3.1. Characterization of membranes
Three different types of electrospun membranes. As shown in Fig. 2A-C, the fibers in all membranes were randomly oriented and stacked with each other to form a 3D porous network structure, which can provide extremely high porosity. Compared with the Celgard PP separator, it is more conducive to the retention of electrolyte and Li+ transmission. Each fiber membrane has a smooth and uniform fiber surface and uniform thickness.
The diameter distribution of the fibers was analyzed by using ImageJ software on SEM images (5000x). 50 fibers were selected in each SEM image and their diameter distribution is shown in Fig. 2D-F. We can see that the fiber diameter of PPH and PPHB fiber is bigger than that of PAN pure fiber. Compared with PPH fiber, the diameter of the PPHB fiber decreased. The average diameter decreased from 661.7 nm to 572.9 nm. According to Shayapat et al., this may be due to the increase of viscosity, surface tension, and electrostatic repulsion of the mixed solution caused by the addition of nanoparticles [42]. It can be seen in the insets of Fig. 2A-C that all composite fibers have a 3D network structure. The crisscross fibers form micro and nano-scale pores, which overlap each other and are conducive to the absorption of liquid electrolytes. Each element is evenly distributed according to the arrangement of fibers, thereby preventing agglomeration during the electrospinning process. It can be seen that uniformity and stability are maintained in the spinning process. As shown in Fig. 2G-I, with the combination of different materials the mass fraction of each element changes in varying degrees, which is completely consistent with the physical process and indirectly proves that there is no chemical reaction in the electrospinning process [43].
The transmission electron microscope images of the fiber membrane are shown in Fig. 3A-C. The microstructures of PPH and PPHB can be seen. Their central regions were dark and the edges were light, showing obvious core@shell structures. The fiber diameter is about 500 nm. The TEM image shown in Fig. 3C shows that some BM particles are successfully embedded in the shell of the fiber. From the selected region electron diffraction (SAED) images, PPHB has bright diffraction spots due to BM doping.
3.2. Mechanical and heat resistance properties
The separator needs certain mechanical properties to prevent its deformation and ensure the working safety of the battery. However, the strength of electrospun membranes is usually low [44]. The stress-strain curve of the fiber membrane is shown in Fig. 4A. The order of the tensile strength values of the membrane is PPH (8.5MPa) > PPHB (5.8 MPa) > PAN (2.9MPa). This sequence shows that the mechanical strength of the core@shell composite fiber membrane prepared by coaxial electrospinning is significantly improved compared with that of the uniaxial fiber membrane. Meanwhile, the addition of BM nanoparticles reduces the mechanical strength and elongation at the break of the membrane, which is due to the stress concentration caused by the BM particles on the surface and becomes the center of crack development [45]. In addition, a large number of particles are only doped into the polymer and do not interact with the polymer interface to form a better force. Therefore, the addition of BM did not improve the tensile strength of the composite membrane. Figure 4B-D shows the uniformity and thickness of self-made membranes. It can be seen that the thickness is about 120–150µm, while the PPBH membrane is more uniform than PPH and PAN. It may be because BM particles have strong insulation and doping in the jet can reduce the electrostatic repulsion, to improve the uniformity of the membrane.
The thermal stability of the separator is also one of the important factors to ensure the safety of the battery. If the separator maintains dimensional integrity at extreme temperatures, it can provide reliable safety [46]. Here, we compared the heat resistance of PAN, PPH, and PPHB membranes with a Celgard PP separator. As shown in Fig. 5A, Celgard PP began to shrink at 100°C. In contrast, all homemade membranes remained unchanged at the same temperature. Celgard PP almost curled up completely after heat treatment at 130 oC for 30 min. PPH membrane began to shrink at this temperature; However, PAN and PPHB membranes can still maintain a complete contour. After heat treatment at 160 oC for 30 min, PPHB began to shrink while PPH showed significant shrinkage. It can be seen from Fig. 5B and 5C that PPHB has the slowest weight loss rate, reflecting stronger heat resistance. The reasons are as follows: (1) polyacrylonitrile fiber can maintain integration at a high temperature (200 oC) [47], and the thermal stability of PAN is higher than that of PP after 100 oC [48]; (2) The heat resistance of PPH is relatively poor and the shrinkage condition of PPH membrane is higher than that of PAN. It is not entirely curled because the core layer of the core@shell structure can be used as a support for the composite membrane to avoid serious thermal shrinkage at a relatively high temperature; (3) Adding BM particles can significantly improve the thermal shrinkage characteristics. Compared with PPH, PPHB has higher thermal dimensional stability, which helps improve the thermal safety of the battery [15, 49].
