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Electrospun polyacrylonitrile nanofibrous membranes with varied fiber diameters and different membrane porosities as lithium-ion battery separators
Electrochimica Acta 236 (2017) 417–423
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Electrospun polyacrylonitrile nanofibrous membranes with varied fiber diameters and different membrane porosities as lithium-ion battery separators Xiaojing Maa,1, Praveen Kollab,1, Ruidong Yangb , Zhao Wangb , Yong Zhaob , Alevtina L. Smirnovab,** , Hao Fonga,b,* a b
Program of Nanoscience and Nanoengineering, South Dakota School of Mines and Technology, Rapid City, SD 57701, USA Department of Chemistry and Applied Biological Sciences, South Dakota School of Mines and Technology, Rapid City, SD 57701, USA
A R T I C L E I N F O
Article history: Received 8 November 2016 Received in revised form 21 March 2017 Accepted 27 March 2017 Available online 30 March 2017 Keywords: Electrospinning polyacrylonitrile nanofibrous membrane lithium-ion battery nonwoven separator
A B S T R A C T
In this study, nine types of polyacrylonitrile (PAN) nanofibrous membranes with varied fiber diameters and different membrane porosities are prepared by electrospinning followed by hot-pressing. Subsequently, these membranes are explored as Li-ion battery (LIB) separators. The impacts of fiber diameter and membrane porosity on electrolyte uptake, Li+ ion transport through the membrane, electrochemical oxidation potential, and membrane performance as LIB separator (during charge/ discharge cycling and rate capability tests of a cathodic half-cell) have been investigated. When compared to commercial Celgard PP separator, hot-pressed electrospun PAN nanofibrous membranes exhibit larger electrolyte uptake, higher thermal stability, wider electrochemical potential window, higher Li+ ion permeability, and better electrochemical performance in LiMn2O4/separator/Li half-cell. The results also indicate that the PAN-based membrane/separator with small fiber diameters of 200– 300 nm and hot-pressed under high pressure of 20 MPa surpasses all other membranes/separators and demonstrates the best performance, leading to the highest discharge capacity (89.5 mA h g1 at C/2 rate) and cycle life (with capacity retention ratio being 97.7%) of the half-cell. In summary, this study has revealed that the hot-pressed electrospun PAN nanofibrous membranes (particularly those consisting of thin nanofibers) are promising as high-performance LIB separators. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction Lithium-ion batteries (LIBs) have been widely utilized as the power source for portable electronic devices and hybrid electric vehicles because of high energy density, long cycle life, low maintenance, and long shelf life (i.e., low self-discharge) [1–5]. The LIB separator, an electrochemically inactive membrane, is placed between cathode and anode of a LIB. The function of LIB separator is to prevent the physical contact of two electrodes while allowing Li+ ions to transport through the liquid electrolyte that fills in the membrane porous structure. For a good LIB separator, it must fulfill
* Corresponding author at: Program of Nanoscience and Nanoengineering, South Dakota School of Mines and Technology, Rapid City, SD 57701, USA. Tel.: +605 3941229, Fax: +605 394-1232. ** Corresponding author. Tel.: +605 394-1890, Fax: +605 394-1232. E-mail addresses: [email protected] (A.L. Smirnova), [email protected] (H. Fong). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.electacta.2017.03.205 0013-4686/© 2017 Elsevier Ltd. All rights reserved.
several requirements including appropriate thickness, suitable pore size, good chemical/thermal stability, and high Li+ ion permeability [5–8]. In commercial LIBs, polyolefin-based (i.e., polyethylene/polypropylene-based) microporous membranes are commonly adopted as the separators due to their reasonably high chemical/thermal stability and relatively good performance. However, some intrinsic limitations of nonpolar polyolefin-based separators (e.g., low electrolyte uptake, poor adhesion property to electrodes, and low ionic transport) considerably restrict their electrochemical performances [9,10]. Recently, highly porous nonwoven mats/membranes (consisting of overlaid fibers) have attracted growing attention as LIB separators. Several techniques such as papermaking [11], solution extrusion [12], melt blowing [13], and electrospinning [14–16] have been investigated to make nonwoven LIB separators. Among these, the electrospinning technique provides a convenient and valuable approach to prepare overlaid membranes with fiber diameters typically being hundreds of nanometers (commonly known as electrospun nanofibers) [17]. The electrospun
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nanofibrous membranes (e.g., nylon, poly(vinylidene fluoride), and polyacrylonitrile nanofibrous membranes) have smaller fiber diameters and higher porosities than the conventional nonwoven membranes, making them promising for high-performance LIB separators [14–17]. For example, electrospun polyacrylonitrile (PAN) nanofibrous membranes were explored as LIB separators; the results indicated that they possessed desired properties such as high ionic transport, small diffusion resistance, good thermal/ chemical/mechanical stability, long cycle life, high electrolyte uptake, and good compatibility with electrodes [18–21]. Research endeavors also suggested that, the nitrile groups in PAN might be able to interact with the Li+ ions in liquid electrolytes and the carbonyl groups in solvent (e.g., propylene carbonate and ethylene carbonate) [22,23]. Furthermore, PAN could also mitigate the formation of Li dendrites during the charging/discharging process of LIBs [20,21]. It is important to note that, electrospun nanofibrous membranes do not have straight-through pores, if they are well prepared. However, tiny particles are able to permeate through these membranes via tortuous pathways. As a result, the membranes show equivalent/apparent pore sizes typically in the range from tens of nanometers to several micrometers. Aselectrospun nanofibrous membranes often have porosities of 85–90%; in other words, the nanofibers only occupy 1/8 of membrane volume. After being mechanically pressed, the porosity can readily be reduced to 75% (i.e., the nanofibers occupy 1/4 of membrane volume); while hot-pressing with relatively high pressure can further reduce the porosity to 50% or even lower. It is obvious that the membranes with different porosities would exhibit different equivalent/apparent pore sizes, leading to different performances as LIB separators. Surprisingly, no studies have been reported regarding the mechanical pressing on the porosities of electrospun nanofibrous membranes and its impact on the LIB separator performances. Moreover, the diameters of fibers in electrospun membranes would also play crucial roles on the equivalent/apparent pore sizes and further on the LIB separator performances; whereas no studies have been reported on this issue either. Therefore, the objective of this study is to explore the morphological/physical properties (particularly fiber diameter and membrane porosity) of hot-pressed electrospun PAN nanofibrous membranes on their performances as LIB separators. In specific, nine types of electrospun PAN nanofibrous membranes with varied fiber diameters and different membrane porosities were prepared; subsequently, the impacts of fiber diameter and membrane porosity on LIB separator performance were explored. The study revealed that the membranes with different fiber diameters and membrane porosities would lead to different performances as LIB separators, while the PAN–S–H membrane/ separator with small fiber diameters of 200–300 nm and hotpressed under high pressure of 20 MPa outperformed all other membranes/separators including the commercial Celgard PP separator.
