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May 23, 2018 - School of Materials Science and Engineering, Wuhan University of Technology, ... for Materials Synthesis and Processing, Wuhan University of.
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Electrospun Polyethylene Terephthalate Nonwoven Reinforced Polypropylene Separator: Scalable Synthesis and Its Lithium Ion Battery Performance Haopeng Cai 1 , Xing Tong 1 , Kai Chen 2 , Yafei Shen 2 , Jiashun Wu 1 , Yinyu Xiang 3 , Zhao Wang 1,4, * ID and Junsheng Li 3, * ID 1 2 3 4

*

School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China; [email protected] (H.C.); [email protected] (X.T.); [email protected] (J.W.) Wuhan Jingce Electronic Technology Co., Ltd., Wuhan 430070, China; [email protected] (K.C.); [email protected] (Y.S.) School of Chemistry, Chemical Engineering and Life Sciences, Wuhan University of Technology, Wuhan 430070, China; [email protected] State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China Correspondence: [email protected] (Z.W.); [email protected] (J.L.); Tel.: +86-027-87756662 (J.L.)  

Received: 18 May 2018; Accepted: 21 May 2018; Published: 23 May 2018

Abstract: A novel polyethylene terephthalate nonwoven reinforced polypropylene composite separator (PET/PP) with high thermal stability and low thermal shrinkage characteristic is developed through a scalable production process. In the composite separator, the electronspun polyethylene terephthalate nonwoven layer improves the electrolyte affinity and can sustain as the barrier layer after the shutdown of the polypropylene layer. Due to its high ionic conductivity, the PET/PP separator shows an excellent discharge capacity. In addition, the superior thermal stability of the separator significantly enhances the safety performance of the separator. Considering the feasibility of the large-scale production of the PET/PP separator and its superior battery performance, we expect that the novel separator could be a promising alternative to the existing commercial separators. Keywords: separator; electronspin; PET nonwoven; polypropylene; lithium ion battery

1. Introduction Due to its high capacity, low self-discharge rate, long cycle life, as well as the feasibility of controlling the discharge performance through the battery management system [1,2], advanced energy devices such as a lithium-ion battery (LIB) are considered as an attractive power source for a wide variety of applications ranging from consumer electronics to electric vehicles. However, compared to their counterparts such as the proton exchange membrane fuel cell [3,4], possible safety concerns arising from the shortage of the LIB have posed a great threat for the practical application of LIBs [5–7]. All the components of the battery should be carefully engineered to improve the safety performance of LIB [8]. The separator is an indispensable component of the LIB, which could retain electrolyte, transport ionic conductor carriers between the two electrodes, prevent the shortage between electrodes, and perform safe deactivation of the LIBs under overcharge, abnormal heating, or mechanical rupture conditions. Polyolefin separators, such as polypropylene (PP) and polyethylene (PE), have been widely used for LIBs because of their superior mechanical and chemical stability. However, polyolefin separators have poor wetting/retaining ability to the electrolyte solutions and will shrink or melt at elevated temperatures, which tends to cause an internal short-circuit [9–14]. Coating the Polymers 2018, 10, 574; doi:10.3390/polym10060574

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polyolefin separators with ceramic powders [15–21] is the most frequently used approach to solve the aforementioned problems. However, the ceramic powders normally have poor combability with the separator and the formation of a physically stable ceramic coating normally relies on the use of polymer binders. The use of the binder would inevitably decrease the porosity of the coating layer and significantly increase the thickness of the separator, which is unfavorable for efficient ion transport through the separator [22–25]. Compositing the polyolefin separator with a second porous polymer layer is another way that has been demonstrated to successfully improve both the electrolyte wettability and thermal stability of the polyolefin separator. Compared to the ceramic modification of the polyolefin separator, the functionalization of polyolefin separator with a porous polymer layer is more advantageous due to the good affinity between the polymer and polyolefin separator [26–29]. The electrospinning technique is a fascinating fiber fabrication technique to produce nanofiber and has been applied to form nonwoven electrolytic membranes with high ionic conductivity and good electrochemical properties [30–32]. In this study, a composite separator with an electronspun microporous polyethylene terephthalate nonwoven (PET) top layer and a commercialized polypropylene membrane as matrix support is prepared with the electrospinning technique. The major advantages of the novel composite separator over the traditional separator can be summarized as follows: (1) The PET layer in the novel composite separator is a dual-functional layer, which improves both the mechanical stability and electrolyte affinity of the composite separator. In contrast, the mechanical reinforcing layer in most commercial polymeric composite separator doesn’t provide additional functionality; (2) In a conventional ceramic coated separator, the use of a binder material is necessary to maintain the integrality of the whole separator, which may sacrifice the ionic conductivity of the separator. As a comparison, the reinforcing PET layer and PP base layer in the novel composite separator have a high compatibility due to their polymeric nature and the formation of the composite separator does not rely on the use of binder. We show that the novel separator has excellent mechanical properties as well as superior electrochemical properties. The effect of the structure parameters (such as the fiber diameter) of the PET nonwoven layer on the mechanical, thermal, and electrochemical properties of the composite separator is carefully studied and optimized. The optimal composite separator obtained in this study shows significantly enhanced stability as well as improved discharge capacity compared to the pristine PP separator. 2. Materials and Methods 2.1. Preparation of Composite Separators Polyethylene terephthalate (PET, MW = 150,000) was bought from yuanfang shanghai company (Shanghai, China) and dried in a vacuum at 60 ◦ C for 24 h before used. The commercialized PP separator (thickness, 20 µm) made by the dry process was kindly supplied from Huilong Co. Ltd. (Wuhan, China). The solvents were reagent grade and used as received. To prepare the solution for electrospun PET membrane fabrication, various amounts of PET were dissolved in the mixture of trifluoroaceticacid/1,2-dichloroethane (8:2, v/v) under stirring at ambient temperature for 12 h. The concentration of PET (w/v) was 16%, 18%, 20%, and 22% respectively [33]. The electrospinning process was carried out with an electrostatic spinning machine (SS-2535H, Ucalery, Beijing, China) using a voltage of 9 kV and a flow rate of 0.19 mm min−1 at 25 ◦ C. The distance between the needle tip and the collector was set to be 15 cm. The commercialized PP separator was fixed on the collector plate with tape to collect the electrospun PET fibrous. After 5 mins of spinning, the PP separator covered with nanofiber nonwoven was aired at room temperature for 48 h to obtain the composite separators. For convenience of expression, the composite separators will be represented by PET/PP-X, where X represents the concentration of the solution used for electronspinning.

