of this study was thus to synthesis and test ZIF8-CNF composite membranes for use as LIB separators in comparison with a commercial tri-layer polymer separator. The synthesis process of the ZIF8-CNF hybrid and effects of ZIF8 on the CNF network pore structure were illustrated. Thermal stability, mechanical properties, thermal expansion, and surface wettability of the membranes were determined. The 2032 type LIB coin cells were fabricated and analyzed for estab lishing the LIB performance with the manufactured separators. This study provides the first comprehensive analysis on the synthesis and use
2. Experimental section 2.1. Preparation of CNFs CNFs were prepared with an established protocol. In brief, bleached wood pulp (i.e., W-50) was added to 48 wt% of sulfuric acid at 45 � C with a string rate of 360 rpm. The pulp to sulfuric acid solution weight ratio was controlled at 1 to10. After 1-h hydrolysis, a large amount of deionized water was poured into the reaction system to terminate the hydrolysis process. The excessive sulfuric acid was removed by centri fuging at 9000 rpm for 30 min. The centrifuge process was repeated three times. Then the precipitant of the reaction suspension was redispersed in deionized water and transferred into dialysis tube with a molecular weight cut-off between 12,000 and 14,000. The dialysis process took about three to five days. Homogenized CNF suspension was obtained by passing the material through a micro-fluidizer processor (M110EH-30 Microfluidics Corp., Westwood, MA, USA) three times at an operation pressure of 202 MPa. The CNF aqueous suspension was freezedried by a Labconco freeze drier at 45 � C under vacuum. 2.2. Preparation of ZIF8- CNF complex The freeze-dried CNFs were re-dispersed in methanol with a weight (g) to volume (mL) ratio of 1: 100. Polyvinylpyrrolidone (PVP, 600 mg) and zinc nitrate hexahydrate (2.8 g) were added with stirring at 500 rpm for 1/2 h and 100 mL of 2-Methyl-1H-imidazole (0.4 mol/L) methanol solution were added (another 15-min stirring). The homogenized reac tion mixture was kept for 12 h at room temperature. The reaction mixture was centrifuged at 10,000 rpm for 15 min and rinsed three times with ethanol to remove excess PVP. The precipitant was re-dispersed in deionized water and homogenized using an ultrasonic homogenizer (MSK-USP-12 N, MTI Corp., Richmond, CA, USA) and finally ZIF8-1CNF material was obtained. Different weight ratios of ZIF8 to CNF hy brids were obtained by adding different amount of reagents propor tionally (e.g., ZIF8-0.5-CNF and ZIF8-2-CNF with half and twice amount of Zn(NO3)2 and 2-Methylimidazole, respectively, compared with these of ZIF8-1-CNF using the same procedure as described above). 2.3. Preparation of ZIF8-CNF composite separator and LIB The ZIF8-CNF nanocomposite membrane was prepared by the vac uum filtration method. The CNF or ZIF8-CNF suspensions were soni cated for about 30 min by an ultrasonic homogenizer (MSK-USP-12 N, MTI Corp., Richmond, USA). The homogenized CNF or ZIF8-CNF sus pensions were filtered through a hydrophilic PVDF membrane (0.65 μm pore size) using a Buchner funnel under vacuum. The obtained wet sheet was detached from the PVDF membrane and inserted between several Whatman filter papers. The samples were vacuum-dried at 95 � C for about 20 min in a vacuum oven (MTI Corporation, Richmond, Califor nia, USA). The LiFePO4 cathode slurry, which was made of 70 wt% LiFePO4, 20 wt% carbon black and 10 wt% PVDF, was coated on the Al foil using a MSK-AFA-III Automatic Thick Film Coater (MTI Corporation, Richmond, California, USA) and dried in vacuum at 120 � C for about 24 h. The 2032 type coin cells were fabricated using LiFePO4 as cathode, lithium chip (Li) as anode, 1.0 M LiPF6 in EC/DMC ¼ 50/50 (v/v) as electrolyte, and two separators including the ZIF8-CNF membrane and a commercial polyproplyene (PP)-polyethlene (PE)-polyproplyene (PP) or PEP separator film (from MTI Corporation, Richmond, CA, USA). A Compact Digital Pressure Controlled Electric Crimping/De-Crimping Machine (MTI Corporation, Richmond, CA, USA) was used to assemble the coin cells with an assembling pressure of 1T in a glove box.
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2.4. Characterization methods
analyzer (TMA) (New Castle, DE, USA) using a film/fiber probe under tensile mode with a temperature range of 20–120 � C at a heating rate of 3 � C/min. The XRD spectra of these samples were measured by the Bruker/Siemens D5000 X-ray diffractometer (Siemens Co., Wittelsba cherplatz, Munich, Germany) under Cu Kα radiation (Voltage: 40 kV, Current: 30 mA, 2θ range: 5-40� ). The Liþ transference number (TN) was calculated based on this formula: tLiþ ¼ [Iss*(ΔV I0R0)]/[I0* (ΔV IssRss)] with the subscript “0” denoting initial, and the subscript “ss” denoting steady state [26]. An 8-channel battery analyzer (MTI Corpo ration, Richmond, CA, USA) was used to test the performance of LIBs made with ZIF8-CNF, pure CNF, and commercial PEP separators. The interfacial resistivity of LIBs with different separators was determined by using electrochemical impedance spectra (EIS) measurement with a CHI6054C chemical work station (CH Instruments, Inc., Austin, TX, USA). The electrochemical stability windows of these separators was determined using the same machine at 0.1 mV s 1 at room temperature.
The morphology of CNFs and ZIF8-CNFs were observed through a JEOL 100CX transmission electron microscope (TEM, JEOL, Inc., Pea body, MA, USA). The morphology of the CNFs, PEP and ZIF8-CNF membranes were studied by a field-emission scanning electron micro scopy (FE-SEM, FEI Company, Hillsboro, OR, USA). The element anal ysis of the ZIF8-CNF separator was conducted using the Energy dispersive X-ray spectroscopy (EDX). The chemical state of ZIF8-CNF membrane was measured by X-ray photoelectron spectroscopy (XPS) equipped with a Specs PHOIBOS-100 spectrometer (SPECS, Berlin, Germany) under the Al Kα irradiation (Voltage: 10 kV, Current: 10 mA). A Mitutoyo Digimatic Indicator (Mitutoyo Corp., Kanagawa, Japan) was used to test the thickness of the separator. The surface wetting contact angle of the separators were measured using an FTA1000 analyzer system (First Ten Angstroms, Inc., Portsmouth, VI, USA). The porosity of the separators was estimated as porosityð%Þ ¼ ðwa wb Þ= ðρe �vs Þ� 100% with wa and wb as weight values before and after electrolyte soaking; and ρe and vs as densities of electrolyte and volume of separa tors [10]. The thermal stability of the CNFs, PEP and ZIF8-CNF sepa rators were measured using a TA Q50 TGA (New Castle, DE, USA) under inert atmosphere between 30 and 600 � C. The derivative thermogravi metric (DTG) curve was used to determine the specific weight-loss rate of each separator. The thermal expansion behavior of the CNF, PEP and ZIF8-CNF separators were determined by a TA Q400 thermomechanical
3. Results and discussion 3.1. Morphological property of CNFs and ZIF8-CNF hybrids The TEM images of CNFs and ZIF8-CNF complex are shown in Fig. 1. Fibrous CNFs with a length of 1–3 μm and a diameter of 10–20 nm were observed in Fig. 1a. With the addition of ZIF8 formulation reagents, ZIF8 crystals were grown on the surface of CNFs (Fig. 1b to d). The synthesis
Fig. 1. TEM images of CNFs and ZIF8-CNF hybrids. (a) CNFs, (b) ZIF8-0.5-CNF, (c) ZIF8-1-CNF, (d) ZIF8-2-CNF, (e) Schematic of the synthesis of ZIF8-CNF hybrids (Hmim - 2-Methylimidazole). 3
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schematic of ZIF8 crystals on the surface of CNFs is shown in Fig. 1e. During the synthesis process, PVP was first coated on the CNFs in methanol [27]. The added Zn2þ was then absorbed on the PVP layer along the length of CNFs. In the nucleation process, the precursors were formed through the reaction of Zn2þ with Hmim. The intermediate product in situ grew along the CNF water suspension in the Ostwald ripening process [28]. The zeta potential of CNFs was about 32.61 mV because of the introduction of sulfate groups on the surface of CNFs during the sulfur acid hydrolysis process [29]. ZIF8 crystals attached to the negatively charged CNFs and formed the ZIF8-CNF complex even tually. ZIF8 crystals helped prevent the aggregation of CNFs and pro mote the homogenization of pore distribution. The micro-morphologies of CNF, PEP and ZIF8-CNF separators are shown in Fig. 2. Slit pores were observed from the commercial PEP separator (Fig. 2a). The length and width of the slit pores were less than 700 nm and 60 nm, respectively, with an aspect ratio of about 10. These special slit pores were formed from the uniaxial stretching method in the preparation procedure, which also have a great influence on the mem brane mechanical property [30]. The slit pore structure is more suitable in the fabrication of high power density battery [5]. The pore size of
pure CNF membrane distributed heterogeneously. For example, several big pores (about 815 nm) were observed in the upper right part of Fig. 2c and d, which could cause the short circuits or self-discharging problem when being used in LIB due to the potential contact of anode and cathode [16]. A relative dense CNF structure was observed in Fig. 2b, which may decrease the ion ionic transportation rate and increase the internal resistance of battery [31]. In addition, the big pores (>2 μm) also may increase the shutdown probability under rigorous conditions such as high charge-discharge rates, and high temperatures [16]. In comparison with the CNF and PEP separators, the ZIF8-CNF separator (Fig. 2e and f) exhibited a tortuous structure and the pores distributed more homogeneously than that of the pure CNF membrane. The pore size of the ZIF8-CNF separator ranged from about 400 to 650 nm. Since these tortuous and interconnected pores can prevent the formation of lithium dendrites during the charging processes, it is more suitable in the long cycle life battery [5]. The size of the ZIF8 particles was about 70 nm. In order to further determine the composition of ZIF8 on the surface of CNFs, the EDX spectra under mapping mode and line mode are pro vided in Fig. 3. The general shape of CNFs and ZIF8 particles can be
Fig. 2. Micro-morphology of CNF, PEP and ZIF8-2-CNF separators. (a) commercial PEP separator, (b), (c)/(d) high magnification and low magnification of pure CNF separator, (e)/(f) high magnification and low magnification of ZIF8-2-CNF separator. 4
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observed from the C-element map (Fig. 3b) because both of them contain carbon atom. N-element and Zn-element (Fig. 3c and d) distributed homogeneously on the surface of CNFs, which confirmed the success synthesis of ZIF8 and also implied that the ZIF8 dispersed uniformly in the ZIF8-CNF system. The EDX spectra of ZIF8-2-CNF separator are shown in Fig. 3e. The quantitative results showed that the atomic ratio of N/Zn was 28.31/7.20 (under unallocated phase), which is approxi mately 3.93 and similar to atom ratio in the formula of ZIF8-Zn (C8H12N4) [32]. This also denoted the successfully synthesized of ZIF8 in the complex. The atomic percentage of carbon was about 62.66%, which is much higher than the corresponding atomic ratio in ZIF8. This phe nomenon was caused by the atom sources of ZIF8 and CNFs.
