Porous membranes of polymer blends and their use as functional separators for electrochemical batteries

This works explores the synthesis of a porous membrane concept combining two polymers: one with a structural role which constitutes the scaffold of the membrane, and the other with the capacity to form a polymer gel with ad-hoc liquids at the pores and surface of the membrane. After liquid infiltration, the structural polymer keeps the mechanical properties and dimensional stability of the membrane constant, i.e. no swelling, for long time periods, while the soluble polymer forms a gel at the pores and membrane surface. In this work, the goal of these membranes is that of a functional separator able to produce semisolid electrolytes by in-situ gel formation upon soaking with liquid electrolytes at electrochemical cells. This functional separator enables the formation of the semisolid electrolyte without modification of the electrochemical cells industrial processes, and avoiding the handling of UHMW gels at an industrial scale. Functional separators comprising polycarbonate (PC) as the structural scaffold and ultrahigh molecular weight (UHMW) polyethylene oxide (PEO) as the soluble polymer for in-situ gel formation are prepared by adapting the non-solvent induced phase separation (NIPS) process to the simultaneous coagulation of the two polymers. The polymer distribution, pore morphology depend on the experimental conditions, especially on the PC/PEO degree of mixing before coagulation. The polymer distribution permits the formation of PEO gel at the pores of the PC scaffold, thus combining the electrochemical properties of UHMW polymer gel electrolytes, with the chemical and mechanical resistance of PC. Moreover the presence of PEO enables the wetting of the PC scaffold (otherwise hydrophobic) with hydrophilic electrolytes such as aqueous ZnCl₂, the Zn eutectic acetamide Zn(TFSI)₂ (ACE-Zn) and the chloroaluminate acetamidine-AlCl₃, selected in this work to showcase the separators’ versatility, which do not do not swell, break, or degrade after months soaked in the electrolytes. Symmetric Zn||Zn cells were built using the electrolyte ACE-Zn soaked in PC/PEO membranes (40 μm) and in a 100 μm glass fiber separator. While the latter short-circuited before 150 cycles, functional separators endured 300 cycles with no short-circuiting, proving the formation of the UHMW PEO gel at the functional separators and concomitant increase in cycling stability, highlighting the success of this approach.