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2.2. Characterization of Composite Separator The thickness of the composite separator was measured by thickness gauge (CH-1-S, Shanghai liuleng). The reported thickness was the average value from 5 individual measurements at different points of the separator. The morphology and the average diameter of nanofiber was characterized and measured by a scanning electron microscope (FE-SEM, S-4800, Hitachi, Tokyo, Japan). The tensile strength of the separator was determined by using a Tensile Tester (Hualong WDW-0.5, Shanghai, China). The sample used for the tensile strength test had a size of 50 × 10 mm and the elongation rate was set to be 5 mm min−1 . The contact angle (CA) measurements were carried out by using a contact angle meter (JC2000D POWEREACH, Shanghai, China). The CA was determined by means of sessile drop method, using electrolyte liquid (1 M lithium hexafluorophosphate (LiPF6 ) dissolved in the mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) (EC:DMC:DEC = 1:1:1, v/v/v) as testing liquids. An electrolyte droplet with a volume of 5 µL was used for each contact angle measurement. The electrolyte uptake of relevant separators was measured by soaking the separators in the electrolyte solution for 30 min. The weight of the separators was taken after removing the electrolyte remaining on the surface of separators with filter paper and the electrolyte uptake was calculated with the following equation: electrolyte uptake(%) = (w − w0 )/w0 × 100

(1)

where W 0 (g) and W (g) are the weight of the dry and wet separators, respectively. DSC scans were performed with a simultaneous TG-DSC system (STA449F3, NETZSCH, Bavaria, Germany) at the rate of 5 ◦ C min−1 under nitrogen purge from 30 ◦ C to 300 ◦ C. TMA tests were performed on a TMA analyze (TMA202, NETZSCH, Bavaria, Germany) at a ramping rate of 5 ◦ C min−1 under nitrogen purge from 30 ◦ C to 300 ◦ C, with a constant external tensile force of 0.02 N. Thermal shrinkage was tested by placing the relevant separators sandwiched by two glasses placed in an oven at various temperatures from 120 ◦ C to 180 ◦ C for 0.5 h, after that, dimensional changes of the separators were measured by the following equation:   shrinkage(%) = Ai − A f /Ai × 100

(2)

where Ai (cm2 ) and Af (cm2 ) are the initial and final area of the relevant separator respectively. 2.3. Electrochenmical Performance of Composite Separator The composite separator and PP separator were punched into circular pieces with diameter 19.6 mm for characterization. The circular separators were sufficiently soaked in liquid electrolyte in an Ar–filled glove box. The electrolyte contained 1M LiPF6 in ethylene carbonate (EC)-ediethyl carbonate (DEC)-ethylmethyl carbonate (EMC) (v/v/v = 1/1/1, LDC3045I). The soaked separators were sandwiched between two stainless steel electrodes and assembled in a tightly sealed test cell. The ionic conductivity of the separators at various temperatures was determined by an AC impedence analysis using an electrochemical workstation (CHI604D, CH Instruments, Shanghai, China) over the frequency range of 0.1 HZ-100 KHZ and under an AC voltage of 5 mV. The conductivity of the separator was calculated from the following equation: σ = d / (R ∗ S)

(3)

where σ (mS cm−1 ) is the ionic conductivity, d (cm) is the thickness of the composite separator, R (Ω) is the bulk resistance, and S (cm2 ) is the area of the electrode. The composite separators for the electrochemical stability measurement were sandwiched between Lithium metal and stainless steel electrodes in a test cell. The electrochemical stability was then measured by using a liner sweep voltammetry from 2.5 V–7 V at 1 mV s−1 . For charge/discharge and

