Failure Mechanisms of High-Voltage Spinel LiNi_0.5Mn_1.5O₄ with Different Morphologies: Effect of Self-Regulation by Lithium Benzimidazole Salt Additive

High voltage (∼5 V) spinel LiNi_0.5Mn_1.5O₄ (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 LiN_xM_yC_zO₂ 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 Ni^2+/3+ and Mn^3+/4+ of LiTFB-modified LNMOs provides excellent electrochemical performance for further high-energy applications.