Recently, ion exchange membranes (IEMs) have attracted significant attention in both aqueous and non-aqueous systems. However, the application of IEMs under non-aqueous conditions remains limited due to the lack of suitable material designs and a detailed understanding of ion transport mechanisms. We here develop a phosphate-functionalized cellulose membrane as a promising non-aqueous IEM, incorporating a phosphonium ionic liquid as an additive to stabilize protons in tetrahydrofuran (THF). Phosphonium cations bearing e- donating alkyl chains stabilize H+, while phosphorylated cellulose provides effective H+ transport pathways in THF. As a result, the prepared IEM exhibits higher ion conductivity and enhanced dimensional stability compared to the commercial Nafion® membrane. The system developed in this study shows strong potential in non-aqueous electrochemistry, such as the lithium-mediated nitrogen reduction reaction.