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cyclability tests, the half-cell was assembled by sandwiching the separator between a lithium anode and a LiFePO4 cathode and then activated by filling with liquid electrolyte. The LiFePO4 cathode (Kejing Zhida Ltd. Co., Shenzhen, China) with an areal density of 1.27 mg cm−2 was prepared using Polymers 2018, 10, xacetylene FOR PEER black REVIEW 4 of 14 its blend with and PVDF binder (HSV 900, Arkema Ltd. Co., Colombes, France) at a ratio of 8:1:1 by weight. The charge/discharge and cyclability tests of cells were performed using battery battery test test equipment equipment (CT2001A, (CT2001A, LAND LAND Electronics, Electronics, Wuhan, Wuhan, China). China). The The discharge discharge current current densities densities were varied from from 0.1 0.1 C C to to 11 C C under underaavoltage voltagerange rangebetween between2.5 2.5VVand and44V.V.The Thecells cellswere werecycled cycledatata were varied afixed fixedcharge/discharge charge/dischargecurrent currentdensity densityofof0.5 0.5CC(1(1CC==170 170mAh mAhgg−−11 ). ). The interfacial resistance between the separator and Lithium metal The interfacial resistance between the separator and Lithium metal electrode electrode was was carried carried out out by by electrochemical electrochemical impedance impedance spectroscopy spectroscopy (EIS) (EIS) over over the the frequency frequency range range of of 0.01 0.01 HZ–100 HZ–100 KHZ KHZ and and under under an an amplitude amplitude of of 55 mV. mV.

3. Results and and Discussion 3.1. Membrane Membrane Characteristics Characteristics of of the the Composite Composite Separators Separators 3.1. As shown shown in in Figure Figure 1, 1, our our synthetic synthetic process process allows allows for for continuous continuous and and large-scale large-scale modification modification As of the the separators, separators, which which is is critical critical for for practical practical production production of of the the separators. separators. To To investigate investigate the the effect effect of of structural structuralparameters parametersofofPET PET performance of the composite separator, PET nanofibers of onon thethe performance of the composite separator, PET nanofibers with with different fiber diameters were prepared by varying the concentration of the solution used for different fiber diameters were prepared by varying the concentration of the solution used for electronspinning. The and thethe concentration of electronspinning. The correlation correlationbetween betweenthe thediameter diameterofofthe thePET PETnanofiber nanofiber and concentration the solution used for electronspinning is shown in Figure 2a. As shown in the figure, slightly varying of the solution used for electronspinning is shown in Figure 2a. As shown in the figure, slightly the solution concentration could result in result obvious the diameter of the obtained fiber. varying the solution concentration could in change obviousinchange in the diameter of the PET obtained The SEM of the pristine PPpristine separator PET/PP separatorseparator on the PET was PET fiber.images The SEM images of the PP and separator andcomposite PET/PP composite onside the PET shown in Figure 2b–f. As can be seen, the PP separator exhibits typical slit pores with the pore side was shown in Figure 2b–f. As can be seen, the PP separator exhibits typical slit pores withsize the in thesize range sub-micrometer. For the PET/PP randomly oriented PET nanofibers pore inofthe range of sub-micrometer. Forseparator, the PET/PP separator, randomly oriented were PET uniformly distributed and formed a three-dimensional porous network onporous the PP separator. The nanofibers were uniformly distributed and formed a three-dimensional network on thepore PP size of the PET nonwoven layer is significantly larger than that for the PP layer, which is beneficial for separator. The pore size of the PET nonwoven layer is significantly larger than that for the PP layer, efficient transportation across separator. across the separator. which is ionic beneficial for efficient ionicthe transportation

Figure 1. (a) Schematic illustration of the electrospinning; (b–d) photography of mass production for Figure 1. (a) Schematic illustration of the electrospinning; (b–d) photography of mass production for composite separator by free-liquid surface electrospinning. composite separator by free-liquid surface electrospinning.

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Figure 2. (a) The fiber diameter of polyethylene terephthalate (PET) nonwoven membrane top layer Figure 2. (a) The fiber diameter of polyethylene terephthalate (PET) nonwoven membrane top layer with different concentrations of PET; SEM image of the polypropylene (PP) separator (b) and with different concentrations of PET; SEM image of the polypropylene (PP) separator (b) and composite composite (c–f): PET/PP-16%, PET/PP-18%, PET/PP-20%, PET/PP-22%. separators separators (c–f): PET/PP-16%, PET/PP-18%, PET/PP-20%, PET/PP-22%.

