Unlocking The Full Capacity of Ni-Rich Layered Oxide Cathodes: The Impact of Lithium Site Defects on Diffusion and Stability

The automotive industry's transition to renewable energy relies, among others, on layered oxides, like LiNi_xCo_yMn_zO₂ (NCM) and LiNi_xCo_yAl_zO₂(NCA), for high-performance applications. These materials promise enhanced capacities within a stable cycling range. While the theoretical specific capacity is above 270 mAh/g, in reality only around 240 mAh/g can be realized under conditions closely resembling real-world applications. Interestingly, specific capacities exceeding 260 mAh/g can be achieved upon very slow cycling, highlighting that capacity loss is caused by sluggish lithium diffusion. A structural characteristic strongly linked to charge transport is the presence of intercalation site defects, including Ni_Li substitutional point defects, which are inherently found in state-of-the-art Ni-rich layered oxide materials. To investigate the effect that these defects have on cycling performance, we have developed a synthesis method for perfectly layered LiNiO₂ (LNO) through sodium to lithium ion exchange. Our analyses, including X-ray techniques and NMR spectroscopy, confirm that this route produces a material entirely devoid of Ni_Li defects. Overall, this method serves as a foundation for re-evaluating the impact of factors that are well recognized for influencing the cyclability, such as nickel content, defect concentration and particle size.<br/>We attribute an ambivalent role to Ni_Li defects, recognizing their negative effect on lithium diffusion, particularly limiting the discharge capacity. However, we also identify a secondary contribution of Ni_Li defects, which is not readily observable in state-of-the-art LNO materials due to the intrinsic presence of substitutional point defects. As the defect concentration approaches zero, the high-voltage regime (high SOC) becomes unstable, as evidenced by increased oxygen release and capacity loss of more than 10%. Owing to the ambivalent nature of Ni_Li defects, we propose a hypothesis that achieving an optimal balance between diffusion and stabilization is crucial to fully harness the material's capacity.