Strategies for Overcoming Ionic Transport Limitations in Polymer Electrolytes
Creators: name: Cazorla, Sebastian Ignacio Calderon
Publisher: name: Purdue University Graduate School
Year: 2024
Subjects: classificationCode: 340307
subject: Structure and dynamics of materials
classificationCode: 340301
subject: Inorganic materials (incl. nanomaterials)
classificationCode: 340210
subject: Solid state chemistry
classificationCode: 340503
subject: Organic chemical synthesis
classificationCode: 340604
subject: Electrochemistry
classificationCode: 340609
subject: Transport properties and non-equilibrium processes
classificationCode: 401602
subject: Composite and hybrid materials
classificationCode: 401609
subject: Polymers and plastics
classificationCode: 401601
subject: Ceramics

description: Solid-polymer electrolytes constitute an attractive alternative to flammable liquid electrolytes, but their low ionic conductivity (σ) and transference number (<i>t</i><sub>+</sub>) are not sufficient to replace current liquid electrolytes. In turn, the rational design of new materials is of ultimate importance to overcome the main limitation to high ionic conductivity: the close relationship between ion transport and polymer segmental relaxation. On one hand, the strategy to overcome such issue is designing new composite polymer electrolytes (CPEs) where ceramic particles can modify the properties of the polymer host by increasing the amorphous fraction, enhance the dissociation of salts, hinder the diffusion of anions, and/or create new Li<sup>+</sup> conduction pathways at the interface ceramic/polymer. One of the main obstacles to achieving higher performance is the limited understanding of transport mechanism and the effect of ceramic filler on the physical properties, ion transport, and interactions with the CPE constituent materials. The dielectric properties of polymers play a critical role in the ability of the polymer to dissolve salts and mediate the electrostatic interactions between the cations and the polymer chain. To further study the effect of the CPE dielectric constant and its impact on ionic conductivity, in this thesis the effect on incorporating High Entropy Oxides (HEO) that possess colossal permittivity into PEO/LiTFSI matrixes is reported. The results show that particles of 700 nm average diameter yield ionic conductivities &gt; 10<sup>‑4</sup> S cm<sup>‑1</sup>. Measurements of the complex dielectric function reveal an increase in the rate of relaxation of the ion-coupled chain dynamics. This is in line with the reduced Tg observed in DSC analysis. DSC also reveals no significant change in the degree of crystallinity and results based on FTIR do not indicate a significant dissociation of Li-salt compared to the PEO-based SPE. Finally, the addition of these high dielectric constant fillers of smaller size produces a radical change in the polymer microstructure because of their integration with the polymer matrix. In summary, these results suggest that the improvement in IC is likely due to the formation of efficient Li-pathways involving fast-moving amorphous polymer. Further studies are needed to determine the effect of the HEO fillers on the bonding interactions between the Li cations and the oxygen groups of the polymer. An additional strategy to overcome limitations in ion-transport of polymer electrolytes was pursued in this work through the design of new polymer structures. Single ion-conducting polymer electrolytes (SICPEs) can restrict the diffusion of anions which is responsible for the development of polarization gradients in rechargeable batteries, under high charge/discharge conditions. The design of poly (Li-FAST-<i>alt</i>-DEG) is intended to regulate other aspects such as Li<sup>+</sup> concentration and free volume of the polymer. Whereas the synthesis of oligomers was successfully accomplished, challenges in the synthetic process hindered the fabrication of the polymer.