separator with the thickness of 25 mm was provided by Celgard LLC (Charlotte, NC). 2.2. Preparation of electrospun PAN nanofibrous membranes PAN was first dissolved in DMF to prepare three solutions with the concentrations being 7, 10, and 12 wt.%, respectively. Note that the reason for selecting these three concentrations of PAN/DMF solution was based upon morphological structures of as-electrospun PAN nanofibers; in specific, the PAN solution with concentration lower than 7 wt.% would result in significant amount of beads and/or beaded nanofibers, while the PAN solution with concentration higher than 12 wt.% would result in thick nanofibers with diameters close to or larger than 1 mm. Prior to electrospinning, each PAN solution was loaded into a 30 mL Luer-LokTM tip plastic syringe having an 18 gauge 90 blunt-end steel needle, and the electrospinning process was carried at room temperature of 22 C and relative humidity of 20%. During electrospinning, a positive high DC voltage of 18 kV was applied to the PAN solution through the needle by using an ES30P high voltage power supply (Gamma High Voltage Research, Inc.). The feed rate of each solution was set at 0.5 mL h1 by using a KDS 200 syringe pump (KD Scientific, Inc.). Electrospun PAN nanofibers were collected on a laboratory-produced rotating drum (with diameter of 25 cm) covered with aluminum foil. The syringe pump was placed on a slider which could move horizontally in front of the collecting drum at the pre-determined speed of 15 cm min1. The obtained electrospun PAN nanofibrous membranes were morphologically uniform with the thicknesses of 60 mm (measured with a micrometer caliper). Subsequently, these membranes were mechanically pressed under three different pressures (i.e., 5, 10, and 20 MPa) at 60 C for 3 min. Therefore, nine types of membranes were prepared, and they were denoted as PAN–D(S/M/L)–P(L/M/ H). Note that D represents the fiber diameter, where S stands for small fiber diameters in the range of 200–300 nm, M stands for medium fiber diameters in the range of 400–500 nm, and L stands for large fiber diameters in the range of 700–800 nm; whereas P represents the applied pressure during the hot-pressing process, where L stands for the low pressure of 5 MPa, M stands for the medium pressure of 10 MPa, and H stands for the high pressure of 20 MPa. 2.3. Characterization of electrospun PAN nanofibrous membranes A Zeiss Supra 40VP field-emission scanning electron microscope (SEM) was employed to examine morphological structures of nine types of electrospun PAN nanofibrous membranes. Prior to SEM examinations, the specimens were sputter-coated with gold to avoid charge accumulation. A TA Instruments Q100 differential scanning calorimeter (DSC) was employed to study the thermal stability of PAN membrane separators, and the DSC curves were recorded from 60 to 400 C with the heating rate of 10 C min1. 2.4. Measurements of electrolyte uptake and membrane porosity
2. Experimental 2.1. Materials Polyacrylonitrile (PAN, Mw = 150,000), N,N-dimethylformamide (DMF), lithium hexafluorophosphate (LiPF6), ethylene carbonate (EC), and diethyl carbonate (DEC) were purchased from Sigma-Aldrich (St. Louis, MO). Lithium foil was purchased from Alfa Aesar (Ward Hill, MA). Carbon-coated LiMn2O4 cathode foil (with active material of 16.6 mg cm2) was purchased from MTI Corporation (Richmond, CA). Celgard polypropelene (Celgard PP)
Electrolyte uptake of a membrane was measured according to Eq. (1) [20,24]: ð1Þ Electrolyte upate ð%Þ ¼ W f W i =W i 100% where Wi and Wf are the weights of a membrane before and after being soaked in liquid LiPF6 electrolyte for 2 h in argon-filled glove box, respectively. Porosity of a membrane was calculated from Eq. (2) [20]: P ¼ ðV V P =V Þ 100%
ð2Þ
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where P is the porosity of membrane, V is the total volume of membrane, and Vp is the volume of PAN nanofibers (i.e., the mass divided by the density (1.184 g cm3) of PAN polymer). The electrolyte uptake and porosity values of each type of electrospun PAN nanofibrous membranes were finally determined from averaging the values of 3 samples. 2.5. Evaluation of electrochemical performance The transport of Li+ ions through electrolyte-soaked separators (including nine types of electrospun membranes and the Celgard PP separator) was indirectly determined by measuring the ionic conductivity of electrolyte in a symmetric coin-cell configuration, comprised of liquid LiPF6 electrolyte-soaked separator that was sandwiched between two stainless-steel electrodes. The ionic conductivity of electrolyte was measured by electrochemical impedance spectroscopy (EIS) using the PARSTAT1 2273 electrochemical impedance analyzer purchased from Princeton Applied Research (Oak Ridge, TN). These impedance measurements were conducted at the applied AC amplitude of 5 mV within the frequency range of 200 kHz–10 mHz. The ionic conductivity value was then calculated from Eq. (3) [20,24]:
s ¼ b=Rb S
ð3Þ
where b is the thickness of a separator (cm), Rb is the electrolytebulk resistance (V), and S is the area of this separator (cm2). Electrochemical stability of each separator was evaluated using linear sweep voltammetry (LSV) by using a potentiostat (580 Battery Test System) purchased from Scribner Associates Inc. (Southern Pines, NC). The electrochemical oxidation behavior was
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tested using a coin-cell comprised of liquid LiPF6 electrolytesoaked membrane/separator that was sandwiched between lithium metal (i.e., counter electrode) and stainless steel (i.e., working electrode). The tests were carried out at the scan rate of 1.0 mV S1 in the voltage window of 3.5–6.0 V vs. Li+/Li. The performance of each membrane as LIB separator was further evaluated upon measuring the rate capability and cycling performance of the cathodic half-cells of LiMn2O4/separator/Li metal in a coin-cell configuration. The rate capabilities at different C-rates of C/2, 1C, 2C, and C/2 (i.e., 0.75, 1.5, 3, and 0.75 mA g1, 3 charge/discharge cycles each), and the cycle life at C/2 for 100 charge/discharge cycles, were studied in the voltage range of 3.0– 4.5 V. All of the coin-cells (CR2025) for the above tests were assembled inside an argon-filled glove box, while each membrane/ separator was thoroughly wetted with the liquid electrolyte of 1 M LiPF6 in EC/DEC (with volume ratio of 1/1). 3. Results and discussion 3.1. Morphological structures and membrane parameters Representative SEM images acquired from nine types of electrospun PAN nanofibrous membranes are shown in Fig. 1. The diameters of PAN nanofibers electrospun from 7, 10, and 12 wt. % PAN solutions were in the ranges of 200–300 nm (top row), 400– 500 nm (middle row), and 700–800 nm (bottom row), respectively. These membranes consisted of randomly overlaid electrospun PAN nanofibers, which were morphologically uniform without microscopically identifiable beads and/or beaded nanofibers. It is necessary to note that, the presence of beads and/or beaded
Fig. 1. SEM images showing the representative morphological structures of nine types of electrospun PAN nanofibrous membranes: (A) PAN–S–L, (B) PAN–S–M, (C) PAN–S–H, (D) PAN–M–L, (E) PAN–M–M, (F) PAN–M–H, (G) PAN–L–L, (H) PAN–L–M, and (I) PAN–L–H.