expected peaks of ZIF8 and CNFs were observed, which include peaks of Zn 2p, O 1s, N1s, and C1s. Both the original peaks and their corre sponding peak deconvolution are shown in Fig. 4c and e. The trivial peak at 159.82 eV was attributed to S 2p [33], which confirmed the existence of residual sulfate groups on the surface of CNFs after the acid hydrolysis process. This also explained why the CNFs aqueous suspension exhibited a negative zeta potential, which promote the attachment of ZIF8 crystals on the surface of CNFs. The peaks at 1022.63 and 1045.68 eV were assigned to Zn 2p 3/2 and Zn 2p ½, respectively (Fig. 4b) [34]. The peak of C 1s was observed at 285.92 eV. Three peaks were observed in the peak deconvolution of C 1s (Fig. 4c). The peaks at 285.39 eV and 287.96 eV were assigned as C–C and C–OH coming from CNFs. The peak of C–N exhibited at 284.46 eV, as introduced by the reagent 2-Methyl-1H-imi dazole. The peak of N 1s was observed at 399.22 eV, which was further decomposed into peaks of N–C and N–H (Fig. 4d) and their corresponding binding energy were 399.20 and 397.5 eV. The peak at
3.2. Elemental property of CNFs and ZIF8-CNF hybrids The XPS spectra of ZIF8-CNF membranes are shown in Fig. 4a. All the
Fig. 3. SEM and EDX image (mapping mode) and spectra of ZIF8-2-CNF separators. (a) ZIF8-2-CNF separator, (b) C-element, (c) N-element, (d) Zn-element, and (e) EDX spectra. 5
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Fig. 4. XPS and FTIR spectra of CNF, ZIF8-CNF, and PEP separator, (a) XPS spectra, (b) Zn 2p regions, (c) C 1s, (d) N 1s, (e) O 1s, and (f) FTIR spectra.
533.12 eV corresponded to O 1s. Three peaks were observed in the peak deconvolution of O 1s region (Fig. 4e). The peaks at 532.24, 532.53 and 531.69 eV were attributed to O–C, O–H and O–Zn, respectively. The FTIR spectra of CNF, ZIF8-modified CNF, and PEP separators are shown in Fig. 4f. The major functional groups of CNFs with a typical cellulose I structure were observed. The region between 3000 and 3660 cm 1 was assigned as the O–H stretching vibration of CNFs. The peak at 3333 and 3297 cm 1 were attributed to the inter-molecular H bonds for 2-OH⋯O-6 and 6-OH⋯O-3 of CNFs [35]. The peak at 2897 cm 1 was assigned to C–H stretching vibration of the CNFs since the major component of cellulose were repeated β(1 → 4) linked D-glucose units with different degree of polymerization. The peak at 1637 cm 1 was considered as the O–H group from absorbed water, which was mainly caused by the hydrophilic characteristic of CNFs. These small peaks at 1429, 1364, and 1315 cm 1 can be attributed to the bending mode of CH2 symmetric vibration, C–H and CH2 wagging vibration at C-6 of CNFs [35]. The peak at 1039 cm 1 was assigned as the C–O stretching vibrations at C-6 of CNFs. Both the characterization peaks of ZIF8 and CNFs were observed in the FTIR spectra of ZIF8-CNFs. The peak at 1583 – N stretching mode and out of plane and 757 cm 1 corresponded to C–
bending mode, respectively [36]. These peaks were not shown in the FTIR spectra of CNFs because they were function groups of ZIF8, con firming the successful synthesis of ZIF8 on the surface of CNFs. Some of the peaks from ZIF8 were overlapped with the peaks from CNFs. This peak shifted to 2925 cm 1 in the FTIR spectra of ZIF8 modified CNF separator, because it overlapped with the peak of C–H symmetric stretching of CH3 in ZIF8 [37]. With the addition of ZIF8 on the surface of CNFs, the peak intensity of O–H group also decreased. Since the majority mechanical property of CNF membrane was provided by the hydrogen bonding among different nano-fibrils, the mechanical prop erty of ZIF8-CNF membrane decreased with the addition of ZIF8 (confirmed by the following tensile strength test). Three major peaks were observed from the FTIR spectra of the PEP separator. The peak at 2931 cm 1 was associated with the C–H group. The peaks at 1456 cm 1 and 1374 cm 1 were attributed to the bending mode of CH3 from PP and bending mode of CH3 or CH2 from PE because the PEP separator was composed of three layer PP-PE-PP material. The peak height ratio of these two peaks can also be used to calculate the ratio of PP and PE in the tri-layer PEP separator.
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3.3. Thermal stability and expansion behavior of CNFs and ZIF8-CNF hybrid membranes
The homogenized distribution of ZIF8 reduced the CNF aggregation in comparison with the pure CNFs separator. PEP separator exhibited a higher Tmax, but the membrane melt at temperature above 200 � C. As shown in the inserted DSC plot in Fig. 5a, two distinctive endothermic peaks corresponding PP and PE were observed. The melting tempera tures of PP and PE were 163 � C and 134 � C, respectively. Thermal expansion behavior of CNF, ZIF8-CNF, and PEP separators, character ized by TMA analysis, is shown in Fig. 5c. No big difference was observed for the coefficients of thermal expansion (CTE) among the CNF and ZIF8-CNF separators (Table 1). When more ZIF8 crystals were synthesized on the surface of CNFs, the CTE slightly decreased from 17.78 ppm/k (pure CNFs separator) to 15.53 ppm/k (ZIF8-2-CNFs separator). The PEP separator became very soft and shrank greatly when the temperature was larger than 73 � C, which made it impossible to determine the CTE because the PEP separator cannot support the pre loading during TMA testing. The corresponding CTE value of PEP separator during this temperature range already reached up to 178 ppm/k. Obviously, the ZIF8-2-CNFs exhibited better thermal expansion behavior than PEP separator. Due to the different components of CNF, ZIF8 modified CNF and PEP separators, CNFs remained stable up to 170 � C (Fig. 5d). This is another advantage for ZIF8-2-CNF separator when being compared with PEP separator.
Thermal stability and expansion behavior have a great influence on the safety and lifetime of battery in rigorous environment (e.g., battery overheating, overcharging, and overcurrent) [38]. The thermal stability properties of CNF, ZIF8-CNF, and PEP separators are shown in Fig. 5. The weight loss before 100 � C was mainly due to the evaporation of water moisture at the surface of CNFs or the residual molecules such as methanol or Hmim in the ZIF8 [39]. The weight loss for CNF, ZIF8-0.5-CNF, ZIF8-1-CNF, and ZIF8-2-CNF separator were 4.175%, 2.229%, 0.615% and 0.323%, respectively, at the low temperature re gion (<100 � C). With the addition of ZIF8 synthesized along the surface of CNFs, the weight loss due to water moisture evaporation also decreased. This was due to the partly removal of hydrogen bonding in the ZIF8 CNF separator, as confirmed by the FTIR spectra. No weight loss was observed for PEP separator at this region because of the hydro phobic nature of PEP. The char yield (or residual) at 600 � C of CNF, ZIF8-0.5-CNF, ZIF8-1-CNF, and ZIF8-2-CNF systems were 10.59%, 23.38%, 48.40%, and 60.16%, respectively (Table 1). Compared with the CNF separator, the increased residual of ZIF8- CNF separator was ascribed to the synthesis of ZIF8 crystals. The residual of PEP separator was near zero (0.32%), because the major component of PEP separator were organic polymers-polyethylene and polypropylene, composing of C, H, and O and being easily degraded at high temperature region. The maximum thermal degradation temperatures (Tmax) and the corre sponding maximum weight loss rate (WLRmax) of CNF, ZIF8-0.5-CNF, ZIF8-1-CNF, ZIF8-2-CNF, and PEP separators were 358 � C (1.35%/min), 320 � C (0.96%/min), 323 � C (0.71%/min), 320 � C (0.47%/min), and 454 � C (2.11%/min), respectively (Table 1). The ZIF8-CNF separator exhibited a slightly lower Tmax, caused by the in crease of surface area and porosity as shown in the SEM image (Fig. 2c).
3.4. Crystal structure of CNFs and ZIF8-CNF hybrids The crystal structures of CNF, ZIF8-CNF, and PEP separators, char acterized using WXRD analysis, are shown in Fig. 6. CNFs showed a cellulose I structure and four characteristic peaks were observed at 14.9� (110), 16.4� (110), 22.5� (002), and 34.4� (004), respectively. The for mation of ZIF8 was confirmed by the characteristic peaks from XRD patterns. The well-defined peaks between 2θ ranges of 5o-33� were assigned to the high crystallinity and ordered porous sodalite structure
Fig. 5. Thermal stability and expansion behavior of CNF, ZIF8-CNF, and PEP separators. (a) TGA curves, (b) DTG curves, (c) TMA curves, and (d) thermal shrinkage at 180 � C. 7
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Table 1 The relevant parameters of the prepared separators and commercialized PEP separator. Samples CNFs ZIF8-0.5-CNFs ZIF8-1-CNFs ZIF8-2-CNFs PEP
Ratio ZIF8
CNFs
– 0.15 0.3 0.6 –
– 1 1 1 –
Thickness (μm)
Porosity (%)
Liþ TN
Contact Angle
WLRmax (%/min)
Residual (%)
CTE (ppm/k)
25 27 29 32 25
42 43 49 55 39
0.39 0.41 0.45 0.50 0.34
8.98� 17.76� 14.3� 13.31� 96.18�
1.35 0.96 0.71 0.47 2.11
10.59 23.38 48.40 60.16 0.32
17.78 16.96 16.12 15.53 178.90
of ZIF8 crystals. The diffraction peaks (2θ) at 7.4� , 10.4� , 12.8� , 14.8� , 16.5� , 18.1� , 24.6� , 26.7� , 29.7� , 30.7� , 31.6� , and 32.5� , were assigned to crystallographic planes of 001, 002, 112, 022, 013, 222, 233, 134, 044, 334, 244, and 235 from ZIF8, respectively, which agreed well with the result in previous published articles [39,40]. Some peaks from different materials were overlapped. For example, the 002 plane from CNFs and 114 plane from ZIF8 overlapped at 22.2� . The peak intensity of ZIF8 increased with more ZIF8 were synthesized on the surface of CNFs. The characteristic peak of PP and PE were clearly observed in the XRD pattern of PEP separator (Fig. 6b). The intensive peaks denoted the high polymerization and crystallinity of PP and PE. A typical α form of PP was observed and the peaks at 14.0� , 16.9� , and 18.6� were assigned to 110, 040, and 130 planes, respectively [41]. The peaks associated with PE were found at 21.6� , and 24.0� , corresponding to 110 and 200 planes [42].
