membranes

Article A Novel Electrospinning Polyacrylonitrile Separator with Dip-Coating of Zeolite and Phenoxy Resin for Li-ion Batteries

Danxia Chen 1, Xiang Wang 1,* , Jianyu Liang 2,*, Ze Zhang 1 and Weiping Chen 1

1 School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China; [email protected] (D.C.); [email protected] (Z.Z.); [email protected] (W.C.) 2 Department of Mechanical Engineering, Worcester Polytechnic Institute, 100 Institute Road, Worcester, MA 01609, USA * Correspondence: [email protected] (X.W.); [email protected] (J.L.)

Abstract: Commercial separators (polyolefin separators) for lithium-ion batteries still have defects such as low thermostability and inferior interface compatibility, which result in serious potential safety distress and poor electrochemical performance. Zeolite/Polyacrylonitrile (Z/PAN) composite separators have been fabricated by electrospinning a polyacrylonitrile (PAN) membrane and then dip-coating it with zeolite (ZSM-5). Different from commercial separators, the Z/PAN composite separators exhibit high uptake, excellent ionic conductivity, and prominent thermal stability. Specifically, the Z/PAN-1.5 separator exhibits the best performance, with a high electrolyte uptake of 308.1% and an excellent ionic conductivity of 2.158 mS·cm−1. The Z/PAN-1.5 separator ◦  may mechanically shrink less than 10% when held at 180 C for 30 min, proving good thermal  stability. Compared with the pristine PAN separator, the Li/separator/LiFePO4 cells with the · −1 Citation: Chen, D.; Wang, X.; Liang, Z/PAN-1.5 composite separator have excellent high-rate discharge capacity (102.2 mAh g at −1 J.; Zhang, Z.; Chen, W. A Novel 7 C) and favorable cycling performance (144.9 mAh·g at 0.5 C after 100 cycles). Obviously, the Electrospinning Polyacrylonitrile Z/PAN-1.5 separator holds great promise in furthering the safety and performance of lithium-ion Separator with Dip-Coating of Zeolite batteries. and Phenoxy Resin for Li-ion Batteries. Membranes 2021, 11, 267. Keywords: separator; electrospinning; polyacrylonitrile; zeolite; lithium-ion batteries https://doi.org/10.3390/ membranes11040267

Academic Editor: Michael 1. Introduction O. Daramola As a high-efficiency energy storage system, lithium-ion batteries have excellent per- formance such as greater energy density, high operational voltage, long cycle life, and low Received: 9 March 2021 self-discharge rate. They have been widely used in mobile phones, intelligent speakers, Accepted: 4 April 2021 Published: 8 April 2021 notebook computers, electric scooters, and other portable electronic equipment [1–6]. With people’s increasing awareness of environmental protection and exhausted fossil oil, electric

Publisher’s Note: MDPI stays neutral bicycles and electric vehicles have become the most promising industries. In recent years, with regard to jurisdictional claims in incidents of spontaneous combustion and even explosion of mobile power supplies, electric published maps and institutional affil- bicycles, and electric vehicles have occurred one after another, which puts forward higher iations. requirements for the safety of lithium-ion batteries [7–10]. Lithium-ion batteries usually are formed from cathodes, anodes, separators, and . As a crucial constituent of the lithium-ion battery, the separator mainly plays a role in isolating cathodes and anodes and allows lithium ions to be transported freely in the lithium-ion battery [11,12]. The low of polyolefin results in the bad thermal stability of polyolefin separa- Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. tors. At the same time, inferior interface compatibility between polyolefin separators and This article is an open access article electrolytes leads to junior electrolyte wettability. Therefore, it is essential to develop a distributed under the terms and separator with prominent thermal stability and excellent wettability for lithium-ion battery conditions of the Creative Commons applications [13,14]. Attribution (CC BY) license (https:// In recent years, researchers have made extensive efforts to compensate for the short- creativecommons.org/licenses/by/ comings of polyolefin separators [15–19]. Electrospinning technology can be used to 4.0/). fabricate fibrous membranes with microporous and nanoporous structures. Electrospun

Membranes 2021, 11, 267. https://doi.org/10.3390/membranes11040267 https://www.mdpi.com/journal/membranes Membranes 2021, 11, 267 2 of 11