3.3. Wetting performance
The affinity between the separator and the electrolyte is very important to improve the performance of the battery. The large retention rate of the electrolyte will effectively enhance the ion transport capacity of the separator [6, 50]. Add a drop of electrolyte to the surface of Celgard PP, PAN, PPH, and PPHB, respectively, to determine the electrolyte permeability in the separator. As shown in Fig. 6A, the electrolyte on the surface of Celgard PP was in the form of droplets, indicating slow electrolyte diffusion and low wettability. In contrast, the upper electrolyte droplets in the electrospun fibrous membrane were rapidly absorbed, showing a very high electrolyte absorption rate because of its high porosity and its material’s extremely high affinity for the electrolyte [51]. The contact angle test results show that the contact angle of Celgard PP was 48.8°, and the contact angles of PAN, PPH, and PPHB were all 0°, which showed that the electrospun fiber membrane has stronger liquid absorption and liquid retention capacity. As shown in Fig. 6B, the wettability of all electrospinning membranes is higher than that of Celgard PP. Figure 6C shows that the electrolyte uptake of PAN is 435.3%, and that of PPH is 763%. It is worth mentioning that the addition of BM can significantly improve the electrolyte absorption rate (the electrolyte absorption rate of PPHB is 872.8%, which is much higher than that of PPH). This is attributed to the strong interaction between the electrolyte and PPHB, which is confirmed by Fig. 7 using DFT calculations. The adsorption energy was calculated using fluorine-containing functional groups (CH2CF2, CH3CHF2, and CHF2CHFCF3) as PVDF-HFP and dimethyl carbonate (DMC), ethylidene carbonate (EC), and ethylmethyl carbonate (EMC) as the electrolyte. The adsorption energy values of AlOOH-DMC (-0.99eV), AlOOH-EC (-1.21eV), and AlOOH-EMC (-0.86eV) are much larger than the adsorption energy values of CH2CF2-DMC (-0.12eV), CH3CHF2-DMC (-0.30eV), CHF2CHFCF3-DMC (-0.27eV), CH2CF2-EC (-0.09eV), CH3CHF2-EC (-0.26eV), CHF2CHFCF3-EC (-0.33eV), CH2CF2-EMC (-0.13eV), CH3CHF2-EMC (-0.30eV), CHF2CHFCF3-EMC (-0.29eV). The PPHB separator has strong adsorption of DMC, EC, and EMC due to the incorporation of Boehmite (AlOOH), which results in better wettability and electrolyte uptake. Appropriate porosity facilitates the retention of electrolytes in the membrane. Generally speaking, the larger the porosity, the higher the retention rate of the electrolyte, and the better the stability of the battery. In conclusion, electrospun membranes have higher porosity and stronger electrolyte uptake than Celgard PP. This is mainly due to the electrospun membrane composed of naturally overlapping fibers with a 3D network structure inside without direct interaction forces. PPHB exhibited the strongest electrolyte affinity because shell structure PVDF-HFP has high polar C-F groups, in addition, the -OH in BM also has exbatteryent compatibility with liquid electrolytes.
