Some of the most remarkable improvements in the cycle life of Li-S batteries could be achieved by nano-structuring of the sulfur/carbon (S/C) cathode material. Meso- and micro-porous particles, carbon nano tubes, and other sophisticated structures are used to actively retain highly soluble polysulfides which are responsible for capacity fading and a short cycle life. In this article we present a detailed 1+1D continuum model of S/C composite cathodes which is based on an ideal structure where all polysulfides are confined within meso- and micro-porous carbon particles. The model allows to correlate particle properties to cell performance and gives interesting insights for cell operation. Most interestingly we identified an additional overpotential caused by a transport of Li ions against a concentration gradient into the particle. Furthermore, the model can be used to find optimal structural and electrolyte properties such as pore volume, sulfur loading, solubility products, and salt concentration.