The rapid and efficient wetting property of a separator toward typical battery electrolytes is The rapid and efficient wetting property of a separator toward typical battery electrolytes is important for the cell assembling process and its electrochemical performance. To quantitatively important for the cell assembling process and its electrochemical performance. To quantitatively evaluate the electrolyte wetting properties of the separators, the static CAs of electrolyte on the evaluate the electrolyte wetting properties of the separators, the static CAs of electrolyte on the separators were measured (Figure 3). The PP separator showed a high electrolyte contact angle of separators were measured (Figure 3). The PP separator showed a high electrolyte contact angle of 44.1◦ 44.1° due to its low polarity. In contrast, the composite separator showed improved electrolyte due to its low polarity. In contrast, the composite separator showed improved electrolyte wettability. wettability. The static electrolyte contact angle for PET/PP-16%, PET/PP-18%, PET/PP-20%, and The static electrolyte contact to angle for PET/PP-16%, PET/PP-18%, PET/PP-20%, andelectrolyte PET/PP-22% was PET/PP-22% was measured be 9.4°, 7.1°, 0°, and 0°, respectively. The decreased contact ◦ ◦ ◦ ◦ measured to be 9.4 , 7.1 , 0 is , and 0 , respectively. decreased electrolyte contact angleon of the PET/PP angle of PET/PP separator originated from theThe presence of polar functional group PET separator is originated from the presence of polar functional group on the PET surface as well as the surface as well as the increased surface roughness brought by the PET nanofiber. The PET/PP increased with surface roughness brought by PET nanofiber. The PET/PP separator with a larger fiber separator a larger fiber diameter hadthe a higher roughness, and thus showed improved electrolyte diameter had a higher roughness, and thus showed improved electrolyte wettability. The decrease wettability. The decrease in CA of the composite separator indicates that the modification of the PP in CA of the composite that the modification of the PP with separator with the PET separator nanofiber indicates was an effective method to enhance theseparator wettability of the thePET PP nanofiber The was electrolyte an effective method to enhance the wettability ofalso the quantified PP separator. The electrolyte separator. uptake capability of the separators was to further evaluate uptake capability of theofseparators was also quantified further evaluate the electrolyte affinity of the electrolyte affinity the separators. Generally, thetoseparator with a lower electrolyte contact the separators. Generally, the separator with a lower electrolyte contact angle had a higher electrolyte angle had a higher electrolyte uptake. The electrolyte uptake was as high as 293% for PET/PP-22%, uptake. The electrolyte uptake as 293%PP for PET/PP-22%, much higher than the 134% for much higher than the 134% for was that as of high the pristine separator. that of the pristine PP separator.

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Figure 3. (a) The contact angle and electrolyte uptake of the PP separator and the composite Figure 3. (a) The contact angle and electrolyte uptake of the PP separator and the composite separators; separators; photography of the PP separator and the composite separators. Images showing the photography of the PP separator and the composite separators. Images showing the electrolyte droplet electrolyte droplet on (b) PP separator; (c) PET/PP-16%; (d) PET/PP-18%; (e) PET-PP-20%; and (f) on (b) PP separator; (c) PET/PP-16%; (d) PET/PP-18%; (e) PET-PP-20%; and (f) PET/PP-22%. PET/PP-22%.

The of the The conductivity conductivity of the separator separator was was measured measured to to evaluate evaluate its its potential potential in in LIB LIB applications applications (Figure 4). Although the total ionic resistance of the PET/PP separators are slightly higher (Figure 4). Although the total ionic resistance of the PET/PP separators are slightly higher than than the the pristine PP separator, the calculated conductivity of the PET/PP separator is higher than pristine PP separator, the calculated conductivity of the PET/PP separator is higher than the the PP PP separator separator due due to to the the increased increasedthickness thicknessof ofthe thePET/PP PET/PP separator separator (Table (Table1). 1).PET/PP-22% PET/PP-22% showed showed the the − 1 among all the separators. The high ionic conductivity of highest highest ionic ionic conductivity conductivity of of 0.782 0.782 mS mScm cm−1 among all the separators. The high ionic conductivity of PET/PP separatorscan canbe becorrelated correlated to to their their high high electrolyte electrolyte affinity. affinity. PET/PP separators

Figure 4. The AC impedance spectra of symmetric cells SS/separator/SS with PP separator and the Figure 4. The AC impedance spectra of symmetric cells SS/separator/SS with PP separator and the PET/PP separators. PET/PP separators.

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Table 1. The thickness, measured resistance, and ionic conductivity of the separators. Table 1. The thickness, measured resistance, and ionic conductivity of the separators.

Separator PP Separator PP/PET-16% PP PP/PET-16% PP/PET-18% PP/PET-18% PP/PET-20% PP/PET-20% PP/PET-22% PP/PET-22%

Thickness/μm 20 Thickness/µm 2920 3029 3130 31 3232

Resistance/Ω 1 1 2.15 Resistance/Ω 3.17 2.15 3.17 2.83 2.83 2.52 2.52 2.21 2.21

Ionic Conductivity 1/mS cm−1 0.503 1 /mS cm−1 Ionic Conductivity 0.495 0.503 0.495 0.573 0.573 0.665 0.665 0.782 0.782

measured at25 25◦ °C. measured at C.

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Furthermore, the the ionic ionic conductivity conductivity of of the the separators separators was was measured measured at at different different temperatures temperatures to to Furthermore, investigate the potential of the separator at high temperature operation (Figure 5). The conductivity investigate the potential of the separator at high temperature operation (Figure 5). The conductivity of the the separators separators generally generally increased increased with with the the rise rise of of the the temperature. temperature. For For the the PP PP separator, separator, its its ionic ionic of −1 at 30 ◦ C to 1.14 mS cm−1 at 90 ◦ C. PET/PP-22% showed conductivity increased from 0.67 mS cm −1 −1 conductivity increased from 0.67 mS cm at 30 °C to 1.14 mS cm at 90 °C. PET/PP-22% showed the the highest ionic conductivity in the whole temperature tested.InInaddition, addition,aagood goodlinear linear relationship relationship highest ionic conductivity in the whole temperature tested. − 1 was found suggesting an an Arrhenius Arrhenius type type kinetics kinetics for for the the ionic ionic transport was found between between the the logσ logσ and and TT−1,, suggesting transport −1 curves of the separators indicated that the through the separator. The similar slope of the logσ vs. T −1 through the separator. The similar slope of the logσ vs. T curves of the separators indicated that the incorporation of of the the PET PET layer layer onto onto PP incorporation PP didn’t didn’t increase increase the the activation activation barrier barrier of of the the ionic ionic transport. transport.

