Understanding the Effect of Surface Morphology on the Properties of Paper-Based Separators for Lithium-Ion Battery Application

The surface morphology of separators is of great importance to the electrode/separator interface compatibility and the final cycle properties of assembled batteries. However, the above structure-performance effects have rarely been investigated. In this research, the microstructure of paper-based separators was tailored by changing the solvents in the polymer coating solutions. Due to the different swelling behaviors between propionylated cellulose fibers and coated benzophenone-attached poly(methyl methacrylate) (P(MMA-co-MABP)), various surface morphologies of the modified paper-based separators were obtained. Interestingly, polymer particles appeared on the surface of the coated paper separator when using DMF as a solvent. In this case, the CPEF-DMF separator exhibited the lowest surface roughness (R_q = 2.9 μm) and interface resistance (252 Ω). Thanks to the more ester groups introduced by the polymer coating, the CPEF-DMF separator displayed the highest ionic conductivity and Li⁺ ion transference number. There was also the lowest PF₅ content in the cycled Li foil assembled by the separator, which could result from the stress dispersion and resistance of Li dendrites by the polymer particles on the surface. MD simulation indicated that the separator had high Li⁺ ion diffusion, transfer, and dissociation behavior, which would facilitate the uniform ion deposition and conduction in the battery. Accordingly, the separator displayed stable cycling over 600 h at 1 mA·cm^-2, and the assembled Li/separator/LiFePO₄ cell showed excellent cycle performance with a discharge capacity of 114.3 mAh·g^-1 after 600 cycles at 1C. This research would enrich the interfacial structure-performance theory in the field of lithium-ion battery application.