fibrous membranes are widely used as separators due to their higher electrolyte absorption rate and excellent wettability with the electrolyte [20–23]. The commonly used for preparing electrospinning membranes are polyvinylidene fluoride (PVDF) and its , polyimide (PI), polyacrylonitrile (PAN), etc. [24]. PVDF has a strong electron- withdrawing group (-C-F-), which is conducive to the ionization of lithium salt in the electrolyte, but its high degree of crystallinity will affect its ionic conductivity [20]. PI has good chemical stability and thermal stability due to its rigid aromatic ring and polar imide ring. It is a with good comprehensive performance [25], but its high cost limits its application. The high melting point of PAN makes it have good thermal stability at high temperatures, and the nitrile group (-CN) in PAN is in favor of the dissociation of lithium ions in the electrolyte. Therefore, PAN is used to prepare separator skeletons by electrospinning due to its super thermal stability, good electrochemical properties, and reasonable price. The biggest problem in the application of fiber membranes in lithium-ion batteries is poor mechanical strength [26–30]. Phenoxy resin is a kind of thermoplastic resin, which does not need a curing agent in actual use. ZSM-5 is an aluminosilicate with uniform micropores and three-dimensional intersecting straight channels, with excellent thermal and chemical stability. Zeolites are mostly embedded by coating or blending [31]. Xiao et al. [32] prepared NaA zeolite- embedded polyethylene terephthalate (PET) nonwoven, and the thermal resistance and ionic conductivity were clearly improved due to the NaA zeolite-embedding. Li et al. [33] blended a polyimide (PI) polymer with a high melting point and zeolite (ZSM-5) to fab- ricate the zeolite/polyimide composite separator via a phase inversion process, with the purpose of enhancing the electrochemical properties and thermal stability of lithium-ion batteries. However, the addition of ZSM-5 significantly reduced the overall strength of the composite separator. It was difficult for ZSM-5 to disperse evenly in the casting liquid. Zhang et al. [34] prepared a zeolite/polyvinylidene fluoride (PVDF) composite separator by a phase inversion process. The result shows that the zeolite/PVDF composite sepa- rator exhibits excellent liquid electrolyte wettability, well-developed microstructure, and superior thermal resistance. The phase inversion process requires a large amount of sol- vent which is difficult to recycle. However, the above-mentioned composite separators made of non-woven fabrics, PI, or other polymers generally have insufficient mechanical strength, which will affect the safety of lithium-ion batteries under high-temperature con- ditions [35,36]. Phenoxy resin is used as an adhesive to improve the mechanical strength of composite separators because of its excellent mechanical properties and non-toxicity. In this study, a PAN fiber membrane was prepared with the electrospinning technique, phenoxy was introduced to improve the mechanical strength of PAN fiber membranes, and the addition of ZSM-5 was used to improve electrochemical performance. The PAN fiber membrane plays the role of backbone support to provide thermal stability for the separator, the phenolic resin acts as a binder to enhance the mechanical strength and ZSM- 5 can improve electrolyte wettability of the composite separator. The results show that the composite separator has excellent mechanical strength and electrochemical properties. The influence of the concentration of ZSM-5 on the mechanical strength and ionic conductivity of the composite separator was researched and selected. Compared with the original PAN fiber membrane and PE separator, the Z/PAN-1.5 composite separator shows enhanced mechanical strength and electrochemical performance.

2. Materials and Methods 2.1. Materials

Polyacrylonitrile (PAN, purity ≥99.9%, Mw = 150,000) was provided by Hefei Sipin Technology Co., Ltd. (Hefei, China). N,N- (DMF) was bought from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Phenoxy resin was obtained from Guangzhou Jiabei Electronic Technology Co., Ltd. (Guangzhou, China). Acetone was purchased from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). Zeolite (ZSM-5) was supplied by Nankai University Chemical Reagent Factory. (Tianjin, Membranes 2021, 11, 267 3 of 11

China). Lithium iron phosphate (LiFePO4) was obtained from Taiwan Likai Power Tech- nology Co., Ltd. (Taiwan, China). Acetylene black was purchased from Tianjin Yiborui Chemical Co., Ltd. (Tianjin, China). Polyvinylidene fluoride (PVDF) was bought from Arkema Fluor Chemical Co., Ltd. (Suzhou, China). The liquid electrolyte (1 mol·L−1 LiPF6, EC/EMC/DMC (1:1:1, volume ratio)) was supplied by Duoduo Chemical Technology Co., Ltd. (Suzhou, China). Commercial PE separator was purchased from SK Innovation for comparison (Changzhou, China).

2.2. Preparation of Electrospun PAN Separator To prepare the solution for electrospun PAN separator preparation, PAN powder was dissolved in DMF under stirring at room temperature for 12 h to obtain a 10 wt% solution. The electrospinning equipment (ELITE) was provided by Beijing Yongkang Leye Technology Development Co., Ltd. (Beijing, China). Then, under a high voltage of 11 kV, enough solution was taken in the syringe and pushed out at a rate of 0.04 mm/min. During the spinning, the distance between the needle tip and collector was kept at 15 cm. After 11 h of spinning, the pure PAN separator was pressed at 120 ◦C for 30 min and dried at 80 ◦C for 12 h to extirpate the remaining . The pure PAN separator is referred to as PAN in subsequent descriptions.