Figure 5. Temperature dependence of ionic conductivity of PP separator and the PET/PP separators. Figure 5. Temperature dependence of ionic conductivity of PP separator and the PET/PP separators.

3.2. Mechanical and Thermal Properties of the Composite Separators 3.2. Mechanical and Thermal Properties of the Composite Separators An applicable separator should also have excellent mechanical properties. The mechanical An applicable separator should also have excellent mechanical properties. The mechanical property of the separator was characterized by the stress–strain curves shown in Figure 6. It was property of the separator was characterized by the stress–strain curves shown in Figure 6. It was previously reported that typical nonwoven PET nanofiber normally has a low tensile strength [34], previously reported that typical nonwoven PET nanofiber normally has a low tensile strength [34], which is because that the mechanical strength of the PET non-woven is mainly contributed by the which is because that the mechanical strength of the PET non-woven is mainly contributed by the intra-molecule interactions between the fibers. Due to the low tensile strength of the PET component, intra-molecule interactions between the fibers. Due to the low tensile strength of the PET component, the PET/PP composite separator showed lower tensile strength than the PP separator and the tensile the PET/PP composite separator showed lower tensile strength than the PP separator and the tensile strength of the PET/PP separator decreased with the increase of the PET component in the composite strength of the PET/PP separator decreased with the increase of the PET component in the composite separator. However, it should be noted that the PET/PP-22%, which exhibited the lowest mechanical separator. However, it should be noted that the PET/PP-22%, which exhibited the lowest mechanical performance, had a tensile strength of 85 MPa, which may be already high enough for practical performance, had a tensile strength of 85 MPa, which may be already high enough for practical battery battery applications [35]. In addition, the PET/PP separator had a high elongation at break, similar to applications [35]. In addition, the PET/PP separator had a high elongation at break, similar to that of that of PP separator. PP separator.

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Figure 6. Stress–strain curves of PP separator and the PET/PP separators. Figure 6. Stress–strain curves of PP separator and the PET/PP separators. Figure 6. Stress–strain curves of PP separator and the PET/PP separators.

DSC curves of the PP separator, the composite separator and granular PET material were DSC ofof thethe PPPP separator, the of composite separator and granular PET material were recorded DSC curves curves separator, the separator PET material were recorded to study the thermal stability thecomposite separators (Figure 7).and It isgranular clearly observed that the PP to study the thermal stability of the separators (Figure 7). It is clearly observed that the PP separator recorded to study the thermal stability of the separators (Figure 7). It is clearly observed that the PP separator gives a single endothermic peak at ~169 °C due to its melting. The PET/PP separator ◦ due to its melting. The PET/PP separator showed two gives a single endothermic peaklocated at ~169 separator gives a single endothermic peak at °C ~169 due°C, towhich its melting. The PET/PP separator showed two endothermic peaks at C ~169 and°C~248 can be assigned to the melting ◦ ◦ C, which can be assigned to the melting of PP and endothermic peaks located at ~169 C and ~248 showed two endothermic peaks at ~169 °Cadvantages and ~248 °C, can beseparator assignedistothat the the melting of PP and PET, respectively. Onelocated of the important ofwhich the PET/PP twoPET, respectively. One of the important advantages of the PET/PP separator is that the two-layer of PP and PET, respectively. One of the important advantages of the PET/PP separator is that the twolayer structure enables a thermal shutdown function that could be used to avoid thermal runaway in structure enables a thermal shutdown function used avoid runaway Li–ion layer enables a thermal shutdown function that be could betoused to thermal avoidshutdown thermal runaway in Li–ionstructure batteries. For the PET/PP separator, thethat PPcould separator could thermally atin~169 °C ◦ C under batteries. For the PET/PP separator, the PP separator could thermally shutdown at ~169 Li–ion the PET/PP separator, PP separator could thermally at ~169 the °C under batteries. abnormal For situations, while the PETthe nanofiber nonwoven membraneshutdown could maintain abnormal while the PET nanofiber nonwoven membrane could maintain integritythe of under abnormal situations, while the248 PET nonwoven could maintain integrity ofsituations, the whole separator up to °C, nanofiber thus allowing for a safemembrane operation of thethe battery. ◦ C, thus allowing for a safe operation of the battery. the whole separator up to 248 integrity of the whole separator up to 248 °C, thus allowing for a safe operation of the battery.

Figure 7. DSC curves of the PP separator, the PET/PP-22% and PET granular materials. Figure 7. DSC curves of the PP separator, the PET/PP-22% and PET granular materials. Figure 7. DSC curves of the PP separator, the PET/PP-22% and PET granular materials.