3.4. Electrochemical performance
Compared with the Celgard PP separator, the electrochemical performance of three kinds of self-made separators was evaluated. First, the electrochemical stability was evaluated. The section of the electrochemical voltammetry curve that does not rise is the electrochemical window. Figure 8A is the linear sweep voltammogram obtained from the battery (SS/separator/Li). The current of Celgard PP, PAN, PPH, and PPHB membranes increased significantly, and the voltages were about 3.9V, 4V, 4V, and 4.8V respectively. This indicates that the electrochemical stability of batteries equipped with a PPHB separator is stronger than that of Celgard PP. The better electrochemical stability is due to the greater compatibility with electrolyte after doping with BM, which inhibits the decomposition of electrolyte components, resulting in a wider electrochemical window [52]. Figure 8B shows the AC impedance spectra of batteries with different separators (SS/separator/SS). The bulk resistance (R) of the separator is the intercept of the curve and the abscissa, and then the ionic conductivity of the separator was calculated by the formula (3), the ionic conductivity of Celgard PP, PAN, PPH, and PPHB was 0.18mS/cm, 1.42mS/cm, 3.02mS/cm, and 3.98mS/cm respectively. It is not difficult to see that the ionic conductivity of the self-made membrane is higher, especially the core@shell structure membrane and BM. This is a result of the following; (1) The high porosity of electrospun membranes facilitates the absorption and maintenance of large amounts of liquid electrolytes; (2) The doped BM particles further improve the lyophilicity of the electrolyte, allowing the effective transport of Li+ in the separator, which greatly increases the efficiency of Li+ migration.
The battery performance of different separators was tested by assembling half-cells (LiFePO4/separator/Li). Figure 9A-C shows the first charge-discharge curves of the battery at a set of different current densities (0.2C, 0.5C, 1.0C, 2.0C, and 5.0C). All batteries show a stable charging and discharging platform. At any current density, the specific capacity of LIBs with PPHB separators is higher than that of LIBs with Celgard PP and PPH separators. As shown in Fig. 9D, the charge-discharge curves of different separator LIBs at 0.2C magnification are shown. The charge-discharge specific capacity of LIBs with PPHB separator is 165.9mAhg− 1 and 164.9mAhg− 1, that of LIBs with PPH separator is 157.92mAhg− 1 and 157.59mAhg− 1, and that of LIBs with Celgard PP is 158.3mAhg− 1 and 155.7mAhg− 1. During the first discharge, the LIBs with electrospinning separator show higher specific capacity than the LIBs with Celgard PP separator at any current density. The first coulombic efficiencies of Celgard PP at different current densities were 97.7% (0.2C), 97.4% (0.5C), 97.0% (1.0C), 96.6% (2.0C) and 87.1% (5.0C) respectively; In contrast, the coulomb efficiencies of PPH were 99.6% (0.2C), 98.4% (0.5C), 98.3% (1.0C), 98.5% (2.0C) and 99.4% (5.0C), respectively; The values of PPHB were 98.9% (0.2C), 98.3% (0.5C), 98.9% (1.0C), 99.7% (2.0C) and 99.1% (5.0C), respectively. Obviously, in the first charge-discharge cycle, the electrospinning separator can give the corresponding battery higher coulomb efficiency. Due to the large swelling phenomenon of PAN in the electrolyte, it is easy to cause a short circuit of the battery, resulting in poor cycle performance. This paper only compares its first charge-discharge curve.
The cycle performance of the battery with these three separators at the rate of 0.2C is shown in Fig. 10A. It is not difficult to see that the battery using the PPHB separator has the advantages of high specific capacity and low attenuation, and the discharge-specific capacity remains around 153 mAhg− 1 after long cycles, while the battery using PPH and Celgard PP with separators are only around 125mAhg− 1 and 110mAhg− 1. Figure 10B shows the cycling performance of batteries with three different separators after four consecutive charges/discharges at different current densities. During the whole process, the batteries using the PPHB separator exhibited higher specific discharge capacity in any case than the batteries using Celgard PP and PPH separators. Especially after cycling at 5.0C, the discharge-specific capacity of the battery using the PPHB separator is 115.73 mAgh− 1, which is 1.15 times that of the battery using the Celgard PP separator. When the current density is changed back to 0.2C, the discharge-specific capacity still maintains 93.3% of the initial value, showing good cycling stability at high current density. For the battery assembled with Celgard PP separator, the discharge-specific capacity maintained only 76.0% of the initial value when cycled back to 0.2C. The above evaluations demonstrate that LIBs with PPHB separators exhibit stronger cycling stability and battery capacity due to the high porosity of the electrospun separators and the addition of BM nanoparticles to the separators. This can enhance the affinity of the electrolyte to the separator, reduce the interfacial resistance between the electrolyte and the electrode, and reduce the energy loss of battery operation.