Zn salts incorporated polyurethane/polyacrylonitrile electrospinning fiber membrane for high porosity polymer electrolyte in Zn ion battery

So far, a large variety of polymer molecule architectures have been explored in the electrolyte field. Polymer electrolytes have gathered research efforts as an interesting alternative to conventional liquid electrolytes due to their advantages of low probability of leakage and low volatility of liquid solvent, lightweight, flexibility, inertness, high durability, and thermal stability. In this work, a polymer electrolyte developed from a polyurethane/polyacrylonitrile (PU/PAN) electrospinning fiber membrane was added with different zinc (Zn) salts, namely, Zn(CH3CO2)2, ZnSO4, and Zn(OTf)2. The samples with the Zn salt presented many different properties; especially, the high Zn(OTf)2 sample showed gradually bundle morphology in its structure. Characterization revealed improved properties in contact angle, water uptake, and thermal resistance. Namely, the 15 wt% Zn(OTf)2) sample exhibited an outstandingly high ionic conductivity of 3.671 mS cm−1, which is 10 times higher than that of the neat PU/PAN membrane.

Zn(OTf) 2 , and ZnSO 4 , were added to the PU/PAN solution with a weight fraction of 0-15 wt%.The solution with zinc salt was stirred overnight until a homogeneous solution was obtained.The suitable blending weight ratio of PU/PAN was then fabricated into non-woven fiber mats.The homogeneous PU/PAN solution was charged into a 5 mL syringe coupled with a needle with an inner diameter of 0.55 mm and placed in a syringe pump with a controlled flow rate of the polymer solution of 1.0 mL/h at a voltage of 15 kV and a needle tip to collector distance of 19 cm.The parameter conditions provide quite fine, perfect fiber morphology for every membrane.Humidity and temperature during the electrospinning were controlled using an environmental chamber, 50% ± 10% and 25 °C ± 2 °C, respectively.The electrospun fiber mats were dried for at least 24 h before further use.The mats are represented as PU/PAN/ZnX-y, where X is the type of Zn salt [A = Zn(CH 3 CO 2 ) 2 , S = ZnSO 4 , or T = Zn(OTf) 2 ] and y is the amount of zinc salt in weight percentage.

Characterization
The morphology of PU/PAN/ZnX-y electrospun fiber mats was determined by scanning electron microscopy (SEM) (Hitachi SU-4800, Japan) equipped with energy dispersive X-ray spectroscopy at a voltage of 3.0 kV and an emission current of 10 mA.The surfaces of the samples were sputter-coated with gold before measurement.The average fiber diameter in all SEM images was measured with ImageJ.
The chemical structures of PU/PAN/ZnX-y were investigated using Fourier-transform infrared spectroscopy (FT-IR; Bruker FT-IR Alpha II, United State) equipped with an attenuated total reflectance accessory.All samples were scanned at a resolution of 4 cm −1 within the spectral range of 4000-500 cm −1 .The obtained results were subtracted from the background spectra.
The mechanical behavior of PU/PAN/ZnX-y was tested at room temperature with a universal testing machine (LLOYD model LF Plus).The specimens with a dimension of 10 mm × 100 mm × 0.02 mm (length × width × depth) were prepared.Gauge length and cross-head speed were 50 mm and 4 mm min −1 , respectively.
The thermal transitions of specimens were investigated using differential scanning calorimetry (DSC; Netzch DSC 3500 Sirius, German).The sample was contained in an aluminum pan, while the reference was determined as empty aluminum pan.The testing condition was set in the range of −100 to 220 °C at a heating rate of 10 °C min −1 under nitrogen.
The thermal degradation and char yield of PU/PAN and zinc salt-incorporated samples were studied using thermo gravimetric analyzer (TGA, Shimadzu).The temperature was operated from room temperature to 700 °C at 20 °C min −1 heating rate in a nitrogen atmosphere.
The dimensional stability of fiber samples was investigated using a modified method from Saisangtham et al. 37 .The fiber was prepared in a circular shape with 19 mm of diameter and then heated in an oven at a temperature of 80, 100, and 120 °C for 1 h each.
The electrospun fiber mat samples were cut into 2 × 2 cm 2 squares.The electrolyte uptake of the samples was determined using Eq.(1).
where W d and W w are the weights of the PU/PAN/ZnX-y electrospun fiber mats before and after immersing in deionized water for 24 h, respectively.The average was calculated from three measurements.