Figure 8 presents the dimensional changes of relevant separators as a function of temperature. Figure 8 presents dimensional changeslower of relevant a function of temperature. As we can see from thethe TMA curves, a slightly degreeseparators of thermalasshrinkage occurred with the Figure 8 presents the dimensional changeslower of relevant separators as a function of temperature. As we can see from the TMA curves, a slightly degree of thermal shrinkage occurred with the PET/PP separator with a larger PET nanofiber. Compared to the PP separator, the maximum length As we can see from the TMA curves, a slightly lower degree of thermal shrinkage occurred with the PET/PP separator with a larger PET nanofiber. Compared to40%). the PP separator, the maximum length reduction of the composite separator was far lower (22% vs. PET/PP with a larger PET nanofiber. Compared the PP separator, the maximum length reductionseparator of the composite separator was far lower (22% vs.to40%). reduction of the composite separator was far lower (22% vs. 40%).

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Figure 8. Thermomechanical behavior of the PP separator and PET/PP separators. Figure 8. Thermomechanical behavior of the PP separator and PET/PP separators. Figure 8. Thermomechanical behavior of the PP separator and PET/PP separators.

To further illustrate the thermal shrinkage and the meltdown behavior of the composite To the thermal shrinkage and the meltdown behavior of the placed composite separators, To further further illustrate the thermal shrinkage and the meltdown of the composite separators, theillustrate relevant separators are sandwiched by two glasses andbehavior then in an oven at the relevant separators are sandwiched by two glasses and then placed in an oven at separators, the relevant separators by shown two glasses and 9, then in an various oven at various temperature from 120 °C to are 180 sandwiched °C for 0.5 h. As in Figure Theplaced PP separator shrank ◦ C to 180 ◦ C for 0.5 h. As shown in Figure 9, The PP separator shrank severely temperature from 120 various 120 °C to 180 °Cfrom for 0.5 h. Astoshown in Figure 9, The PP separator shrank severelytemperature (33%) with from the color changing white semi-transparent, whereas the composite (33%) with the color changing from whitefrom to semi-transparent, whereas the composite severely (33%) withlittle the in color changing white to semi-transparent, whereas the separators composite separators changed dimension. changed little in dimension. separators changed little in dimension.

Figure 9. Photography of changes in the separator profile as a function of temperature. Figure 9. Photography of changes in the separator profile as a function of temperature. Figure 9. Photography of changes in the separator profile as a function of temperature.

To directly prove the thermal shutdown function of the separator, the PET/PP-22% was To directly prove the and thermal shutdown function of for the0.5 separator, the PET/PP-22% was sandwiched by two glasses placed in an oven at 170 °C h. The morphology of PET/PPTo directly prove the thermal shutdown function of the separator, the PET/PP-22% was sandwiched two glasses placed in an oven at 170 h. from The morphology PET/PP22% after theby shutdown wasand characterized (Figure 10). It °C canfor be0.5 seen the surfaces of and cross◦ sandwiched by two glasses and placed in(Figure an oven 170combined for 0.5 h.the The morphology of 22% after the shutdown was as characterized 10). at Itand can beCseen from surfaces and section that the PP separator a base membrane melted with the surface of thecrossPET PET/PP-22% after the shutdown wasporous characterized (Figure It canwith befilm seen froma the section that membrane, the PP separator as a the base membrane melted and 10). combined theinto surface of surfaces the PET nonwoven turning ionic conductive polymer non-porous and cross-section that the PP separator as a base membrane melted and combined with the surface of nonwoven membrane, the porous ionic of conductive into shortage a non-porous insulating layer betweenturning the electrodes. The closure the pores polymer preventedfilm possible of the the PETand nonwoven membrane, turning the conductive polymer film intoshortage a non-porous insulating layer between electrodes. Theporous closure of the pores prevented possible of the battery increased the the safety performance of the ionic battery. All composite separators present similar insulating layer between the electrodes. The closure of the pores prevented possible shortage of battery and increased safety performance the battery. All separators function present similar morphology with the the same heating process, of demonstrating thecomposite effective shutdown of the the battery and increased the safety performance of the battery. All composite separators present similar morphology with the same heating process, demonstrating the effective shutdown function of the composite separator. morphology with the same heating process, demonstrating the effective shutdown function of the composite separator. composite separator.

