In this work, we successfully use ultralong γ-MnOOH microrods to synthesize ultralong spinel LiNi0.5Mn1.5O4 microrods through high temperature solid state reaction, which allows to drastically increase the content of residual Mn3+ ions (up to 61.3%) along with oxygen deficiency (up to 0.44) in spinel structure and improves the rate, cycling and discharge capacity of LiNi0.5Mn1.5O4 materials significantly. The excessive theoretical capacity (185 mAhg−1) is even observed at low charge and discharge rate due to the additional enhanced contribution of the Mn3+/Mn4+ redox couples. The LNMO-800 shows a little capacity decay with the increasing of the rate from 0.5 C to 1 C and 2 C, the discharge capacities were 145 mAhg−1, 144.6 mAhg−1 and 142 mAhg−1, respectively. Even at a high rate of 10 C, it still deliveres a capacity of 125.3 mAhg−1. After 1000 cycles at 1 C, the discharge capacity can still reach 120 mAhg−1, corresponding to a capacity retention of 82.3%. The high temperature (55 °C) tests also demonstrate its excellent structural stability. The LiNi0.5Mn1.5O4 microrod with ultrahigh Mn3+ content should be a promising choice for future high energy power applications.