## 2 Experimental Materials and Methods
### PU/PAN Diaphragm Performance Testing and Structural Characterization  
(1) Morphological characterization Scanning electron microscopy (SEM: GeminiSEM 300, Zeiss, Jena, Germany) was used to observe the surface morphology and determine the fiber diameter and pore size of the PU-based lithium-ion battery diaphragm. The diaphragm samples were first cut, then gold sprayed for 2 min. The freshly prepared PU-based fiber diaphragm film was placed under the microscope for morphological characterization. (2) Porosity test An n-butanol aspiration technique was used to determine the diaphragm porosity. First, the diaphragm was cut into 2 cm \(\times\) 2 cm test samples, then the mass of the dry film without n-butanol was measured. The dry diaphragm was then soaked in an n-butanol solution for 3 h. The residual n-butanol on the surface of the diaphragm was removed with paper, and the mass of the diaphragm was measured. The formula used to calculate the diaphragm porosity was as follows: \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Number** & **PU/PAN Solution Mass** & **Stirring Time (h)/Temperature (\({}^{\circ}\)C)** & **Needle Inner Diameter (G)** & **Rotating Speed (r/min)** \\  & **Fraction (\%)** & & & **(r/min)** \\ \hline
1 & 14\% & 8 h/80 \({}^{\circ}\)C & 28 & 4200 \\
2 & 14\% & 8 h/80 \({}^{\circ}\)C & 26 & 4200 \\
3 & 16\% & 8 h/80 \({}^{\circ}\)C & 28 & 4200 \\
4 & 16\% & 8 h/80 \({}^{\circ}\)C & 26 & 4200 \\
5 & 18\% & 8 h/80 \({}^{\circ}\)C & 28 & 4200 \\
6 & 18\% & 8 h/80 \({}^{\circ}\)C & 26 & 4200 \\
7 & 20\% & 8 h/80 \({}^{\circ}\)C & 28 & 4200 \\
8 & 20\% & 8 h/80 \({}^{\circ}\)C & 26 & 4200 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Preparation schemes of PU/PAN nanofiber separators with different mass fractions. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Number** & **PU/PAN Ratio** & **Stirring Time (h)/Temperature (\({}^{\circ}\)C)** & **Needle Inner Diameter (G)** & **Rotating Speed (r/min)** \\ \hline
1 & 4:6 & 8 h/80 \({}^{\circ}\)C & 28 & 4200 \\
2 & 4:6 & 8 h/80 \({}^{\circ}\)C & 26 & 4200 \\
3 & 5:5 & 8 h/80 \({}^{\circ}\)C & 28 & 4200 \\
4 & 5:5 & 8 h/80 \({}^{\circ}\)C & 26 & 4200 \\
5 & 7:3 & 8 h/80 \({}^{\circ}\)C & 28 & 4200 \\
6 & 7:3 & 8 h/80 \({}^{\circ}\)C & 26 & 4200 \\
7 & 8:2 & 8 h/80 \({}^{\circ}\)C\[P(\%)=\frac{w_{w}-w_{d}}{\rho_{b}\times v} \tag{1}\]  
where \(P\) is the porosity of the diaphragm (%); \(w_{w}\) is the weight after wetting (g); \(w_{d}\) is the dry weight before wetting (g); \(\rho_{b}\) is the density of n-butanol (g/cm\({}^{3}\)); and \(v\) is the volume of the dried septum (cm\({}^{3}\)). Absorption rate test  
The diaphragm was dried under a vacuum, and the dry weight was recorded. The diaphragm was then immersed in the electrolyte, and the electrolyte was dried with 1 mol/L lithium hexafluorophosphate (LiPF\({}_{6}\)) and ethylene carbonate (EC)/methyl ethyl carbonate (EMC)/dimethyl carbonate (DMC) (mass fraction ratio = 1/1/1) for 10 h. The residual electrolyte attached to the surface was removed with paper, and the diaphragm was weighed again. After measuring the mass of the wet film, the absorption rate was calculated as follows:  
\[Uptake(\%)=\frac{w_{1}-w_{0}}{w_{0}}\% \tag{2}\]  
where \(Uptake\) is the liquid absorption rate of the diaphragm (%); \(w_{1}\) is the weight after wetting (g); \(w_{0}\) is the dry weight before wetting (g). Ion conductivity test  
An electrochemical workstation was used to measure the ionic conductivity of the commercial Celgard 2400 diaphragm, which consists of PU-based fiber films. A sample of the diaphragm was placed between two steel sheets, and a button cell in the order of steel sheet/diaphragm/steel sheet was assembled in a super clean glove box on an electrochemical workstation. A scan frequency range from 1 MHz to 1 Hz with an amplitude of 5 mV was applied, and the intercept between the high-frequency region and the horizontal axis was expressed as the bulk resistance of the diaphragm.