The wettability of the fiber mats by an electrolyte was observed with electrolyte droplets placed on their surfaces and confirmed using an in-house contact angle analyzer.The electrospun fiber mats were dried in a vacuum overnight to remove all moisture.Contact angle measurements were conducted within 5 s by placing one drop of DI water on the samples.The final contact angles were obtained as the average of three measurements at room temperature.www.nature.com/scientificreports/A potentiostat/galvanostat (PSTrace4 PalmSens, The Netherlands) was utilized for investigating the electrochemical properties.The measurements were performed on an applied 10 mV AC potential from 1 MHz to 1 Hz.The SS/Polymer electrolyte/SS cell was constructed by inserting the fiber mats between stainless steel blocking electrodes.The membrane thickness and active area were approximately 100-300 µ m and 1.54 cm 2 , respectively.Transference number measurement was performed by the DC polarization method using chronoamperometry.A polarization voltage of 10 mV was applied across the sample, and the initial maximum current I 0 and steadystate current I s were recorded.Equation (2) was used to calculate ionic conductivity (δ).The average value was from three measurements.
where d refers to the thickness of the fiber membrane, R b is the bulk ionic resistance of the membrane, which can be obtained from the plot, and S is the area of electrodes connected within the membrane.The Zn 2+ transference number (t + ) was calculated by the Bruce-Vincent method using Eq.(3) 38,39 : where ∆V represents the testing step potential; I 0 and I s are the currents at the initial and steady states, respectively; and R 0 and R S represent the cell resistance before and after polarization, respectively.
The dissolution of zinc salt in a highly porous polymer electrolyte was evaluated by immersing the membrane in 20 ml of DI water for 24 h.Then, the obtained solutions were brought to test by inductively coupled plasma (ICP-OES, Shimadzu, ICPE900, Kyoto, Japan) and the concentration of zinc in water was calculated in parts per million (ppm).
The voltage response on the electrochemical compatibility of PU/PAN/ZnX-y fiber mats was investigated and recorded as a function of cycle at room temperature for long-term zinc charge/discharge cycles using a Neware testing system (Shenzhen Neware CT-4008, China).The charge-discharge cycles of the CR2032 coin cell with symmetric Zn/polymer electrolyte/Zn cells and adding a drop of DI water were performed to get quasi-solid polymer electrolyte at a current density of 0.25 mA cm −2 .The CR2032 coin cells were assembled by sandwiching the polymer electrolyte between Zn foil and a VNO electrode in deionized water to measure the potential range.The cathode NVO fabrication is explained in the supplementary file.Then, the coin cells equipped with Zn/PU/PAN/NVO and Zn/PU/PAN/ZnT-15/NVO adding a drop of 0.5 M Zn(OTf) 2 solution were brought for rate performance testing.The setting conditions for different current densities were 0.1, 0.2, 0.4, 0.6, 0.8, 1.0, 2.0, and 5.0 A g -1 , then returned to 0.2 A g −1 .

Morphology of the Zn salt incorporated PAN/PU electrospun fiber mats with different types of Zn salts and ratios
The electrospun membrane was prepared by electrospinning at 1 mL h −1 polymer solution feeding rate, 15 kV applied voltage, and 19 cm distance from the needle tip to the collector.The morphologies of the PAN/PU electrospun membranes with different Zn salts, i.e., Zn(OTf) 2 , (Zn(CH 3 CO 2 ) 2 ), or (ZnSO 4 ), at 0-15 wt% were characterized by SEM.Shear force, which drives the polymer solution at the needle wall, was fixed at a constant rate.
