Two types of structure- and morphology-controlled spinel Li[Ni0.5Mn1.5]O4 (LNMO) are prepared and systematically investigated as 5V, high-rate, and long-life cathode materials for rechargeable Li-ion batteries. The octahedral LNMO particles (1μm or 1–5μm mixed sizes) are prepared through a heat-treatment at 850°C after a hydrothermal reaction, and their performance is compared with that of one-dimensional LNMO nanorods (100–200nm and 1–3μm in diameter and length, respectively), which are synthesized via a two-step method consisting of a hydrothermal reaction followed by solid-state Li and Ni implantation. They show high single crystallinities with an ordered (P4332) and disordered (Fd-3 m) phase for the nanorods and octahedral particles, respectively. Rietveld refinement of X-ray and neutron diffraction, FT-IR, SEM, and TEM are employed to study their phases and microstructures. Galvanostatic studies reveal that overall battery performances of the octahedral LNMO particles are superior to those of the LNMO nanorods. In particular, the disordered octahedral LNMO particles that are composed of mixed particle sizes ranging from of 1 to 5μm show not only the best rate capability and specific discharge capacity but also an excellent cycle stability with a capacity retention of 89% (corresponding to specific discharge capacity of 105mAhg−1) at a 10C cycling rate, even after 1000 cycles. This remarkable performance is attributed to the structural stability, while the highest electrode tap density (1.59gcm−3) in combination with efficient packing resulted in the coexistence of various particle sizes that can provide a shortened pathway for lithium ions between particles.