The tensile stress of PEP separator at machine direction was slightly larger than CNF based separator, however, the tensile stress of ZIF8-CNF separator was much larger than the corresponding value of PEP sepa rator at transverse direction or diagonal direction, which is shown in the small insert plot of Fig. 7a. The anisotropic property of PEP separator was caused by the utilization of dry process with a uniaxial stretching method during the separator manufacturing process [4]. The contact angles in the initial 60 s for CNF, ZIF8-CNF, and PEP separators are shown in Fig. 7b. It is obvious to observe that CNF based separator exhibited better surface wettability than that of the PEP separator. At the initial time (0s), the contact angles of DMC solvent, which is widely used as the solvent in electrolyte, on the surface of CNF, ZIF8-0.5-CNF, ZIF8-1-CNF, ZIF8-2-CNF, and PEP separators were 8.98� , 17.76� , 14.3� , 13.31� , and 96.18� , respectively, as shown in Fig. 7b and Table 1. After 60 s, the contact angle of PEP separator gradually decreased to 90.6� . In comparison, the DMC droplet wicked into ZIF8-2CNF separator at 42s with a contact angle of 0� . The contact angle of PEP separator did not change greatly in the initial 60 s, indicating that the electrolyte was not easily absorbed by the PEP separator. Since the speed of surface wetting is associated with the inner resistance of battery [4], the replacement of ZIF8-2-CNF separator with PEP separator can help decrease the battery internal resistance and improve the battery performance.
3.5. Mechanical and surface wettability of CNFs and ZIF8-CNF hybrids Mechanical properties of separators have a great impact on the safety and durability of li-ion batteries. During the battery winding and fabrication process, the separator has to sustain compression and tension induced by mechanical handling. In the service life of battery, the separator also needs to withstand potential cyclic deformation of the electrodes and penetration of dendritic growth [43,44]. The tensile strain–stress curves of CNF, ZIF8-CNF, and PEP separators are shown in Fig. 7a. The maximum tensile stresses at break for CNF, ZIF8-0.5-CNF, ZIF8-1-CNF, and ZIF8-2-CNF separators were 101.6, 94.1, 85.4, and 77.2 MPa, respectively. The mechanical property of CNF separator was mainly provided by the formation of strong hydrogen bonding among the cellulose nano-fibrils during the drying process [45]. The decreased tensile strength with the additional synthesis of ZIF8 was due to the combination of two effects. First, some hydrogen bonding sites were removed, which are illustrated in the FTIR spectra. Second, the synthesis of ZIF8 on the surface of CNFs prevented the aggregation of CNFs during the drying process. In contrast to the isotropic property of CNF based separator, PEP separator exhibited anisotropic property with the maximum tensile stress at break for PEP separator at machine, diagonal and transverse directions of 150.6, 22.14, and 15.18 MPa, respectively.
3.6. Performance of LIBs with CNFs and ZIF8-CNF hybrid membranes The cycling performance of LIBs using ZIF8-2-CNF, pure CNF and PEP separators are shown in Fig. 8a. The discharge capacity and capacity retention of ZIF8-2-CNF separator after 100th cycle were about 112 mAh g 1 and 88.2%, which were higher than the corresponding values of PEP separator (102 mAh g 1 and 80.3%). This was due to the excellent surface wettability of ZIF8-CNF separator, good porosity (Table 1) and wetting speed (Fig. 7b) compared with PEP separator. In addition, ZIF8-2-CNF separator exhibited larger discharge capacity retention than CNF separator. The better cycling performance of ZIF8-2CNF separator was mainly attributed to the improved nano-porous structure [15]. Both the pore and pore size of ZIF8-2-CNF separator distributed more homogeneously, which improved the drawbacks of
Fig. 6. XRD patterns of CNF, ZIF8-CNF, and PEP separators. (a) CNFs and ZIF8-CNF separators, (b) PEP separators. 8
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Fig. 7. Mechanical property and surface wettability of CNF, ZIF8-CNF, and PEP separators, (a) tensile strain–stress curves, (b) change of contact angles in the initial 60 s with DMC as the reference solvent.
CNF separator-dense membrane (Fig. 2b) and big pores (Fig. 2c and d). The initial 5 and specific 10th, 20th, 30th, 40th, and 50th charge-discharge cycles of LIB assembled with ZIF8-2-CNF separator are shown in Fig. 8b and c. No obvious irreversible capacity loss and no abnormal charge-discharge profiles were observed at this stage. This was mainly due to the good electrolyte retention of the ZIF8-2-CNF separator. The relevant charge-discharge profiles of CNF separator are shown in Fig. 8d and a small capacity loss was observed between 10th and 20th cycles. For PEP separator (Fig. 8e), an irreversible capacity loss was observed at the first 20th cycles. The ZIF8-2-CNF separator exhibited better cycling performance in comparison of the PEP sepa rator, indicating that ZIF8-CNFs can be considered as a good alternative material to replace PEP. The rate capacity of LIB with ZIF8-2-CNF and PEP separators was evaluated at 8 different rates and the results are shown in Fig. 8f. Batteries with ZIF8-2-CNF separator exhibited better rate capacity than that of PEP separator. A downward trend was observed for the discharge capacity of LIBs with these two types of separators when rate was increased from 200 to 5000 mA g 1. This phenomenon was due to the formation of membrane at the surface of active materials in the electrode when Li ion transporting in and out of electrode, which is consistent with previous reports [46]. The discharge capacity of LIB with ZIF8-2-CNF and PEP separators was about 132 and 129 mA g 1 at a current density of 0.2 C, and the corresponding value decreased to approximately 82 and 51 mA g 1 at a current density of 8 C. However, the difference of discharge capacity between ZIF8-2-CNF and PEP separators became larger with the increase of rate. For example, the gap of discharge capacity at a current density of 1 C was about 10 mAh g 1, whereas the gap increased to about 30 mAh g 1 at a current density of 8 C. The ion conductivity and interfacial resistivity had a great in fluence on the LIB performance including charge and discharge capac ity, rate capacity, and cycle life [44]. EIS measurements were used to determine the interfacial compatibility of LIBs with ZIF8-CNF, CNF and PEP separators and the Nyquist plots are shown in Fig. 8g. LIBs assem bled with ZIF8-CNF hybrid membrane exhibited a lower interfacial re sistivity than that of the CNF, and PEP separators, mainly due to the good electrochemical interface stability and fast liquid electrolyte wet ting behavior of ZIF8-CNF separator as illustrated in Fig. 7 [18]. The interfacial resistivity also decreased when more ZIF8 material was synthesized on the surface of CNFs. The corresponding interfacial re sistivity of ZIF8-2-CNF, CNF and PEP separators was about 170 Ω, 210 Ω and 320 Ω, respectively, after 100 cycles. The linear sweep voltammo grams (LSV) of these separators are illustrated in Fig. 8h. ZIF8-2-CNF and CNF separator exhibited good electrochemical stability and no obvious decomposition of the main components in LIB was observed before 4.4 V, which are comparable with the conventional PEP sepa rator. The ion conductivity (λ) of ZIF8-2-CNF and PEP separators was calculated by using a simple formula λ ¼ T=RA (T: thickness of
separator, R: resistivity, A: the area of separator) [4]. The ion conduc tivities of ZIF8-2-CNF and PEP separators were approximately 1:41 � 10 3 S cm 1 , and 0:64 � 10 3 S cm 1 . The Liþ TN was estimated (Fig. S2) using the same equation from Chi et al. [26]. The Liþ TN of ZIF8-2-CNF, CNF and PEP separators were 0.50, 0.39, and 0.34, respectively (Table 1). The higher Liþ TN of ZIF8-2-CNF separator was mainly due to the improved porous structure with the addition of ZIF8 and better surface wettability of CNF. The higher ion conductivity, Liþ transference number, and lower interfacial resistivity of ZIF8-2-CNF separator contributed to the better charge and discharge capacity and rate capacity. The combination of SEM images and elemental mapping analysis were used to analyze the stability of prepared separators. The ZIF8-2-CNFs separator remained stable after cycling test as illustrated in Fig. S1. The observed small irregular particles (Figs. S1a and S1b) on the PEP and CNF separators were due to the residual electrolyte-LiPF6. ZIF-8 had a cubic structure and the residual electrolyte-LiPF6 had an irregular structure as shown in Fig. S1c. The P-elemental map (Fig. S1e) confirmed the homogeneous distribution of electrolyte in the ZIF8-2-CNFs separator. 4. Conclusions ZIF8 crystals were successfully synthesized on the surface of CNFs, and the ZIF8-CNF composite separator was fabricated using a vacuum filtration method. The use of ZIF8 in the CNF system improved the pore structure of composite separators as compared with the pure CNF separator. The introduction of ZIF8 effectively prevented the aggrega tion of CNFs, leading to more homogeneously distributed pores. In addition to the environmentally friendly advantage of CNFs, ZIF8-CNF composite separators also exhibited better thermal stability, and ther mal dimensional stability than those of commercial PEP separator. ZIF8CNF composite separator was thermally stable up to 200 � C, helping improve the safety of LIBs under the rigorous environment. ZIF8-CNF composite separators had better surface wettability and fast wetting speed, which can help decrease the electrolyte filling time and battery interior resistance. The ZIF8-CNF composite separators had more isotropic mechanical properties. LIBs made with ZIF8-CNF nano composite separator exhibited better discharge capacity retention, cycling performance and rate capacity. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Fig. 8. Performance of LIBs made with ZIF8-CNF, CNF and PEP separators. (a) Cycle performance at a current density of 0.5 C, (b) capacity-voltage curves com parison, (c) capacity-voltage curves with ZIF8-CNF separator, (d) capacity-voltage curves with CNF separator, (e) capacity-voltage curves with PEP separator, (f) rate capacity, (g) nyquist plots and (h) LSV of LIBs using ZIF8-CNF, pure CNF and PEP separators.
