Traditional polyolefin separators tend to perform poorly against heat-triggering reactions causing potentially catastrophic issues in lithium-ion batteries (LIBs). In this study, we developed a high temperature resistant, flexible, mechanically robust and highly ionically conductive composite separator made from electrochemically stable polymer encased with nanodiamonds (NDs). We systematically study the effect of ND concentration in the polymer matrix of the membrane to improve the thermal, mechanical and electrochemical properties of the composites. When tested in LIB with high-voltage LiNiMnCoO₂ (NMC), these novel separators exhibited substantially superior performance when compared with commercial polyolefin separator membranes. These cells exhibited closed-to-theoretical capacity (200mAhg^-1_NCM), excellent rate capability and minimal degradation for over 150 cycles. Our study showcased the capability of ND powder as a nano-sized filler materials for the fabrication of composite membranes with superior properties, and opened up the broad possibilities of composite membrane fabrications by varying the polymer composition, micro-structure, nanoparticle surface chemistry and concentrations.