Introduction

Many investigations have been devoted to gel polymer electrolytes (GPEs) in the rechargeable polymer lithium-ion battery, due to their high ionic conductivity, wide electrochemical window, and dramatic improvement of safety property [1–5]. There are many ways to get GPEs including electrospinning technique, phase inversion method, solvent casting technique, etc. In comparison to other membrane preparation techniques, electrospinning is simpler, rapider, and more inexpensive [6, 7]. It is clear that matrix is the most important factor affecting the performance of GPEs. Some polymer hosts have been developed and investigated such as poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyvinyl chloride, polymethyl methacrylate, polyurethane, etc. Among these, PVDF-HFP has received much attention because of its high electrochemical stability and good affinity with liquid electrolyte [8, 9]. It comprises an amorphous and a crystalline phase [10–13]. The research of Saito et al. [14] implies that amorphous domain of microporous membrane influence the ionic conductivity and electrolyte reservation of corresponding polymer electrolyte. Therefore, PVDF-HFP has a high dielectric constant and electron withdrawing fluorine atoms in the backbone structure due to addition of an amorphous phase of HFP group into the main constituent VDF blocks.

However, the GPE membrane based on PVDF-HFP has poor mechanical strength. Blending is a simple and effective method to improve the properties of porous films. In recent years, some investigations on PVDF-HFP blend matrix for polymer electrolyte have been carried out. For example, Ding et al. [15] prepared PVDF-HFP/poly(methyl methacrylate) (PMMA) blend matrix and the ionic conductivity of corresponding polymer electrolyte exceeding 1.0 × 10−3 S cm−1 at room temperature. The value was more than the standard of practical application for lithium-ion batteries [16].

Thermoplastic polyurethane (TPU) is a linear segmented block copolymer composed of both hard segments and soft segments. The hard segments are interconnected by interchain hydrogen bonds to act as a mechanical support for the polymer matrix. While the soft segments dissolve alkali metal without formation of ionic cluster and help to capture large amount of liquid electrolytes.

We have done some research [17], which was the first trial of making TPU/PVDF-HFP fiber membranes. In comparison to pure TPU and pure PVDF-HFP films, the blend fibrous films show more noticeable electrochemical characteristic and mechanical performance. We would like to continue our efforts to develop TPU/PVDF-HFP porous fibrous films by electrospinning using different concentration polymer solutions. In order to investigate the influence of various polymer concentration stresses on the TPU/PVDF-HFP fiber membranes, membrane morphology, ionic conductivity, charge and discharge capacity, and mechanical properties will be examined systemically. In this study, we expect to provide a deep investigation and insight on the preparation of TPU/PVDF-HFP microporous fiber membranes with prominent electrochemical and mechanical performance.

Experimental

Materials

PVDF-HFP (Knyar Flex, LBG) and TPU (Yantai Wanhua, 1190A) were dehydrated in a vacuum drying oven at 80 °C for 24 h before using. LiClO4·3H2O (AR, Sinopharm Chemical Reagent Co., Ltd.) was dried in vacuum oven at 140 °C for 72 h. Liquid electrolyte (1.0 M) was made by dissolving a certain weights of LiClO4 in ethylene carbonate (EC, Shenzhen Capchem Technology Co., Ltd)/propylene carbonate (PC, Shenzhen Capchem Technology Co., Ltd) (1/1, v/v). Acetone and N,N-dimethylforamide (DMF) were analytical purity and used without further purification.

Preparation of TPU/PVDF-HFP porous fibrous film

Appropriate amount of TPU and PVDF-HFP powder (1/1, wt/wt) were dissolved in the mixture of an acetone/N,N-dimethylacetamide (1/1, wt/wt). TPU/PVDF-HFP solutions of varying concentration from 6 to 14 wt% were then stirred continuously. The solutions were electrospun under high voltage of 24.5 kV at room temperature. Porous fibrous film was obtained on the collector plate; then, it was removed under reduced pressure and dried at 80 °C for 12 h in a vacuum desiccator.

Preparation of gel polymer electrolyte

The dried porous fibrous film was soaked in 1 M LiClO4–EC/PC liquid electrolyte solution for 1 h in an Ar-filled glove box at room temperature. After activation, the membrane was removed from the electrolyte solution and excess electrolyte solution of swelled film was wiped with a filter paper.

