Ab initio study of effects of interstitial species on electrochemical and mechanical properties of prussian blue analogues

Prussian blue analogue (PBA) materials have found extensive use as Faradaic electrodes for both ion separations and energy storage during the past decade. The micro-porous framework lattice of PBA allows intercalation and deintercalation of cations by reducing and oxidizing redox-active centers on the lattices, respectively. PBAs received extra attention from research communities due to their ability to intercalate cations of various types and their superior rate capability. However, the interactions between interstitial species and framework lattice and their effects on intercalation reaction kinetics are understood mainly at a phenomenological level. In this study, we investigate electronic and mechanical interactions between framework lattice and interstitial species using theoretical methods that combine ab initio calculation, ensemble theory, and machine learning models. Accordingly, the first part of this thesis introduces the details of employed methods in later chapters. In the second part, we consider an anhydrous PBA lattice in which cation-cation interactions are captured by density functional theory (DFT) calculations. Based on the energy difference between lattices with various cation ordering within body-centered sites, we identified two classes of cation ordering, vacancy-pair free configurations (VPFCs) and vacancy-pair configurations (VPCs), with the former being energetically more stable than the latter. We incorporate the knowledge of degeneracy of VPFCs with a grand canonical ensemble (GCE) theory to give more accurate predictions of the equilibrium potential of nickel hexacyanoferrate (NiHCF) electrode than previous theories. Because cation ordering can be disturbed by movements of interstitial water molecules, in the third part, we include interstitial water molecules in our ab initio calculations by sampling molecular ordering and orientations. The results are transformed into training datasets for XGBoost machine learning models, which efficiently learn the configurational energy landscape of hydrated PBA lattices. By quantifying the accuracy of the XGBoost model with many-particle atomic features, we show that lattice-interstitials interactions are cation-specific, and the arrangement of interstitial species depends on two factors: bare ionic size and cations' hydrophilicity. The last part examines the effects of interstitial species on the electronic and mechanical properties of hydrated PBA lattices. We use grand potential analysis to predict equilibrium potentials for NiHCF electrode intercalating Na-, K-, and Cs-ions at various concentrations of solute and temperature. The results agree well with experiments and imply possibilities of fine-tuning ion-selectivity of PBA electrodes. By investigating noncolinear band structures of ground states of hydrated lattices, we show that interstitial water reduces bandgap and raises Fermi level to induce a semiconductor/metal transition. Finally, we perform density functional perturbation theory (DFPT) calculations to find that firm cation-ligand coordination causes the concurrent synchronous lattice vibration.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Mechanical Engineering

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2022

Author dc:creator

• Liu, Sizhe

Contributors dc:contributor

• Smith, Kyle
• Aluru, Narayana
• Ertekin, Elif
• Schleife, André

• Faradaic deionization
• ion separation
• Prussian blue analogue
• first-principle calculation
• XGBoost
• grand potential
• phonon
• noncolinear magnetism

Statement dc:rights

• Copyright 2021 Sizhe Liu

Language dc:language

en

Handle dc:identifier

http://hdl.handle.net/2142/113310

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F113310