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nanofibers in an electrospun nanofibrous membrane is a common issue/problem, which has negative effect on advantageous properties of the membrane. In this study, the pre-determined PAN concentrations (being 7 wt.% or higher) could effectively avoid the formation of beads and/or beaded nanofibers; this was in agreement with previously reported studies [25,26]. The PAN nanofibers in the membranes that were hot-pressed under low pressure (i.e., 5 MPa) were able to well retain their shapes/ morphologies without distinguishable deformations (left column), whereas the PAN nanofibers in the membranes that were hotpressed under high pressure (i.e., 20 MPa) appeared flattened (right column). Evidently, the hot-pressing process would change the porosity of an electrospun PAN nanofibrous membrane, further leading to different apparent/equivalent pore sizes. Membrane parameters of different separators (including the nine electrospun membranes and the Celgard PP separator) were measured, and the results are summarized in Table 1. In general, the porosity values of electrospun membranes were considerably higher than the value of Celgard PP separator (50.2%). With the increase of hot-pressing pressure, the porosity values of electrospun membranes would be decreased, suggesting that the equivalent/apparent pore sizes would be reduced. Upon hotpressing under high pressure (i.e., 20 MPa), the porosity values of PAN–S–H, PAN–M–H, and PAN–L–H membranes/separators were 54.7%, 52.6%, and 55.7%, respectively. Meantime, the hotpressing process also had significant influence on electrolyte uptake. In specific, the electrolyte uptake values of PAN–S–H, PAN– M–H, and PAN–L–H membranes/separators were 336, 300, and 289%, respectively. It is well known that higher values of porosity and electrolyte uptake are generally desired for LIB separators. Even though the hot-pressing process reduced both porosity and electrolyte uptake values of electrospun membranes (i.e., PANbased separators), the values acquired from the nine types of PANbased separators were still substantially higher than the values acquired from the Celgard PP separator. Moreover, the electrolyte uptake value of PAN-based separators was also affected by the variation of fiber diameters. For example, the PAN-based separator with the smallest fiber diameters (i.e., 200–300 nm) had the highest electrolyte uptake value. In specific, the electrolyte uptake values of PAN–S–L, PAN–S–M, and PAN–S–H membranes/separators were 363, 350, and 329%, respectively; whereas the electrolyte uptake value of Celgard PP separator was merely 233%. Note that the values of thickness and ionic conductivity of electrolyte-soaked electrospun membranes were similar to the values of Celgard PP separator. Nevertheless, both parameters of the PAN–S–H membrane/separator were appreciably better than those of the Celgard PP separator, suggesting that the PAN–S–H membrane/separator might have higher LIB performance.
Fig. 2. DSC thermograms acquired from the electrospun PAN nanofibrous membranes and from the Celgard PP separator.
3.2. Thermal stability DSC was employed to investigate the thermal stability of electrospun PAN nanofibrous membranes. For comparison, the thermal stability of Celgard PP separator was also studied. As shown in Fig. 2, the thermogram acquired from Celgard PP separator had an endothermic peak centered at 163 C, corresponding to the melting temperature of PP polymer. For the nine electrospun PAN nanofibrous membranes, the acquired thermograms were almost identical; and a typical thermogram is shown in Fig. 2. This thermogram had an exothermic peak centered at 303 C, which was attributed to the stabilization of PAN polymer (i.e., the nucleophilic attack on nitrile groups followed by instantaneous cyclization into extended conjugated structure) [26,27]. The DSC results indicated that the PAN nanofibrous membranes had better thermal stability than the Celgard PP separator. As a result, these membranes would be able to better retain their morphological structures during the operation. Furthermore, in a thermal run-away incident, the LIB device with such a PAN-based separator would be substantially more stable. 3.3. Electrochemical performance Nyquist plots of the symmetric cells (i.e., stainless steel/ separator/stainless steel) for the Celgard PP and PAN-based separators in the liquid electrolyte are shown in Fig. 3A. The ionic conductivity of the electrolyte-soaked separator was measured from the bulk-resistance value (i.e., the x-intercept of extrapolated Nyquist plot at high-frequency end of real z). As shown in Table 1,
Table 1 Membrane parameters acquired from the nine types of electrospun PAN nanofibrous membranes as well as from the Celgard PP separator. Each datum showing the average value of three measurements and the associated standard deviation. Separator
Thickness (mm)
Porosity (%)
Electrolyte uptake (%)
Ionic conductivity (mS cm1)
PAN–S–L PAN–S–M PAN–S–H PAN–M–L PAN–M–M PAN–M–H PAN–L–L PAN–L–M PAN–L–H Celgard PP
27.2 1.1 24.5 0.8 23.6 0.7 27.1 1.1 23.7 1.0 22.8 0.8 28.1 1.3 25.4 1.6 23.5 0.9 25.2 0.7
61.0 2.5 57.7 2.5 54.7 1.8 59.6 2.3 53.8 3.0 52.6 1.8 60.4 1.9 57.2 2.9 55.6 1.7 50.2 1.6
363 15 350 14 336 9 350 12 310 13 300 11 329 13 313 15 289 9 233 7
0.93 0.04 0.98 0.04 1.06 0.03 0.74 0.03 0.88 0.04 0.95 0.03 0.59 0.03 0.60 0.03 0.62 0.02 0.80 0.02
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Fig. 3. (A) Nyquist plots of the symmetric cells (i.e., stainless steel/separator/stainless steel) for the PAN-based and Celgard PP separators soaked in liquid electrolyte, and (B) Linear sweep voltammograms of the cells (i.e., Li metal/separator/stainless steel) to study oxidative behaviors of the PAN-based and Celgard PP separators in liquid electrolyte within the electrochemical window of 3.5–6 V (the inset showing the enlarged area within the window of 3.75–5 V).
