## 3 Results and Discussion
### Analysis of the Thermal Stability  
The thermal stability of the diaphragm is critical to the safety of lithium-ion batteries and is an extremely important indicator. Batteries often charge and discharge at high rates, and the internal temperature of the battery is prone to rise. This results in a severe contraction of the lithium-ion battery diaphragm, which can cause a short circuit in direct contact with the positive and negative electrodes of the battery. To compare the thermal stability of the Celgard 2400 and PU/PAN fiber diaphragms, a thermal weight loss analysis was conducted with the results shown in Figure 6. The Celgard 2400 diaphragm started to show significant thermal decomposition at about 400 to 700 \({}^{\circ}\)C, with a residual mass retention rate of no more than 2%, while thermal weight loss of PU/PAN fiber diaphragms occurred from 310 \({}^{\circ}\)C to 450 \({}^{\circ}\)C. This part of the mass loss is the thermal decomposition of PU caused by the destruction of the structure at high temperatures and was then stable until the temperature reached 700 \({}^{\circ}\)C. The final mass retentions of the PU, 18% PU:PAN = 8:2, and 18% PU:PAN = 7:3 diaphragms were 5.4%, 17.2%, and 32.1%, respectively, which showed that PAN addition did not reduce the thermal stability of PU/PAN fiber diaphragms and could improve their residual mass retention. So, the addition of PAN will reduce the mass loss of the composite diaphragm and improve the thermal stability of the diaphragm, meeting the thermal stability requirements of lithium-ion battery separators. The DSC results for the Celgard 2400 and PU/PAN fiber diaphragms are shown in Figure 7. The heat absorption peak of the Celgard 2400 diaphragm appeared at 118 \({}^{\circ}\)C, indicating that the melting point of the Celgard 2400 diaphragm was 118 \({}^{\circ}\)C, while the heat absorption peaks of the PU, 18% PU:PAN = 8:2, and 18% PU:PAN = 7:3 diaphragms appeared at 300 \({}^{\circ}\)C, 308 \({}^{\circ}\)C, and 314 \({}^{\circ}\)C, respectively, indicating that their melting points were 300 \({}^{\circ}\)C, 308 \({}^{\circ}\)C, and 314 \({}^{\circ}\)C, respectively. In summary, the thermal stabilities of the PU/PAN fiber diaphragms were better than that of the commercial Celgard 2400 diaphragm because the polymer chain segments in the composite diaphragms were separated, while in the commercial diaphragm they were in a taut state.  
Figure 5: Impedance diagram of the commercial Celgard 2400 and PU/PAN fiber diaphragms.  
The thermal dimensional stability of the diaphragm is another important indicator of battery safety performance. Portions of the Celgard 2400 and PU/PAN fiber diaphraggrams were cut into samples of the same size and shape, and then subjected to a heat treatment at 160 \({}^{\circ}\)C for 30 min.  
The changes in the dimensions of the different diaphraggrams are shown in Figure 8. After the heat treatment, the commercial Celgard 2400 diaphragm was no longer a film, while there were no significant dimensional changes in the PU/PAN fiber diaphragms. The thermal dimensional stability of the PU/PAN fiber diaphragms was significantly better than that of the commercial Celgard 2400 diaphragm, compared to polyacrylonitrile (PAN)/styrene-isoprene-styrene (SIS) composite fiber membranes prepared by [PERSON] et al. [24] using electrostatic spinning, PAN/SIS composite fiber membranes curled after heat treatment at 160 \({}^{\circ}\)C. PU-based fiber membranes possess advantages as candidates for lithium-ion battery separators.  
Figure 6: Thermal weight loss analysis of the commercial Celgard 2400 and PU/PAN fiber diaphragms.  
Figure 7: The DSC test results for the commercial Celgard 2400 and PU/PAN fiber diaphragms.