Densely packed submicron polyhedral LiNi0.5Mn1.5O4 material with disordered Fd 3 ¯ m structure was synthesized via a modified sol–gel method. The as-synthesized material has a high tap density of 2.15 g cm−3, guaranteeing a high volumetric energy density for high power batteries. Electrochemical properties were investigated in both a LiNi0.5Mn1.5O4/Li half-cell and a LiNi0.5Mn1.5O4/graphite full-cell. The LiNi0.5Mn1.5O4/Li half-cell exhibits a superior cycle stability and rate capability. Here the LiNi0.5Mn1.5O4 material can deliver capacity retentions of 86% at 25 °C and 75% at 55 °C within 1000 cycles for a charge–discharge rate of 1 C. At a much higher rate of 10 C, a discharge capacity of 95 mAh g−1 can be still obtained. The LiNi0.5Mn1.5O4/graphite full-cell delivers a stable discharge capacity of 130.2 mAh g−1 at 0.2 C, corresponding to a discharge energy density as high as 576.2 Wh kg−1. After 100 cycles, the full cell can maintain a working voltage of 4.55 V and capacity retention of 84.6%. The excellent cycle stability is attributed to the dense structure, large particle size, low specific surface area and less exposed (110) facets, which dramatically reduce irreversible surface chemical reactions and manganese dissolution.