both the increase of hot-pressing pressure and the decrease of fiber diameter would result in the increase of ionic conductivity of the electrolyte-soaked separator. Among nine types of electrospun PAN nanofibrous membranes, the membrane consisting of the thinnest nanofibers (i.e., 200–300 nm) exhibited the highest ionic conductivity. In specific, the ionic conductivity values of electrolyte-soaked PAN–S–L, PAN–S–M, and PAN–S–H membranes/ separators were 0.93, 0.98, and 1.06 mS cm1, respectively. Note that among all types of membranes/separators including the Celgard PP separator, the PAN–S–H membrane/separator had the highest ionic conductivity value of 1.06 mS cm1. Furthermore, the PAN–S–H membrane/separator was also thinner and more porous
than the Celgard PP separator, which would be more beneficial to the LIB performance as well. The electrochemical stability of the cells (i.e., Li metal/ separator/stainless steel) was tested via the linear sweep voltammetry (Fig. 3B). For the Celgard PP separator, the anodic current was observed at 4.0 V vs. Li+/Li, probably due to microshort circuit upon the formation of lithium dendrite and the oxidation of PP polymer [19]. It was evident that, all of the nine PAN-based membranes/separators had substantially higher electrochemical stability than the Celgard PP separator, and there was no identifiable oxidation peaks for these cells below 4.5 V vs. Li+/Li, suggesting that the PAN-based membranes/separators would be more suitable for LIBs by having higher electrochemical stability in
Fig. 4. Initial charge/discharge voltage profiles at the C-rate of 0.5C for the coin half-cells (i.e., LiMn2O4/separator/Li metal) with separators being (A) PAN–S–L, PAN–S–M, PAN–S–H, and Celgard PP, (B) PAN–M–L, PAN–M–M, PAN–M–H, and Celgard PP, and (C) PAN–L–L, PAN–L–M, PAN–L–H, and Celgard PP. (D) Specific capability of the cells with the separators being PAN–S–H, PAN–M–H, PAN–L–H, and Celgard PP at charge/discharge rates of 0.5, 1, 2, and 0.5C (i.e., 0.75, 1.5, 3, and 0.75 mA g1, 3 cycles at each current).
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a wider potential range than the PP-based separator. Among these separators, the PAN–L–H separator exhibited the highest onset oxidation potential and the lowest oxidation current. The larger diameter and better compactness of PAN–L–H separator (due to high-pressure hot-pressing) could have improved mechanicalshielding against micro-dendrite growth. On the other hand, the developments of high-voltage cathode materials for LIBs has attracted significant attention [28]; the newly developed PANbased separators would be particularly advantageous for making LIB devices with high-voltage cathode materials. 3.4. Battery performance The developed PAN membranes/separators were tested in the coin half-cell configuration with LiMn2O4 cathode and Li metal reference electrode in the presence of liquid LiPF6 electrolyte (Fig. 4A–C). The galvanostatic charge/discharge curves at the constant current of 0.5C (i.e., 0.75 mA g1) were recorded for the PAN-based separators and compared to the curves acquired from the Celgard PP separator. The charge/discharge curves for all the cells had stable plateaus, while the cells with the PAN–S–H separator exhibited the highest capacity. For the cells with PANbased separators hot-pressed at high pressure (i.e., 20 MPa), the specific capacity values of three cells with the PAN–S–H, PAN–M– H, and PAN–L–H separators were 89.5, 83.1, and 81.6 mA h g1, respectively, which were significantly higher than that of the cell with the Celgard PP separator (75.1 mA h g1). Based on the experimental results, it is obvious that the fiber diameter has significant impact on the specific capacity of electrochemical cell. In general, the specific capacities of the cells with the PAN-based separators consisting of thin nanofibers (i.e., 200–300 nm) were higher; and the specific capacity values of the cells with PAN–S–H, PAN–S–M, and PAN–S–L separators were 89.5, 80.4, and 72.6 mA h g1, respectively. As previously speculated, the cell with the PAN–S–H separator had the highest discharge capacity of 89.5 mA h g1, which was due to high porosity, large electrolyte uptake, and better Li+ ion transport (revealed by ionic conductivity measurements of electrolyte-socked membranes) through the PAN–S–H separator. Fig. 4D depicts specific capabilities for the electrochemical cells with PAN–S–H, PAN–M–H, PAN– L–H, and Celgard PP separators at constant current charge/ discharge rates of 0.5, 1, 2, and 0.5C (3 cycles at each C-rate). In general, the capacity retention ratios of all types of cells were stable, while the cell with PAN–S–H separator had the largest discharge capacity at different C-rates of 0.5, 1, 2, and 0.5C. In particular, the electrochemical cell with PAN–S–H separator exhibited the discharge capacity values of 89.5, 36.5, 14.8, and 89.5 mA h g1 at the C-rates of 0.5, 1, 2, and 0.5C, respectively; and the capacity advantage of the PAN-based separators over Celgard PP separator was more significant at lower rate (0.5C) than higher rate (2C). As the C-rate became higher, batteries equipped with all types of separators experienced the decreasing of utilized capacity caused by the higher resistance; while the overall resistance in a battery was contributed by all components including the separator, electrolyte, electrodes, and interfaces. Therefore, the advantage resulted from PAN-based separator would become less significant at higher C-rate [29,30]. On the other hand, this phenomenon could also be related to the inherent mass/ionic transport limitations caused by high loading in the working electrode (i.e., carboncoated LiMn2O4 with active material at 16.6 mg cm2) of electrochemical half-cells [30,31]. Cycling performance was evaluated at the constant current charge/discharge rate of 0.5C for 100 cycles, and the results are shown in Fig. 5. Overall, the PAN-based separators prepared under high hot-pressing pressure (i.e., 20 MPa) resulted in superior battery cycling performance. The cells with the PAN–S–H separator
Fig. 5. Cycling performance/stability of the coin half-cells (i.e., LiMn2O4/separator/ Li metal) with the PAN–S–H, PAN–M–H, PAN–L–H, and Celgard PP separators at the constant current charge/discharge rate of 0.5 C.