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Acknowledgement
[19] K.S. Park, Z. Ni, A.P. C^ ot� e, J.Y. Choi, R. Huang, F.J. Uribe-Romo, H.K. Chae, M. O’Keeffe, O.M. Yaghi, Proc. Natl. Acad. Sci. U.S.A. 103 (2006) 10186–10191. [20] H. Wu, W. Zhou, T. Yildirim, J. Appl. Comput. Sci. 129 (2007) 5314–5315. [21] Q. Song, S. Nataraj, M.V. Roussenova, J.C. Tan, D.J. Hughes, W. Li, P. Bourgoin, M. A. Alam, A.K. Cheetham, S.A. Al-Muhtaseb, Energy Environ. Sci. 5 (2012) 8359–8369. [22] G. Lu, J.T. Hupp, J. Appl. Comput. Sci. 132 (2010) 7832–7833. [23] K. Yang, K. Yang, S. Chao, J. Wen, Y. Pei, Z. Pei, Chem. Commun. 54 (2018) 9817–9820. [24] S. Aguado, J. Canivet, D. Farrusseng, Chem. Commun. 46 (2010) 7999–8001. [25] W. Sun, X. Zhai, L. Zhao, Chem. Eng. J. 289 (2016) 59–64. [26] M. Chi, L. Shi, Z. Wang, J. Zhu, X. Mao, Y. Zhao, M. Zhang, L. Sun, S. Yuan, Nano Energy 28 (2016) 1–11. [27] D. Zhang, P. Liu, S. Xiao, X. Qian, H. Zhang, M. Wen, Y. Kuwahara, K. Mori, H. Li, H. Yamashita, Nanoscale 8 (2016) 7749–7754. [28] M. Jian, B. Liu, R. Liu, J. Qu, H. Wang, X. Zhang, RSC Adv. 5 (2015) 48433–48441. [29] H.V. Lee, S.B.A. Hamid, S.K. Zain, Sci. World J. 2014 (2014) 20. [30] J. Zhang, L. Yue, Q. Kong, Z. Liu, X. Zhou, C. Zhang, Q. Xu, B. Zhang, G. Ding, B. Qin, Y. Duan, Q. Wang, J. Yao, G. Cui, L. Chen, Sci. Rep. 4 (2014) 3935. [31] B. Weng, F. Xu, M. Alcoutlabi, Y. Mao, K. Lozano, Cellulose 22 (2015) 1311–1320. [32] Y. Grosu, G. Renaudin, V. Eroshenko, J.M. Nedelec, J.P.E. Grolier, Nanoscale 7 (2015) 8803–8810. [33] D. Pan, L. Wang, Z. Li, B. Geng, C. Zhang, J. Zhan, L. Yin, L. Wang, New J. Chem. 42 (2018) 5083–5089. [34] F. Tian, A.M. Cerro, A.M. Mosier, H.K. Wayment-Steele, R.S. Shine, A. Park, E. R. Webster, L.E. Johnson, M.S. Johal, L. Benz, J. Phys. Chem. C 118 (2014) 14449–14456. [35] S.Y. Oh, D.I. Yoo, Y. Shin, H.C. Kim, H.Y. Kim, Y.S. Chung, W.H. Park, J.H. Youk, Carbohydr. Res. 340 (2005) 2376–2391. [36] Y. Hu, H. Kazemian, S. Rohani, Y. Huang, Y. Song, Chem. Commun. 47 (2011) 12694–12696. [37] P. Cheng, Y.H. Hu, J. Phys. Chem. C 118 (2014) 21866–21872. [38] Q. Wang, P. Ping, X. Zhao, G. Chu, J. Sun, C. Chen, J. Power Sources 208 (2012) 210–224. [39] A. Schejn, L. Balan, V. Falk, L. Aranda, G. Medjahdi, R. Schneider, CrystEngComm 16 (2014) 4493–4500. [40] H. Kaur, G.C. Mohanta, V. Gupta, D. Kukkar, S. Tyagi, J. Drug Deliv. Sci. Technol. 41 (2017) 106–112. [41] J.-H. Lin, Y.-J. Pan, C.-F. Liu, C.-L. Huang, C.-T. Hsieh, C.-K. Chen, Z.-I. Lin, C.W. Lou, Materials 8 (2015) 5496. [42] X. Zhang, H. Wu, S. Guo, Y. Wang, RSC Adv. 5 (2015) 99812–99819. [43] A. Sheidaei, X. Xiao, X. Huang, J. Hitt, J. Power Sources 196 (2011) 8728–8734. [44] X. Huang, J. Solid State Electrochem. 15 (2011) 649–662. [45] I. Sir� o, D. Plackett, Cellulose 17 (2010) 459–494. [46] P. Ramadass, B. Haran, R. White, B.N. Popov, J. Power Sources 123 (2003) 230–240.
This collaborative study was carried out with support from Louisiana Board of Regents, Baton Rouge, Louisiana, United States [LEQSF(201718)-RD-A-01; LEQSF(2018-19)-ENH-DE-06], and National Institute of Forest Science,Seoul, Republic of Korea. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2020.227878. References [1] J.B. Goodenough, K.-S. Park, J. Appl. Comput. Sci. 135 (2013) 1167–1176. [2] S. Goriparti, E. Miele, F. De Angelis, E. Di Fabrizio, R. Proietti Zaccaria, C. Capiglia, J. Power Sources 257 (2014) 421–443. [3] H. Li, Z. Wang, L. Chen, X. Huang, Adv. Mater. 21 (2009) 4593–4607. [4] P. Arora, Z. Zhang, Chem. Rev. 104 (2004) 4419–4462. [5] S.S. Zhang, J. Power Sources 164 (2007) 351–364. [6] K. Xu, Chem. Rev. 114 (2014) 11503–11618. [7] P. Verma, P. Maire, P. Nov� ak, Electrochim. Acta 55 (2010) 6332–6341. [8] Y. Xiang, J. Li, J. Lei, D. Liu, Z. Xie, D. Qu, K. Li, T. Deng, H. Tang, ChemSusChem 9 (2016) 3023–3039. [9] R.J. Moon, A. Martini, J. Nairn, J. Simonsen, J. Youngblood, Chem. Soc. Rev. 40 (2011) 3941–3994. [10] X. Sun, Q. Wu, X. Zhang, S. Ren, T. Lei, W. Li, G. Xu, Q. Zhang, Cellulose 25 (2018) 1103–1115. [11] A. Isogai, T. Saito, H. Fukuzumi, Nanoscale 3 (2011) 71–85. [12] X. Sun, C. Mei, A.D. French, S. Lee, Y. Wang, Q. Wu, Cellulose 25 (2018) 5071–5087. [13] Q. Xu, Q. Kong, Z. Liu, X. Wang, R. Liu, J. Zhang, L. Yue, Y. Duan, G. Cui, ACS Sustain. Chem. Eng. 2 (2014) 194–199. [14] G. Ding, B. Qin, Z. Liu, J. Zhang, B. Zhang, P. Hu, C. Zhang, G. Xu, J. Yao, G. Cui, J. Electrochem. Soc. 162 (2015) A834–A838. [15] L. Jabbour, R. Bongiovanni, D. Chaussy, C. Gerbaldi, D. Beneventi, Cellulose 20 (2013) 1523–1545. [16] J. Zhang, Z. Liu, Q. Kong, C. Zhang, S. Pang, L. Yue, X. Wang, J. Yao, G. Cui, ACS Appl. Mater. Interfaces 5 (2013) 128–134. [17] Q. Xu, Q. Kong, Z. Liu, J. Zhang, X. Wang, R. Liu, L. Yue, G. Cui, RSC Adv. 4 (2014) 7845–7850. [18] S.-J. Chun, E.-S. Choi, E.-H. Lee, J.H. Kim, S.-Y. Lee, S.-Y. Lee, J. Mater. Chem. 22 (2012) 16618–16626.
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Zeolitic imidazolate framework-cellulose nanofiber hybrid membrane as Li-Ion battery separator: Basic membrane property and battery performance Xiuxuan Sun a, Meichun Li a, Suxia Ren b, Tingzhou Lei b, Sang-Young Lee c, Sunyoung Lee d, Qinglin Wu a, * a
School of Renewable Natural Resources, Louisiana State University AgCenter, Baton Rouge, LA, 70803, United States Key Biomass Energy Laboratory of Henan Province, Zhengzhou, 45008, China c Department of Energy Engineering, School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan, 689-798, South Korea d Department of Forest Products, National Institute of Forest Science, Seoul, 02455, Republic of Korea b
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� ZIF8-CNF membrane synthesized and used as lithium-ion battery (LIB) separator. � More homogeneously distributed pores in the manufactured membrane. � Better thermal, mechanical and surface wetting property of the membrane. � LIB’s discharge retention stability, cycling performance and rate capacity improved. � ZIF8-CNF nanocomposite membrane as a future sustainable LIB separator material.
A R T I C L E I N F O
A B S T R A C T
Keywords: Cellulose nanofibers Zeolitic imidazolate framework Li-ion battery separator Membrane property Battery performance
Zeolitic imidazolate framework-8 (ZIF8) concept was introduced in the fabrication of li-ion battery (LIB) sepa rator for the first time. The ZIF8 crystals were synthesized on the surface of cellulose nanofibers (CNFs) by mixing 2-methylimidazole (Hmim) with zinc nitrate hexahydrate (Zn) in methanol. ZIF8- or Zn (mim)2-CNF composite membrane was fabricated and the effect of ZIF8 on the pore structure of CNF network was investigated. The pores of the ZIF8-CNF membrane distributed more homogeneously than that of the pure CNF membrane. The porosity increased from 42% (pure CNF membrane) to 55% (composite membrane). The successful synthesis of ZIF8 crystals on CNF surface was confirmed through elemental composition and crystalline structure analysis of the composite. Compared with polymer based separator, the ZIF8-2-CNF membrane exhibited better thermal stability, mechanical property (isotropic vs. anisotropic), thermal expansion behavior (15.53 vs. 178.90 ppm/k), and surface wettability (13.31� vs. 96.18� ). The LIB fabricated with ZIF8-CNF membrane had a comparable cycling stability and a better discharge retention stability (88.3% vs 80.2%) in comparison with these of a trilayer commercial polymer membrane. The environmentally friendly ZIF8-CNF nanocomposite membrane can be considered as a good alternative for manufacturing LIBs.
* Corresponding author. E-mail address: [email protected] (Q. Wu). https://doi.org/10.1016/j.jpowsour.2020.227878 Received 1 March 2019; Received in revised form 27 January 2020; Accepted 10 February 2020 Available online 22 February 2020 0378-7753/© 2020 Elsevier B.V. All rights reserved.
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1. Introduction
of ZIF8-CNF composite membranes in LIBs.