Figure 2 shows the morphologies and fiber diameters of all Zn-incorporated PU/PAN electrospun membranes.The pure PU/PAN membrane without Zn salt reveals a smooth and random fiber morphology with an average fiber diameter of 535 ± 75 nm.The PU/PAN/ZnA-y fiber mats presented a slight decrease in fiber diameter compared to the neat PU/PAN and a decrease in fiber content with increasing Zn(CH 3 CO 2 ) 2 content.In Fig. 2a-d, the fiber diameters of PU/PAN/ZnA-5, PU/PAN/ZnA-10, and PU/PAN/ZnA-15 were 511 ± 95, 478 ± 106, and 337 ± 84 nm, respectively.The decrease in fiber diameter is due fiber extension morphology from the addition of the salt molecule in the polymer solution.Next, ZnSO 4 salt was incorporated to synthesize PU/ PAN/ZnS-5, PU/PAN/ZnS-10, and PU/PAN/ZnS-15, as shown in Fig. 2e-g.The fiber diameter decreased in PU/ PAN/ZnS.However, the average fiber diameter was not clearly decreased by the addition of ZnSO 4 salt.The fiber diameter of ZnS-5 was decreased to 310 ± 73 nm compared with that of the non-salt fiber, and the fiber diameter appeared stable with measured sizes of 367 ± 84 and 381 ± 70 nm as the ZnSO 4 salt contents increased to 10 and 15wt%, respectively.This turnaround in trend toward increasing fiber diameter from salt addition may be due to the high amount of salt in polymer fiber and fiber connections.Moreover, the fiber morphologies of PUPAN with Zn(OTf) 2 at ratios of ZnT-5, ZnT-10, and ZnT-15 are exhibited in Fig. 2h-j.The ZnT samples exhibited decreasing fiber diameter, a similar trend to Zn(CH 3 CO 2 ) 2 salt-included samples in 5% and 10% salt ratios, but ZnT-3 exhibited increasing fiber diameter with a measured size of 597 ± 205 nm.The size distribution of all electrospun fiber mats is illustrated in Table S1.The comparison of average fiber diameter is clearly presented in Fig. 2k.Besides, the zinc element mappings were investigated to confirm the presence of zinc from zinc salt by SEM-equipped energy dispersive X-ray spectroscopy (EDS) and presented as in Fig. 2l-n.
The decreasing fiber diameter due to the addition of another material in polymer solution was reported previously 32 that system, MXene as filler materials were encapsulated by elongated polymer chains that affected the fiber diameter decrease.Furthermore, Fan et.al., studied LiBr salt incorporated in electrospinning solution 40 , which results in an increase in the conductivity of the spinning solution and surface charge density.When the Taylor cone was generated by electric field force, the whipping jets at transition distance were unstable and split into more filaments, making ultrafine fibers.As for the different morphology of ZnT-15, the average fiber diameter increased and presented alignment into the membrane.The larger size distribution was determined from Vol.:(0123456789) www.nature.com/scientificreports/PU/PAN/ZnT-15.This phenomenon is probably due to the high dielectric constant of PU/PANZnT-15 as can be seen in Figure S3.The effect of dielectric constant on alignment fiber was proposed by Sun et al. 41 .The dielectric constant influences the instability function of the available free charge in the solution.Then, the oriented fibers are formed.The dielectric constants of zinc salt-modified fibers are reported in Table S2 and Fig. S3, which reveal that ZnT-15 has the highest dielectric constant, which may be due to the increase in Zn(OTf) 2 that also increases the charge carriers in the polymer membrane 42 .In addition, the positive correlation between dielectric constant and ionic conductivity was studied and reported by Muchakayala et.al 43 .Interestingly, PU/PAN/ZnT-15 has a fluffy fiber structure, similar to the fluffy 3D structure reported by Juhasz et al. describing the interaction between solvent (DMF) and salt and charge 44 .The preparation process for the membrane is displayed in Fig. 1, which explains the ion transport within the polymer structure through O in urethane (-COONH-) and N in the acrylonitrile (-CN) group 34,45 .

FT-IR analysis of the Zn salt incoporated PU/PAN electrospun fiber mat chemical structures
The chemical structures of Zn salt-incoporated PU/PAN electrospun fiber mats were confirmed by FT-IR, as shown in Fig. 3a.The spectra of the PU/PAN fiber presented broad N-H streching and bending vibrations of the urethane linkage amide group around 3330 and 1528 cm −1 , respectively.The absorption peak at 2240 cm −1 is assigned to the nitrile (−CN) group of the PAN matrix.The occurrence of the characteristic band at 1724 cm −1 confirms the C = O stretching bands of the carbonate group in polycarbonate.Meanwhile, the overlap peak at 1724-1670 cm −1 can be attributed to the urethane and carbonate carbonyl groups of the PU 28 .The Zn(CH 3 CO 2 ) 2 modified PU/PAN fiber (Fig. 3b) presented peaks at 1067, 690, and 615 cm −1 , which correspond to the deformation vibration of Zn-O, acetate anion twisting, and scissoring, respectively 21,46 .The results revealed nonapparent bands of chemical bonding from specific sides of the PU/PAN fiber and zinc acetate salt.Next, the ZnSO 4 -modified PU/PAN fibers (Fig. 3c) displayed S = O stretching at 1100 and 870 cm −1 from the sulfate ion.
