Influence of Nanoparticle Morphology on Ion Release and Biological Impact of Nickel Manganese Cobalt Oxide (NMC) Complex Oxide Nanomaterials

Abstract

Lithium intercalation compounds such as nickel manganese cobalt oxides (Li x Ni y Mn z Co1–y–z O2, 0 < x, y, z < 1, or NMCs) are complex transition metal oxides of increasing interest in nanoscale form for applications in electrochemical energy storage and as tunable catalysts. These materials exhibit sheetlike structures that expose low-energy basal planes and higher-energy edge planes in relative amounts that vary with the nanoparticle morphology. Yet there is little understanding of how differences in nanoparticle morphology and exposed crystal planes affect the biological impact of this class of technologically relevant nanomaterials. We investigated how changing nanoparticle morphology from two-dimensional (001)-oriented nanosheets to three-dimensional nanoblocks affects the release of ions and the resulting biological impact using Shewanella oneidensis MR-1 as a model organism. NMC nanoparticles were synthesized in sheetlike morphology and then converted to block morphologies by heating, leading to two morphologies of identical chemical composition that were compared to a commercially available NMC. Ion dissolution studies reveal that NMC nanomaterials release transition metal ions incongruently (Ni > Co > Mn) in amounts that vary with nanoparticle morphology. However, when normalized by the specific surface areas, the rates of release of each transition metal from flakes, blocks, and commercial material were equivalent. Similarly, the impact on S. oneidensis MR-1 was different when using mass-based dosing but was nearly identical using surface-area-normalized exposure dosing. Our results show that even though nanosheets and nanoblocks expose different crystal faces with significantly different surface energies, the rate of ion release is independent of the distribution of crystal faces exposed and depends only on the total surface area exposed. These data suggest that the key protonation steps that control release of transition metals do not depend on the degree of coordination of the initially exposed surface, providing insights into the molecular-level factors that influence the environmental impact of complex metal oxide nanomaterials. Our results have significant implications for establishing methodologies to assess toxicity of reactive nanomaterials.

DFT and Thermodynamics Calculations of Surface Cation Release in LiCoO2

“…To further characterize the bulk structure, we performed an experimental Raman study and a computational vibrational analysis. The Raman spectrum of bulk LCO in the range 400 -700 cm −1 is shown in Figure 3b surface being predominant as shown by experiments 10,16 and DFT calculations. 29 Moreover, first-principles thermodynamics 13 show that the LCO(001) surface exhibits two distinct ter- In order to build structural models for the surface, two base unit cells were used.…”

DFT and thermodynamics calculations of surface cation release in LiCoO2

“…To further characterize the bulk structure, we performed an experimental Raman study and a computational vibrational analysis. The Raman spectrum of bulk LCO in the range 400 -700 cm −1 is shown in Figure 3b Bulk LCO can be cleaved to expose (001), (104), and (110) surfaces, 62,63 with the polar (001) surface being predominant as shown by experiments 10,16 and DFT calculations. 29 Moreover, first-principles thermodynamics 13 show that the LCO(001) surface exhibits two distinct ter- In order to build structural models for the surface, two base unit cells were used.…”

“…The calculations yield insight into how the dissolution process will stop after time, which has been experimentally investigated across a wide range of Li-ion battery cathode material compositions based on LCO and related CMOs. 10,15,16 We now turn to discuss the effect of surface Co-vacancy density on ∆G 1 and ∆G . Table 5 shows ∆G 1 and ∆G values for Co vacancy densities ranging from 25% down to 6.25%.…”

“…10 With no established economic incentive to recycle LCO-based batteries, 11 they can contribute to electronic waste in landfills. 12 When exposed to different environmental conditions, CMO nanomaterials undergo transformations that are only beginning to be explored, [13][14][15][16][17] and which could have as-yet unknown biological impacts. [18][19][20] Therefore, molecular-level understanding of how CMOs transform outside of their operational settings and under variable envrionemntal conditions is critical.…”

“…Such a scheme was recently applied in the case of binary metal oxide materials, 24,25 where high-throughput experimental works were used to inform hazard and O 2 (NMC) 17 will partially dissolve in aqueous media, releasing transition metals such as Co and Ni to solution. The metals released from these CMOs have shown toxicity effects in Shewanella oneidensis MR-1 bacteria, 10,16 the eukaryotic water flea Daphnia magna, 26 and have also been shown to potentially impact the lipid ordering in model cell membranes. 27,28 The mechanism for the release of metals from NMC has been studied with experimental and density functional theory (DFT) methods.…”