Mechanism study of Fe penetration behavior on structural stability of lithium-rich layered co-free cathode materials

Abstract

Lithium-rich layered oxides (LLOs) have emerged as promising cathode materials for lithium-ion batteries due to their high capacity and low cost. However, their practical application is hindered by inherent structural instability and poor cycling performance. Addressing the cost and environmental concerns associated with Co reliance in conventional cathode materials, this work investigates a series of Co-free Li₂₈Fe_lNi_mMn_20-l-mO₄₈ (LF₂NMO for l = 2, m = 5; LF₄NMO for l = 4, m = 4; LF₆NMO for l = 6, m = 3) systems. The influences of Fe-doping concentration and the penetrated-Fe number into the Li₂MnO₃ domain on delithiated structures, lattice stress, oxidation processes of transition metal (TM) ions and oxygen ions, oxygen Bader charge evolution, oxygen release enthalpy and structural stability are systematically examined using DFT calculations. Results demonstrate that the LF₄NMO-P(1Fe)(the number 1 in 1Fe is the penetrated-Fe number into the Li₂MnO₃ domain) composition, exhibits the optimal structural stability. Upon the full delithiation it shows the smallest volume change of 0.23%, minimal lattice strain, and the least variation in Mn-O bond lengths among the studied systems. Furthermore, LF₄NMO possesses the highest oxygen release enthalpy, indicating the superior oxygen stability. This originates from its more gradual change in oxygen Bader charge during oxidation and the participation of more boundary oxygen ions into the charge compensation, effectively suppressing the excessive oxygen oxidation. Critically, the LF₄NMO composition mitigates interphase strain, thereby preventing phase transformation and oxygen release. This study provides a theoretical foundation for the structural design of high-performance Co-free LLOs cathode materials.