The considerable increase of the peak at approximately 1100 cm −1 can be observed, probably due to the feature of the modifier being very intensive absorption caused by S = O stretching 47 .Then, the spectrum of the Zn(OTf) 2 modified PU/PAN fiber (Fig. 3d) showed peaks at 1242 and 1176 cm −1 assigned to the symmetric and asymmetric CF 3− stretching vibrational modes.The shoulder peak at approximately 1650 cm −1 confirms the occurrence of Zn 2+ -bonded C = O 48 .The triflate ions are confirmed by the peak at 1033 cm −1 related to the symmetrical stretching vibration mode of SO 3 [υ s (SO 3 )] and a peak at approximately 640 cm −1 corresponding to the bending of C-F and SO 3 groups and CS symmetrical stretching 21,49 .In addition, the elemental analysis of Zn salt-modified membranes PU/PAN/ZnA-15, PU/PAN/ZnS-15, and PU/PANZnT-15 was performed by elemental mapping and energy dispersive X-ray spectroscopy.In Fig. S5 (a-c), the Zn element is dispersed all over the mapping area, and the C, N, and O elements, the major contents in the polymer chain, can be observed in Fig. S4d-f.In addition, the S element is detected on the membranes ZnS and ZnT, and the F element can be seen in the Zn(OTf) 2 modified membrane (Fig. S4f).The polar functional group interaction with the salt from the salt-incorporated fiber membranes can support ionic conduction in high-porous polymer electrolytes.Then, some of the functional groups affect other membrane properties to improve electrochemical performance and battery safety.

Thermal properties of the Zn salt included PU/PAN electrospun fiber mats
Differential scanning calorimetry (DSC) was used to further investigate the microphase separation of the Zn salt using PU/PAN electrospun fiber mats.The thermal behavior of the materials was examined by scanning the samples from -90 °C to 200 °C at a heating rate of 20 °C•min −1 under nitrogen on an aluminum pan.The DSC thermograms of the zinc salt-modified PU/PAN electrospun fiber mats with different zinc salts are illustrated in Fig. 4a.The glass transition temperature (T g ) of PAN fiber was reported to be 100 °C50 .The PU/PAN sample is presented as line a, which indicates its glass transition temperature (T g ) to be  28 .The transition at approximately T g of PAN for PU/PAN/ZnT-5 presents high area merging, which refers to good dissolution of Zn(OTf) 2 salt into the polymer matrix and the appearance of C-F, which probably increases the amount of H bond in the polymer chain.
In addition, the thermograms of all Zn-salt modified samples present a higher area of endothermic peak at close to 100 °C than PU/PAN, particularly for PU/PAN/ZnS and PU/PAN/ZnT.There is the possibility that the hygroscopic nature of these samples, as expressed by the effect of moisture absorption, leads to good water uptake.
The thermal stability of the fabricated PU/PAN and PU/PAN/ZnX-y salt-incorporated electrospun membranes was evaluated by thermogravimetric analysis at 30 °C-700 °C under N 2 at a heating rate of 20 °C min −1 (Fig. 4b).The plot of the PU/PAN membrane reveals degradation temperature at 5 wt% loss (T d,5 ) at 297 °C, and the weight loss corresponds to high decomposition of the PU/PAN membrane at approximately 350 °C.Then, the decomposition rate is decreased until a char residual of 40 wt% remains.The T d,5 values of the Zn(CH 3 CO 2 ) 2 incorporated samples (ZnA-y) seem to decrease to 256 °C, 238 °C, and 226 °C for PU/PAN/ZnA-5, PU/PAN/ ZnA-10, and PU/PAN/ZnA-15, respectively.The early decomposition loss is due to the volatile component and water.Meanwhile, the char residual weights of PU/PAN/ZnA-y are higher than those of PU/PAN.Next, the T d,5 values of PU/PAN/ZnS-5, PU/PAN/ZnS-10, and PU/PAN/ZnS-15 are 278 °C, 275 °C, and 260 °C, respectively.This decreasing trend of T d,5 with increasing Zn salt may be related to the decrease in the polymer:salt ratio, that is, polymer contents.The TGA thermogram of PU/PAN/ZnT presents quite high weight loss at approximately 100 °C, which may be because the absorbed moisture due to its hygroscopic nature evaporated.Nevertheless, the decomposition slope of the PU/PAN/ZnT sample after moisture evaporation seems slower than that of the other samples.There is the possibility that the C-F from Zn(OTf) 2 delays the degradation process due to the high bonding energy (C-F = 485 J mol −1 , C−C=347 J mol −1 ) and fluorinated atom.Moreover, the char residual of PU/ PAN/ZnT is developed from neat PU/PAN, particularly from the Zn(OTf) 2 salt addition.Changing degradation temperature and loss of absorbed moisture in the modified fibers have been reported in the literature 51,52 .In the same way, different degradation temperatures between the pristine fiber and the addition of electroactive salt fillers were proposed by Tiwari and Maiti 51 .
