Tailoring surface entropy gradient towards 4.6 V ultrahigh-nickel cathodes with durable cationic and anionic redox

Abstract

Nickel-rich layered transition metal oxides are intriguing cathode materials for lithium-ion batteries because of high energy density, but they suffer from structural degradation at high voltages, caused by lattice distortion, cation migration/dissolution, and lattice oxygen loss. To address these degradation issues, herein we report a surface entropy-gradient strategy to construct a LiNi_0.93Mn_0.02Mg_0.015Al_0.015Co_0.01Mo_0.01O_1.99F_0.01 cathode featuring concentration gradients of Ni/Co/Mo/F/O elements at the primary particle surfaces. Comprehensive microscopic and spectroscopic characterizations, combined with theoretical calculations, reveal that this engineered gradient structure establishes a progressive strengthening mechanism driven by increasing configurational entropy from bulk to surface, thereby significantly enhancing structural stability and electrochemical reversibility. Specifically, the entropy-gradient configuration effectively mitigates the irreversible O3-to-O1 phase transition, promoting lithium-ion diffusion; simultaneously, it inhibits Ni migration and dissolution while suppressing excessive oxygen oxidation, thereby substantially improving the reversibility of both cationic and anionic redox reactions upon deep (de)lithiation. Under high cut-off voltage of 4.6 V, the formulated cathode retains 91.9% of its initial capacity (229.9 mAh g^-1) after 100 cycles, outperforming the conventional high-nickel counterparts. This study highlights the entropy-gradient engineering as an innovative methodology to upgrade ultrahigh-nickel cathodes under high-voltage operation.