Membrane characterization

The morphological examination of the films was made by scanning electron microscope (SEM, Hitachi S-3500N, Japan) after sputtering gold on one side of the films. The thermal characterization of membranes was measured by differential scanning calorimeter (DSC-7, Perkin-Elmer Co., USA) at a heating and cooling rate of 20 °C min−1 under nitrogen atmosphere from 25 to 200 °C. The crystallinity of the samples was calculated using the Eq. (1) from the DSC curves [18].

$$ {X}_c\left(\%\right)=\frac{\varDelta H}{\varDelta {H}_m\varphi}\times 100 $$
(1)

where ΔH and ΔH m represent the melting enthalpy of blend membrane and PVdF with 100 % crystallinity, respectively. The value of ΔH m is 104.7 J/g [19]. φ is the measuring weight fraction of PVDF.

The electrolyte uptake was assessed by measuring the weight of the dried and wetted sample. Electrolyte uptake was calculated according to the following formula [20]:

$$ \mathrm{Uptake}\left(\%\right)=\frac{M_{\mathrm{wet}}-{M}_{\mathrm{dry}}}{M_{\mathrm{dry}}}\times 100 $$
(2)

where M dry and M wet represent the weights of the dry and wet membrane, respectively.

The mechanical strength of the membrane was evaluated by universal testing machine (UTM, Instron Instruments) at a loading of ~5 mm min−1. The dimensions of the sheet used were ~2 cm × 5 cm × ~150–250 μm (width × length × thickness).

The ionic conductivity of the TPU/PVDF-HFP porous fibrous films were examined by the AC impedance method of the SS/PE/SS blocking cell, using Zahner Zennium electrochemical analyzer over a frequency range of 0.1–1 MHz. Thickness of 100 μm, an area of 1.96 cm2 of the sample was prepared for impedance measurement. Thus, the ionic conductivity was calculated from the following Eq. (3):

$$ \delta =\frac{h}{R_bS} $$
(3)

where δ is ionic conductivity, R b is the bulk resistance, and h and S represent the thickness and area of film, respectively.

Cell assembly and performance characteristics

Charge–discharge cycling tests of Li/PE/LiFePO4 cell were conducted using an automatic charge–discharge unit, Neware battery testing system (model BTS-51, ShenZhen, China), at a constant current density of 0.1 C and within the voltage range of 2.5–4.2 V at room temperature.

Results and discussion

Morphology and structure

Figure 1 shows SEM images of electrospun membranes prepared from 7 to 14 wt% polymer solutions. All of the films show network of interlaid and nearly straightened tubular structure fibers. It is important to note that the fibers have smooth surface, and there are no bead formation, which may be attributed to the hydrogen bonds’ product between the strong electron-withdrawing functional group (-C-F) which is in the backbone structure of PVDF-HFP and amino group (–NH) which is in the hard segments of TPU. The average fiber diameters (AFDs) are 410, 400, 300, 250, 280, 350, 500, and 750 nm corresponding to Fig. 1 (7–14 %), respectively.

Fig. 1
figure 1

SEM images of electrospun membranes based on TPU/PVDF-HFP (1/1, w/w)

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There are several factors contributing to the fiber morphology of electrospun film: the distance between the needle and the collection plate, properties of polymer and solvent, the voltage used for electrospinning, and the concentration of the polymer. In the present study, the first three factors in the above list were remained the same while electrospinning and hence cannot bring the morphological differences. Figure 1 (7 %) shows that the electrospun fiber film is very adhesive and uneven, which may be due to incomplete evaporation of the solvent and loose deposition of the fiber on the collector. With the increase of concentration, the situation will be better. However, as the concentration up to 11 %, the AFD increased with the increase of polymer solution concentration. It implied that the surface tension and viscosity of the solution increased with the polymer concentration increase; electrostatic stresses would not allow the droplets to elongate and be deposited uniform fibers. Kim et al. had reported that decreasing in AFDs can result in an increase in electrolyte uptake [21]. It is obviously seen that the fiber of 10 % is homogeneous and slender, so the film has prominent electrochemical properties and mechanical properties as followed.