2.3. Preparation of Composite Separator First, a certain amount of phenoxy resin was dissolved in the mixture of DMF/acetone (1:2, v/v) to obtain a 5 wt% phenoxy resin solution. To prepare the solution for dipping, varying amounts of ZSM-5 were dissolved in 5 wt% phenoxy resin solution under ultrasonic for 12 h. The concentrations of ZSM-5 (w/v) were 0%, 0.5%, 1%, 1.5%, and 2%, respectively. The previously prepared pure PAN separator was immersed in the ZSM-5 dispersion with different contents for 20 min and then taken out. The composite separators were first dried at room temperature until there was no obvious liquid on the surface and then dried in an oven at 60 ◦C for 12 h. For identification, composite partitions containing 0 wt%, 0.5 wt%, 1 wt%, 1.5 wt%, and 2 wt% ZSM-5 were defined as Z/PAN-0, Z/PAN-0.5, Z/PAN-1, and Z/PAN-1.5, Z/PAN-2, respectively.

2.4. Characterization of Composite Separator In the range of 400–4000 cm−1, the Fourier transform infrared spectrum (FTIR, Nexus, Therno Nicolet, Waltham, MA, USA) was applied to demonstrate the presence of phenolic oxygen and ZSM-5 on the separator. The morphologies of composite separators were inves- tigated by scanning electron microscopy (SEM, JSM-IT300, JEOL, Tokyo, Japan) after sput- tering a thin layer of gold. The tensile strengths of the separator samples (50 mm × 10 mm) were tested by using a universal testing apparatus (Instron5967, Instron Pty Ltd., Norwood, MA, USA), at a speed of 5 mm·min−1. The contact angles of the separators and electrolytes were measured with a contact angle meter (OCA20, Dataphysics, Filderstadt, Germany). The electrolyte uptake of the separators was calculated by measuring the weight change before and after being soaked in the electrolyte for 2 h, and then calculated by the equation:

Electrolyte uptake (%) = (M − M0)/M0 × 100 (1)

where M0 is the weight of the separators without being soaked and M is the weight of the separators after being soaked in an electrolyte. The thermal stability of the separators was analyzed by a thermogravimetric analyzer– differential scanning calorimeter (TGA–DSC, STA449F3, Netzsch, Selb, Germany) in the temperature range from 40 ◦C to 800 ◦C at a heating rate of 10 ◦C·min−1. The thermal shrinkage of separators was checked by placing membranes in a drying oven at different temperatures from 120 to 180 ◦C for 30 min. After being heated, the dimensional change was measured by the equation:

Shrinkage (%) = (A0 − A)/ A0 × 100 (2) Membranes 2021, 11, 267 4 of 11

2 2 where A0 (cm ) and A (cm ) are the areas of the separator before and after being heated, respectively.

2.5. Electrochemical Performance of the Composite Separator The composite separator and PE separator were stamped into discs with a diameter of 19 mm for use. The thickness of the PAN and composite separator used is 45 µm in the test. The electrochemical stability, ionic conductivity, and interfacial resistance (Rint) were measured by using an electrochemical workstation (Ivium Stats, Ivium Technologies, Eindhoven, Netherlands). The thickness of the separator samples was measured with a thickness gauge (CH-1-S, Shanghai liuleng, China). The Rint between the separator sample and lithium metal electrode was carried out by electrochemical impedance spectroscopy (EIS). The ionic conductivity of the separators was also determined by electrochemical impedance spectroscopy (EIS), with two stainless steel electrodes. The tested frequency ranged from 0.1 Hz to 1 MHz with a signal amplitude of 5 mV. The ionic conductivity was calculated by the equation: σ = d/(R × S) (3) where d is the thickness of the separators sample, R is the bulk resistance and S is the area of the electrode. The electrochemical stability was measured in a cell of lithium metal/separator/ stainless steel by using liner sweep voltammetry from 2.5 V to 6.0 V at 5 mV·s−1. For rate performances and cyclability tests, a CR2032 coin-type cell was assembled by sandwiching the separator between a lithium anode and a LiFePO4 cathode and then adding a liquid electrolyte. The measurements were performed by battery-testing equipment (CT2001A, LAND Electronics, Wuhan, China). The cell was cycled at a fixed charge/discharge current density of 0.5 C for 100 cycles. Rate capability test was applied to the discharge of 0.2, 0.5, 1, 2, 3, 5, 7, and 0.2 C, under 2.5–4.2 V.

