458651 Theoretical Consideration of Nanostructured Magnetite As an Electrode Material for Li-Ion Energy Storage
Thursday, November 17, 2016: 2:12 PM
Golden Gate 5 (Hilton San Francisco Union Square)
Christianna N. Lininger1, Mark S. Hybertsen2 and Alan C. West1, (1)Chemical Engineering, Columbia University, New York, NY, (2)Center for Functional Nanomaterials, Brookhaven National Lab, Upton, NY
Theoretical Consideration of Nanostructured Magnetite as an Electrode Material for Li-ion Energy Storage C. N. Liningera, M. S. Hybertsenb, A. C. Westa aDepartment of Chemical Engineering, Columbia University, New York, NY 10027, USA
bCenter for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY 11973, USA
The electrode material in a Li-ion battery must allow for reversible intercalation of lithium, and there are a range of materials capable of this behavior. Magnetite, Fe3O4, is a low cost, environmentally inert, iron oxide that allows for intercalation into its crystal lattice. Upon full reduction, magnetite has a high theoretical capacity against Li/Li+ at 926 mAh/g as opposed to the current commercial cathode materials operating in the 100-300 mAh/g range.1–3 Due to its low cost, low environmental impact, high theoretical capacity, and well defined nanoscale synthesis method, magnetite is a model material to push the state of the art in lithium-ion batteries.5
Magnetite is an inverse spinel transition metal oxide. The iron in magnetite takes on two valence states, Fe2+ and Fe3+, spread over two coordination environments, tetrahedral 8a and octahedral 16d. The tetrahedral iron is all Fe3+, while the octahedral iron centers are comprised of half Fe2+ and half Fe3+.7–9 The oxygen in magnetite forms a robust close packed cubic array at 32e sites, which was recently found to act as an invariant scaffold around which structural rearrangements occur during lithiation.10 Due to the multivalent irons in magnetite, it is capable of accepting multiple electrons per iron during discharge. Multiple-electron transfer (MET) materials are of great interest to the battery community as the ability of a single cation center to accept multiple electrons greatly increases energy density.10 However, as a result of facilitating multiple electrons, the metal center goes through oxidation state changes and structural rearrangements occur within the material. Phase transformations can be irreversible and are known to affect cyclability and limit the current rates that conversion-materials can be operated under.10Due to the complexity of MET materials, there exists a need for fundamental research into the electronic and ionic transport processes that are occurring in these MET materials during discharge.
Magnetite can be synthesized as a nanomaterial, with spherical nanoparticles ranging from 6-12 nm in a low-temperature synthetic method that does not involve extraneous synthetic processes.4,5 Upon manufacture into an electrode, the magnetite nanoparticles are observed to agglomerate into micron-sized agglomerates.6 We have developed continuum-level models for the electrochemical performance of this material including both nanoscale and agglomerate-scale physics. Neither exclusively nanoscale or agglomerate-scale simulations were able to accurately capture the experimentally observed electrochemical performance of magnetite.6,11In the agglomerate, ionic transport occurs through nanoporous pathways in-between the aggregated nanoparticles. On this length scale, the size of the diffusing species and the pores through which the diffusion occurs are both in the nano regime. On the nanoscale, solid-state diffusion occurs as lithium inserts into the crystalline magnetite. Depending on concentration, this causes structural rearrangements. The crystalline diffusivity can vary by orders of magnitude as a function of the state of discharge, due to these structural rearrangements that occur during lithiation.
We have used density functional theory to explore structural rearrangements that occur in magnetite during lithiation between zero and one equivalents. We have established that structural rearrangements include the movement of tetrahedral iron to an octahedral coordination environment at concentrations as low as half an equivalent of lithium. We have also established that at this concentration, the lithium has lower energy in octahedral coordinated sites as compared to tetrahedral coordination. While evidence pointed to such changes in coordination, in all previous literature, the lithium concentration at which it occurred had not been established.
Additionally, we are using DFT to explore i) the diffusivity of lithium in crystalline solid magnetite and ii) the lithium open circuit potential in the dilute concentration limit. The use of DFT to understand the kinetic resistances to the structural rearrangements that we have shown occur at low levels of lithiation will give insights into relaxation times observed for this material. Both the DFT determined solid-state diffusivity and the theoretically predicted open-circuit potential will aid in informing the limiting ionic and electronic processes occurring in magnetite for its uses as an electrode material in lithium-ion batteries.
This work was supported as part of the Center for Mesoscale Transport Properties, an Energy Frontier Research Center supported by the U. S. Department of Energy, Office of Science, Basic Energy Sciences, under award #DE-SC0012673. C. N. L. acknowledges the support of the National Science Foundation Graduate Research Fellowship under Grant No. DGE-11-44155. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. This research used resources of the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704. The Yeti Shared HPC Cluster at Columbia University was also used, which includes support from Empire State Development's Division of Science, Technology, and Innovation under contract number C090171.
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