Block Copolymer Electrolytes: Thermodynamics, Ion Transport, and Use in Solid-State Lithium/Sulfur Cells
Nanostructured block copolymer electrolytes containing an ion-conducting block and a modulus-strengthening block are of interest for applications in solid-state lithium metal batteries. These materials can self-assemble into well-defined microstructures, creating conducting channels that facilitate ion transport. The overall objective of this dissertation is to gain a better understanding of the behavior of salt-containing block copolymers, and evaluate their potential for use in solid-state lithium/sulfur batteries. Anionically synthesized polystyrene-b-poly(ethylene oxide) (SEO) copolymers doped with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt were used as a model system. This thesis investigates the model system on several levels: from fundamental thermodynamic studies to bulk characterization and finally device assembly and testing.
First, the thermodynamics of neat and salt-containing block copolymers was studied. The addition of salt to these materials is necessary to make them conductive, however even small amounts of salt can have significant effects on their phase behavior, and consequently their ion-transport and mechanical properties. As a result, the effect of salt addition on block copolymer thermodynamics has been the subject of significant interest over the last decade. A comprehensive study of the thermodynamics of block copolymer/salt mixtures over a wide range of molecular weights, compositions, salt concentrations and temperatures was conducted. The Flory-Huggins interaction parameter was determined by fitting small angle X-ray scattering data of disordered systems to predictions based on the random phase approximation (RPA). Experiments on neat block copolymers revealed that the Flory-Huggins parameter is a strong function of chain length. Experiments on block copolymer/salt mixtures revealed a highly non-linear dependence of the Flory-Huggins parameter on salt concentration. These findings are a significant departure from previous results, and indicate the need for improved theories for describing thermodynamic interactions in neat and salt-containing block copolymers.
Next, the effect of molecular weight on ion transport in both homopolymer and copolymer electrolytes were studied over a wide range of chain lengths. Homopolymer electrolytes show an inverse relationship between conductivity and chain length, with a plateau in the infinite molecular weight limit. This is due to the presence of two mechanisms of ion conduction in homopolymers; the first mechanism is a result of the segmental motion of the chains surrounding the salt ions, creating a liquid-like environment around the ion while the second mechanism of ion conduction is attributed to diffusion of the entire polymer chain with coordinated ions.
Equilibrated block copolymer electrolytes exhibit a non-monotonic dependence on molecular weight, decreasing with increasing molecular weight in the small molecular weight limit before increasing when molecular weight exceeds about 10 kg mol-1
The effect of morphology on ion transport was studied by conducting simultaneous impedance and X-ray scattering experiments as the block copolymer electrolyte transitioned from an ordered lamellar structure to a disordered phase. The ionic conductivity increased discontinuously through the transition from order to disorder. A simple framework for quantifying the magnitude of the discontinuity was presented.
Finally, block copolymer electrolytes were examined specifically for use in high energy density solid state lithium/sulfur batteries. Such materials have been shown to form a stable interface with lithium metal anodes, maintain intimate contact upon cycling, and have sufficiently high shear moduli to retard dendrite formation. Having previously satisfied the concerns associated with the lithium metal anode, the compatibility of the sulfur cathode was explored. The sulfur cathode presents many unique challenges, including the generation of soluble lithium polysulfides (Li2Sx, 2 ≤ x ≤ 8) during discharge. The solubility of such species in block copolymers and their effect on morphology was examined. The lithium polysulfides were found to exhibit similar solubility in the block copolymers as in typical organic electrolytes, however induced unusual and unexpected phase behavior in the block copolymers.
Inspired by successful efforts to physically confine the soluble lithium polysulfides via nanostructured carbon-sulfur composites in the cathode, our nanostructured block copolymer electrolytes were employed in full electrochemical cells with a lithium metal anode and sulfur cathode. Different cathode compositions, electrolyte additives, and cell architectures were tested. Surprisingly, the polysulfides diffused readily from the cathode through the block copolymer electrolyte, and the normally robust SEO|Li metal interface was detrimentally affected their presence during cycling. The polysulfides appeared to change the mechanical properties of the electrolyte such that intimate contact with the lithium metal was lost. Several promising strategies to overcome this problem were investigated and offer exciting avenues for improvement for future researchers.