In addition to thermal decomposition, thermal dimensional stability is a crucial thermal property.The shape shrinkage or dimensional change of samples can affect batteries in a bad ways such as electrical short circuits or thermal runaway.Figures S4a and S4b show the highly porous polymer electrolytes heat-treated at 80 °C and 120 °C, respectively.The electrolyte membrane samples were cut into 19 mm diameter circles and brought to heat in an oven.The samples after heat treatment were drawn into 19 mm diameter circle shapes, which equal the original cutting shape for electrolyte membrane applications and measured by a standard scale ruler for dimensional change studies.The morphology after heat treatment is presented in Fig. 5a-j.After 80 °C for 1 h of heat treatment, all samples' appearances were observed to retain their initial shapes, that is, without shrinkage.Next, continuous heat treatment at 100 °C for another 1 h was performed on the same samples.With higher temperatures and longer times, most polymer electrolyte membranes showed shape resistance to dimensional change; however, the pristine PU/PAN and ZnS-10 were observed to gradually change shape.Finally, after 120 °C of heat treatment, the dimensional change of the unmodified PU/PAN membrane decreased by 12.1%.Percentages of dimensional shrinkage were plotted in Fig. 5k.The dimensional change at 120 °C of the PU/PAN sample is clearly higher than that of other Zn salt-modified samples.The Zn(CH 3 CO 2 ) 2 modified PU/PAN membranes present lower shrinkage than the other membranes; moreover, the SEM images confirm the retained straight fiber shape as shown in Fig. 5b-d.At the same level of heat treatment, the dimensional shrinkage of all ZnT samples was measured to be 7.8%, which is lower than that of PU/PAN.Finally, the ZnSO 4 -modified PU/PAN membranes are observed to have shrinkage of approximately 7.8%-10.0%;however, these values are lower than 12.1% of PU/ PAN.Based on the shrinkage results, it is undeniable that the Zn salts clearly enhance the dimensional shrinkage resistance of the membranes.SEM can confirm the different morphologies of the fibers after heat treatment, such as curl and connecting fibers.In addition, the enhancement of shrinkage resistance of the fiber membrane may relate to adding functional salts such as Zn(OTf) 2 .For example, adding the Zn(OTf) 2 salt acts as a temperature retarder that corresponds to C-F, as we clearly found evidence from the FT-IR results.The sample's appearance after 100 °C of heat treatment, which is above the high operation temperature of batteries, is shown in Fig. 5l.

Mechanical properties of Zn salt incorporated PU/PAN electrospun fiber mats
The mechanical properties of highly porous polymer electrolytes influence the safety of use and large-scale manufacture of batteries.The obvious folding and rolling appearances of highly porous polymer electrolyte www.nature.com/scientificreports/membranes are presented in Fig. 6a-h.The high salt ratio sample is folded in several angles with no broken point; moreover, it can be brought to rolling up approximately 1 cm in diameter of the glass rod.These apparent flexibilities indicate their acceptable utility in real-life applications.In addition, their mechanical properties were evaluated using a universal testing machine.One mechanical characteristic of electrospun membranes is their tensile strength, which determines the membrane's ability to withstand mechanical stress under various applications.In this work, the effects of the addition of Zn salt on the tensile strength and Young's modulus of the electrospun membranes are exhibited in Fig. 6i,j.As can be observed in these plots, the addition of Zn(CH 3 CO 2 ) 2 in PU/PAN has no clear improvement in the mechanical properties of the membranes.The ZnSO 4 -incorporated PU/PAN membrane presents slightly increasing mechanical properties at 5 wt% ZnSO 4 in PU/PAN, and then the properties decrease when adding more ZnSO 4 .Both the tensile strength and Young's modulus of the polymer membrane gradually improve by adding Zn(OTf) 2 salt.The improvement in the PU/PAN/ZnT samples is probably due to the addition of the functional group, as shown in the FT-IR results, which increases hydrogen bonding in the whole polymer chain.