exhibited the best capacity retention ratio of 97.7%, while the capacity retention ratios of the cells with PAN–M–H and PAN–L–H separators were 96.1% and 96.2%, respectively. Hence, the capacity retention ratios of the cells with PAN-based separators (consisting of thin nanofibers and hot-pressed under high pressure) were significantly higher than that of the cell with Celgard PP separator (88.6%). Such a difference on capacity retention ratio between the cells with PAN-based and Celgard PP separators could be attributed to the discrepancy on retainability of liquid electrolyte (thus the improvement of Li+ ion transport across the separator’s porous network). The higher values of porosity and electrolyte uptake of the PAN-based separators (in comparison to the Celgard PP separator) would help to retain larger amount of electrolyte in the separators, to facilitate better Li+ ion transport during the cycling tests. The better cycling performance for the cells with PAN-based separators (than the Celgard PP separator) could be explained in terms of lower Li+ ion transfer resistance, better specific capability, and higher stability of the PAN-based separators as evidenced from their electrochemical stability [19]. 4. Conclusions In this study, nine types of electrospun PAN nanofibrous membranes with varied fiber diameters and different membrane porosities were prepared; subsequently, these membranes were explored as LIB separators. In specific, the impacts of fiber diameter and membrane porosity on electrolyte uptake, Li+ ion transport, electrochemical oxidation limit, and specific capacity were investigated. The results indicated that the PAN-based membranes with different fiber diameters and porosities would lead to different electrochemical performances as LIB separators, and the PAN–S–H membrane with small fiber diameters of 200– 300 nm and hot-pressed under high pressure of 20 MPa outperformed others separators including the commercial Celgard PP separator. The electrochemical cells with PAN–S–H separator exhibited the best discharge capacity of 89.5 mA h g1 and the capacity retention ratio of 97.7% due to its high porosity and large electrolyte uptake. Overall, the hot-pressed electrospun PAN nanofibrous membranes (particularly those consisting of thin nanofibers) had larger electrolyte uptake, higher thermal stability, wider potential windows, better Li+ ion transport capability, and higher electrochemical performance, as compared to the commercial Celgard PP separator.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrospun polyacrylonitrile nanofibrous membranes with varied fiber diameters and different membrane porosities as lithium-ion battery separators Xiaojing Maa,1, Praveen Kollab,1, Ruidong Yangb , Zhao Wangb , Yong Zhaob , Alevtina L. Smirnovab,** , Hao Fonga,b,* a b
Program of Nanoscience and Nanoengineering, South Dakota School of Mines and Technology, Rapid City, SD 57701, USA Department of Chemistry and Applied Biological Sciences, South Dakota School of Mines and Technology, Rapid City, SD 57701, USA
A R T I C L E I N F O
Article history: Received 8 November 2016 Received in revised form 21 March 2017 Accepted 27 March 2017 Available online 30 March 2017 Keywords: Electrospinning polyacrylonitrile nanofibrous membrane lithium-ion battery nonwoven separator
A B S T R A C T
In this study, nine types of polyacrylonitrile (PAN) nanofibrous membranes with varied fiber diameters and different membrane porosities are prepared by electrospinning followed by hot-pressing. Subsequently, these membranes are explored as Li-ion battery (LIB) separators. The impacts of fiber diameter and membrane porosity on electrolyte uptake, Li+ ion transport through the membrane, electrochemical oxidation potential, and membrane performance as LIB separator (during charge/ discharge cycling and rate capability tests of a cathodic half-cell) have been investigated. When compared to commercial Celgard PP separator, hot-pressed electrospun PAN nanofibrous membranes exhibit larger electrolyte uptake, higher thermal stability, wider electrochemical potential window, higher Li+ ion permeability, and better electrochemical performance in LiMn2O4/separator/Li half-cell. The results also indicate that the PAN-based membrane/separator with small fiber diameters of 200– 300 nm and hot-pressed under high pressure of 20 MPa surpasses all other membranes/separators and demonstrates the best performance, leading to the highest discharge capacity (89.5 mA h g1 at C/2 rate) and cycle life (with capacity retention ratio being 97.7%) of the half-cell. In summary, this study has revealed that the hot-pressed electrospun PAN nanofibrous membranes (particularly those consisting of thin nanofibers) are promising as high-performance LIB separators. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction Lithium-ion batteries (LIBs) have been widely utilized as the power source for portable electronic devices and hybrid electric vehicles because of high energy density, long cycle life, low maintenance, and long shelf life (i.e., low self-discharge) [1–5]. The LIB separator, an electrochemically inactive membrane, is placed between cathode and anode of a LIB. The function of LIB separator is to prevent the physical contact of two electrodes while allowing Li+ ions to transport through the liquid electrolyte that fills in the membrane porous structure. For a good LIB separator, it must fulfill
* Corresponding author at: Program of Nanoscience and Nanoengineering, South Dakota School of Mines and Technology, Rapid City, SD 57701, USA. Tel.: +605 3941229, Fax: +605 394-1232. ** Corresponding author. Tel.: +605 394-1890, Fax: +605 394-1232. E-mail addresses: [email protected] (A.L. Smirnova), [email protected] (H. Fong). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.electacta.2017.03.205 0013-4686/© 2017 Elsevier Ltd. All rights reserved.
several requirements including appropriate thickness, suitable pore size, good chemical/thermal stability, and high Li+ ion permeability [5–8]. In commercial LIBs, polyolefin-based (i.e., polyethylene/polypropylene-based) microporous membranes are commonly adopted as the separators due to their reasonably high chemical/thermal stability and relatively good performance. However, some intrinsic limitations of nonpolar polyolefin-based separators (e.g., low electrolyte uptake, poor adhesion property to electrodes, and low ionic transport) considerably restrict their electrochemical performances [9,10]. Recently, highly porous nonwoven mats/membranes (consisting of overlaid fibers) have attracted growing attention as LIB separators. Several techniques such as papermaking [11], solution extrusion [12], melt blowing [13], and electrospinning [14–16] have been investigated to make nonwoven LIB separators. Among these, the electrospinning technique provides a convenient and valuable approach to prepare overlaid membranes with fiber diameters typically being hundreds of nanometers (commonly known as electrospun nanofibers) [17]. The electrospun
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nanofibrous membranes (e.g., nylon, poly(vinylidene fluoride), and polyacrylonitrile nanofibrous membranes) have smaller fiber diameters and higher porosities than the conventional nonwoven membranes, making them promising for high-performance LIB separators [14–17]. For example, electrospun polyacrylonitrile (PAN) nanofibrous membranes were explored as LIB separators; the results indicated that they possessed desired properties such as high ionic transport, small diffusion resistance, good thermal/ chemical/mechanical stability, long cycle life, high electrolyte uptake, and good compatibility with electrodes [18–21]. Research endeavors also suggested that, the nitrile groups in PAN might be able to interact with the Li+ ions in liquid electrolytes and the carbonyl groups in solvent (e.g., propylene carbonate and ethylene carbonate) [22,23]. Furthermore, PAN could also mitigate the formation of Li dendrites during the charging/discharging process of LIBs [20,21]. It is important to note that, electrospun nanofibrous membranes do not have straight-through pores, if they are well prepared. However, tiny particles are able to permeate through these membranes via tortuous pathways. As a result, the membranes show equivalent/apparent pore sizes typically in the range from tens of nanometers to several micrometers. Aselectrospun nanofibrous membranes often have porosities of 85–90%; in other words, the nanofibers only occupy 1/8 of membrane volume. After being mechanically pressed, the porosity can readily be reduced to 75% (i.e., the nanofibers occupy 1/4 of membrane volume); while hot-pressing with relatively high pressure can further reduce the porosity to 50% or even lower. It is obvious that the membranes with different porosities would exhibit different equivalent/apparent pore sizes, leading to different performances as LIB separators. Surprisingly, no studies have been reported regarding the mechanical pressing on the porosities of electrospun nanofibrous membranes and its impact on the LIB separator performances. Moreover, the diameters of fibers in electrospun membranes would also play crucial roles on the equivalent/apparent pore sizes and further on the LIB separator performances; whereas no studies have been reported on this issue either. Therefore, the objective of this study is to explore the morphological/physical properties (particularly fiber diameter and membrane porosity) of hot-pressed electrospun PAN nanofibrous membranes on their performances as LIB separators. In specific, nine types of electrospun PAN nanofibrous membranes with varied fiber diameters and different membrane porosities were prepared; subsequently, the impacts of fiber diameter and membrane porosity on LIB separator performance were explored. The study revealed that the membranes with different fiber diameters and membrane porosities would lead to different performances as LIB separators, while the PAN–S–H membrane/ separator with small fiber diameters of 200–300 nm and hotpressed under high pressure of 20 MPa outperformed all other membranes/separators including the commercial Celgard PP separator.