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Figure 10. FE-SEM photograph for PET/PP-22% after shutdown. Figure 10. FE-SEM photograph for PET/PP-22% after shutdown. Figure 10. FE-SEM photograph for PET/PP-22% after shutdown. 3.3. Electrochemical Performance 3.3. Electrochemical Performance As PET/PP-22% showed the highest ionic conductivity and electrolyte uptake, it is expected that 3.3. Electrochemical Performance As PET/PP-22% showed the highest ionic conductivity and electrolyte uptake, it is expected the separator could possess high battery performance. To demonstrate this hypothesis, the charge– that As thePET/PP-22% separator could possess high battery performance. To demonstrate hypothesis, showed conductivity andPET/PP electrolyte uptake,using itthis is expected that4 discharge behaviors of the of the thehighest half-cellionic assembled with the separator the LiFePO the charge–discharge behaviors ofbattery the of performance. the half-cell assembled with the PET/PP separator using the separator could possess high To demonstrate this hypothesis, the charge– cathode were investigated. Figure 11a,b revealed that the discharge capacity of the cells gradually the LiFePObehaviors were investigated. Figure 11a,b revealed that the discharge capacity of the4 4 cathodeof discharge the of half-cellrate. assembled PET/PP separator thedischarge LiFePO decreased with the increase of the discharge The cellswith withthe PET/PP-22% achievedusing a high cells gradually decreased with the increase of discharge rate. The cells with PET/PP-22% achieved cathode of were 11a,brate revealed discharge of the cells gradually capacity 161investigated. mAh g−1 at theFigure discharge 0.1 C;that at 1the C, the cell keptcapacity 74% of the discharge capacity −1 of adecreased high discharge capacity of 161 mAh g at the discharge rate of 0.1 C; at 1 C, the cell discharge kept 74% with the of discharge rate. cells high at 0.1 C, which wasincrease much higher than the cellThe with thewith PP PET/PP-22% separator at achieved a relevanta discharge rate. of the discharge capacity at 0.1 C, which was much higher than the cell with the PP separator at −1 at the capacity of 161 mAh g discharge rate of 0.1 C; at 1 C, the cell kept 74% of the discharge capacity Furthermore, the rate performance of the separators was investigated (Figure 11c). The PET/PP-22% aatrelevant discharge rate. Furthermore, the rate performance of the separators was investigated 0.1 C, improved which wasdischarge much higher than compared the cell with a relevant discharge rate. showed capacity to the the PP PP separator separator atover the whole rate range (Figure 11c). The PET/PP-22% showed improved discharge capacity compared to the PP separator Furthermore,demonstrating the rate performance ofpotential the separators was investigated (Figure 11c). applications. The PET/PP-22% investigated, the high of PET/PP-22% for practical battery over the whole rate range investigated, potential of PET/PP-22% practical showed improved discharge capacitydemonstrating compared to the the high PP separator over the wholefor rate range battery applications. investigated, demonstrating the high potential of PET/PP-22% for practical battery applications.

Figure 11. Charge and discharge profiles of cells assembled (a) PP and (b) PET/PP-22%; (c) rate capability of the cells with PP separator and PET/PP-22% at different C-rates (LE, 2.5–4.0 V); (d) Cycle Figure 11. 11. Charge Charge and discharge profiles of cells cells assembled assembled (a) (a) PP PPand and(b) (b)PET/PP-22%; PET/PP-22%; (c) (c) rate rate performance of cellsand withdischarge PP separator and of PET/PP-22%. Figure profiles capability of the cells with PP separator and PET/PP-22% at different C-rates (LE, 2.5–4.0 V); (d) Cycle capability of the cells with PP separator and PET/PP-22% at different C-rates (LE, 2.5–4.0 V); (d) Cycle performance of cells cellswith with PPseparator separator and PET/PP-22%. Figure 11d showed the cyclability ofand thePET/PP-22%. PP separator and the PET/PP-22%. Both the PP separator performance of PP

and PET/PP-22% exhibited stable discharge capacity up to at least 50 cycles with little degradation. Figure 11don showed the cyclability of the PP separator and the PET/PP-22%. Both the PP separator The cell based PET/PP-22% had a discharge capacity of 149 mAh g−1 after 50 cycles, higher than −1 and PET/PP-22% exhibited stable discharge capacity up to at least 50 cycles with little degradation. 134 mAh g for cell with PP separator. The improved discharge performance of the PET/PP-22% is The cell based on PET/PP-22% had a discharge capacity of 149 The mAhexcellent g−1 after rate 50 cycles, higher than believed to be contributed by its excellent electrolyte affinity. performance and −1 134 mAh g for cell with PP separator. The improved discharge performance of the PET/PP-22% is believed to be contributed by its excellent electrolyte affinity. The excellent rate performance and