Physical properties and water contact angle of high porous polymer electrolytes
It is undeniable that the interaction between polymer electrolytes and liquid water is one of the crucial properties for determining the desirable performance of ion movement in polymer electrolytes.Contact angle measurement is a direct way to explain the wettability of our samples.A drop of 2 μL of water was placed on the samples, and within 5 s, the contact angles were recorded.Figure 7a illustrates the contact angles after averaging three measurements at ambient conditions.The angle between the water droplet and the PU/PAN membrane is approximately 103°.All the PU/PAN/ZnA modified samples present a high contact angle compared with PU/PAN without Zn salt, whereas the contact angle is reduced when PU/PAN is incorporated with ZnSO 4 .The contact angle of PU/PAN/ZnS-5 is reduced to 54°.Meanwhile, there is an upward trend in contact angles for PU/PAN/ZnS-10 and PU/PAN/ZnS-15.The increasing trend with increasing amounts of ZnSO 4 in the ZnS samples is probably due to the agglomeration of the hydrophilic group.In addition, the Zn salts used in this work contain in their structures the hydrophobic −CF 3 group and the hydrophilic group -SO 3 − .In addition, the FT-IR results reveal the increment of the OH stretching group, increasing the hydrophilic group in the PU/PAN/ZnT sample.The more hydrophilic the material is, the higher the interaction with water.
Water uptake, the water absorption ability of a material, is shown in Fig. 7b (pink square sign).The porous polymer electrolyte membrane samples were immersed in deionized water for 24 h, and then the differences in weight before and after water immersion were measured for all samples.The porosity values of the highly porous polymer electrolyte membranes were calculated and are presented in Fig. 7b (green circle sign).The water uptake of PU/PAN is reported to be approximately 1,332%, which is significantly higher than the 100% water uptake of the poly(2,6-dimethyl-1,4-phenylene oxide)-trimethylamine separator for a Zn air system 53 .The PU/PAN/ ZnA samples present an increasing amount of liquid uptake with increasing Zn(CH 3 CO 2 ) 2 content, and water uptake is up to 1,773% at the highest Zn(CH 3 CO 2 ) 2 content.In Fig. 7b, this increase in uptake value seems to go hand in hand with increasing porosity in the fiber structure, which is probably related to the decrease in fiber diameter when adding Zn(CH 3 CO 2 ) 2 salt, as shown in Fig. 2.This is because the space area between fibers is enlarged when minimizing fiber diameter size.The membrane including ZnSO 4 presents a gradual increase in uptake percentages early, and then the uptake decreases when adding more ZnSO 4 .
Like the PU/PAN/ZnA samples, the PU/PAN/ZnT samples show an increasing uptake value when the salt content is increased.The increase in uptake value is related to the increase in porosity of the membrane.To the best of our knowledge, the porosity of a membrane directly affects the reservoir area for liquid uptake because the liquid would fill up between pore structures.Moreover, the high hydrophilicity groups, such as -OH, as shown in Fig. 3, on the fiber surface probably reduce the adsorption time, as explained by the contact angle measurements.

Ionic conductivities of the high porous polymer electrolytes
The real utilization of polymer electrolyte is as a medium between two electrodes.In this position, the highly porous polymer electrolyte should present enough ionic conductivity.The crucial role of high porous polymer electrolytes in energy storage devices is to conduct ion charge migration.EIS usually provides data for real resistance and imaginary capacitance through the ability of a circuit to resist the flow of electrical current.The ionic conductivities of the highly porous polymer electrolyte membranes sandwiched between two stainless steel blocking electrodes were measured by AC impedance spectroscopy under setting conditions of 10 mV from 100 kHz to 0.01.The Nyquist plot, shown in Fig. 8a, presents the impedance relationship between the imaginary part (Z″) on the y axis and the real part (Z′) on the x axis.In the plot, the internal resistance of the testing cell can be clearly observed on the x axis.The unmodified PU/PAN membrane has a higher internal resistance than the other Zn salt membranes.From this plot, it is undeniable that one of the benefits of Zn salts is to reduce membrane resistance.Moreover, the effect of the anion and adding salt is shown on the left-hand side, shifting on the x axis in this plot.The ionic conductivity (δ) of materials can be calculated using Eq. ( 2). 