separator with the thickness of 25 mm was provided by Celgard LLC (Charlotte, NC). 2.2. Preparation of electrospun PAN nanofibrous membranes PAN was first dissolved in DMF to prepare three solutions with the concentrations being 7, 10, and 12 wt.%, respectively. Note that the reason for selecting these three concentrations of PAN/DMF solution was based upon morphological structures of as-electrospun PAN nanofibers; in specific, the PAN solution with concentration lower than 7 wt.% would result in significant amount of beads and/or beaded nanofibers, while the PAN solution with concentration higher than 12 wt.% would result in thick nanofibers with diameters close to or larger than 1 mm. Prior to electrospinning, each PAN solution was loaded into a 30 mL Luer-LokTM tip plastic syringe having an 18 gauge 90 blunt-end steel needle, and the electrospinning process was carried at room temperature of 22 C and relative humidity of 20%. During electrospinning, a positive high DC voltage of 18 kV was applied to the PAN solution through the needle by using an ES30P high voltage power supply (Gamma High Voltage Research, Inc.). The feed rate of each solution was set at 0.5 mL h1 by using a KDS 200 syringe pump (KD Scientific, Inc.). Electrospun PAN nanofibers were collected on a laboratory-produced rotating drum (with diameter of 25 cm) covered with aluminum foil. The syringe pump was placed on a slider which could move horizontally in front of the collecting drum at the pre-determined speed of 15 cm min1. The obtained electrospun PAN nanofibrous membranes were morphologically uniform with the thicknesses of 60 mm (measured with a micrometer caliper). Subsequently, these membranes were mechanically pressed under three different pressures (i.e., 5, 10, and 20 MPa) at 60 C for 3 min. Therefore, nine types of membranes were prepared, and they were denoted as PAN–D(S/M/L)–P(L/M/ H). Note that D represents the fiber diameter, where S stands for small fiber diameters in the range of 200–300 nm, M stands for medium fiber diameters in the range of 400–500 nm, and L stands for large fiber diameters in the range of 700–800 nm; whereas P represents the applied pressure during the hot-pressing process, where L stands for the low pressure of 5 MPa, M stands for the medium pressure of 10 MPa, and H stands for the high pressure of 20 MPa. 2.3. Characterization of electrospun PAN nanofibrous membranes A Zeiss Supra 40VP field-emission scanning electron microscope (SEM) was employed to examine morphological structures of nine types of electrospun PAN nanofibrous membranes. Prior to SEM examinations, the specimens were sputter-coated with gold to avoid charge accumulation. A TA Instruments Q100 differential scanning calorimeter (DSC) was employed to study the thermal stability of PAN membrane separators, and the DSC curves were recorded from 60 to 400 C with the heating rate of 10 C min1. 2.4. Measurements of electrolyte uptake and membrane porosity
2. Experimental 2.1. Materials Polyacrylonitrile (PAN, Mw = 150,000), N,N-dimethylformamide (DMF), lithium hexafluorophosphate (LiPF6), ethylene carbonate (EC), and diethyl carbonate (DEC) were purchased from Sigma-Aldrich (St. Louis, MO). Lithium foil was purchased from Alfa Aesar (Ward Hill, MA). Carbon-coated LiMn2O4 cathode foil (with active material of 16.6 mg cm2) was purchased from MTI Corporation (Richmond, CA). Celgard polypropelene (Celgard PP)
Electrolyte uptake of a membrane was measured according to Eq. (1) [20,24]: ð1Þ Electrolyte upate ð%Þ ¼ W f W i =W i 100% where Wi and Wf are the weights of a membrane before and after being soaked in liquid LiPF6 electrolyte for 2 h in argon-filled glove box, respectively. Porosity of a membrane was calculated from Eq. (2) [20]: P ¼ ðV V P =V Þ 100%
ð2Þ
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where P is the porosity of membrane, V is the total volume of membrane, and Vp is the volume of PAN nanofibers (i.e., the mass divided by the density (1.184 g cm3) of PAN polymer). The electrolyte uptake and porosity values of each type of electrospun PAN nanofibrous membranes were finally determined from averaging the values of 3 samples. 2.5. Evaluation of electrochemical performance The transport of Li+ ions through electrolyte-soaked separators (including nine types of electrospun membranes and the Celgard PP separator) was indirectly determined by measuring the ionic conductivity of electrolyte in a symmetric coin-cell configuration, comprised of liquid LiPF6 electrolyte-soaked separator that was sandwiched between two stainless-steel electrodes. The ionic conductivity of electrolyte was measured by electrochemical impedance spectroscopy (EIS) using the PARSTAT1 2273 electrochemical impedance analyzer purchased from Princeton Applied Research (Oak Ridge, TN). These impedance measurements were conducted at the applied AC amplitude of 5 mV within the frequency range of 200 kHz–10 mHz. The ionic conductivity value was then calculated from Eq. (3) [20,24]:
s ¼ b=Rb S
ð3Þ
where b is the thickness of a separator (cm), Rb is the electrolytebulk resistance (V), and S is the area of this separator (cm2). Electrochemical stability of each separator was evaluated using linear sweep voltammetry (LSV) by using a potentiostat (580 Battery Test System) purchased from Scribner Associates Inc. (Southern Pines, NC). The electrochemical oxidation behavior was
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tested using a coin-cell comprised of liquid LiPF6 electrolytesoaked membrane/separator that was sandwiched between lithium metal (i.e., counter electrode) and stainless steel (i.e., working electrode). The tests were carried out at the scan rate of 1.0 mV S1 in the voltage window of 3.5–6.0 V vs. Li+/Li. The performance of each membrane as LIB separator was further evaluated upon measuring the rate capability and cycling performance of the cathodic half-cells of LiMn2O4/separator/Li metal in a coin-cell configuration. The rate capabilities at different C-rates of C/2, 1C, 2C, and C/2 (i.e., 0.75, 1.5, 3, and 0.75 mA g1, 3 charge/discharge cycles each), and the cycle life at C/2 for 100 charge/discharge cycles, were studied in the voltage range of 3.0– 4.5 V. All of the coin-cells (CR2025) for the above tests were assembled inside an argon-filled glove box, while each membrane/ separator was thoroughly wetted with the liquid electrolyte of 1 M LiPF6 in EC/DEC (with volume ratio of 1/1). 3. Results and discussion 3.1. Morphological structures and membrane parameters Representative SEM images acquired from nine types of electrospun PAN nanofibrous membranes are shown in Fig. 1. The diameters of PAN nanofibers electrospun from 7, 10, and 12 wt. % PAN solutions were in the ranges of 200–300 nm (top row), 400– 500 nm (middle row), and 700–800 nm (bottom row), respectively. These membranes consisted of randomly overlaid electrospun PAN nanofibers, which were morphologically uniform without microscopically identifiable beads and/or beaded nanofibers. It is necessary to note that, the presence of beads and/or beaded
Fig. 1. SEM images showing the representative morphological structures of nine types of electrospun PAN nanofibrous membranes: (A) PAN–S–L, (B) PAN–S–M, (C) PAN–S–H, (D) PAN–M–L, (E) PAN–M–M, (F) PAN–M–H, (G) PAN–L–L, (H) PAN–L–M, and (I) PAN–L–H.