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Figure 11d showed the cyclability of the PP separator and the PET/PP-22%. Both the PP separator 11 of 14 and PET/PP-22% exhibited stable discharge capacity up to at least 50 cycles with little degradation. The cell based on PET/PP-22% had a discharge capacity of 149 mAh g−1 after 50 cycles, higher than cycling of PET/PP-22% indicates its high potential practical battery applications. A 134 performance mAh g−1 for cell with PP separator. The improved dischargefor performance of the PET/PP-22% list of performance parametersbyofitsthe PET/PP-22% separator andexcellent their comparison withand typical is believed to be contributed excellent electrolyte affinity. The rate performance separators reported recently is shown in Table S1. From the comparison, it can be seen clearly cycling performance of PET/PP-22% indicates its high potential for practical battery applications. A list that the PET/PP-22% a superiorseparator mechanical property and excellent electrochemical of performanceseparator parametersexhibits of the PET/PP-22% and their comparison with typical separators reported recently is shown in Table S1. From the comparison, it can be seen clearly that the PET/PP-22% performance. separator a superior mechanical property and excellent electrochemical performance. In order exhibits to further understand the excellent electrochemical performance of PET/PP-22%, the In order to further understand the excellent electrochemical performance of PET/PP-22%, the EIS EIS spectra of the cells with PP or PET/PP-22% were recorded before and after charge–discharge spectra of the cells with PP or PET/PP-22% were recorded before and after charge–discharge cycles cycles (Figure 12). The intercept of Nyquist curves at the Z’ axis indicates the combination resistance (Figure 12). The intercept of Nyquist curves at the Z’ axis indicates the combination resistance (RZ ) (RZ) which is related to the ionic resistance of the electrolyte. The semicircle in the high frequency which is related to the ionic resistance of the electrolyte. The semicircle in the high frequency area area represents the resistance through the solid electrolyte interface (SEI) layers (RSEI) [36]. As represents the resistance through the solid electrolyte interface (SEI) layers (RSEI ) [36]. As determined determined the plots, Nyquist thePET/PP-22% cell with PET/PP-22% had lower RZ1st after both 1stΩ) cycle from thefrom Nyquist the plots, cell with had a lower RZ aafter both cycle (1.17 and(1.17 Ω) and (0.35 the cell with the PP(1.88 separator (1.88 Ω and 0.37 Ω, respectively). 50th50th cyclecycle (0.35 Ω) thanΩ) thethan cell with the PP separator Ω and 0.37 Ω, respectively). Furthermore, Furthermore, thecell Rctwith of the cell with PET/PP-22% was the alsocell lower the cell with the PP the Rct of the PET/PP-22% separator was separator also lower than with than the PP separator in all separator alllow cases. TheRctlow Rz cell andbased Rct ofon the cell based could on PET/PP-22% could be ascribed to the cases. in The Rz and of the PET/PP-22% be ascribed to the higher porosity, electrolyte uptake, and better electrolyte wettability of the separator. higher porosity, electrolyte uptake, and better electrolyte wettability of the separator. Polymers 2018, 10, x FOR PEER REVIEW

Figure 12. The variation in AC impedance cycle-50th)ofofcells cells assembled with PP separator Figure 12. The variation in AC impedancespectra spectra (1st (1st cycle-50th) assembled with PP separator and PET/PP-22%. and PET/PP-22%.

Finally, thethe electrochemical of the thePP PPseparator separator and composite separator Finally, electrochemicalstability stabilitywindow window of and thethe composite separator werewere evaluated by by linear (Figure13). 13). AAvery verylow low background current evaluated linearsweep sweepvoltammetry voltammetry (Figure background current was was measured below 4.5 4.5 V, V, followed incurrent currentflow flow which indicated measured below followedby byaasmall small increase increase in which indicated the the onsetonset of of electrochemical decomposition ofthethe electrolyte. Considering the almost fact that all of the electrochemical decomposition of electrolyte. Considering the fact that all of almost the commercial + ), the electrochemical stability electrode materials an electrode potential below 4.5 V (Vs. Li/Libelow commercial electrodehave materials have an electrode potential 4.5 V (Vs. Li/Li+), the of PP/PET-22% is highofenough for commercial electrochemical stability PP/PET-22% is high battery enoughapplications. for commercial battery applications.

Finally, the electrochemical stability window of the PP separator and the composite separator were evaluated by linear sweep voltammetry (Figure 13). A very low background current was measured below 4.5 V, followed by a small increase in current flow which indicated the onset of electrochemical decomposition of the electrolyte. Considering the fact that almost all of the commercial materials have an electrode potential below 4.5 V (Vs. Li/Li+),12 ofthe Polymers 2018, 10,electrode 574 14 electrochemical stability of PP/PET-22% is high enough for commercial battery applications.

Figure13. 13.Liner Linersweep sweepvoltammetry voltammetryofofSS/PP/Li SS/PP/Li and and SS/PET/PP-22%/Li SS/PET/PP-22%/Li cell. Figure cell.

4. Conclusions A facile electrospinning process, which can be easily scaled up for industrial production, is developed to prepare a composite PET/PP separator. A precursor solution with a concentration of 22% is identified as the optimal solution for the preparation of the separator. The optimized PET/PP separator exhibits excellent thermal stability with little dimensional change at a temperature up to 180 ◦ C. In addition, the separator has a shutdown function, which could effectively improve the safety performance of the separator. The PET/PP separator also inherits excellent electrolyte affinity from the PET component and therefore shows improved electrochemical performance compared to the commercial PP separator. Our results show that the novel electronspun PET/PP separator can function as a competitive alternative to the existing polyolefin separator and may find enormous applications in different battery applications. Supplementary Materials: The following supplementary materials are available online at www.mdpi.com. Table S1. Comparison of the properties of the PET/PP separator with recently reported separators. Author Contributions: Conceptualization, H.C., Z.W. and J.L.; Experiments and analysis, H.C., X.T., J.W., K.C., Y.X. and Y.S.; Draft Preparation, X.T., J.L.; Supervision, H.C. and J.L.; Funding Acquisition, H.C. and J.L. Funding: This work is financially supported by National Nature Science Foundation of China (61376064) and the Fundamental Research Funds for the Central Universities (WUT: 2018-IB-026). Conflicts of Interest: The authors declare no conflict of interest.

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