The performance of lithium manganese phosphate as a lithium-ion battery cathode material is improved by collective and cooperative strategies including Fe substitution, carbon coating, and the assembly of carbon-coated LiMn1−x Fe x PO4 nanocrystals into a highly dense packing of monodisperse microboxes. These strategies are implemented experimentally by a facile and scalable synthesis method. The dense packing allows the conductive carbon coating to be interconnected into a continuous three-dimensional network for electron conduction. The porosity in the packed structure forms the complementary network for Li+ transport in the electrolyte. The primary particles are nanosized and Fe-substituted to improve the effectiveness of Li+ insertion and extraction reactions in the solid phase. The reduction of transport resistance external and internal to the nanocrystals yields a Li storage host with good rate performance (116 mAh g−1 at 5 C discharge rate where C = 170 mA g−1) and cycle stability (95% retention of initial capacity in 50 cycles). Electrochemical impedance spectroscopy and morphology examination of the cycled microboxes reveal a robust packed structure with stable surfaces.