3. Results FT-IR spectrums of the electrospun PAN separator, phenoxy resin/PAN separator, and Z/PAN-1.5 separators were recorded to confirm the smooth incorporation of phenoxy resin and ZSM-5 zeolite, as shown in Figure1. In the infrared spectrum curve of PAN, the peak at 2243 cm−1 and 1453 cm−1 can be vested in the stretching vibration of the -CN bond and the bending vibration of -CH2-[29]. After immersing in 5 wt% phenoxy resin solution, a new peak appeared at 1238 cm−1, which is the characteristic absorption peak of the ether bond connected to the benzene ring, and the absorption peaks of the benzene skeleton at 1508 cm−1 and 1454 cm−1. Besides this, a T-O-T (T=Si or Al) bending vibration peak of ZSM-5 appeared at 553 cm−1, indicating that ZSM-5 is normally embedded into the Z/PAN-1.5 separators [33]. The above indicated that phenoxy resin and ZSM-5 were successfully introduced into the Z/PAN-1.5 separators. Membranes 20212021,, 1111,, x 267x FORFOR PEERPEER REVIEWREVIEW 55 of of 11 11

FigureFigure 1. 1. FTFT-IR--IIR spectra spectra of of electrospun electrospun PAN PAN separator, separator, phenoxy phenoxy resin/PAN resin/PAN separator separator,,, and and Z/PAN Z/PAN--- 1.5 separators.

FigureFigure 22 depictsdepicts the the surface surface morphology morphology of of the thethe electrospun electrospun PAN PAN separator separator and and differ- dif- ferentferentent composite compositecomposite separators separatorsseparators after afterafter immersing immersingimmersing in 5 inin wt% 55 wt% phenoxy phenoxy resin resinresin solution. solution.solution. It is observed ItIt isis ob-ob- servedservedin Figure inin2 FigureFigurea that the 2a2a surfacethatthat thethe of surfacesurface PAN fiber ofof PANPAN is smooth fiberfiber is andis smoothsmooth uniform andand without uniformuniform beads. withoutwithout According beads.beads. Accordingto Figure S1, to itFigure can be S1, seen it can the be average seen the diameter average of diameter the fibers of inthe the fibers membrane in the membrane is 180 nm. isisPAN 180180 fibers nm.. PAN were fibers simply were lapped simply together lapped to together form a PANto form separator a PAN afterseparator heat treatment.after heat treatment.treatment.Although theAlthoughAlthough large number thethe largelarge of voidsnumbernumber in theofof voidsvoids structure inin thethe of PANstructurestructure separators ofof PANPAN can separatorsseparators better absorb cancan betterthe electrolyte, absorb the its electrolyte, poor mechanical its poor properties mechanical cannot properties reach the cannot minimum reach requirements the minimum for requirementsrequirementsuse. Figure2b–f forfor presents use.use. FigureFigure that phenoxy2b–ff presentspresents resin formsthatthat phenopheno a certainxy resin bonding forms effect a certain on the bonding surface effectof PAN on fibers, the surface which of helps PAN to fibers, enhance which the helps mechanical to enhance properties thethe mechanicalmechanical of PAN separators. propertiesproperties As ofof PANthe concentration separators. As of the ZSM-5 concentration increases, of the ZSM composite--5 increase separatorss,, thethe compositecomposite exhibits separatorseparator more and exhib-exhib- more itsitsparticles. moremore andand After moremore embedding particles.particles. a AfterAfter few ZSM-5 embedd particlesinging aa ffew into ZSM the composite--5 particles separators, intointo thethe compositecomposite the sepa- separators,separators,rators’ absorption thethe separatorsseparators of the electrolyte’’ absorption is enhanced of thethe electrolyte due to the isis enhanced multistage due pore to structure the multistage inside poreZSM-5 structure nanoparticles insideside ZSM [37]. --5 nanoparticles [37][37]..

Figure 2. SEM micrographs for the surface of electrospun PAN separator and composite separator with different ZSM-5 concentra- FigureFigure 2. SEM 2. micrographsSEM micrographs for the for surface the surface of electrospun of electrospun PAN separator PAN separator and composite and composite separator separator with different with different ZSM-5 concentra- ZSM- tions:tions: ((a)) PAN,PAN, ((b)) Z/PANZ/PAN--0, (cc)) Z/PANZ/PAN--0.5, (d)) Z/PANZ/PAN--1, (e)) Z/PANZ/PAN--1.5 and (ff)) Z/Z/ PANPAN--2. 5 concentrations: (a) PAN, (b) Z/PAN-0, (c) Z/PAN-0.5, (d) Z/PAN-1, (e) Z/PAN-1.5 and (f) Z/ PAN-2.

