Phase Evolution and Degradation Modes of R3̄ m Li_xNi_{1- y- z}Co_yAl_zO₂ Electrodes Cycled Near Complete Delithiation

Practical utilization of energy densities near the theoretical limit for R3̄m layered oxide positive electrode materials is dependent on the stability of the electrochemical performance of these materials at or near full delithiation. To develop new chemistries and novel approaches toward the improvement of the electrochemical performance of these materials at such high states of charge, a robust understanding of the failure mechanisms limiting current materials is necessary. Thorough analysis of Li_xCo_1-yAl_yO₂ and Li_xNi_1-yAl_yO₂ as well as Li_xNi_0.8Co_0.2O₂ and Li_xNi_0.8Co_0.15Al_0.05O₂ (1 ≥ x ≥ 0 and 0.2 ≥ y ≥ 0) enabled the identification of key relationships between the transition metal chemistry of the electrode, its structural stability, and cycling characteristics at or near complete delithiation (4.75 V). Extensive characterization of these materials was achieved by a multitude of physical and electrochemical techniques to investigate the relative importance of surface vs bulk phenomena. The resulting insights derived from these analyses highlight the importance of the intrinsic structural and mechanical stability of the electrode when highly delithiated and establish guidelines for identifying positive electrode materials with improved high state of charge performance. Particularly important is the contrasting electrochemical impact of Al substitution into LiCoO₂- and LiNiO₂-based materials, which is shown to likely arise from the enhanced propensity for Al ions to migrate to the tetrahedral site in Co-rich compounds at high states of delithiation.