Transition from fossil energy to sustainable energy and wide-spread application of portable electronics, and electric vehicles promote a rapid growth of Li-ion battery (LIB) industry [1]. Currently, considerable effort is being made to improve electrode materials [2,3], separators [4, 5], and electrolyte systems [6,7] for the LIB to meet the increasing market demands of LIBs with large energy density and improved rate capabilities. LIB separators play a critical role in keeping anode and cathode isolation and good ion transmission rate. Currently, polymer separators such as polypropylene (PP), Polyethylene (PE), or the hybrids are extensively used in the LIB industry. The limitations of the polymer separators include poor surface wettability, high thermal expansion, and low thermal stability [8]. In order to solve these problems, alternative materials and coated polymers are considered. Cellulose is the most abundant renewable bio-polymer in the world [9]. Cellulose nanofibers (CNFs) are prepared from cellulose using me chanical defibrillation, acid hydrolysis, and/or TEMPO treatments [10, 11]. CNF membranes exhibit better thermal stability, good electrolyte wettability and excellent thermal expansion behavior in comparison with traditional polymer membrane [12]. Different types of cellulose-based separators have been fabricated, including microfibrillar cellulose/polysulfonamide separator through a paper making method [13], and polyborate coated cellulose nonwoven separator through the electrospinning combined with dip-coating method [14,15]. For example, the discharge capacity of LIB assembled with cellulose/poly vinylidene fluoride-hexafluoropropylene (PVDF-HFP) composite sepa rator was 115 mA h g 1 at 1.0 C, which is larger than 106 mA h g 1 from LIB with traditional PP separator [16]. The LIBs, using LiCoO as active material of cathode, graphite as anode, and cellulose/polysulfonamide composite membrane as separator, exhibited a capacity retention ratio of 85% after one hundred cycles and worked well with a normal cycling profile at high temperature [13]. Cellulose/polydopamine separator exhibited excellent mechanical strength and ion conductivity (0:95 � 10 3 S cm 1 ), which is favorable for the long-term reliable performance of LIBs [17]. However, major drawbacks about CNF-based membranes are heterogeneous distribution of pores. Large pores (>1 μm) often exist in some areas of the separator, while other areas of the separator are too densely packed due to strong hydrogen bonding among cellulose fibrils [10]. In order to prevent dendritic lithium penetration through sepa rator pores, some general requirements of pore size are less than 1 μm [4]. Chun et al. used different solvent mixture systems in the CNF membrane making process to improve the pore structure of CNF sepa rators, leading to a satisfactory ion conductivity (0.75 mS cm 1 ) [18]. Zeolitic imidazolate framework-8 (ZIF8) is a subclass of metalorganic framework (MOF), which was originally discovered and used in the hydrogen storage media [19]. The 2-methylimidazole (Hmim) zinc complex, designated as Zn (mim)2 or ZIF8 [20], exhibits excellent thermal and chemical stability. The material is widely used in gas sep aration [21], selective sensor [22], drug delivery [23], and catalytic applications [24]. The pore size of ZIF8-based composite (e.g., ZIF8-CNF complex) can be controlled through different solvent systems at different temperatures [25]. The tunable pore size of the ZIF8 complex provides a possible strategy to facilitate the Li-ion flow through the separator and improve the interior battery resistance and electrolyte filling time [4]. The objective of this study was thus to synthesis and test ZIF8-CNF composite membranes for use as LIB separators in comparison with a commercial tri-layer polymer separator. The synthesis process of the ZIF8-CNF hybrid and effects of ZIF8 on the CNF network pore structure were illustrated. Thermal stability, mechanical properties, thermal expansion, and surface wettability of the membranes were determined. The 2032 type LIB coin cells were fabricated and analyzed for estab lishing the LIB performance with the manufactured separators. This study provides the first comprehensive analysis on the synthesis and use
2. Experimental section 2.1. Preparation of CNFs CNFs were prepared with an established protocol. In brief, bleached wood pulp (i.e., W-50) was added to 48 wt% of sulfuric acid at 45 � C with a string rate of 360 rpm. The pulp to sulfuric acid solution weight ratio was controlled at 1 to10. After 1-h hydrolysis, a large amount of deionized water was poured into the reaction system to terminate the hydrolysis process. The excessive sulfuric acid was removed by centri fuging at 9000 rpm for 30 min. The centrifuge process was repeated three times. Then the precipitant of the reaction suspension was redispersed in deionized water and transferred into dialysis tube with a molecular weight cut-off between 12,000 and 14,000. The dialysis process took about three to five days. Homogenized CNF suspension was obtained by passing the material through a micro-fluidizer processor (M110EH-30 Microfluidics Corp., Westwood, MA, USA) three times at an operation pressure of 202 MPa. The CNF aqueous suspension was freezedried by a Labconco freeze drier at 45 � C under vacuum. 2.2. Preparation of ZIF8- CNF complex The freeze-dried CNFs were re-dispersed in methanol with a weight (g) to volume (mL) ratio of 1: 100. Polyvinylpyrrolidone (PVP, 600 mg) and zinc nitrate hexahydrate (2.8 g) were added with stirring at 500 rpm for 1/2 h and 100 mL of 2-Methyl-1H-imidazole (0.4 mol/L) methanol solution were added (another 15-min stirring). The homogenized reac tion mixture was kept for 12 h at room temperature. The reaction mixture was centrifuged at 10,000 rpm for 15 min and rinsed three times with ethanol to remove excess PVP. The precipitant was re-dispersed in deionized water and homogenized using an ultrasonic homogenizer (MSK-USP-12 N, MTI Corp., Richmond, CA, USA) and finally ZIF8-1CNF material was obtained. Different weight ratios of ZIF8 to CNF hy brids were obtained by adding different amount of reagents propor tionally (e.g., ZIF8-0.5-CNF and ZIF8-2-CNF with half and twice amount of Zn(NO3)2 and 2-Methylimidazole, respectively, compared with these of ZIF8-1-CNF using the same procedure as described above). 2.3. Preparation of ZIF8-CNF composite separator and LIB The ZIF8-CNF nanocomposite membrane was prepared by the vac uum filtration method. The CNF or ZIF8-CNF suspensions were soni cated for about 30 min by an ultrasonic homogenizer (MSK-USP-12 N, MTI Corp., Richmond, USA). The homogenized CNF or ZIF8-CNF sus pensions were filtered through a hydrophilic PVDF membrane (0.65 μm pore size) using a Buchner funnel under vacuum. The obtained wet sheet was detached from the PVDF membrane and inserted between several Whatman filter papers. The samples were vacuum-dried at 95 � C for about 20 min in a vacuum oven (MTI Corporation, Richmond, Califor nia, USA). The LiFePO4 cathode slurry, which was made of 70 wt% LiFePO4, 20 wt% carbon black and 10 wt% PVDF, was coated on the Al foil using a MSK-AFA-III Automatic Thick Film Coater (MTI Corporation, Richmond, California, USA) and dried in vacuum at 120 � C for about 24 h. The 2032 type coin cells were fabricated using LiFePO4 as cathode, lithium chip (Li) as anode, 1.0 M LiPF6 in EC/DMC ¼ 50/50 (v/v) as electrolyte, and two separators including the ZIF8-CNF membrane and a commercial polyproplyene (PP)-polyethlene (PE)-polyproplyene (PP) or PEP separator film (from MTI Corporation, Richmond, CA, USA). A Compact Digital Pressure Controlled Electric Crimping/De-Crimping Machine (MTI Corporation, Richmond, CA, USA) was used to assemble the coin cells with an assembling pressure of 1T in a glove box.
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2.4. Characterization methods
analyzer (TMA) (New Castle, DE, USA) using a film/fiber probe under tensile mode with a temperature range of 20–120 � C at a heating rate of 3 � C/min. The XRD spectra of these samples were measured by the Bruker/Siemens D5000 X-ray diffractometer (Siemens Co., Wittelsba cherplatz, Munich, Germany) under Cu Kα radiation (Voltage: 40 kV, Current: 30 mA, 2θ range: 5-40� ). The Liþ transference number (TN) was calculated based on this formula: tLiþ ¼ [Iss*(ΔV I0R0)]/[I0* (ΔV IssRss)] with the subscript “0” denoting initial, and the subscript “ss” denoting steady state [26]. An 8-channel battery analyzer (MTI Corpo ration, Richmond, CA, USA) was used to test the performance of LIBs made with ZIF8-CNF, pure CNF, and commercial PEP separators. The interfacial resistivity of LIBs with different separators was determined by using electrochemical impedance spectra (EIS) measurement with a CHI6054C chemical work station (CH Instruments, Inc., Austin, TX, USA). The electrochemical stability windows of these separators was determined using the same machine at 0.1 mV s 1 at room temperature.