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As depicted in Figure 3, the tensile strength of the composite separators that were As depicted in Figure 3, the tensile strength of the composite separators that were immersAsed depicted in the inphenoxy Figure 3resin, the solution tensile strength was effectively of the composite improved. separators The tensile that strength were immersed in the phenoxy resin solution was effectively improved. The tensile strength immersedrose from in2.6 the MPa phenoxy for the resin PAN solution separator was to effectively 13.4 MPa improved.for the Z/PAN The- tensile0 composite strength separa- rose rose from 2.6 MPa for the PAN separator to 13.4 MPa for the Z/PAN-0 composite separa- fromtor. The 2.6 MPamechanical for the PANstrength separator of the toPAN 13.4 separator MPa for the is basically Z/PAN-0 facilitat compositeed by separator. the bonding The tor. The mechanical strength of the PAN separator is basically facilitated by the bonding mechanicalof fibers to each strength other of. theSince PAN the separatorfibers were is basicallybonded by facilitated the addition by the of bondingphenoxy of resin fibers at of fibers to each other. Since the fibers were bonded by the addition of phenoxy resin at tothe each laps other. together, Since slippage the fibers and were breakage bonded of bythe the fibers addition were effectively of phenoxy prevented. resin at the At laps the together,the laps together, slippage andslippage breakage and ofbreakage the fibers of werethe fibers effectively were prevented.effectively prevented. At the same At time, the same time, with the combination of ZSM-5, the tensile strength of the separator dropped withsame the time, combination with the combin of ZSM-5,ation theof ZSM tensile-5, strengththe tensile of strength the separator of the droppedseparator a droppe bit, butd a bit, but the tensile strength of the Z/PAN-1.5 separator was 13 Mpa, which qualified for thea bit, tensile but the strength tensile of strength the Z/PAN-1.5 of the Z/PAN separator-1.5 separator was 13 Mpa, was which 13 Mpa, qualified which forqualified practical for practical battery applications. batterypractical applications. battery applications.

Figure 3. The stress–strain curves of different separators. FigureFigure 3.3. TheThe stress–strainstress–strain curvescurves ofof differentdifferent separators.separators. Figure 4 shows the Nyquist plot of the AC impedance of different liquid-electrolyte- Figure 4 shows the Nyquist plot of the AC impedance of different liquid-electrolyte- soakedFigure separators.4 shows In the the Nyquist high-frequency plot of the region, AC impedance the intercept of different of the fitted liquid-electrolyte- curve on the soakedsoaked separators.separators. In In the the high-frequency high-frequency region, region, the the intercept intercept of theof the fitted fitted curve curve on the on real the real axis stands for the bulk resistance (Rb). The Rb of PAN and Z/PANs (from 0 to 2) sep- real axis stands for the bulk resistance (Rb). The Rb of PAN and Z/PANs (from 0 to 2) sep- axisarators stands is 1.10, for the 1.27, bulk 2.23, resistance 1.42, 1.04 (Rb,). and The 2.14 Rb of Ω, PAN respectively. and Z/PANs The (from ionic 0 conductivity to 2) separators of arators is 1.10, 1.27, 2.23, 1.42, 1.04, and 2.14 Ω, respectively. The ionic conductivity of isPAN, 1.10, Z/PAN 1.27, 2.23,-0 and 1.42, Z/PAN 1.04,-1.5 and separators 2.14 Ω, respectively.is 1.982 mS·cm The−1, 1.712 ionic mS·cm conductivity−1, 2.158 ofmS·cm PAN,−1 PAN, Z/PAN-0 and Z/PAN-1.5 separators is 1.982· mS·cm−1 −1, 1.712 ·mS·cm−1 −1, 2.158 mS·cm· −1−1 Z/PAN-0respectively. and The Z/PAN-1.5 ionic conductivity separators (Table is 1.982 1) mSof thecm Z/PAN, 1.712-1.5 mS separatorcm , 2.158 is higher mS cm than respectively.respectively. TheThe ionicionic conductivityconductivity (Table(Table1 )1) of of the the Z/PAN-1.5 Z/PAN-1.5 separator separator is is higher higher than than that of the Z/PAN-0 separator, which is ascribed to the multistage pore structure inside thatthat ofof thethe Z/PAN-0Z/PAN-0 separator, whichwhich isis ascribedascribed toto thethe multistagemultistage porepore structurestructure insideinside ZSM-5 nanoparticles allowing more liquid electrolyte to be stored, as well as facilitating ZSM-5ZSM-5 nanoparticlesnanoparticles allowingallowing moremore liquidliquid electrolyteelectrolyte toto bebe stored,stored, asas wellwell asas facilitatingfacilitating the migration of lithium-ion. Considering the satisfactory tensile strength and excellent thethe migrationmigration ofof lithium-ion.lithium-ion. ConsideringConsidering thethe satisfactorysatisfactory tensiletensile strengthstrength andand excellentexcellent ion conductivity, the Z/PAN-1.5 separator was chosen as a typical sample of the Z/PAN ionion conductivity,conductivity, thethe Z/PAN-1.5Z/PAN-1.5 separatorseparator waswas chosenchosen asas aa typicaltypical sample sample of of the the Z/PAN Z/PAN separators for succeeding studies. separatorsseparators forfor succeedingsucceeding studies.studies.

FigureFigure 4.4. ACAC impedanceimpedance spectraspectra ofof thethe symmetricsymmetric stainless-steelstainless-steel cellscells ofof differentdifferent separators.separators. Figure 4. AC impedance spectra of the symmetric stainless-steel cells of different separators.

