From lattice to kinetics

Singh, Apeksha (2025): From lattice to kinetics: molecular engineering of covalent organic frameworks for rapid energy storage. Dissertation, LMU München: Fakultät für Chemie und Pharmazie

Abstract

Covalent organic frameworks (COFs) have garnered considerable interest as materials for energy storage systems (ESSs) owning to their robust porous architecture, flexibility in selecting redox-active building blocks, and their well-defined pores to facilitate directional and accelerated charge as well as ion transport. While conventional electrodes are frequently tied to scarce and environmentally damaging mining practices, traditional liquid electrolytes introduce inherent safety hazards, and together, these limitations highlight the promise of next-generation technologies derived from abundant, sustainable resources that ensure cleaner and safer energy storage. COFs unite ultralight architectures with exceptional tunability, porosity, and reliance on earth-abundant elements, placing them among one of the most promising candidates for next-generation sustainable energy storage. This thesis centers on the development of COF-based battery components, and investigation on the influence of their molecular architecture on the electrochemical dynamics and overall rate performance of the battery system: In the first study, two Na-ion quasi-solid-state electrolytes (QSSEs) incorporating anionic COFs, TpPaSO3Na and Tp(PaSO3Na)2, were investigated as solid scaffolds, differing in pore width and sulfonate group density. The ionic liquid, N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide (Pyr13FSI), was incorporated at varying mass fractions as the liquid component, yielding COF-based ionogel composites with impressive thermal stability (~375–431 °C), high ionic conductivities (σi ~10–3 S cm–1), and elevated sodium-ion transference numbers (tNa+ ~ 0.67–0.79). Ab inito molecular dynamics (AIMD) simulations highlight how tailoring nanochannel dimensions and the concentration of anionic moieties along the pore walls governs sodium-ion transport, revealing a synergistic mechanism in which sodium-ions migrate via hopping along the anionic COF backbone while simultaneously undergoing “vehicle-type” transport through the solvated ionic liquid phase. The second project shifts focus towards designing a bipolar 2D COF electrode, engineered for high-rate kinetics to enhance electrochemical performance, and evaluated across diverse Li-ion battery configurations. A novel highly crystalline bipolar-type WTTF-COF, was synthesized by integrating p-type electro-active N,N,N′,N′-tetrakis(4-aminophenyl)-1,4-phenylenediamine (W) and 4,4′,4″,4′″-([2,2'-bi(1,3-dithiolylidene)]-4,4′,5,5′-tetrayl)tetrabenzaldehyde (TTF) molecular building blocks via n-type imine linkages. The combination of electron-rich and electron-deficient redox functionalities, along with π-π interactions between the COF layers, lead to a 12 e– dual-ion redox chemistry per unit cell, corresponding to a high theoretical capacity of 315 mAh g−1. In a Li-ion half-cell configuration, the WTTF electrode delivered an efficient pseudocapacitive dominated charge-storage mechanism, with reversible specific capacities of 271 mAh g−1 at 0.1 A g−1 (~0.3C rate) and 66 mAh g−1 at 5.0 A g−1 (~16C rate) within a stable wide potential window of 0.1–3.6 V versus Li/Li+, storing both Li+ and PF6− anions as charge carriers. Distinct diffusion pathways and diffusion coefficients for Li+ and PF6− transport are resolved through electrochemical impedance spectroscopy (EIS) and theoretical modeling, revealing the intrinsic kinetic advantages of the framework. Furthermore, symmetric all-organic dual-ion full cells demonstrate the bipolar versatility of WTTF-COF, operating effectively as both cathode and anode, with enhanced pseudocapacitive/capacitive charge-storage dynamics, retaining reversibility and stability at scan rates as high as 200 mV s−1. Building upon the insights gained from the first two projects, we further probed the electrochemical kinetics of a novel bipolar PyTTF-COF electrode, focusing on the role of the COF scaffold as well as the influence of electrolyte environment, including ionic composition and concentration. To this end, we designed a 2D imine linked PyTTF-COF, synthesized by integrating a p-type 4,4',4'',4'''-([2,2'-bi(1,3-dithiolylidene)]-4,4',5,5'-tetrayl)tetraaniline (TTF-NH2) building unit, and a novel n-type 7,7',7'',7'''-(pyrene-1,3,6,8-tetrayl)tetrakis(benzo[c][1,2,5]thiadiazole-4-carbaldehyde) (PyBT-CHO) monomer, forming a donor–acceptor (D–A) framework with a low optical band gap of ~1.84 eV, and a total 16 e− dual-ion redox chemistry per unit cell. To investigate the dual-ion redox dynamics of PyTTF-COF, Li-ion half-cells were assembled by employing the PyTTF electrode to systematically probe the role of electrolyte composition. When paired with 1 м LiPF6 or LiTFSI electrolytes, the electrode exhibited a broad operating window of 0.1–3.6 V vs. Li/Li⁺. Strikingly, LiTFSI enabled far superior pseudocapacitive charge-storage kinetics and ion transport compared to LiPF6, as reflected in specific capacities of 286 mAh g−1 and 184 mAh g−1 at 0.3 A g−1 (~1C rate), respectively. Beyond anion identity, concentration effects proved equally decisive; tuning LiTFSI from 1 to 3 м established clear correlations between salt content, ion-storage dynamics, and interfacial charge-transfer resistance, ultimately tailoring the overall redox-behavior of COF-based dual-ion batteries.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Keywords: Covalent organic frameworks, Li-ion batteries, Na-ion batteries
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 540 Chemie
• Fakultäten: Fakultät für Chemie und Pharmazie
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 16. Dezember 2025
• 1. Bericht­erstatter:in: Bein, Thomas