A polyhedral structured LiNi0.5Mn1.5O4 spinel (denoted as LNMO-P) is synthesized by using a polymer auxiliary method. The results of XRD, FT-IR, Raman, SEM and TEM measurements indicate that the LNMO-P exhibits a polyhedral structure with the size of approximately 2 μm and a major phase of P4332. Extreme condition testing, which means successively testing the rate capability, low-rate cyclability at 25 and 55 °C with the same half-cell as well as the high-rate cyclability at 25 and 55 °C with another half-cell, is also introduced to evaluate the electrochemical properties of the materials. The results indicate that the LNMO-P exhibits an acceptable power density for pure electric vehicle, a higher energy density and excellent cyclability for both low-rate and high-rate cycling at 25 °C; this might be attributed to the fact that the polyhedral structure is favor to lithium ion diffusion and suppression of lattice expansion. Although the capacity retention fades largely under high-rate cycling at 55 °C, this is ascribed to the synergistic effect of the electrolyte decomposition and corrosion reaction rather than the reason of LNMO-P itself. In brief, such a material is quite qualified for the actual application in electric vehicle even in extreme conditions.