Cathode materials consisting of porous pyrite nano-octahedra encapsulated by uniform carbon nanocages exhibit very high energy density, superior rate capability (reversible capabilities of 439, 340, and 256 mA h g(-1) at 1C, 2.5C, and 5C, respectively) and stable cycling performance (ca. 10% capacity loss after 50 cycles at 0.5C with a capacity retention of 495 mA h g(-1)).