DSC analysis

Figure 2 displays DSC curves, and the following observations were made. Firstly, all the blends with different TPU/PVDF-HFP contents show almost similar melting temperatures, but the melting enthalpy of the eletrospun TPU/PVDF-HFP (10 %) membrane is much lower than the others. Secondly, as shown in Table 1, the ΔH f of eletrospun TPU/PVDF-HFP (10 %) membrane is 15.11 J/g. According to the Eq. (1), the crystallinity of the membrane was 20.14 %. In the electrospinning process, a high electric power is applied (in the range of kilovolts) to overcome the surface tension of the polymer drops. The repulsion between molecules in the surface of the drops increases with increasing the concentration of the solution. The greater the force applied to the molecular chain, the more the molecular structure is destroyed. The regularity and symmetry of the molecular chains are destroyed, and the crystallinity is decreased. Therefore, the crystallinity decreased from 40.09 to 20.14 % as the polymer concentration increased from 6 to 10 %. However, as the concentration up to 11 %, surface tension and viscosity of the solution increases with the polymer concentration increase, it can prevent electrostatic stresses from elongating the droplets. The crystallinity increases with decrease of the external force on the molecular chain. Therefore, the crystallinity increased from 20.14 to 30.30 % as the polymer concentration increased from 10 to 14 %. What’s more, growth of polymer crystallinity can decrease the free volume of amorphous region and hinder the migration of Li+ ions. Therefore, lower crystallinity of membrane can provide a favorable condition for higher conductivity enhancement. The results suggested that the electropun TPU/PVDF-HFP (10 %) film may have the excellent ionic conductivity.

Fig. 2
figure 2

DSC curves of TPU/PVDF-HFP films

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Table 1 Melting enthalpy and crystallinity of membranes
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Mechanical properties

Figure 3 presents the stress–strain curves of polymer membranes. As our previous investigation [18], pure TPU is ductile elastomer, which can improve both elongation at break and tensile strength in TPU/PVDF-HFP blends. The mechanical test dates of the membranes are also summarized in Table 2. The results of mechanical strength test show that both the membrane (10 %) and membrane (13 %) possessed the highest stress of 9.8 MPa, but the latter corresponding value of elongation at break was smaller. These marked enhancements in the mechanical properties are due to finer morphology, good compatibility, and stronger interfacial adhesion between PVDF-HFP and TPU. Comprehensive comparison, the membrane (10 %) is more suitable for gel polymer electrolyte material of lithium-ion battery.

Fig. 3
figure 3

Stress–strain curves of the electrospun TPU/PVDF-HFP membranes

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Table 2 Tensile strength and elongation at break of membranes
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Electrolyte uptake

The variations of the uptake behavior of the TPU/PVDF-HFP membranes were presented in Fig. 4. The percentage of electrolyte uptake can be calculated according to Eq. (2). The fibrous film (10 %) shows an electrolyte uptake of about 345 % within 2 min. After 14 min, it is found that the electrolyte uptake of these eight carves become stabile and the sample with 10 % TPU/PVDF-HFP owns the highest electrolyte uptake percentage.

Fig. 4
figure 4

The uptake behavior of the electrospun fibrous films

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The absorption of large quantities of liquid electrolyte by the composite membranes mainly results from the high porosity of the membranes and the high amorphous content of the polymer. The AFD significantly affected the physical properties of the fibrous membranes such as their porosity, pore size and specific surface area, and so on. In the certain volume, the pores will increase when the AFD decreases. Saito et al. [14] studied the influence of electrolyte uptake by pore structure. They found that electrolyte uptake improved with an increase in the number of pore. Compared with the other concentration solutions, the 10 % fibers display significantly thin AFDs; in addition, its size distributions are much narrower, so it always has the higher electrolyte uptake percentage.

Ionic conductivity

Figure 5 illustrates the impedance spectra of TPU/PVDF-HFP-based fibrous polymer electrolyte with different concentrations. It is a typical AC impedance for gel polymer electrolyte. According to the Eq. (3) δ = h/R b S, the conductivity of membranes was calculated and given in Table 3. It can be seen that ionic conductivity increased from 0.84 × 10−3 S cm−1 to the maximum 6.62 × 10−3 S cm−1 as polymer concentration increase from 6 to 10 %. However, the ionic conductivity decreased from 6.62 × 10−3 S cm−1 to 0.21 × 10−3 S cm−1 as the polymer concentration increase from 10 to 15 %. The TPU/PVDF-HFP (10 %) fibrous polymer electrolyte film has an ionic conductivity of 6.62 × 10−3 S cm−1. It is much higher than the value 1 × 10−4 S cm−1 reported by Kuo et al. [22] for TPU–PAN incorporating LiClO4/propylene carbonate (PC) gel polymer electrolyte system and the value 3.2 × 10−3 S cm−1 reported by Wu et al. [23] for TPU–PVDF/LiClO4 (9 wt%) soaked by EC/PC of 0.1 M LiClO4 polymer electrolyte system.

