High voltage (∼5 V) spinel LiNi0.5Mn1.5O4 (LNMO) has attracted great attention because of its ultrahigh voltage plateau, which can be used as a cathode to reduce pressure in battery management systems. Moreover, compared with layered LiNxMyCzO2 materials, LNMO only requires little amounts of Ni, is cobalt-free for maintaining energy density, is inexpensive, and is lightweight. This study demonstrates two types of primary particles with different morphologies: rectangular and pentahedral. The pentahedron-shaped LNMO has lower surface energy owing to the formation of high valence Ni on the surface, thereby causing gas evolution and a loss in cycle retention, a direct Ni2+/Ni4+ reaction. Conversely, rectangular-shaped LNMO with higher Mn3+ content exhibits a stable electrochemical reaction, which provides a higher surface energy that prevents ethylene carbonate (EC) decomposition on the surface, and thereby, excellent performance is obtained, a parallel reaction of Mn3+/ Mn4+ and Ni2+/ Ni3+. By adding a lithium salt additive, trifluoromethyl benzimidazole (LiTFB), a self-regulation of Ni and Mn ion valences leads to a key reaction on both pentahedral (surface disordering effect) and rectangular (preventing Jahn-Teller distortion effect) LNMO morphologies. The two-electron transfer in the reactions of Ni2+/3+ and Mn3+/4+ of LiTFB-modified LNMOs provides excellent electrochemical performance for further high-energy applications.