54Testing cells with the high porous polymer electrolyte membrane were tested by EIS, which is presented as the Nyquist plots in Fig. 8a and the ionic conductivity values are presented in Fig. 8b.The intercept at the x axis point represents electrolyte resistance (Rs), which can be used to calculate ionic conductivity (δ) with Eq. ( 3).The PU/PAN membrane presents the highest ohmic resistances compared with the other membranes and offers the lowest ionic conductivity of 0.140 ± 0.019 mS cm −1 .This value is close to the conductivity of a conventional system glass fiber membrane in an aqueous electrolyte which was 0.144 mS cm −1 .The resistance value drastically decreases when the membrane is incorporated with Zn salt.In pink square Fig. 8b, the Zn(CH 3 CO 2 ) 2 incorporated membranes present increasing ionic conductivity: PU/PAN/ZnA-5, PU/PAN/ZnA-10, and PU/PAN/ ZnA-15 are 0.196 ± 0.034, 0.557 ± 0.061, and 0.747 ± 0.141 mS cm −1 , respectively.The ionic conductivities of the PU/PAN/ZnS samples are increased relative to PU/PAN without Zn salt, in the range of 1.072 ± 0.029 to 1.676 mS cm −1 .However, the trend of ionic conductivity seems to level off at PU/PAN/ZnS-15.This stable value is possibly due to the high zinc ion dissolution, which is consistent to the result of zinc dissolution concentration in Fig. 8c.The membranes modified with Zn(OTf) 2 have the highest value of 3.671 ± 0.373 mS cm −1 taken from PU/ PAN/ZnT-15.Based on these results, salt concentration can affect the ionic conductivity of a polymer electrolyte, similar to the explanation by Bao et al. 28 .Moreover, the polar functional group on the polymer chain affects coordination with the alkaline ion to support their ion conduction 55 .As can be seen in the ionic conductivity results, the PU/PAN/ZnS samples represent higher ionic conductivity than all the PU/PAN/ZnA membranes.The enhancement is due to the appeareance of the polar group -S-, as found in the FT-IR results, coordinating with the alkaline Zn ion.In addition, the PU/PAN/ZnT samples, which contain both polar groups of CF and SO 3 , provide higher ionic conductivity than the PU/PAN/ZnS membrane.The ionic conductivity values of this work and some GPEs are compared and shown in Table 1.
Next, the ion transference number (t Zn 2+ ), green circle in Fig. 8b, was characterized by assembling the Zn symmetrical cell and calculating the result by using Eq. ( 3).The ion transference number is the contribution of Zn 2+ as an cation to the overall ionic conductivity, which implies concentration polarization 38 .The DC polarization current versus time plot is presented in Fig. S6.From the calculated transference numbers, the membranes incorporated with Zn(CH 3 CO 2 ) 2 and Zn(OTf) 2 present higher transference numbers than the pristine PU/PAN, indicating that the ionic conductivities of PU/PAN/ZnA, and PU/PAN/ZnT have contributed more Zn 2+ ions than pristine PU/PAN.Meanwhile, the ZnSO 4 modified membrane has the lowest ion transference number.The possible reasons for the enhancement of the ion transference number of Zn(CH 3 CO 2 ) 2 and Zn(OTf) 2 may be due to the size of the anion molecule: a bulky anion can decrease the coordination number of H 2 O surrounding the Zn 2+ ion, resulting in improved ionic conductivity 11 .Figure 8c presents the result of zinc dissolution concentration, which can be observed by the high amount of zinc concentration in the PU/PAN/ZnS samples.This result is consistent with the high interaction between water molecules and ZnSO 4 .Moreover, these bulky anions are supposed to hinder themselves in transportation, which eventually results in increasing the cation ratio for overall ionic conductivity.Conversely, SO 4  2− (in ZnS) is a compact anion, which is easier to transport and can be counted in overall ionic conduction.www.nature.com/scientificreports/ The cycling charge-discharge plots at room temperature of pristine PU/PAN and Zn salt-modified PU/PAN are displayed in Figs.9a-d and S7a-f.The symmetrical (Zn//Zn) cells were long-term tested with a current density of 0.25 mA cm −2 , and then the responding voltage signals were plotted versus testing time.In Fig. 9a, the voltage profile of pristine PU/PAN shows quite a fluctuation signal in the first 20 h, and then the signal becomes stable; however, the polarized voltage is presented after 100 h.The PU/PAN/ZnA and PU/PAN/ZnS samples show some fluctuation signals during the long-run testing; in addition, the highest unstable signal is observed at over 50 cycles from PU/PAN/ZnA-10.The Zn(CH 3 CO 2 ) 2 in PU/PAN/ZnA has produced an acidiclike electrolyte from saturated water, and the acidic condition affects Zn anode dissolution and corrosion, which may have a negative effect on battery performance 57 .Of these samples, only deionized water uptake may not be enough for smooth electrochemical stability.Finally, the voltage profile of the PU/PAN/ZnT samples is observed in Figs.S6e-f, displaying a less fluctuating signal than any other samples.These constant voltage profiles reveal the exceptional Zn plating/stripping and unchanging within the internal of the operating cell, which may result from high ionic conductivity and ion transference number of the PU/PAN/ZnT sample 12 .Moreover, the SEM image verification of zinc electrode in Fig. 9e-h confirms that the zinc electrode assembling with PU/PAN and PU/PAN/ZnA-15 contains some zinc dendrite structure, whereas the zinc electrode assembling with PU/PAN/ ZnT-15 present a smoother surface.The outstanding result in Zn plating/stripping of Zn(CF 3 SO 3 ) 2 has been reported in liquid salt electrolyte 7,57 .