Fig. 5
figure 5

Impedance spectra of gel polymer electrolytes

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Table 3 Bulk resistance, thickness, surface area, and ionic conductivity of polymer electrolytes
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The formation of stable LiF restricts the use of PVDF-HFP-based polymer electrolytes. However, the addition of TPU causes the hydrogen bond generation between the two polymer matrixes. Therefore, Li+ ions in the blend membrane can freely migrate to improve the ionic conductivity. It was reported that the electrolyte solution introduced in the polymer was trapped in the pores and then penetrated into the polymer chains for swelling the polymer network [14]. The content of carrier ions and their mobility domain contributed to the ionic conductivity. The ionic conductivity increased with the decrease in AFD, which resulted from the higher electrolyte uptake. Therefore, the high ionic conductivity for 10 % sample is attributed to arise from the combined influence of higher content of electrolyte (>400 %) incorporated into the pores of the membrane and augmented lithium ion mobility in the gelled state. Moreover, low crystallinity can increase the mobility of lithium ion to result higher conductivity of electrolyte.

Evaluation in Li/LiFePO4 cell

In the following study, the cell prototypes were assembled in sequence of the three components, i.e., the lithium anode, electrospun TPU/PVDF-HFP membrane as the electrolyte, and the LiFePO4 as cathode. The performance was assessed under 0.1 C rate at room temperature.

Figure 6 shows the first charge–discharge curves of Li/GPE/LiFePO4 cells. The membrane (10 %) delivers a charge capacity of 167.23 mAh g−1, which is about 98.37 % of the theoretical capacity of LiFePO4. The charge and discharge capacity of the cells increased from 152.65 and 151.73 to 167.23 and 163.49 mAh g−1 as polymer solution concentration increased from 6 to 10 %, then decreased to 159.99 and 157.46 mAh g−1 as the polymer solution concentration further increased to 14 %. The charge capacity was influenced by uniform morphology of membranes, AFDs, higher electrolyte uptake, and fully interconnected pores.

Fig. 6
figure 6

First charge–discharge capacities of GPEs based on electrospun TPU/PVDF-HFP membrane

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The Li cell with GPEs incorporating EC/PC has been evaluated for cycleability character under the 0.1 C rate at 25 °C, and the results are shown in Fig. 7. The membrane (10 %) has highest dischange capacities in the whole 50 cycles. The GPE with 10 % TPU/PVDF-HFP achieves a discharge capacity of 163.49 mAh g−1; after 50 cycles, its residual capacity is 154.41 mAh g−1. In other words, there is no evident capacity fading of the cell-based GPEs after 50 cycles. It suggests that all the cells exhibit a better reversibility. This study displays that electrospun TPU/PVDF-HFP membrane (10 %) based GPE is more suitable for Li/LiFePO4 cells under 0.1 C rate at 25 °C.

Fig. 7
figure 7

The cycle performance (discharge capacities) of GPE based on electrospun TPU/PVdF-HFP membranes

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Conclusions

GPEs based on fibrous TPU/PVDF-HFP blend membranes were prepared by electrospinning the 6–14 % polymer solutions in DMF/acetone (1:1, wt/wt) at room temperature. The optimum blend composition has been observed for the electrolyte with 10 % of TPU/PVDF-HFP (1:1, wt/wt). Firstly, TPU/PVDF-HFP (10 %) based gel polymer electrolyte has a high ionic conductivity of 6.62 × 10−3 S cm−1. Besides, the GPE has good mechanical stability and strength to allow safe operation in rechargeable lithium polymer batteries. What’s more, after 50 cycles, the cell shows a very stable discharge behavior and little capacity loss under current constant voltage conditions, at the 0.1 C rate of 25 °C. Considering these results, the cell with 10 % TPU/PVDF-HFP-based gel polymer electrolyte appears as a promising candidate for rechargeable lithium-ion batteries.