Lithium sulfur battery interlayers were made from mesoporous carbon nanofibers with large tunable mesopores (17-50+ nm). The nanofibers were prepared by blending and electrospinning two immiscible polymers, polyacrylonitrile (PAN) and cellulose diacetate (CDA) with dimethylformamide as the solvent. Humidity control during the electrospinning process made adjustments to the size of the meso-scaled phase separation domains of the sacrificial CDA. The resultant fiber mat was carbonized and activated in air to create micropores in mesoporous carbon nanofibers (MPCNF). The pore characteristics of the activated MPCNFs were examined and then tested as interlayers in Li-sulfur batteries. First, a fundamental study was conducted regarding the effectiveness of the interlayer versus its own weight and the overall sulfur composition along with a characterization of the pore filling and impedance. Secondly, the effect of micropores and three different pore widths of large mesopores in interlayers on battery performance was studied which revealed that the largest mesopores played a critical role in improving rate capability. Cells with MPCNF interlayers with 50+ nm pores and 70:30 sulfur: Super P composite cathodes had a discharge capacity of over similar to 1100 mAh g(-1) sulfur for 100 cycles at 0.5C and exhibited improved high charge rate capabilities with 850mAh g(-1) at 3C. (C) 2016 The Electrochemical Society. All rights reserved.