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

Abstract

Lithium nickel manganese cobalt oxides (NMCs) are promising cathode materials for high-performance lithium-ion batteries. Although these materials are commonly cycled within mild voltage windows (up to 4.3 V vs Li/Li + ), operation at high voltages (>4.7 V vs Li/Li + ) to access additional capacity is generally avoided due to severe interfacial and chemomechanical degradation. At these high potentials, NMC degradation is caused by exacerbated electrolyte decomposition reactions and non-uniform buildup of chemomechanical strains that result in particle fracture. By applying a conformal graphene coating on the surface of NMC primary particles, we find significant enhancements in the high-voltage cycle life and Coulombic efficiency upon electrochemical cycling. Postmortem X-ray diffraction, X-ray photoelectron spectroscopy, and electron microscopy suggest that the graphene coating mitigates electrolyte decomposition reactions and reduces particle fracture and electrochemical creep. We propose a relationship between the spatial uniformity of lithium flux and particle-level mechanical degradation and show that a conformal graphene coating is well-suited to address these issues. 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.

Characterizing and Mitigating Chemomechanical Degradation in High-Energy Lithium-Ion Battery Cathode Materials

“…(h,i) High-voltage cycling promotes (h) electrochemical creep and particle fracture after high-voltage cycling for primary particles of NCM-523 (scale bar is 2 μm) and (i) intergranular fracture in secondary particles of LiNi 0.8 Co 0.15 Al 0.05 O 2 (NCA). Reproduced with permission from refs and , respectively. Copyright 2021 American Chemical Society and 2018 Royal Society of Chemistry, respectively.…”

“…The extent of chemomechanical degradation is also tied to the operating voltage of the cell. High SOCs can induce severe volumetric changes to the unit cell of layered oxides, which exacerbates the stress buildup that precedes particle fracture in both primary (Figure h) and secondary particles (Figure i). , Moreover, at high SOCs, the partially filled Li layers present a lower barrier to TM slab sliding, suggesting that electrochemical creep and secondary particle fracture may become more facile when materials are overcharged.…”

“…(e) In comparison to electrodes that rely on nonuniform distributions of carbon black particles for charge compensation, conductive particle coatings, such as graphene, can reduce chemomechanical degradation by delocalizing cycling-induced strain. Reproduced with permission from ref . Copyright 2021 American Chemical Society.…”

“…Conformal graphene coatings are a notable example since they act as barrier layers that reduce the accumulation of electrolyte decomposition products while also supplying electrons for charge transfer reactions uniformly across the surface of primary particles (Figure e). In contrast to aggregates of carbon black particles, which may be nonuniformly distributed on active material surfaces, conformal electron conduction paths delocalize SOC heterogeneity within primary particles, leading to reduced particle cracking and electrochemical creep …”

Reduction of Capacity Fading in High-Voltage NMC Batteries with the Addition of Reduced Graphene Oxide

“…Furthermore, lithium extraction causes compositional heterogeneity (or spatial gradients) within the active material particle, which is a consequence of the non-uniform distributions of the interfacial chemical products, surface phases, and reaction sites. Spatial gradients in the lithium concentration induce internal strains, which are manifested as particle cracking and intergranular fracture [ 15 ]. Unreacted lithium ingredients, such as the oxide form of Li O, are sometimes located on the surfaces of the active materials during both low- and high-voltage operation.…”