Enhancing nanostructured nickel-rich lithium-ion battery cathodes via surface stabilization

Abstract

Layered, nickel-rich lithium transition metal oxides have emerged as leading candidates for lithium-ion battery (LIB) cathode materials. High-performance applications for nickel-rich cathodes, such as electric vehicles and grid-level energy storage, demand electrodes that deliver high power without compromising cell lifetimes or impedance. Nanoparticle-based nickel-rich cathodes seemingly present a solution to this challenge due to shorter lithium-ion diffusion lengths compared to incumbent micrometer-scale active material particles. However, since smaller particle sizes imply that surface effects become increasingly important, particle surface chemistry must be well characterized and controlled to achieve robust electrochemical properties. Moreover, residual surface impurities can disrupt commonly used carbon coating schemes, which result in compromised cell performance. Using x-ray photoelectron spectroscopy, here we present a detailed characterization of the surface chemistry of LiNi0.8Al0.15Co0.05O2 (NCA) nanoparticles, ultimately identifying surface impurities that limit LIB performance. With this chemical insight, annealing procedures are developed that minimize these surface impurities, thus improving electrochemical properties and enabling conformal graphene coatings that reduce cell impedance, maximize electrode packing density, and enhance cell lifetime fourfold. Overall, this work demonstrates that controlling and stabilizing surface chemistry enables the full potential of nanostructured nickel-rich cathodes to be realized in high-performance LIB technology.

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

“…Following synthesis, the NMC particles were uniformly coated with graphene and ethyl cellulose (GrEC) using a previously established solution-phase method. 25,27 The coating conformality was confirmed using SEM and transmission electron microscopy (TEM) (Figure 1a−c), which revealed the presence of a percolating network of graphene flakes throughout the electrode, consistent with prior work. 25,27,28 After fabricating electrodes with the GrEC powder (NMC-GrEC), the electrode was heated to pyrolyze the ethyl cellulose (EC) polymer, which largely volatilizes the EC, compacts the electrode, and generates a carbon residue that helps form a percolating, electrically conductive network among the graphene-coated NMC particles.…”

“…25,27 The coating conformality was confirmed using SEM and transmission electron microscopy (TEM) (Figure 1a−c), which revealed the presence of a percolating network of graphene flakes throughout the electrode, consistent with prior work. 25,27,28 After fabricating electrodes with the GrEC powder (NMC-GrEC), the electrode was heated to pyrolyze the ethyl cellulose (EC) polymer, which largely volatilizes the EC, compacts the electrode, and generates a carbon residue that helps form a percolating, electrically conductive network among the graphene-coated NMC particles. 24,25,27−30 Subsequent Raman spectroscopy of the electrode confirmed that the ratio of the D and G peaks decreased (at 1350 and 1580 cm −1 , respectively) (Figure S3), suggesting that EC decomposition resulted in a more graphitic carbon network that facilitates efficient charge transfer.…”

“…38 In contrast, the C 1s spectrum for the pristine NMC-GrEC electrode surface is dominated by the C−C bonding character, consistent with previous reports for pyrolyzed graphene-ethyl cellulose films. 25,29,39,40 Similarly, the Mn 3p and the Co 3p peaks are initially prominent in the Li 1s XPS spectra for both pristine samples but significantly decrease in intensity after cycling (Figure S10). These trends suggest that an organic-rich SEI layer formed during cycling that attenuated the transition metaloxygen signal from the NMC bulk (Figure 3), consistent with the galvanostatic cycling and the EIS results.…”

“…Consequently, no film delamination was observed during casting and drying, which was consistent with previous reports. 25,27 The average active material loadings for both the NMC-CBPVDF and the NMC-GrEC electrodes were maintained at ∼3 mg cm −2 . Electrode discs were punched out and calendered with approximately 6 MPa applied pressure.…”

“…Here, we demonstrate a strategy to enhance the high-voltage cycle life and Coulombic efficiency of NMC cathodes via a conformal, conductive graphene coating. Although graphene coatings have been widely employed to improve the cycle life of Ni-rich cathode materials, most reports explore the effect of graphene coatings under mild operating voltage windows that avoid severe degradation mechanisms. − In this work, we use a high upper cutoff voltage not only to accelerate interfacial degradation, including electrolyte decomposition reactions, but also to drive chemomechanical degradation at high SOCs, such as particle fracture or electrochemical creep. Additionally, by utilizing submicron primary particles in this study, we circumvent issues of strain buildup within large secondary particle structures and ensure that exposed NMC surfaces can be uniformly coated with graphene.…”