Elucidating and Mitigating High-Voltage Interfacial Chemomechanical Degradation of Nickel-Rich Lithium-Ion Battery Cathodes Via Conformal Graphene Coating

Abstract

Layered-type, nickel-rich lithium nickel manganese cobalt oxides (NMCs) are well-established cathode materials for high-performance lithium-ion batteries. These materials exhibit stable cycling within mild voltage windows (up to 4.3 V vs. Li/Li+) but can be charged to higher potentials (4.8 V vs. Li/Li+) to access additional capacity. However, high-voltage operation is generally avoided due to severe interfacial and chemomechanical degradation, such as electrolyte decomposition reactions and non-uniform buildup of chemomechanical strains that can result in particle fracture and compromised cyclability.In this work, we find that a conformal graphene coating enables significant enhancements in the cycle life and coulombic efficiency of NMC cathodes during high voltage electrochemical cycling. Postmortem surface chemical analysis reveals that graphene-coated NMC electrodes exhibit reduced spectral intensities corresponding to electrolyte decomposition products, suggesting that the interfacial graphene layer limits parasitic electrode-electrolyte interactions. Furthermore, global and local structural analysis shows that the graphene coating mitigates mechanical degradation, which was evidenced by reduced microstrain, particle fracture, and electrochemical creep. Based on these observations, we propose a mechanistic relationship between the spatial uniformity of lithium flux and primary particle-level mechanical degradation, and show that a conformal graphene coating is well-suited to address these issues associated with high voltage chemomechanical degradation. Overall, these results delineate a pathway for rationally mitigating high-voltage chemomechanical degradation of nickel-rich cathodes that can be applied to existing and emerging classes of battery materials.https://doi.org/10.1021/acsaem.1c01995