To understand the redox reaction, CV can be performed in three electrode formations to understand the redox reaction of electrochemical cells, where the potential is supplied between a reference and working electrode and the current is measured between the working and counter electrodes.The effects of highly porous polymer electrolyte membranes were evaluated from the cyclic voltammograms using the full-cell vanadium based (NVO) cathode and Zn anode (NVO//Zn).The SEM elemental mapping of NVO coated on graphite paper was utilized to confirm the cathode material as Fig. S8a-e.The experiments were performed at a 10 mV s −1 scan rate.The highest Zn salt contents (15% w/w) from each Zn salt modified membrane were tested and reported in Fig. 9i-l.Figure 9g,h, PU/PAN and PU/PAN/ZnA-15 show an unclear peak of the oxidation reaction, but it seems to reveal a small peak at around 1.0 V.While PU/PAN/ZnS-15 and PU/PAN/ZnT-15 (Fig. 9k,l) clearly present an oxidation peak at around 1.1 and 1.3 V, respectively, which is related to the extraction of zinc ion.However, the peaks that represent the reduction reaction are around 0.5-0.7 V for all samples, indicating the insertion of zinc ions on the electrode.The complete CV cycle indicates the plenary cell reaction.However, the insufficient charge carrier of the pristine PU/PAN membrane shows an unclear oxidation reaction, as shown in the CV curve.Besides, the rate performance of the samples was tested at different current densities from 0.1 to 5.0 A g −1 for full cell battery Zn/Polymer electrolyte/NVO.In Fig. S9, the average capacity at 0.1 A g −1 was 335 and 120 mAh g -1 for cell assembled with PU/PAN/ZnT15 and PU/PAN, respectively.The sufficient ion carrier part of PU/PAN/ZnT15 may be the cause of its higher capacity than PU/PAN.However, the plots shows capacity fading after current density returns to 0.2 A g −1 due to the morphology change and loss of contact of active material to the carbon substrate 58 .

Conclusions
The Zn salt incorporated PU/PAN electrospinning fiber membranes were manufactured by electrospinning.The Zn salts used in this work, (Zn(CH 3 CO 2 ) 2 ), (ZnSO 4 ), and (Zn(OTf) 2 ), resulted in enhancements in porous morphology, thermal shrinkage properties, and ionic conductivity.The effects of Zn salt can determine the fiber morphology and porous structure, as we proposed in this study, which explains that high dielectric constant salt and solvent provide good fiber producibility.The addition of various Zn salts affected the interaction between fiber and a small amount of water; especially, Zn(OTf) 2 drastically improved the wettability of the membrane, as presented in contact angle measurements.The crucial parameter of polymer electrolyte, ionic conductivity, was improved with adding Zn salt, and it can be suggested that the addition of Zn salt increases Zn 2+ as charge carriers.However, the amount of ZnSO 4 in the PU/PAN membrane shows a decrease in ion transference number due to the effect of compact molecules from sulfate ions.Also, the Zn symmetrical charge-discharge (stripping/ plating) could confirm the stable signal from the Zn(OTf) 2 membrane.Especially the CV plot of the PU/PAN/ ZnT-15 membrane provides a good circle of redox reactions.Based on this study, the Zn(OTf) 2 membrane shows promise as a polymer electrolyte as it provides better performance for use as a water-uptake polymer electrolyte than the pristine membrane and other Zn salts.Promising PU/PAN/ZnT membranes or Zn(OTf) 2 incorporated in other materials should be developed further in future work.

Figure 7 .
Figure 7. Representative (a) contact angle values and (b) water uptake (pink square) and porosity (green circle) values of the PU/PAN/Zn salt electrospun membranes.

Figure 8 .
Figure 8. Representative (a) Nyquist plots of stainless-steel blocking symmetric cell with high porous membrane electrolyte tested.(b) Plot of ionic conductivity and transference number versus sample with different zinc salt incorporated membrane.(c) Concentration of dissolution zinc with different zinc salt incorporated membrane.

Table 1 .
Review of other polymer electrolyte for Zn ion batteries.