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As listed in Table1, the electrolyte uptake of the PAN separator is the highest at 385.7%,As and listed those in Table of the 1, Z/PAN-0 the electrolyte separator uptake and of Z/PAN-1.5 the PAN separatorseparator areis the 195.2% highest and at 308.1%,385.7%, respectively. and those of It the can Z/PAN be seen-0 that separator phenoxy and resin Z/PAN has-1.5 blocked separator part of are the 195.2% pores and of the308.1%, PAN respectively. separator, which It can reducesbe seen that the porosityphenoxy ofresin the has composite blocked part separator. of the pores However, of the thePAN special separator, micropore which structure reduces ofthe ZSM-5 porosity allows of the the composite composite sepa separatorrator. However, to absorb the more spe- electrolyte,cial micropore so the structure electrolyte of ZSM uptake-5 allows of the the Z/PAN-1.5 composite separator separator has to decreasedabsorb more a little. electro- A separatorlyte, so the with electrolyte high wettability uptake of is the essential Z/PAN for-1.5 batteries, separator as has it facilitates decreased battery a little. assembly A separa- intor battery with high applications wettability [38 ].is Theessential contact for anglebatteries, of an as electrolyte it facilitates on battery different assembly separators in bat- is exhibited in Figure5. Generally, the smaller the contact angle the better the wettability tery applications [38]. The contact angle of an electrolyte on different separators is exhib- of the separator. The PE separator shows a high electrolyte contact angle of 48.8◦, while ited in Figure 5. Generally, the smaller the contact angle the better the wettability of the PAN and composite separators show improved electrolyte wettability. The contact angles separator. The PE separator shows a high electrolyte contact angle of 48.8°, while PAN of PAN, Z/PAN-0, and Z/PAN-1.5 separator are 20.4◦, 21.3◦, and 17.9◦, respectively. The and composite separators show improved electrolyte wettability. The contact angles of lower contact angles of the PAN separator and composite separators originate from their PAN, Z/PAN-0, and Z/PAN-1.5 separator are 20.4°, 21.3°, and 17.9°, respectively. The rough nanofiber surface. lower contact angles of the PAN separator and composite separators originate from their rough nanofiber surface. Table 1. The ionic conductivity, electrolyte uptake, and porosity of different separators soaked with electrolytes at room temperature. Table 1. The ionic conductivity, electrolyte uptake, and porosity of different separators soaked with electrolytesIonic at room Conductivity, temperature. mS ·cm−1 Electrolyte Uptake, % Porosity, % IonicPE Conductivity, mS·cm 0.592−1 Electrolyte Uptake, 45.2 % Porosity, 41.4 % PE PAN0.592 1.98245.2 385.741.4 73.2 Z/PAN-0 1.712 195.2 55.5 PAN 1.982 385.7 73.2 Z/PAN-1.5 2.158 308.1 68.3 Z/PAN-0 1.712 195.2 55.5 Z/PAN-1.5 2.158 308.1 68.3

FigureFigure 5. 5.Digital Digital photographsphotographs of contact angle angle of of an an electrolyte electrolyte on on PE PE separator, separator, PAN PAN separator, separator, Z/PAN-0 separator, and Z/PAN-1.5 separator. Z/PAN-0 separator, and Z/PAN-1.5 separator.

TheThe thermal thermal stability stability of of the the separators separators is is one one of of the the important important indicators indicators of of battery battery safety.safety. A A PE PE separator separator will will shrink shrink severely severely at at high high temperatures, temperatures, causing causing a a short short circuit circuit insideinside the the batterybattery andand serious safety safety issues issues [39] [39.]. As As prese presentednted in inFigure Figure 6a6, a,the the PAN PAN sep- separator,arator, Z/PAN Z/PAN-0,-0, and and Z/PAN Z/PAN-1.5-1.5 separators separators show show an an endothermic endothermic peak atat aboutabout 309 309◦ C°C duedue to to PAN PAN melting. melting. Among Among DSC DSC curves, curves, the the PAN PAN separator separator has has a a higher higher endothermic endothermic peak,peak, because because the the molecular molecular chain chain structure structure of of PAN PAN is is more more rigid rigid than than phenoxy phenoxy resin, resin, a a PANPAN separator separator will will absorb absorb more more heat heat per per unit unit time time when when it it reaches reaches the the decomposition decomposition temperature than the Z/PAN-0 separator. The melting temperature of PE separators is