The morphology of CNFs and ZIF8-CNFs were observed through a JEOL 100CX transmission electron microscope (TEM, JEOL, Inc., Pea body, MA, USA). The morphology of the CNFs, PEP and ZIF8-CNF membranes were studied by a field-emission scanning electron micro scopy (FE-SEM, FEI Company, Hillsboro, OR, USA). The element anal ysis of the ZIF8-CNF separator was conducted using the Energy dispersive X-ray spectroscopy (EDX). The chemical state of ZIF8-CNF membrane was measured by X-ray photoelectron spectroscopy (XPS) equipped with a Specs PHOIBOS-100 spectrometer (SPECS, Berlin, Germany) under the Al Kα irradiation (Voltage: 10 kV, Current: 10 mA). A Mitutoyo Digimatic Indicator (Mitutoyo Corp., Kanagawa, Japan) was used to test the thickness of the separator. The surface wetting contact angle of the separators were measured using an FTA1000 analyzer system (First Ten Angstroms, Inc., Portsmouth, VI, USA). The porosity of the separators was estimated as porosityð%Þ ¼ ðwa wb Þ= ðρe �vs Þ� 100% with wa and wb as weight values before and after electrolyte soaking; and ρe and vs as densities of electrolyte and volume of separa tors [10]. The thermal stability of the CNFs, PEP and ZIF8-CNF sepa rators were measured using a TA Q50 TGA (New Castle, DE, USA) under inert atmosphere between 30 and 600 � C. The derivative thermogravi metric (DTG) curve was used to determine the specific weight-loss rate of each separator. The thermal expansion behavior of the CNF, PEP and ZIF8-CNF separators were determined by a TA Q400 thermomechanical
3. Results and discussion 3.1. Morphological property of CNFs and ZIF8-CNF hybrids The TEM images of CNFs and ZIF8-CNF complex are shown in Fig. 1. Fibrous CNFs with a length of 1–3 μm and a diameter of 10–20 nm were observed in Fig. 1a. With the addition of ZIF8 formulation reagents, ZIF8 crystals were grown on the surface of CNFs (Fig. 1b to d). The synthesis
Fig. 1. TEM images of CNFs and ZIF8-CNF hybrids. (a) CNFs, (b) ZIF8-0.5-CNF, (c) ZIF8-1-CNF, (d) ZIF8-2-CNF, (e) Schematic of the synthesis of ZIF8-CNF hybrids (Hmim - 2-Methylimidazole). 3
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schematic of ZIF8 crystals on the surface of CNFs is shown in Fig. 1e. During the synthesis process, PVP was first coated on the CNFs in methanol [27]. The added Zn2þ was then absorbed on the PVP layer along the length of CNFs. In the nucleation process, the precursors were formed through the reaction of Zn2þ with Hmim. The intermediate product in situ grew along the CNF water suspension in the Ostwald ripening process [28]. The zeta potential of CNFs was about 32.61 mV because of the introduction of sulfate groups on the surface of CNFs during the sulfur acid hydrolysis process [29]. ZIF8 crystals attached to the negatively charged CNFs and formed the ZIF8-CNF complex even tually. ZIF8 crystals helped prevent the aggregation of CNFs and pro mote the homogenization of pore distribution. The micro-morphologies of CNF, PEP and ZIF8-CNF separators are shown in Fig. 2. Slit pores were observed from the commercial PEP separator (Fig. 2a). The length and width of the slit pores were less than 700 nm and 60 nm, respectively, with an aspect ratio of about 10. These special slit pores were formed from the uniaxial stretching method in the preparation procedure, which also have a great influence on the mem brane mechanical property [30]. The slit pore structure is more suitable in the fabrication of high power density battery [5]. The pore size of
pure CNF membrane distributed heterogeneously. For example, several big pores (about 815 nm) were observed in the upper right part of Fig. 2c and d, which could cause the short circuits or self-discharging problem when being used in LIB due to the potential contact of anode and cathode [16]. A relative dense CNF structure was observed in Fig. 2b, which may decrease the ion ionic transportation rate and increase the internal resistance of battery [31]. In addition, the big pores (>2 μm) also may increase the shutdown probability under rigorous conditions such as high charge-discharge rates, and high temperatures [16]. In comparison with the CNF and PEP separators, the ZIF8-CNF separator (Fig. 2e and f) exhibited a tortuous structure and the pores distributed more homogeneously than that of the pure CNF membrane. The pore size of the ZIF8-CNF separator ranged from about 400 to 650 nm. Since these tortuous and interconnected pores can prevent the formation of lithium dendrites during the charging processes, it is more suitable in the long cycle life battery [5]. The size of the ZIF8 particles was about 70 nm. In order to further determine the composition of ZIF8 on the surface of CNFs, the EDX spectra under mapping mode and line mode are pro vided in Fig. 3. The general shape of CNFs and ZIF8 particles can be
Fig. 2. Micro-morphology of CNF, PEP and ZIF8-2-CNF separators. (a) commercial PEP separator, (b), (c)/(d) high magnification and low magnification of pure CNF separator, (e)/(f) high magnification and low magnification of ZIF8-2-CNF separator. 4
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observed from the C-element map (Fig. 3b) because both of them contain carbon atom. N-element and Zn-element (Fig. 3c and d) distributed homogeneously on the surface of CNFs, which confirmed the success synthesis of ZIF8 and also implied that the ZIF8 dispersed uniformly in the ZIF8-CNF system. The EDX spectra of ZIF8-2-CNF separator are shown in Fig. 3e. The quantitative results showed that the atomic ratio of N/Zn was 28.31/7.20 (under unallocated phase), which is approxi mately 3.93 and similar to atom ratio in the formula of ZIF8-Zn (C8H12N4) [32]. This also denoted the successfully synthesized of ZIF8 in the complex. The atomic percentage of carbon was about 62.66%, which is much higher than the corresponding atomic ratio in ZIF8. This phe nomenon was caused by the atom sources of ZIF8 and CNFs.
expected peaks of ZIF8 and CNFs were observed, which include peaks of Zn 2p, O 1s, N1s, and C1s. Both the original peaks and their corre sponding peak deconvolution are shown in Fig. 4c and e. The trivial peak at 159.82 eV was attributed to S 2p [33], which confirmed the existence of residual sulfate groups on the surface of CNFs after the acid hydrolysis process. This also explained why the CNFs aqueous suspension exhibited a negative zeta potential, which promote the attachment of ZIF8 crystals on the surface of CNFs. The peaks at 1022.63 and 1045.68 eV were assigned to Zn 2p 3/2 and Zn 2p ½, respectively (Fig. 4b) [34]. The peak of C 1s was observed at 285.92 eV. Three peaks were observed in the peak deconvolution of C 1s (Fig. 4c). The peaks at 285.39 eV and 287.96 eV were assigned as C–C and C–OH coming from CNFs. The peak of C–N exhibited at 284.46 eV, as introduced by the reagent 2-Methyl-1H-imi dazole. The peak of N 1s was observed at 399.22 eV, which was further decomposed into peaks of N–C and N–H (Fig. 4d) and their corresponding binding energy were 399.20 and 397.5 eV. The peak at
3.2. Elemental property of CNFs and ZIF8-CNF hybrids The XPS spectra of ZIF8-CNF membranes are shown in Fig. 4a. All the
Fig. 3. SEM and EDX image (mapping mode) and spectra of ZIF8-2-CNF separators. (a) ZIF8-2-CNF separator, (b) C-element, (c) N-element, (d) Zn-element, and (e) EDX spectra. 5
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Fig. 4. XPS and FTIR spectra of CNF, ZIF8-CNF, and PEP separator, (a) XPS spectra, (b) Zn 2p regions, (c) C 1s, (d) N 1s, (e) O 1s, and (f) FTIR spectra.
533.12 eV corresponded to O 1s. Three peaks were observed in the peak deconvolution of O 1s region (Fig. 4e). The peaks at 532.24, 532.53 and 531.69 eV were attributed to O–C, O–H and O–Zn, respectively. The FTIR spectra of CNF, ZIF8-modified CNF, and PEP separators are shown in Fig. 4f. The major functional groups of CNFs with a typical cellulose I structure were observed. The region between 3000 and 3660 cm 1 was assigned as the O–H stretching vibration of CNFs. The peak at 3333 and 3297 cm 1 were attributed to the inter-molecular H bonds for 2-OH⋯O-6 and 6-OH⋯O-3 of CNFs [35]. The peak at 2897 cm 1 was assigned to C–H stretching vibration of the CNFs since the major component of cellulose were repeated β(1 → 4) linked D-glucose units with different degree of polymerization. The peak at 1637 cm 1 was considered as the O–H group from absorbed water, which was mainly caused by the hydrophilic characteristic of CNFs. These small peaks at 1429, 1364, and 1315 cm 1 can be attributed to the bending mode of CH2 symmetric vibration, C–H and CH2 wagging vibration at C-6 of CNFs [35]. The peak at 1039 cm 1 was assigned as the C–O stretching vibrations at C-6 of CNFs. Both the characterization peaks of ZIF8 and CNFs were observed in the FTIR spectra of ZIF8-CNFs. The peak at 1583 – N stretching mode and out of plane and 757 cm 1 corresponded to C–
bending mode, respectively [36]. These peaks were not shown in the FTIR spectra of CNFs because they were function groups of ZIF8, con firming the successful synthesis of ZIF8 on the surface of CNFs. Some of the peaks from ZIF8 were overlapped with the peaks from CNFs. This peak shifted to 2925 cm 1 in the FTIR spectra of ZIF8 modified CNF separator, because it overlapped with the peak of C–H symmetric stretching of CH3 in ZIF8 [37]. With the addition of ZIF8 on the surface of CNFs, the peak intensity of O–H group also decreased. Since the majority mechanical property of CNF membrane was provided by the hydrogen bonding among different nano-fibrils, the mechanical prop erty of ZIF8-CNF membrane decreased with the addition of ZIF8 (confirmed by the following tensile strength test). Three major peaks were observed from the FTIR spectra of the PEP separator. The peak at 2931 cm 1 was associated with the C–H group. The peaks at 1456 cm 1 and 1374 cm 1 were attributed to the bending mode of CH3 from PP and bending mode of CH3 or CH2 from PE because the PEP separator was composed of three layer PP-PE-PP material. The peak height ratio of these two peaks can also be used to calculate the ratio of PP and PE in the tri-layer PEP separator.
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3.3. Thermal stability and expansion behavior of CNFs and ZIF8-CNF hybrid membranes
The homogenized distribution of ZIF8 reduced the CNF aggregation in comparison with the pure CNFs separator. PEP separator exhibited a higher Tmax, but the membrane melt at temperature above 200 � C. As shown in the inserted DSC plot in Fig. 5a, two distinctive endothermic peaks corresponding PP and PE were observed. The melting tempera tures of PP and PE were 163 � C and 134 � C, respectively. Thermal expansion behavior of CNF, ZIF8-CNF, and PEP separators, character ized by TMA analysis, is shown in Fig. 5c. No big difference was observed for the coefficients of thermal expansion (CTE) among the CNF and ZIF8-CNF separators (Table 1). When more ZIF8 crystals were synthesized on the surface of CNFs, the CTE slightly decreased from 17.78 ppm/k (pure CNFs separator) to 15.53 ppm/k (ZIF8-2-CNFs separator). The PEP separator became very soft and shrank greatly when the temperature was larger than 73 � C, which made it impossible to determine the CTE because the PEP separator cannot support the pre loading during TMA testing. The corresponding CTE value of PEP separator during this temperature range already reached up to 178 ppm/k. Obviously, the ZIF8-2-CNFs exhibited better thermal expansion behavior than PEP separator. Due to the different components of CNF, ZIF8 modified CNF and PEP separators, CNFs remained stable up to 170 � C (Fig. 5d). This is another advantage for ZIF8-2-CNF separator when being compared with PEP separator.
