Polystyrene-b-polyethylene oxide (PS-b-PEO) nanostructured polymer electrolytes: Synthesis and electrochemical characterization

Considerable research has been dedicated to the development of a solid, solvent-free electrolyte for battery cells. The advantages of a solid polymer electrolyte over current liquid systems include lower environmental, health, and safety hazards, lower predicted material and processing costs, and greater freedom in battery configurations [1], which are crucial for electric-vehicle applications. For ionic conduction in a cell, the polymer electrolyte has to promote the dissociation and solvation of an ionic salt. Current research employs lithium salts due to their high mobility and solubility in polymeric hosts such as polyethylene oxide (PEO) [2], [3]. Early work by Michel Armand on the use of polymers in battery applications demonstrated that only the amorphous domain of PEO showed appreciable ionic mobility [4], [5]. Furthermore, for practical applications, it is desirable that the battery system operate at ambient temperature. However, PEO crystallizes at temperatures below 65°C, and its low mobility in the crystalline phase results in unacceptable ionic conductivity values.; In addition to high ionic conductivity at room temperature, it is desirable to have high modulus solids as electrolytes. Low modulus promotes the rapid deterioration of cell performance in PEO-containing lithium batteries due to the growth of Li dendrites. Theoretical simulations have shown that increasing the electrolyte modulus results in suppressed dendrite growth [6]. Consequently, two major problems associated with PEO electrolytes in rechargeable lithium batteries are its inability to satisfy simultaneously the criteria of high ionic conductivity and high modulus requirements.; The goal of this work is to develop and study the behavior of a solid-state electrolyte by combining several desirable attributes into the design of a diblock copolymer system by utilizing its phase separation into microdomains. It is desired that the material satisfy both the high ionic conductivity and dimensional stability requirements at operating temperature and have higher efficiency than current battery cells. The chosen system is a polystyrene-b-polyethylene oxide (PS-b-PEO) diblock copolymer in which PEO exists as the minor phase in a polystyrene matrix. The role of the polystyrene is to maintain the mechanical integrity of the electrolyte while polyethylene oxide serves as the conducting phase. To achieve the goals of this project, polymer synthesis, necessary physical characterization, preparation of electrolyte samples, and AC impedance spectroscopy measurements were carried out.; This dissertation presents research results conducted on PS-b-PEO diblock copolymer electrolytes. Synthesis of the diblock systems was done using high-vacuum anionic polymerization while gel permeation chromatography (GPC) and nuclear magnetic resonance (NMR) were used to determine the molecular weight and volume fraction of each block. After synthesis, polymer-electrolyte samples were made by doping the polymer samples with lithium bis(trifluoromethylsulfonyl)imide, Li[N(SO2CF3)2], with varying EO/Li ratios.; The freeze-dried polymer/salt mixtures were then pressed into Teflon disks with diameter of 4.8 mm and thickness of 1.0 mm. Electrochemical measurements were made on the pellets using AC impedance spectroscopy to determine the ionic conductivity of the samples at temperatures ranging from 60 to 120°C. Transmission electron microscopy (TEM) and rheology experiments were conducted by Mr. Gomez and Dr. Singh, respectively. TEM results indicate a weakly ordered lamella-phase morphology, while AC impedance spectroscopy reveals significant ionic conductivity of 10-4 S/cm at temperatures above 90°C, while the samples maintained an elastic modulus of 0.1 GPa.; The results presented here show that between 80 and 120°C, our PS-b-PEO polymer electrolytes exhibit ionic conductivity values in the range of theoretically predicted values without any special processing. The elec...