In an effort to increase the thermomechanical stability of lithium-ion battery separators, thermoset membranes (TMs) are a viable alternative to commercial polyolefin separators. We present an efficient and scalable method to produce thin TMs via photopolymerization-induced phase separation (PIPS) in ambient conditions. The pore size is controllable and tuneable by varying the ratio between propylene carbonate (PC) and tetraethylene glycol (TEG) as porogens. The TMs maintain dimensional stability above 200 °C and display sufficient mechanical stiffness. By incorporating a small amount of a thiol monomer, the brittleness of the TMs was suppressed, and a high Young's modulus was achieved (880 MPa). The ionic conductivity of the optimized TMs was around 1 mS cm^-2, with a low MacMullin number, N_M (4.9). In symmetrical Li/Li cells, the TMs behaved similarly to the commercial PE reference, effectively suppressing short circuits for 1000+ hours although continuous overpotential build-up and electrolyte consumption eventually led to cell failure. In LiFePO₄/Li half-cells, similar rate capabilities were achieved for the TMs compared to the reference showing its viability as a separator material.