Thermal stability and expansion behavior have a great influence on the safety and lifetime of battery in rigorous environment (e.g., battery overheating, overcharging, and overcurrent) [38]. The thermal stability properties of CNF, ZIF8-CNF, and PEP separators are shown in Fig. 5. The weight loss before 100 � C was mainly due to the evaporation of water moisture at the surface of CNFs or the residual molecules such as methanol or Hmim in the ZIF8 [39]. The weight loss for CNF, ZIF8-0.5-CNF, ZIF8-1-CNF, and ZIF8-2-CNF separator were 4.175%, 2.229%, 0.615% and 0.323%, respectively, at the low temperature re gion (<100 � C). With the addition of ZIF8 synthesized along the surface of CNFs, the weight loss due to water moisture evaporation also decreased. This was due to the partly removal of hydrogen bonding in the ZIF8 CNF separator, as confirmed by the FTIR spectra. No weight loss was observed for PEP separator at this region because of the hydro phobic nature of PEP. The char yield (or residual) at 600 � C of CNF, ZIF8-0.5-CNF, ZIF8-1-CNF, and ZIF8-2-CNF systems were 10.59%, 23.38%, 48.40%, and 60.16%, respectively (Table 1). Compared with the CNF separator, the increased residual of ZIF8- CNF separator was ascribed to the synthesis of ZIF8 crystals. The residual of PEP separator was near zero (0.32%), because the major component of PEP separator were organic polymers-polyethylene and polypropylene, composing of C, H, and O and being easily degraded at high temperature region. The maximum thermal degradation temperatures (Tmax) and the corre sponding maximum weight loss rate (WLRmax) of CNF, ZIF8-0.5-CNF, ZIF8-1-CNF, ZIF8-2-CNF, and PEP separators were 358 � C (1.35%/min), 320 � C (0.96%/min), 323 � C (0.71%/min), 320 � C (0.47%/min), and 454 � C (2.11%/min), respectively (Table 1). The ZIF8-CNF separator exhibited a slightly lower Tmax, caused by the in crease of surface area and porosity as shown in the SEM image (Fig. 2c).
3.4. Crystal structure of CNFs and ZIF8-CNF hybrids The crystal structures of CNF, ZIF8-CNF, and PEP separators, char acterized using WXRD analysis, are shown in Fig. 6. CNFs showed a cellulose I structure and four characteristic peaks were observed at 14.9� (110), 16.4� (110), 22.5� (002), and 34.4� (004), respectively. The for mation of ZIF8 was confirmed by the characteristic peaks from XRD patterns. The well-defined peaks between 2θ ranges of 5o-33� were assigned to the high crystallinity and ordered porous sodalite structure
Fig. 5. Thermal stability and expansion behavior of CNF, ZIF8-CNF, and PEP separators. (a) TGA curves, (b) DTG curves, (c) TMA curves, and (d) thermal shrinkage at 180 � C. 7
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Table 1 The relevant parameters of the prepared separators and commercialized PEP separator. Samples CNFs ZIF8-0.5-CNFs ZIF8-1-CNFs ZIF8-2-CNFs PEP
Ratio ZIF8
CNFs
– 0.15 0.3 0.6 –
– 1 1 1 –
Thickness (μm)
Porosity (%)
Liþ TN
Contact Angle
WLRmax (%/min)
Residual (%)
CTE (ppm/k)
25 27 29 32 25
42 43 49 55 39
0.39 0.41 0.45 0.50 0.34
8.98� 17.76� 14.3� 13.31� 96.18�
1.35 0.96 0.71 0.47 2.11
10.59 23.38 48.40 60.16 0.32
17.78 16.96 16.12 15.53 178.90
of ZIF8 crystals. The diffraction peaks (2θ) at 7.4� , 10.4� , 12.8� , 14.8� , 16.5� , 18.1� , 24.6� , 26.7� , 29.7� , 30.7� , 31.6� , and 32.5� , were assigned to crystallographic planes of 001, 002, 112, 022, 013, 222, 233, 134, 044, 334, 244, and 235 from ZIF8, respectively, which agreed well with the result in previous published articles [39,40]. Some peaks from different materials were overlapped. For example, the 002 plane from CNFs and 114 plane from ZIF8 overlapped at 22.2� . The peak intensity of ZIF8 increased with more ZIF8 were synthesized on the surface of CNFs. The characteristic peak of PP and PE were clearly observed in the XRD pattern of PEP separator (Fig. 6b). The intensive peaks denoted the high polymerization and crystallinity of PP and PE. A typical α form of PP was observed and the peaks at 14.0� , 16.9� , and 18.6� were assigned to 110, 040, and 130 planes, respectively [41]. The peaks associated with PE were found at 21.6� , and 24.0� , corresponding to 110 and 200 planes [42].
The tensile stress of PEP separator at machine direction was slightly larger than CNF based separator, however, the tensile stress of ZIF8-CNF separator was much larger than the corresponding value of PEP sepa rator at transverse direction or diagonal direction, which is shown in the small insert plot of Fig. 7a. The anisotropic property of PEP separator was caused by the utilization of dry process with a uniaxial stretching method during the separator manufacturing process [4]. The contact angles in the initial 60 s for CNF, ZIF8-CNF, and PEP separators are shown in Fig. 7b. It is obvious to observe that CNF based separator exhibited better surface wettability than that of the PEP separator. At the initial time (0s), the contact angles of DMC solvent, which is widely used as the solvent in electrolyte, on the surface of CNF, ZIF8-0.5-CNF, ZIF8-1-CNF, ZIF8-2-CNF, and PEP separators were 8.98� , 17.76� , 14.3� , 13.31� , and 96.18� , respectively, as shown in Fig. 7b and Table 1. After 60 s, the contact angle of PEP separator gradually decreased to 90.6� . In comparison, the DMC droplet wicked into ZIF8-2CNF separator at 42s with a contact angle of 0� . The contact angle of PEP separator did not change greatly in the initial 60 s, indicating that the electrolyte was not easily absorbed by the PEP separator. Since the speed of surface wetting is associated with the inner resistance of battery [4], the replacement of ZIF8-2-CNF separator with PEP separator can help decrease the battery internal resistance and improve the battery performance.
3.5. Mechanical and surface wettability of CNFs and ZIF8-CNF hybrids Mechanical properties of separators have a great impact on the safety and durability of li-ion batteries. During the battery winding and fabrication process, the separator has to sustain compression and tension induced by mechanical handling. In the service life of battery, the separator also needs to withstand potential cyclic deformation of the electrodes and penetration of dendritic growth [43,44]. The tensile strain–stress curves of CNF, ZIF8-CNF, and PEP separators are shown in Fig. 7a. The maximum tensile stresses at break for CNF, ZIF8-0.5-CNF, ZIF8-1-CNF, and ZIF8-2-CNF separators were 101.6, 94.1, 85.4, and 77.2 MPa, respectively. The mechanical property of CNF separator was mainly provided by the formation of strong hydrogen bonding among the cellulose nano-fibrils during the drying process [45]. The decreased tensile strength with the additional synthesis of ZIF8 was due to the combination of two effects. First, some hydrogen bonding sites were removed, which are illustrated in the FTIR spectra. Second, the synthesis of ZIF8 on the surface of CNFs prevented the aggregation of CNFs during the drying process. In contrast to the isotropic property of CNF based separator, PEP separator exhibited anisotropic property with the maximum tensile stress at break for PEP separator at machine, diagonal and transverse directions of 150.6, 22.14, and 15.18 MPa, respectively.
3.6. Performance of LIBs with CNFs and ZIF8-CNF hybrid membranes The cycling performance of LIBs using ZIF8-2-CNF, pure CNF and PEP separators are shown in Fig. 8a. The discharge capacity and capacity retention of ZIF8-2-CNF separator after 100th cycle were about 112 mAh g 1 and 88.2%, which were higher than the corresponding values of PEP separator (102 mAh g 1 and 80.3%). This was due to the excellent surface wettability of ZIF8-CNF separator, good porosity (Table 1) and wetting speed (Fig. 7b) compared with PEP separator. In addition, ZIF8-2-CNF separator exhibited larger discharge capacity retention than CNF separator. The better cycling performance of ZIF8-2CNF separator was mainly attributed to the improved nano-porous structure [15]. Both the pore and pore size of ZIF8-2-CNF separator distributed more homogeneously, which improved the drawbacks of
Fig. 6. XRD patterns of CNF, ZIF8-CNF, and PEP separators. (a) CNFs and ZIF8-CNF separators, (b) PEP separators. 8
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Fig. 7. Mechanical property and surface wettability of CNF, ZIF8-CNF, and PEP separators, (a) tensile strain–stress curves, (b) change of contact angles in the initial 60 s with DMC as the reference solvent.
CNF separator-dense membrane (Fig. 2b) and big pores (Fig. 2c and d). The initial 5 and specific 10th, 20th, 30th, 40th, and 50th charge-discharge cycles of LIB assembled with ZIF8-2-CNF separator are shown in Fig. 8b and c. No obvious irreversible capacity loss and no abnormal charge-discharge profiles were observed at this stage. This was mainly due to the good electrolyte retention of the ZIF8-2-CNF separator. The relevant charge-discharge profiles of CNF separator are shown in Fig. 8d and a small capacity loss was observed between 10th and 20th cycles. For PEP separator (Fig. 8e), an irreversible capacity loss was observed at the first 20th cycles. The ZIF8-2-CNF separator exhibited better cycling performance in comparison of the PEP sepa rator, indicating that ZIF8-CNFs can be considered as a good alternative material to replace PEP. The rate capacity of LIB with ZIF8-2-CNF and PEP separators was evaluated at 8 different rates and the results are shown in Fig. 8f. Batteries with ZIF8-2-CNF separator exhibited better rate capacity than that of PEP separator. A downward trend was observed for the discharge capacity of LIBs with these two types of separators when rate was increased from 200 to 5000 mA g 1. This phenomenon was due to the formation of membrane at the surface of active materials in the electrode when Li ion transporting in and out of electrode, which is consistent with previous reports [46]. The discharge capacity of LIB with ZIF8-2-CNF and PEP separators was about 132 and 129 mA g 1 at a current density of 0.2 C, and the corresponding value decreased to approximately 82 and 51 mA g 1 at a current density of 8 C. However, the difference of discharge capacity between ZIF8-2-CNF and PEP separators became larger with the increase of rate. For example, the gap of discharge capacity at a current density of 1 C was about 10 mAh g 1, whereas the gap increased to about 30 mAh g 1 at a current density of 8 C. The ion conductivity and interfacial resistivity had a great in fluence on the LIB performance including charge and discharge capac ity, rate capacity, and cycle life [44]. EIS measurements were used to determine the interfacial compatibility of LIBs with ZIF8-CNF, CNF and PEP separators and the Nyquist plots are shown in Fig. 8g. LIBs assem bled with ZIF8-CNF hybrid membrane exhibited a lower interfacial re sistivity than that of the CNF, and PEP separators, mainly due to the good electrochemical interface stability and fast liquid electrolyte wet ting behavior of ZIF8-CNF separator as illustrated in Fig. 7 [18]. The interfacial resistivity also decreased when more ZIF8 material was synthesized on the surface of CNFs. The corresponding interfacial re sistivity of ZIF8-2-CNF, CNF and PEP separators was about 170 Ω, 210 Ω and 320 Ω, respectively, after 100 cycles. The linear sweep voltammo grams (LSV) of these separators are illustrated in Fig. 8h. ZIF8-2-CNF and CNF separator exhibited good electrochemical stability and no obvious decomposition of the main components in LIB was observed before 4.4 V, which are comparable with the conventional PEP sepa rator. The ion conductivity (λ) of ZIF8-2-CNF and PEP separators was calculated by using a simple formula λ ¼ T=RA (T: thickness of
separator, R: resistivity, A: the area of separator) [4]. The ion conduc tivities of ZIF8-2-CNF and PEP separators were approximately 1:41 � 10 3 S cm 1 , and 0:64 � 10 3 S cm 1 . The Liþ TN was estimated (Fig. S2) using the same equation from Chi et al. [26]. The Liþ TN of ZIF8-2-CNF, CNF and PEP separators were 0.50, 0.39, and 0.34, respectively (Table 1). The higher Liþ TN of ZIF8-2-CNF separator was mainly due to the improved porous structure with the addition of ZIF8 and better surface wettability of CNF. The higher ion conductivity, Liþ transference number, and lower interfacial resistivity of ZIF8-2-CNF separator contributed to the better charge and discharge capacity and rate capacity. The combination of SEM images and elemental mapping analysis were used to analyze the stability of prepared separators. The ZIF8-2-CNFs separator remained stable after cycling test as illustrated in Fig. S1. The observed small irregular particles (Figs. S1a and S1b) on the PEP and CNF separators were due to the residual electrolyte-LiPF6. ZIF-8 had a cubic structure and the residual electrolyte-LiPF6 had an irregular structure as shown in Fig. S1c. The P-elemental map (Fig. S1e) confirmed the homogeneous distribution of electrolyte in the ZIF8-2-CNFs separator. 4. Conclusions ZIF8 crystals were successfully synthesized on the surface of CNFs, and the ZIF8-CNF composite separator was fabricated using a vacuum filtration method. The use of ZIF8 in the CNF system improved the pore structure of composite separators as compared with the pure CNF separator. The introduction of ZIF8 effectively prevented the aggrega tion of CNFs, leading to more homogeneously distributed pores. In addition to the environmentally friendly advantage of CNFs, ZIF8-CNF composite separators also exhibited better thermal stability, and ther mal dimensional stability than those of commercial PEP separator. ZIF8CNF composite separator was thermally stable up to 200 � C, helping improve the safety of LIBs under the rigorous environment. ZIF8-CNF composite separators had better surface wettability and fast wetting speed, which can help decrease the electrolyte filling time and battery interior resistance. The ZIF8-CNF composite separators had more isotropic mechanical properties. LIBs made with ZIF8-CNF nano composite separator exhibited better discharge capacity retention, cycling performance and rate capacity. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Fig. 8. Performance of LIBs made with ZIF8-CNF, CNF and PEP separators. (a) Cycle performance at a current density of 0.5 C, (b) capacity-voltage curves com parison, (c) capacity-voltage curves with ZIF8-CNF separator, (d) capacity-voltage curves with CNF separator, (e) capacity-voltage curves with PEP separator, (f) rate capacity, (g) nyquist plots and (h) LSV of LIBs using ZIF8-CNF, pure CNF and PEP separators.