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temperature than the Z/PAN-0 separator. The melting temperature of PE separators ◦ isgenerally generally 130 130 °CC which which is ismuch much lower lower than than that that of PAN of PAN separators separators and and composite composite sepa- separators.generallyrators. It 130can It can °Cbe seen bewhich seen from is from much the theresul lower resultsts ofthan the of that theTG TGcurvesof PAN curves in separators Figure in Figure 6b andthat6b that compositethe thequality quality sepa- of the ◦ ofrators.three the three differentIt can different be separatorsseen separatorsfrom thebegins resul begins tots decr of totheease decrease TG significantly curves significantly in Figure at around 6b at that around 300 the °C. quality300The qualityC. of The the of qualitythreeZ/PAN different of-0 Z/PAN-0 and Z/PANseparators and-1.5 Z/PAN-1.5 beginsseparators to decr separatorsdecreaseease significantlys more decreases slowly at more aroundthan slowlythat 300 of the°C. than PANThe that quality separator of the of ◦ PANZ/PANduring separator- 0300 and °C Z/PAN during–400 °C.-1.5 300–400 T separatorshe temperatureC. The decrease temperature hass notmore reached slowly has not the than reached decomposition that of the the decomposition PAN temperature separator temperatureduringof the phenoxy300 °C of– the400 resin phenoxy°C. structure, The temperature resin an structure,d the hasphenoxy andnot reached the resin phenoxy is thea certain decomposition resin obstacle is a certain in temperature decomposi- obstacle inoftion decomposition. the. Withphenoxy the temperatureresin With structure, the temperature rising, and thethe phenoxyphenoxy rising, the resinresin phenoxy graduallyis a certain resin decomposes gradually obstacle in decomposes decomposi-when reach- whentioning. theWith reaching decomposition the temperature the decomposition temperature rising, the temperature of phenoxy the phenoxy resin of the resingradually phenoxy, and decomposesthe resin, quality and decline thewhen quality sreach- faster declinesingthan the before fasterdecomposition. To than further before. temperature invest To furtherigate theof investigate thethermal phenoxy shrinkage the resin thermal, ofand shrinkagethe the composite quality ofthe decline separators, composites faster the separators,thanseparator before thes. amplesTo separator further are investsandwiched samplesigate are the by sandwiched thermal two glasses shrinkage by and two then of glasses the parke composite andd in then an separators,oven parked at a in series anthe ◦ ◦ ovenseparatortemperature at a series samples from temperature are120 sandwiched °C tofrom 180 °C 120 by for twoC 30 to min glasses 180. AsC anddisplayed for 30then min. parke in As Figured displayed in an7, The oven inPE at Figure separator a series7, Thetemperatureshrank PE separator acute fromly ( shrank88 120%) °Cwith acutely to its180 color °C (88%) for vary 30 with ingmin its from. As color displayed white varying to transparent, in from Figure white 7, Thewhereas to transparent,PE separator the PAN whereasshrankseparator acute the basically PANly (88 separator%) did with not basicallyits change color ,vary didand noting the change,from composite white and separatorsto the transparent, composite varied separatorswhereas little in the size varied PAN. The littleseparatorcolor in size.of the basically The Z/PAN color did-1.5 of not theseparator change Z/PAN-1.5 ,was and separatoronly the compositeslightly was yellow. only separators slightly Obviously, varied yellow. the little Obviously, excellent in size. thether-The excellentcolormal staof the thermalbility Z/PAN of stabilitythe-1.5 composite separator of the compositeseparator was only separatorisslightly supposed yellow. is supposedto further Obviously, to the further safetythe excellent the of safetythe corre-ther- of the corresponding battery. malsponding stability battery. of the composite separator is supposed to further the safety of the corre- sponding battery.

FigureFigure 6. (6.a )(a DSC) DSC and and (b )(b TG) TG curves curves of of the the PAN PAN separator, separator, the the Z/PAN-0 Z/PAN and-0 and Z/PAN-1.5 Z/PAN-1.5 separator. separator. Figure 6. (a) DSC and (b) TG curves of the PAN separator, the Z/PAN-0 and Z/PAN-1.5 separator.

Figure 7. Thermal shrinkage of the separators observed at different temperatures. FigureFigure 7. 7.Thermal Thermal shrinkage shrinkage of of the the separators separators observed observed at at different different temperatures. temperatures.