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nanofibers in an electrospun nanofibrous membrane is a common issue/problem, which has negative effect on advantageous properties of the membrane. In this study, the pre-determined PAN concentrations (being 7 wt.% or higher) could effectively avoid the formation of beads and/or beaded nanofibers; this was in agreement with previously reported studies [25,26]. The PAN nanofibers in the membranes that were hot-pressed under low pressure (i.e., 5 MPa) were able to well retain their shapes/ morphologies without distinguishable deformations (left column), whereas the PAN nanofibers in the membranes that were hotpressed under high pressure (i.e., 20 MPa) appeared flattened (right column). Evidently, the hot-pressing process would change the porosity of an electrospun PAN nanofibrous membrane, further leading to different apparent/equivalent pore sizes. Membrane parameters of different separators (including the nine electrospun membranes and the Celgard PP separator) were measured, and the results are summarized in Table 1. In general, the porosity values of electrospun membranes were considerably higher than the value of Celgard PP separator (50.2%). With the increase of hot-pressing pressure, the porosity values of electrospun membranes would be decreased, suggesting that the equivalent/apparent pore sizes would be reduced. Upon hotpressing under high pressure (i.e., 20 MPa), the porosity values of PAN–S–H, PAN–M–H, and PAN–L–H membranes/separators were 54.7%, 52.6%, and 55.7%, respectively. Meantime, the hotpressing process also had significant influence on electrolyte uptake. In specific, the electrolyte uptake values of PAN–S–H, PAN– M–H, and PAN–L–H membranes/separators were 336, 300, and 289%, respectively. It is well known that higher values of porosity and electrolyte uptake are generally desired for LIB separators. Even though the hot-pressing process reduced both porosity and electrolyte uptake values of electrospun membranes (i.e., PANbased separators), the values acquired from the nine types of PANbased separators were still substantially higher than the values acquired from the Celgard PP separator. Moreover, the electrolyte uptake value of PAN-based separators was also affected by the variation of fiber diameters. For example, the PAN-based separator with the smallest fiber diameters (i.e., 200–300 nm) had the highest electrolyte uptake value. In specific, the electrolyte uptake values of PAN–S–L, PAN–S–M, and PAN–S–H membranes/separators were 363, 350, and 329%, respectively; whereas the electrolyte uptake value of Celgard PP separator was merely 233%. Note that the values of thickness and ionic conductivity of electrolyte-soaked electrospun membranes were similar to the values of Celgard PP separator. Nevertheless, both parameters of the PAN–S–H membrane/separator were appreciably better than those of the Celgard PP separator, suggesting that the PAN–S–H membrane/separator might have higher LIB performance.
Fig. 2. DSC thermograms acquired from the electrospun PAN nanofibrous membranes and from the Celgard PP separator.
3.2. Thermal stability DSC was employed to investigate the thermal stability of electrospun PAN nanofibrous membranes. For comparison, the thermal stability of Celgard PP separator was also studied. As shown in Fig. 2, the thermogram acquired from Celgard PP separator had an endothermic peak centered at 163 C, corresponding to the melting temperature of PP polymer. For the nine electrospun PAN nanofibrous membranes, the acquired thermograms were almost identical; and a typical thermogram is shown in Fig. 2. This thermogram had an exothermic peak centered at 303 C, which was attributed to the stabilization of PAN polymer (i.e., the nucleophilic attack on nitrile groups followed by instantaneous cyclization into extended conjugated structure) [26,27]. The DSC results indicated that the PAN nanofibrous membranes had better thermal stability than the Celgard PP separator. As a result, these membranes would be able to better retain their morphological structures during the operation. Furthermore, in a thermal run-away incident, the LIB device with such a PAN-based separator would be substantially more stable. 3.3. Electrochemical performance Nyquist plots of the symmetric cells (i.e., stainless steel/ separator/stainless steel) for the Celgard PP and PAN-based separators in the liquid electrolyte are shown in Fig. 3A. The ionic conductivity of the electrolyte-soaked separator was measured from the bulk-resistance value (i.e., the x-intercept of extrapolated Nyquist plot at high-frequency end of real z). As shown in Table 1,
Table 1 Membrane parameters acquired from the nine types of electrospun PAN nanofibrous membranes as well as from the Celgard PP separator. Each datum showing the average value of three measurements and the associated standard deviation. Separator
Thickness (mm)
Porosity (%)
Electrolyte uptake (%)
Ionic conductivity (mS cm1)
PAN–S–L PAN–S–M PAN–S–H PAN–M–L PAN–M–M PAN–M–H PAN–L–L PAN–L–M PAN–L–H Celgard PP
27.2 1.1 24.5 0.8 23.6 0.7 27.1 1.1 23.7 1.0 22.8 0.8 28.1 1.3 25.4 1.6 23.5 0.9 25.2 0.7
61.0 2.5 57.7 2.5 54.7 1.8 59.6 2.3 53.8 3.0 52.6 1.8 60.4 1.9 57.2 2.9 55.6 1.7 50.2 1.6
363 15 350 14 336 9 350 12 310 13 300 11 329 13 313 15 289 9 233 7
0.93 0.04 0.98 0.04 1.06 0.03 0.74 0.03 0.88 0.04 0.95 0.03 0.59 0.03 0.60 0.03 0.62 0.02 0.80 0.02
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Fig. 3. (A) Nyquist plots of the symmetric cells (i.e., stainless steel/separator/stainless steel) for the PAN-based and Celgard PP separators soaked in liquid electrolyte, and (B) Linear sweep voltammograms of the cells (i.e., Li metal/separator/stainless steel) to study oxidative behaviors of the PAN-based and Celgard PP separators in liquid electrolyte within the electrochemical window of 3.5–6 V (the inset showing the enlarged area within the window of 3.75–5 V).
both the increase of hot-pressing pressure and the decrease of fiber diameter would result in the increase of ionic conductivity of the electrolyte-soaked separator. Among nine types of electrospun PAN nanofibrous membranes, the membrane consisting of the thinnest nanofibers (i.e., 200–300 nm) exhibited the highest ionic conductivity. In specific, the ionic conductivity values of electrolyte-soaked PAN–S–L, PAN–S–M, and PAN–S–H membranes/ separators were 0.93, 0.98, and 1.06 mS cm1, respectively. Note that among all types of membranes/separators including the Celgard PP separator, the PAN–S–H membrane/separator had the highest ionic conductivity value of 1.06 mS cm1. Furthermore, the PAN–S–H membrane/separator was also thinner and more porous
than the Celgard PP separator, which would be more beneficial to the LIB performance as well. The electrochemical stability of the cells (i.e., Li metal/ separator/stainless steel) was tested via the linear sweep voltammetry (Fig. 3B). For the Celgard PP separator, the anodic current was observed at 4.0 V vs. Li+/Li, probably due to microshort circuit upon the formation of lithium dendrite and the oxidation of PP polymer [19]. It was evident that, all of the nine PAN-based membranes/separators had substantially higher electrochemical stability than the Celgard PP separator, and there was no identifiable oxidation peaks for these cells below 4.5 V vs. Li+/Li, suggesting that the PAN-based membranes/separators would be more suitable for LIBs by having higher electrochemical stability in
Fig. 4. Initial charge/discharge voltage profiles at the C-rate of 0.5C for the coin half-cells (i.e., LiMn2O4/separator/Li metal) with separators being (A) PAN–S–L, PAN–S–M, PAN–S–H, and Celgard PP, (B) PAN–M–L, PAN–M–M, PAN–M–H, and Celgard PP, and (C) PAN–L–L, PAN–L–M, PAN–L–H, and Celgard PP. (D) Specific capability of the cells with the separators being PAN–S–H, PAN–M–H, PAN–L–H, and Celgard PP at charge/discharge rates of 0.5, 1, 2, and 0.5C (i.e., 0.75, 1.5, 3, and 0.75 mA g1, 3 cycles at each current).
