First-Principles and Thermodynamics Study of Compositionally Tuned Complex Metal Oxides: Cation Release from the (001) Surface of Mn-Rich Lithium Nickel Manganese Cobalt Oxide

Abstract

Compositional tuning of nanoscale complex metal oxides (CMOs) can lead to enhanced performance and favorable properties for a variety of energy-related applications. However, investigations of the nanoscale CMOs used in energy storage technologies demonstrate that these nanomaterials may have an adverse biological impact, highlighting a fundamental knowledge gap between nanomaterial design and the structure and properties at the end of life. CMO nanomaterials can enter the environment due to improper disposal, where they undergo subsequent (as of yet poorly understood) nanoscale transformations that may affect biological response and, ultimately, environmental fate. This points to the need for studies at the nano-bio interface that can be used to shape rules for the redesign of CMOs: materials that are are potentially more benign by design and serve as examples of sustainable nanotechnology. The example given here is to enrich lithium nickel manganese cobalt oxide, Liₓ(NiᵧMn₂Co₁₋ᵧ₋₂)O₂ (NMC), with Mn to create a family of materials that are less expensive and potentially less toxic to a wide range of organisms. In this paper, we investigate the structure and electronic states of Mn-rich NMC at the density functional theory (DFT) level to elucidate the interplay of redox properties, oxidation state, and coordination environment of a compositionally tuned CMO. We find that the oxidation states of Ni and Co remain mostly unaffected while Mn exists as both Mn²⁺ and Mn⁴⁺. Our models show that the ratio of Mn²⁺ and Mn⁴⁺ varies with changes in the coordination environment, such as the identity of neighboring atoms and surface OH group coverage. The surface metal release properties of Mn-rich NMC compositions are predicted using a DFT + solvent ion model and show that Mn-rich NMC compositions are inherently more prone to dissolution than NMC and that this is attributed to the changes in oxidation state of the transition metals in Mn-rich NMC.