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

• The capacity decay of Ni-rich cathodes is firmly established to correlate directly with the degree of delithiation , rather than with the nickel content or the cut-off voltage. • The irreversible H2-H3 phase transition and intergranular cracks cannot be the primary triggers for capacity loss in Ni-rich cathodes. • The defining role of the reconstruction evolving properties of the Ni-rich cathode particle surface layer is highlighted for capacity decay. 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 x Co y Mn 1- x - y O 2 (NCM) cathodes. Electrochemical performance tests show that two Ni-rich cathodes with different nickel contents including LiNi 0.6 Co 0.2 Mn 0.2 O 2 (NCM622) and LiNi 0.9 Co 0.05 Mn 0.05 O 2 (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.