Reduction-Induced Surface Reconstruction Mechanism of Layered Oxide Cathodes in Lithium-Ion Batteries

The crystal structural transformation from a layered to a rocksalt-type phase is a key degradation pathway for layered oxide cathode materials such as Li[NiMnCo]O₂ (NMC) and Li[NiCoAl]O₂ (NCA) in lithium-ion batteries. This process involves irreversible oxygen loss and transition metal (TM) migration. Traditionally, the degradation mechanism has primarily been attributed to the thermodynamic instability of highly delithiated (oxidized) cathodes at high voltages (> 4.3 V vs. Li/Li⁺). In such cases, the high vacancy concentration in lithium layers exceeding a critical value or the oxidation of active materials beyond manageable valence state triggers TM cations to migrate from TMO₂ slabs into the lithium layers. The TM migration induces local coordination changes that stabilize the overall structure but lead to irreversible oxygen release. Furthermore, the instability of the highly oxidized active materials results in parasitic reactions with the electrolyte, accelerating degradation.
Existing theories primarily explain the oxygen loss mechanism as occurring during the charging process at high voltages. However, the battery degradation also occurs within seemingly stable voltage ranges. The layered-to rocksalt surface reconstruction, which involves oxygen loss and stoichiometric changes, has been observed in NMC cathodes even within the stable electrochemical window of commercial electrolytes (i.e., < 4.3 V vs. Li/Li⁺), though the origin of such degradation remains unclear.
In this study, for the first time, we reveal a reduction-induced surface oxygen loss mechanism in layered lithium TM oxides, which occurs within a “safe” potential range, well within the electrochemical window of commercial electrolytes. Through a systematic study of NMC cathodes cycled under conditions with various discharge cut-off voltages (DCOVs) and a constant charge cut-off voltage (4.3 V vs. Li/Li⁺), we demonstrate that the degradation of NMC primarily occurs at the particle surface near the end of discharge, particularly below 3.0 V. Density functional theory (DFT) calculations show that reduction-induced surface reconstruction is feasible at potentials between 2.0–3.0 V (vs. Li/Li⁺) in the regions with fewer TM-coordinated oxygen anions. The potential for oxygen loss depends on the local coordination of surface oxygen anions, with higher potentials observed for oxygens bonded with fewer Mn cations.
Additionally, the reduction-induced surface degradation reaction leads to severe electrolyte decomposition at the particle surface, forming an organic-rich cathode-electrolyte interface (CEI) layer and gaseous byproducts. The combined effect of the ion-blocking rocksalt phase and the high-resistance CEI layer significantly increases charge-transfer resistance (R_ct) under cycling with lower DCOVs, resulting in poor cycling performance. Our findings establish a novel surface degradation pathway for commercial electrode materials, providing insight into battery degradation even under seemingly stable conditions.


References
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2) S. Jeon, G. Lim, H. Park, M.K. Cho, Y.E. Lee, J.-K. Yoo, S.-H. Yu, M. Lee, J. Kim*, and J. Hong*, under review.
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Acknowledgments
This study was supported by Korea Institute for Advancement of Technology (KIAT) grant funded by the Korea Government (MOTIE) (RS-2024-00419413, HRD Program for Industrial Innovation).