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a wider potential range than the PP-based separator. Among these separators, the PAN–L–H separator exhibited the highest onset oxidation potential and the lowest oxidation current. The larger diameter and better compactness of PAN–L–H separator (due to high-pressure hot-pressing) could have improved mechanicalshielding against micro-dendrite growth. On the other hand, the developments of high-voltage cathode materials for LIBs has attracted significant attention [28]; the newly developed PANbased separators would be particularly advantageous for making LIB devices with high-voltage cathode materials. 3.4. Battery performance The developed PAN membranes/separators were tested in the coin half-cell configuration with LiMn2O4 cathode and Li metal reference electrode in the presence of liquid LiPF6 electrolyte (Fig. 4A–C). The galvanostatic charge/discharge curves at the constant current of 0.5C (i.e., 0.75 mA g1) were recorded for the PAN-based separators and compared to the curves acquired from the Celgard PP separator. The charge/discharge curves for all the cells had stable plateaus, while the cells with the PAN–S–H separator exhibited the highest capacity. For the cells with PANbased separators hot-pressed at high pressure (i.e., 20 MPa), the specific capacity values of three cells with the PAN–S–H, PAN–M– H, and PAN–L–H separators were 89.5, 83.1, and 81.6 mA h g1, respectively, which were significantly higher than that of the cell with the Celgard PP separator (75.1 mA h g1). Based on the experimental results, it is obvious that the fiber diameter has significant impact on the specific capacity of electrochemical cell. In general, the specific capacities of the cells with the PAN-based separators consisting of thin nanofibers (i.e., 200–300 nm) were higher; and the specific capacity values of the cells with PAN–S–H, PAN–S–M, and PAN–S–L separators were 89.5, 80.4, and 72.6 mA h g1, respectively. As previously speculated, the cell with the PAN–S–H separator had the highest discharge capacity of 89.5 mA h g1, which was due to high porosity, large electrolyte uptake, and better Li+ ion transport (revealed by ionic conductivity measurements of electrolyte-socked membranes) through the PAN–S–H separator. Fig. 4D depicts specific capabilities for the electrochemical cells with PAN–S–H, PAN–M–H, PAN– L–H, and Celgard PP separators at constant current charge/ discharge rates of 0.5, 1, 2, and 0.5C (3 cycles at each C-rate). In general, the capacity retention ratios of all types of cells were stable, while the cell with PAN–S–H separator had the largest discharge capacity at different C-rates of 0.5, 1, 2, and 0.5C. In particular, the electrochemical cell with PAN–S–H separator exhibited the discharge capacity values of 89.5, 36.5, 14.8, and 89.5 mA h g1 at the C-rates of 0.5, 1, 2, and 0.5C, respectively; and the capacity advantage of the PAN-based separators over Celgard PP separator was more significant at lower rate (0.5C) than higher rate (2C). As the C-rate became higher, batteries equipped with all types of separators experienced the decreasing of utilized capacity caused by the higher resistance; while the overall resistance in a battery was contributed by all components including the separator, electrolyte, electrodes, and interfaces. Therefore, the advantage resulted from PAN-based separator would become less significant at higher C-rate [29,30]. On the other hand, this phenomenon could also be related to the inherent mass/ionic transport limitations caused by high loading in the working electrode (i.e., carboncoated LiMn2O4 with active material at 16.6 mg cm2) of electrochemical half-cells [30,31]. Cycling performance was evaluated at the constant current charge/discharge rate of 0.5C for 100 cycles, and the results are shown in Fig. 5. Overall, the PAN-based separators prepared under high hot-pressing pressure (i.e., 20 MPa) resulted in superior battery cycling performance. The cells with the PAN–S–H separator
Fig. 5. Cycling performance/stability of the coin half-cells (i.e., LiMn2O4/separator/ Li metal) with the PAN–S–H, PAN–M–H, PAN–L–H, and Celgard PP separators at the constant current charge/discharge rate of 0.5 C.
exhibited the best capacity retention ratio of 97.7%, while the capacity retention ratios of the cells with PAN–M–H and PAN–L–H separators were 96.1% and 96.2%, respectively. Hence, the capacity retention ratios of the cells with PAN-based separators (consisting of thin nanofibers and hot-pressed under high pressure) were significantly higher than that of the cell with Celgard PP separator (88.6%). Such a difference on capacity retention ratio between the cells with PAN-based and Celgard PP separators could be attributed to the discrepancy on retainability of liquid electrolyte (thus the improvement of Li+ ion transport across the separator’s porous network). The higher values of porosity and electrolyte uptake of the PAN-based separators (in comparison to the Celgard PP separator) would help to retain larger amount of electrolyte in the separators, to facilitate better Li+ ion transport during the cycling tests. The better cycling performance for the cells with PAN-based separators (than the Celgard PP separator) could be explained in terms of lower Li+ ion transfer resistance, better specific capability, and higher stability of the PAN-based separators as evidenced from their electrochemical stability [19]. 4. Conclusions In this study, nine types of electrospun PAN nanofibrous membranes with varied fiber diameters and different membrane porosities were prepared; subsequently, these membranes were explored as LIB separators. In specific, the impacts of fiber diameter and membrane porosity on electrolyte uptake, Li+ ion transport, electrochemical oxidation limit, and specific capacity were investigated. The results indicated that the PAN-based membranes with different fiber diameters and porosities would lead to different electrochemical performances as LIB separators, and the PAN–S–H membrane with small fiber diameters of 200– 300 nm and hot-pressed under high pressure of 20 MPa outperformed others separators including the commercial Celgard PP separator. The electrochemical cells with PAN–S–H separator exhibited the best discharge capacity of 89.5 mA h g1 and the capacity retention ratio of 97.7% due to its high porosity and large electrolyte uptake. Overall, the hot-pressed electrospun PAN nanofibrous membranes (particularly those consisting of thin nanofibers) had larger electrolyte uptake, higher thermal stability, wider potential windows, better Li+ ion transport capability, and higher electrochemical performance, as compared to the commercial Celgard PP separator.
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