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The transformations of complex metal oxides in aqueous settings must be studied to form a chemical understanding of how technologically relevant nanomaterials impact the environment upon disposal. Owing to the inherent heterogeneity and structural complexity of the ternary intercalation material Li(NixMnyCo1-x-y)O2 (NMC), the mechanisms of chemical processes at the solid–water interface are challenging to model. Here, density functional theory (DFT) + solvent ion methodology is used to study the energetics of stepwise release of two surface metals following unique pathways. The study spans different combinations of metal removal and also considers unique patterns of defects formed by modeling the NMC surface in supercells. The approach here also considers the equilibration of the surface with the surroundings between successive metal removals. A key finding is that a second metal removal prefers to proceed at a metal lattice site adjacent to the initial defect, and this is attributed in part to how the resulting slab with two metal vacancies maintains the most antiferromagnetic couplings between the remaining Ni/Mn. 

Nanoscale complex metal oxides have transformed how technology is used around the world. A ubiquitous example is the class of electroreactive cathodes used in Li-ion batteries, found in portable electronics and electric cars. Lack of recycling infrastructure and financial drivers contribute to improper disposal, and ultimately, introduction of these materials into the environment. Outside of sealed operational conditions, it has been demonstrated that complex metal oxides can transform in the environment, and cause negative biological impact through leaching of cations into aqueous phases. Using a combined DFT and thermodynamics methodology, insights into the mechanism and driving forces of cation release can be studied at the molecular-level. Here, we describe design principles that can be drawn from previous collaborative research on complex metal oxide dissolution of the Li(Ni y Mn z Co 1−y−z )O 2 family of materials, and go on to posit ternary complex metal oxides in the delafossite structure type with controlled release behavior. Using equistoichiometric formulations in the delfossite structure, we use DFT and thermodynamics to model cation release. The release trends are discussed in terms of lattice stability, solution chemistry/solubility limits, and electronic/magnetic properties. Intercalation voltages are calculated and discussed as a predictive metric for potential functionality of the model materials. 

Lithium intercalation compounds, such as the complex metal oxide, lithium nickel manganese cobalt oxide (LiNi x Mn y Co 1−x−y O 2 , herein referred to as NMC), have demonstrated their utility as energy storage materials. In response to recent concerns about the global supply of cobalt, industrially synthesized NMCs are shifting toward using NMC compositions with enriched nickel content. However, nickel is one of the more toxic components of NMC materials, meriting investigation of the toxicity of these materials on environmentally relevant organisms. Herein, the toxicity of both nanoscale and microscale Ni-enriched NMCs to the bacterium, Shewanella oneidensis MR-1, and the zooplankton, Daphnia magna , was assessed. Unexpectedly, for the bacteria, all NMC materials exhibited similar toxicity when used at equal surface area-based doses, despite the different nickel content in each. Material dissolution to toxic species, namely nickel and cobalt ions, was therefore modelled using a combined density functional theory and thermodynamics approach, which showed an increase in material stability due to the Ni-enriched material containing nickel with an oxidation state >2. The increased stability of this material means that similar dissolution is expected between Ni-enriched NMC and equistoichiometric NMC, which is what was found in experiments. For S. oneidensis , the toxicity of the released ions recapitulated toxicity of NMC nanoparticles. For D. magna , nickel enrichment increased the observed toxicity of NMC, but this toxicity was not due to ion release. Association of the NMC was observed with both S. oneidensis and D. magna. This work demonstrates that for organisms where the major mode of toxicity is based on ion release, including more nickel in NMC does not impact toxicity due to increased particle stability; however, for organisms where the core composition dictates the toxicity, including more nickel in the redesign strategy may lead to greater toxicity due to nanoparticle-specific impacts on the organism. 

We describe a strategy of integrating quantum mechanical (QM), hybrid quantum mechanical/molecular mechanical (QM/MM) and MM simulations to analyze the physical properties of a solid/water interface. This protocol involves using a correlated ab initio (CCSD(T)) method to first calibrate Density Functional Theory (DFT) as the QM approach, which is then used in QM/MM simulations to compute relevant free energy quantities at the solid/water interface using a mean-field approximation of Yang et al. that decouples QM and MM thermal fluctuations; gas-phase QM/MM and periodic DFT calculations are used to determine the proper QM size in the QM/MM simulations. Finally, the QM/MM free energy results are compared with those obtained from MM simulations to directly calibrate the force field model for the solid/water interface. This protocol is illustrated by examining the orientations of an alkyl amine ligand at the gold/water interface, since the ligand conformation is expected to impact the chemical properties ( e.g. , charge) of the solid surface. DFT/MM and MM simulations using the INTERFACE force field lead to consistent results, suggesting that the effective gold/ligand interactions can be adequately described by a van der Waals model, while electrostatic and induction effects are largely quenched by solvation. The observed differences among periodic DFT, QM/MM and MM simulations, nevertheless, suggest that explicitly including electronic polarization and potentially charge transfer in the MM model can be important to the quantitative accuracy. The strategy of integrating multiple computational methods to cross-validate each other for complex interfaces is applicable to many problems that involve both inorganic/metallic and organic/biomolecular components, such as functionalized nanoparticles.