Delithiation coupling with surface reconstruction during capacity degradation in Ni-rich layered cathodes

Abstract

Surface reconstruction and mechanical failure play key roles in the capacity loss of Ni-rich cathodes, yet their intertwining influences are still not completely elucidated. Herein, this work deconvolutes the primary-secondary relationships between surface reconstruction and mechanical failure in affecting capacity decay for LiNi_xCo_yMn_1-x-yO₂ (NCM) cathodes. Electrochemical performance tests show that two Ni-rich cathodes with different nickel contents including LiNi_0.6Co_0.2Mn_0.2O₂ (NCM622) and LiNi_0.9Co_0.05Mn_0.05O₂ (NCM9055) in the same delithiation state exhibit similar initial discharge specific capacities and capacity retentions after cycling, which unveils that capacity decay is directly related to the degree of delithiation. In contrast to NCM622, the deep delithiation triggers the typical H2-H3 phase transition of NCM9055, leading to higher internal strain and more severe mechanical degradation during the similar capacity fading process. Such discrepancies in structural degradations disclose that the H2-H3 phase transition and the intergranular cracking cannot be the primary causes for capacity degradation. Impressively, the resemblance in surface reconstruction evolution for two cathodes after cycling further reveals that the capacity fading is strongly dependent on the reconstruction evolving properties of the cathode particle surface layer. This work offers valuable insights and further understanding of electrochemical performance degradation, which serve to facilitate Ni-rich cathode material design improvements.