A new synthesis approach has been developed to control the morphology of Mn- and Ni-containing carbonate precursors. Through a controlled release of the precipitate, the carbonate precursors form in a “non-classical crystallization” process similar to the natural crystallization of CaCO3, resulting in shapes tunable from microcubes to microspheres. The oriented attachment in the mesocrystal precursors leads to uniform particles with high tap density. Further lithiation of the shape-controlled carbonates produces the final 5 V spinel cathode LiMn1.5Ni0.5O4 with superior electrochemical properties in lithium-ion cells, while maintaining the high tap density and offering high volumetric energy density.