Polydopamine-mediated multistep SiO2 surface modification of polypropylene separators for sodium-ion batteries

Commercial polypropylene (PP) separators offer low cost and established manufacturability, but their limited electrolyte affinity and thermal dimensional stability can restrict their performance in sodium-ion batteries. In this study, a commercial Celgard 2400 separator was modified through a multistep route comprising polydopamine (PDA) deposition, L-lysine-assisted SiO 2 colloid treatment, and a final 1,1′-carbonyldiimidazole-assisted interfacial treatment. A binder-assisted PP-SiO 2 separator was used as a physical-coating control. SEM, EDS, and ATR-FTIR supported the formation of a relatively dispersed SiO 2 -containing surface layer with retained visible microporous features and a modified interfacial chemical environment. PP-PDA-SiO 2 exhibited a total thickness of 26.90 ± 0.36 μm and apparent area shrinkage ratios of 18.42 ± 1.44% at 140 °C and 61.17 ± 0.55% at 170 °C. Its transverse maximum engineering stress reached 16.12 ± 0.38 MPa, while the estimated terminal strain decreased to 58.1 ± 3.8%. The apparent electrolyte contact angle decreased to 20.63 ± 0.80° after 10 s. Its effective ionic conductivity was 0.780 ± 0.038 mS cm −1 , compared with 0.556 ± 0.012 mS cm −1 for pristine PP. PP-SiO 2 showed faster apparent wetting and a higher effective ionic conductivity of 0.878 ± 0.056 mS cm −1 , while PP-PDA-SiO 2 exhibited the more favorable overall electrochemical response. It retained 114.33 ± 2.28 mAh g −1 after 50 cycles at 100 mA g −1 with a capacity retention of 87.99 ± 0.20%, and delivered 88.97 ± 2.10 mAh g −1 at 500 mA g −1 . Equivalent-circuit fitting yielded the lowest combined R 1 + R 2 resistance contribution of 0.1169 ± 0.0067 kΩ for PP-PDA-SiO 2 . These results show that separator performance depends on the combined effects of surface-layer morphology, thickness, retained pore features, electrolyte wettability, separator-level ionic transport, and interfacial electrochemical response. The observed improvements are attributed to the complete multistep modification route, while the independent contribution of each processing step remains unresolved.