MembranesMembranes2021 2021, 11, 11, 267, x FOR PEER REVIEW 99 of of 11 11

FigureFigure8 a8a depicts depicts the the equivalent equivalent circuit circuit and and the the AC AC impedance impedance spectra spectra of of different different separators.separators. It It is is clear clear that that the the Z/PAN-1.5 Z/PAN-1.5 separators separators (about (about 331 331Ω Ω)) show show much much smaller smaller R Rintint thanthan the the PAN PAN separator separator (about (about 439 439Ω Ω)) and and Z/PAN-0 Z/PAN-0 separator separator (about (about 471 471Ω Ω).). The The smaller smaller thethe RRintint, the the better better the the interface interface compatibility. compatibility. Obviously, Obviously, the the Z/PAN Z/PAN-1.5-1.5 separator separator has has the thebest best interface interface compatibility compatibility with with electrodes. electrodes. The Thereason reason should should be attributed be attributed to the to supe- the superiorrior electrolyte electrolyte wettability wettability and and preservation preservation of ofthe the ZSM ZSM-5-5 embedd embeddeded separator, separator, which which is isconducive conducive to to favorable favorable contact contact between between the the separator and electrodes.electrodes. TheThe electrochemicalelectrochemical stabilitystability windowwindow ofof the separator is is evaluated evaluated by by linear linear sweep sweep voltammetry voltammetry as as shown shown in inFig Figureure 8b8.b. It Itis isobvious obvious that that the the elect electrochemicalrochemical window window of the of theZ/PAN Z/PAN-1.5-1.5 separator separator looks lookslike that like of that the of PAN the separator, PAN separator, indicating indicating that the that Z/PAN the Z/PAN-1.5-1.5 separator separator is stable is enough stable enoughto meetto the meet battery thebattery applications. applications. According According to Figure toFigure 8c, the8 c,discharge the discharge capacities capacities of the ofcells the with cells different with different separ separatorsators reduce reduce gradually gradually with withthe raising the raising of discharge of discharge current current den- density.sity. The The discharge discharge capacities capacities of cells of cells using using the thePAN PAN separator separator,, Z/PAN Z/PAN-0-0 separator separator and −1 −1 −1 andZ/PAN Z/PAN-1.5-1.5 separator separator are 155.5 are 155.5 mAh·g mAh−1,· 141.2g , 141.2 mAh·g mAh−1, and·g 152.5, and mAh·g 152.5 mAh−1 at ·0.2g C,at respec- 0.2 C, respectively.tively. It is evident It is evident that the that Z/PAN the Z/PAN-1.5-1.5 separator separator possesses possesses much higher much capacity higher capacityretention retentionthan the thanPAN the separator PAN separator when the when discharge the discharge current currentdensities densities changechange from 0.2 from C to 0.2 7 CC. −1 toThe 7 C.specific The specificdischarge discharge capacity capacity of the Z/PAN of the-1.5 Z/PAN-1.5 separator separatoris 102.2 m isAh·g 102.2−1, the mAh Z/PAN·g ,- the1.5 Z/PAN-1.5separator still separator maintains still 67% maintains of the 67%discharge of the capacity discharge at capacity 0.2 C when at 0.2 the C whendischarge the dischargecurrent densities current densitiescycle at 7 cycleC. At atthe 7 C.same At thetime, same the time,Z/PAN the-1.5 Z/PAN-1.5 separator separatoralmost possesses almost possesseslittle capacity. little capacity.Figure 8d Figure prove8sd the proves cycling the performance cycling performance of batteries of batteries combined combined with dif- withferent different separators separators at 0.5 C. atIt 0.5can C.be Itseen can that be seenthe Z/PAN that the-1.5 Z/PAN-1.5 separator exhibit separators better exhibits cycle betterstability cycle up stabilityto 100 cycles up to with 100 cycleslittle degradation with little degradation when cycled when at a permanent cycled at a current permanent den- currentsity of 0.5 density C. The of 0.5superior C. The rate superior performance rate performance and cycling and performance cycling performance of the Z/PAN of the-1.5 Z/PAN-1.5separator demonstrate separator demonstrate its high feasibility its high for feasibility actual battery for actual applications. battery applications.

FigureFigure 8. (a) 8. The(a) Theinterfacial interfacial AC impedance AC impedance spectra spectra of cells of cells assembled assembled with with different different separators; separators; (b) Linear (b) Linear scan scan voltammograms voltammo- (LSV)grams curves (LSV) of different curvesof separators; different separators;(c) Rate capability (c) Rate of capability the cells ofwith the different cells with separators different separatorsat the current at the densities current range densities from 0.2 C to range7 C; (d from) Cycle 0.2 performance C to 7 C; (d) Cycleof the performancecells with different of the cellsseparators with different under 0.5 separators C. under 0.5 C.

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4. Conclusions In this paper, electrospinning technology is used to prepare a PAN separator. After phenoxy resin and ZSM-5 treatment, a Z/PAN-1.5 separator with a tensile strength of 13 MPa is prepared. Due to the excellent thermal stability of the PAN matrix and ZSM-5, the Z/PAN-1.5 separator exhibits satisfactory thermal durability, which is good for the safety of batteries. Simultaneously, it also has superb electrolyte wettability and is low interface resistance. More importantly, the battery assembled with the Z/PAN-1.5 separator shows stable cycle performance and excellent rate performance. In general, the novel composite separator can function as an excellent performance separator, a good substitute for actual Li-ion batteries.

Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/membranes11040267/s1, Figure S1: The size distribution of membrane fiber. Author Contributions: Conceptualization, X.W.; investigation, D.C., X.W., J.L., Z.Z., W.C.; writing— original draft preparation, D.C.; writing—review and editing, X.W., D.C.; supervision, X.W., J.L.; All authors have read and agreed to the published version of the manuscript. Funding: The work was supported by the National Natural Science Foundation of China (51672201). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Informed consent was obtained from all subjects involved in the study. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Conflicts of Interest: The authors declare no conflict of interest.

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