Relieving Stress Concentration through Anion–Cation Codoping toward Highly Stable Nickel-Rich Cathode

Abstract

Nickel-rich LiNi_0.8Co_0.15Al_0.015O₂ (NCA) with excellent energy density is considered one of the most promising cathodes for lithium-ion batteries. Nevertheless, the stress concentration caused by Li⁺/Ni²⁺ mixing and oxygen vacancies leads to the structural collapse and obvious capacity degradation of NCA. Herein, a facile codoping of anion (F^-)-cation (Mg²⁺) strategy is proposed to address these problems. Benefiting from the synergistic effect of F^- and Mg²⁺, the codoped material exhibits alleviated Li⁺/Ni²⁺ mixing and demonstrates enhanced electrochemical performance at high voltage (≥4.5 V), outperformed the pristine and F^-/Mg²⁺ single-doped counterparts. Combined experimental and theoretical studies reveal that Mg²⁺ and F^- codoping decreases the Li⁺ diffusion energy barrier and enhances the Li⁺ transport kinetics. In particular, the codoping synergistically suppresses the Li⁺/Ni²⁺ mixing and lattice oxygen escape, and alleviates the stress-strain accumulation, thereby inhibiting crack propagation and improving the electrochemical performance of the NCA. As a consequence, the designed Li_0.99Mg_0.01Ni_0.8Co_0.15Al_0.05O_0.98F_0.02 (Mg1+F2) demonstrates a much higher capacity retention of 82.65% than NCA (55.69%) even after 200 cycles at 2.8-4.5 V under 1 C. Furthermore, the capacity retention rate of the Mg1+F2||graphite pouch cell after 500 cycles is 89.6% compared to that of the NCA (only 79.4%).