A simple, low-cost modification of lithium-sulfur (Li-S) cells by placing a treated carbon paper between the sulfur electrode and the separator has been investigated to significantly improve the performance of Li-S cells. The treated carbon paper was prepared by an alcohol-alkaline/thermal treatment of a commercial Toray carbon paper, introducing hydroxyl functional groups and micro-cracks on the carbon fibers in the carbon paper, which enhances the hydrophilicity and increases surface areas of the carbon paper matrix. The modified Li-S cells deliver a higher initial capacity of 1651 mAh g-1 at 1.5-2.8 V at a rate of C/5 compared to the cells without any interlayer or with an untreated carbon paper interlayer. The cells with the treated carbon paper offer additional improvement in performance when the discharge cut-off voltage is raised to 1.8 V: 1057, 1002, and 929 mAh g-1 after 100 cycles, respectively, at C/5, C/2, and 1 C rates. The improved cell performance is attributed to the 3D architecture of the carbon paper interlayer, serving as a conductive skeleton for trapping and depositing dissolved sulfur-containing active materials, as confirmed by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). The novel configuration presented here offers a low-cost approach to overcome the persistent problems of Li-S cells.