Synthesis, structure and electrochemical performance of poly(ethylenimine)-based electrolytes.
Abstract
Ion conducting polymers have found application as the electrolyte host in lithium batteries and proton exchange membrane fuel cells as a result of desirable processing and adequate ionic conductivity. Poly(ethylenimine) (PEI) is an example of an ion conducting polymer whose potential has not been fully explored as an electrolyte host. Through previously unexplored chemical modifications to the PEI structure and/or addition of ionic conductivity promoting small molecules, the physical properties and ion conduction of PEI-based electrolytes have been improved. Addition of a cyanoethyl (-CH2CH2CN) side chain to the nitrogen of the PEI repeat units resulted in a new polymer called poly((N-cyanoethyl)ethylenimine) (PCEEI). This polymer, when complexed with lithium triflate, has altered ionic conductivity from PEI and poly(acrylonitrile). Infrared spectroscopy has also revealed a significant coordination of lithium ion to the nitrile of the PCEEI side chain in PCEEI:LiTf electrolytes and has been quantified and compared with LiTf speciation data. In another polymer electrolyte system studied, PEI was cross-linked with tetraethylene glycol diacrylate (TEG) and plasticized with diglyme to produce gel-type electrolytes. With dissolved LiTf, high room temperature ionic conductivity was observed in this system (10-4 S/cm). Infrared spectroscopy allowed analysis of lithium ion coordination to the individual gel components and correlation of the ionic conductivity to LiTf speciation. A lithium polymer battery with lithium and LiV3O8 electrodes was designed, and a variety of PEI-based polymers were tested as the electrolyte host including poly(N-methylethylenimine) (PMEI), cross-linked PMEI, PCEEI, cross-linked PCEEI and diglyme infused TEG cross-linked PEI. Good to excellent first discharge capacities were observed. Proton exchange membranes consisting of PEI with a di-aldehyde based cross-linker in the presence of varying compositions of H3PO4 gave membranes with high proton conductivity (10-2 S/cm) at 150° C. The chemical structure of these electrolytes was studied with infrared and nuclear magnetic resonance spectroscopies, and, when viewed in light of the conductivity data, revealed a system whose mechanism of ion conduction changes at different phosphoric acid compositions. Membrane electrode assemblies containing the cross-linked PEI-H3PO 4 membranes were incorporated and tested in a fuel cell and resulted in performance comparable to a commercially available membrane.
Collections
- OU - Dissertations [9305]