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Cathode materials consisting of porous pyrite nanooctahedra encapsulated by uniform carbon nanocages exhibit very high energy density, superior rate capability (a reversible capability 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).