Abstract:

Objective The conventional liquid electrolyte is easy to leak and flammable, which brings potential safety risks to the actual application of lithium metal batteries. Replacing liquid electrolyte with all-solid-state electrolyte has become one of the most feasible methods. However, solid polymer electrolytes are limited by low ionic conductivity and poor mechanical strength. For solving the two problems of solid polymer electrolyte at the same time, nanofiber membranes with high strength are used for modification.

Method Dendritic sulfonated polyethersulfone nanofibers (SPES) were prepared by electrospinning technology. They were introduced into polyethylene oxide (PEO) to prepare composite solid electrolytes and applied in high-performance all-solid-state lithium metal batteries. The influences of spinning solution concentration, salt addition, electrospinning voltage and receiving distance on fiber morphologies were explored and analyzed. Moreover, the influences of SPES nanofiber membrane on the crystallinity, ionic conductivity, mechanical properties, and electrochemical properties of composite solid electrolyte were also studied under the optimal spinning process.

Results When the spinning solution concentration was 23%, the electrospinning voltage was 30 kV and the receiving distance was 15 cm, the obtained SPES nanofibers had the best morphology among them. Based on obtaining the optimal spinning solution concentration of ordinary SPES nanofibers at 23%, the influence of ammonium tetrabutyl hexafluorophosphate on fiber morphologies were investigated. It was found that the optimum parameters for preparing dendritic SPES nanofibers were salt dosage of 2%, electrospinning voltage of 30 kV and receiving distance of 15 cm. After the nanofibers and PEO were constructed into the composite electrolytes, both the ordinary SPES nanofibers and the dendritic SPES nanofibers caused the crystallization peak of PEO in the composite electrolyte be smaller than that of pure PEO electrolyte, indicating that the interlaced nanofibers were conducive to destroying the crystallization zone of PEO matrix. The destruction of nanofibers with two structures to the crystallinity of PEO was also reflected by the ionic conductivity of the electrolyte. At 30 ℃, the ionic conductivity of the electrolyte containing ordinary SPES nanofibers was 6.92×10^-5 S/cm. The ionic conductivity of the electrolyte containing dendritic SPES nanofibers was as high as 8.13×10^-5 S/cm at 30 ℃, which is even 1.4 times that of pure PEO electrolyte (5.62×10^-5 S/cm). In addition, the ordinary SPES nanofiber membranes and the dendritic SPES nanofiber membrane can provide skeleton support for the PEO matrix, and the mechanical strength of the electrolyte containing the two types of fiber membranes was as high as 4.8 MPa and 5.1 MPa, respectively. In the lithiumi/lithium symmetric battery, the electrolytes composed of ordinary SPES nanofiber membrane and dendritic SPES nanofiber membrane could maintain the battery cycling for 180 h and 198 h, respectively. But the pure PEO electrolyte had a short circuit during a 65 h cycle at 0.1 mA·h/cm². When LiFePO₄/Li was assembled with an electrolyte containing of the dendritic SPES nanofiber membranes, the electrolyte enabled the battery to maintain a high specific discharge capacity of 128.6 mA·h/g after 400 cycles.

Conclusion From the tested results, it can be seen that both the ordinary SPES nanofiber membrane and the dendritic SPES nanofiber membrane can damage the crystalline region of the PEO matrix to a certain extent, thereby greatly enhancing the ionic conductivity of the prepared composite solid electrolyte. In addition, as the support skeleton of PEO matrix, both fiber membranes can improve the mechanical strength of composite solid electrolyte. However, the modification effect of dendritic SPES nanofiber membrane on electrolyte is more excellent. This is because dendritic SPES nanofiber has more branch fibers than SPES nanofiber, which destroys the crystalline region of PEO to a greater extent and is more helpful for constructing the enough three-dimenstional ion transport pathway. Therefore, the dendritic SPES nanofiber membrane modified electrolyte can better meet the actual application requirements of high-performance all-solid-state lithium metal batteries.

Key words: composite solid electrolyte, lithium metal battery, electrospinning, sulfonated polyethersulfone fiber, polyethylene oxide, nanofiber