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Acknowledgement
[19] K.S. Park, Z. Ni, A.P. C^ ot� e, J.Y. Choi, R. Huang, F.J. Uribe-Romo, H.K. Chae, M. O’Keeffe, O.M. Yaghi, Proc. Natl. Acad. Sci. U.S.A. 103 (2006) 10186–10191. [20] H. Wu, W. Zhou, T. Yildirim, J. Appl. Comput. Sci. 129 (2007) 5314–5315. [21] Q. Song, S. Nataraj, M.V. Roussenova, J.C. Tan, D.J. Hughes, W. Li, P. Bourgoin, M. A. Alam, A.K. Cheetham, S.A. Al-Muhtaseb, Energy Environ. Sci. 5 (2012) 8359–8369. [22] G. Lu, J.T. Hupp, J. Appl. Comput. Sci. 132 (2010) 7832–7833. [23] K. Yang, K. Yang, S. Chao, J. Wen, Y. Pei, Z. Pei, Chem. Commun. 54 (2018) 9817–9820. [24] S. Aguado, J. Canivet, D. Farrusseng, Chem. Commun. 46 (2010) 7999–8001. [25] W. Sun, X. Zhai, L. Zhao, Chem. Eng. J. 289 (2016) 59–64. [26] M. Chi, L. Shi, Z. Wang, J. Zhu, X. Mao, Y. Zhao, M. Zhang, L. Sun, S. Yuan, Nano Energy 28 (2016) 1–11. [27] D. Zhang, P. Liu, S. Xiao, X. Qian, H. Zhang, M. Wen, Y. Kuwahara, K. Mori, H. Li, H. Yamashita, Nanoscale 8 (2016) 7749–7754. [28] M. Jian, B. Liu, R. Liu, J. Qu, H. Wang, X. Zhang, RSC Adv. 5 (2015) 48433–48441. [29] H.V. Lee, S.B.A. Hamid, S.K. Zain, Sci. World J. 2014 (2014) 20. [30] J. Zhang, L. Yue, Q. Kong, Z. Liu, X. Zhou, C. Zhang, Q. Xu, B. Zhang, G. Ding, B. Qin, Y. Duan, Q. Wang, J. Yao, G. Cui, L. Chen, Sci. Rep. 4 (2014) 3935. [31] B. Weng, F. Xu, M. Alcoutlabi, Y. Mao, K. Lozano, Cellulose 22 (2015) 1311–1320. [32] Y. Grosu, G. Renaudin, V. Eroshenko, J.M. Nedelec, J.P.E. Grolier, Nanoscale 7 (2015) 8803–8810. [33] D. Pan, L. Wang, Z. Li, B. Geng, C. Zhang, J. Zhan, L. Yin, L. Wang, New J. Chem. 42 (2018) 5083–5089. [34] F. Tian, A.M. Cerro, A.M. Mosier, H.K. Wayment-Steele, R.S. Shine, A. Park, E. R. Webster, L.E. Johnson, M.S. Johal, L. Benz, J. Phys. Chem. C 118 (2014) 14449–14456. [35] S.Y. Oh, D.I. Yoo, Y. Shin, H.C. Kim, H.Y. Kim, Y.S. Chung, W.H. Park, J.H. Youk, Carbohydr. Res. 340 (2005) 2376–2391. [36] Y. Hu, H. Kazemian, S. Rohani, Y. Huang, Y. Song, Chem. Commun. 47 (2011) 12694–12696. [37] P. Cheng, Y.H. Hu, J. Phys. Chem. C 118 (2014) 21866–21872. [38] Q. Wang, P. Ping, X. Zhao, G. Chu, J. Sun, C. Chen, J. Power Sources 208 (2012) 210–224. [39] A. Schejn, L. Balan, V. Falk, L. Aranda, G. Medjahdi, R. Schneider, CrystEngComm 16 (2014) 4493–4500. [40] H. Kaur, G.C. Mohanta, V. Gupta, D. Kukkar, S. Tyagi, J. Drug Deliv. Sci. Technol. 41 (2017) 106–112. [41] J.-H. Lin, Y.-J. Pan, C.-F. Liu, C.-L. Huang, C.-T. Hsieh, C.-K. Chen, Z.-I. Lin, C.W. Lou, Materials 8 (2015) 5496. [42] X. Zhang, H. Wu, S. Guo, Y. Wang, RSC Adv. 5 (2015) 99812–99819. [43] A. Sheidaei, X. Xiao, X. Huang, J. Hitt, J. Power Sources 196 (2011) 8728–8734. [44] X. Huang, J. Solid State Electrochem. 15 (2011) 649–662. [45] I. Sir� o, D. Plackett, Cellulose 17 (2010) 459–494. [46] P. Ramadass, B. Haran, R. White, B.N. Popov, J. Power Sources 123 (2003) 230–240.
This collaborative study was carried out with support from Louisiana Board of Regents, Baton Rouge, Louisiana, United States [LEQSF(201718)-RD-A-01; LEQSF(2018-19)-ENH-DE-06], and National Institute of Forest Science,Seoul, Republic of Korea. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2020.227878. References [1] J.B. Goodenough, K.-S. Park, J. Appl. Comput. Sci. 135 (2013) 1167–1176. [2] S. Goriparti, E. Miele, F. De Angelis, E. Di Fabrizio, R. Proietti Zaccaria, C. Capiglia, J. Power Sources 257 (2014) 421–443. [3] H. Li, Z. Wang, L. Chen, X. Huang, Adv. Mater. 21 (2009) 4593–4607. [4] P. Arora, Z. Zhang, Chem. Rev. 104 (2004) 4419–4462. [5] S.S. Zhang, J. Power Sources 164 (2007) 351–364. [6] K. Xu, Chem. Rev. 114 (2014) 11503–11618. [7] P. Verma, P. Maire, P. Nov� ak, Electrochim. Acta 55 (2010) 6332–6341. [8] Y. Xiang, J. Li, J. Lei, D. Liu, Z. Xie, D. Qu, K. Li, T. Deng, H. Tang, ChemSusChem 9 (2016) 3023–3039. [9] R.J. Moon, A. Martini, J. Nairn, J. Simonsen, J. Youngblood, Chem. Soc. Rev. 40 (2011) 3941–3994. [10] X. Sun, Q. Wu, X. Zhang, S. Ren, T. Lei, W. Li, G. Xu, Q. Zhang, Cellulose 25 (2018) 1103–1115. [11] A. Isogai, T. Saito, H. Fukuzumi, Nanoscale 3 (2011) 71–85. [12] X. Sun, C. Mei, A.D. French, S. Lee, Y. Wang, Q. Wu, Cellulose 25 (2018) 5071–5087. [13] Q. Xu, Q. Kong, Z. Liu, X. Wang, R. Liu, J. Zhang, L. Yue, Y. Duan, G. Cui, ACS Sustain. Chem. Eng. 2 (2014) 194–199. [14] G. Ding, B. Qin, Z. Liu, J. Zhang, B. Zhang, P. Hu, C. Zhang, G. Xu, J. Yao, G. Cui, J. Electrochem. Soc. 162 (2015) A834–A838. [15] L. Jabbour, R. Bongiovanni, D. Chaussy, C. Gerbaldi, D. Beneventi, Cellulose 20 (2013) 1523–1545. [16] J. Zhang, Z. Liu, Q. Kong, C. Zhang, S. Pang, L. Yue, X. Wang, J. Yao, G. Cui, ACS Appl. Mater. Interfaces 5 (2013) 128–134. [17] Q. Xu, Q. Kong, Z. Liu, J. Zhang, X. Wang, R. Liu, L. Yue, G. Cui, RSC Adv. 4 (2014) 7845–7850. [18] S.-J. Chun, E.-S. Choi, E.-H. Lee, J.H. Kim, S.-Y. Lee, S.-Y. Lee, J. Mater. Chem. 22 (2012) 16618–16626.
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