Dual Substitution Strategy in Co-Free Layered Cathode Materials for Superior Lithium Ion Batteries

A dual substitution strategy is introduced to Co-free layered material LiNi_0.5Mn_0.5O₂ by partially replacing Li and Ni with Na and Al, respectively, to achieve a superior cathode material for lithium ion batteries. Na⁺ ion functions as a "pillar" and a " cationic barrier" in the lithium layer while Al³⁺ ion plays an auxiliary role in stabilizing structure and lattice oxygen to improve the electrochemical performance and safety. The stability of lattice oxygen comes from the binding energy between the Ni and O, which is larger due to higher valences of Ni ions, along with a stronger Al-O bond in the crystal structure and the "cationic barrier" effect of Na⁺ ion at the high-charge. The more stable lattice oxygen reduces the cation disorder in cycling, and Na⁺ in the Li layer squeezes the pathway of the transition metal from the LiM₂ (M = metal) layer to the Li layer, stabilizing the layered crystal structure by inhibiting the electrochemical-driven cation disorder. Moreover, the cathode with Na-Al dual-substitution displays a smaller volume change, yielding a more stable structure. This study unravels the influence of Na-Al dual-substitution on the discharge capacity, midpoint potential, and cyclic stability of Co-free layered cathode materials, which is crucial for the development of lithium ion batteries.