TY  - THES
AU  - Daniel, Davis Thomas
AU  - Granwehr, Josef
AU  - Pich, Andrij
TI  - Electron paramagnetic resonance spectroscopic investigations of organic radical polymer batteries
PB  - RWTH Aachen
VL  - Dissertation
M1  - FZJ-2025-00535
SP  - pages 1 Online-Ressource : Illustrationen
PY  - 2024
N1  - Dissertation, RWTH Aachen, 2024
AB  - Organic radical polymer batteries (ORBs) represent a viable pathway to a more sustainable energy storage technology compared to conventional Li-ion batteries which utilise metal oxides as cathodes. ORBs feature rapid charging capabilities, long cycle life and can be constructed from non–toxic raw materials. Furthermore, the use of polymers extend the scope of their application to fields which conventional batteries fail to cater to. For further materials and cell development towards competitive energy and power densities, a deeper understanding of redox processes in an ORB is essential. Presently, realisation of high energy densities in ORB is challenged by the need to use large amount of conductive additives to the cathode composite for ensuring sufficient electronic conductivity. Optimisation of cathode composition necessitates the formulation of specific methods and investigative protocols, tailored to ORBs, which can aid in their practical implementation and transition to the next generation of all–organic batteries. In this thesis, a Li-ORB, which consists of lithium as the anode and a poly(2,2,6,6- tetramethyl-1-piperidinyloxy methacrylate) (PTMA)–carbon black (CB) composite as the cathode is investigated. The main technique which is applied is electron paramagnetic resonance (EPR) spectroscopy, which detects unpaired electrons and is therefore particularly amenable to the study of redox processes. Furthermore, EPR results are complemented with nuclear magnetic resonance spectroscopic, electrochemical and theoretical methods. First, individual components of the ORB such as the active material and the conductive additive are studied using routine EPR techniques such as continuous wave (CW) EPR. Then, the interactions between these components such as active material–electrolyte interactions and active material–carbon black contact, are investigated using advanced pulsed EPR techniques. Finally, in operando EPR is used to probe the whole PTMA–ORB cell system. Investigation into the interaction between the active material, TEMPO ((2,2,6,6- tetramethylpiperidin-1-yl)oxyl) methacrylate (PTMA monomer) and the electrolyte revealed that the microenvironment of the radical species differs depending on the solvent used for the electrolyte. In case of linear carbonates, Li nuclei preferentially bind to the radical species while in case of cyclic carbonates, a significantly smaller fraction of lithium bound radical species is found. The findings indicate that ionic transport in ORBs may be crucially influenced by the redox unit and the composition of solvent system used for preparing the electrolyte. In this aspect, EPR hyperfine spectroscopic techniques provide very localised information about Li–ion solvation and its interaction with the redox unit. Two relevant electron spin processes, electron spin exchange and electron spin relaxation are exploited to bridge electrochemical performance with EPR observables. T1 was found to be a good indicator of the electronic contact quality between CB and the active material. A method based on the combination of Laplace inversion and T1 relaxation was used to monitor the contact quality and its dependence on the ratio of CB to active material in the composite samples. Within the same composite sample, different fractions of radicals with varying contact quality with the CB were identified. A hypothesis for the origin of different relaxation modes was devised. It suggests that the electrode composition may locally affect the quality of electronic contact between the active material and CB. The method was also applied to unravel a possible pathway for isolated redox units to participate in the main electron transport mechanism in ORBs i.e. electron hopping. While the isolated redox units were found to have high affinity for carbon black, loss in contact with the conductive additive due to dissolution into the electrolyte was shown by Laplace–inverted pulsed EPR relaxation. CW EPR indicated that the cross–linked PTMA polymers adopt conformations which lead to an increase in radical–radical contacts. In operando EPR further revealed that such conformational changes may be influenced by rate of charging and discharging. At high charge rates, regions of the cathode film were rendered electrochemically inactive which could be made active again at slower charging rates. Electrochemical capacity showed correlation with the radical concentration and, EPR linewidth allowed for a quantification of state of charge dependent electron hopping (or spin exchange) rates. Transient changes in the radical density also caused a variation in the observed g values, indicating that g values can be used as a parameter to validate theoretical simulations of the organic cathode at different states of charge. Density functional theory (DFT) was used to calculate g tensors for radical polymers, which served as training data for a machine learning (ML) based approach to predict g values. The ML based approach was found to be a viable alternative to computationally–demanding DFT calculations, thereby offering a more scalable method to predict g–values for systems with a larger size and complexity.
KW  - DFT (Other)
KW  - EPR (Other)
KW  - NMR (Other)
KW  - batteries (Other)
KW  - electrolyte degradation (Other)
KW  - machine learning (Other)
KW  - organic radical poylmer batteries (Other)
KW  - polymers (Other)
KW  - sustainable energy storage (Other)
LB  - PUB:(DE-HGF)11
DO  - DOI:10.18154/RWTH-2024-05180
UR  - https://juser.fz-juelich.de/record/1037190
ER  -