Intrinsic mechanism of Co/Mn elemental manipulation in enhancing the cycling stability of single-crystal ultrahigh-nickel layered cathodes

Abstract

Ultrahigh-nickel layers suffer from poor cycling stability. Ultrahigh-nickel single-crystal cathodes with Co/Mn elements doped in the transition metal layer are considered promising to overcome this challenge via effectively mitigating the microcrack issue. However, the intrinsic mechanism by which Co/Mn elements enhance the cycling stability of single-crystal ultrahigh-nickel cathodes, as well as the dominant role of structural evolution and surface side reactions in the cycling fading of single-crystal cathodes, remains unclear. Herein, the single-crystal LiNiO₂ (SC-N) with manipulated Co/Mn doping as LiNi_0.9Co_0.1O₂ (SC-NC) and LiNi_0.9Mn_0.1O₂ (SC-NM) oxides are synthesized to reveal the correlation between crystal structural evolution and electrochemical performance. The alleviated intensity and delayed onset potential of H2/H3 phase transition in the SC-NM cathode effectively mitigate the abrupt anisotropic lattice collapse, thereby enhancing the morphology integrity of the particles. Despite a higher cation mixing degree, the excellent structural stability derived from the reversible H2/H3 phase transition also provides favorable kinetics for repeated lithiation/delithiation. As a result, the Co-free SC-NM cathode can exhibit unconventional cycling stability with a capacity retention of 93.8% after 100 cycles at 0.5C between 2.7 and 4.3 V compared to the SC-N and SC-NC cathodes with capacity retention values of 71.7% and 81.1%, respectively under the same condition. This study emphasizes the importance of regulating the crystal structure evolution via Co/Mn manipulation in constructing high-performance single-crystal ultrahigh-nickel layered cathodes.