Polymer Electrolytes and the Limiting Current
Lightweight, safe energy storage options are a critical tool in enabling implementation of renewable energy options. Typical lithium-ion batteries use a liquid electrolyte composed of ethylene carbonate (EC), dimethyl carbonate (DMC), and a lithium salt, lithium hexafluorophosphate (LiPF6). These batteries use graphite anodes, of which a large fraction is an electrochemically inactive support. One important direction in the energy storage field is to replace graphite anodes with solid lithium metal anodes to improve the energy density of the battery. However, lithium metal is incompatible with common electrolytes, and it is limited by its propensity for dendrite growth, which can lead to cell failure. Therefore, we are in search of an electrolyte that is safe against lithium metal and can prevent dendrite growth. We focus on polymer electrolytes, especially those based on poly(ethylene oxide) (PEO), including block copolymer electrolytes composed of PEO and polystyrene (PS), which microphase separate into PEO-rich ion-conducting regions and PS-rich mechanically-rigid regions that suppress dendrite growth.
In this Dissertation, a variety of techniques are used to characterize various electrolyte options, from liquids (Chapter 5), to homopolymers, which contain only one type of monomer (Chapter 6), to block copolymers, which contain two (Chapters 7-10). Chapter 1 provides a general introduction to polymers, polymer electrolytes, and characterization techniques. Chapters 2¬¬–4 detail the techniques used in this Dissertation. Nuclear magnetic resonance (NMR) spectroscopy (Chapter 2) probes the local environments of nuclei and can shed light on microphase separation in block copolymers, and pulsed-field gradient NMR measures the self-diffusion of nuclei, which is relevant to ion transport. Small angle X-ray scattering (SAXS) (Chapter 3) provides information about block copolymer morphology and phase behavior. Electrochemical characterization (Chapter 4) probes the ion transport and thermodynamics of electrolytes and concentrated solution theory, developed by John Newman, enables theoretical prediction of electrolyte behavior.
In Chapter 5, we use oligomeric liquid glyme-based electrolytes to demonstrate the impact of local frictional interactions, quantified by Stefan-Maxwell diffusion coefficients calculated using concentrated solution theory, on ion transport. We define factors to quantify the importance of ion-ion and ion-solvent interactions, and find that in fluorinated electrolytes, ion-ion interactions dominate even at very low salt concentrations, bringing into question the validity of ideal solution assumptions even in dilute electrolytes. Chapter 6 applies these methods to solid systems with high-molecular weight PEO / LiTFSI electrolytes, providing the first characterization of such systems at salt concentrations above r = 0.30 lithium ions per ethylene oxide moiety. We observe a salt solubility limit at r = 0.50, impacting the maximum applicable current density—the limiting current—and a two-phase region between r = 0.28 and 0.50, the electrochemical consequences of which have not been explored previously.
Chapters 7 and 8 move into PS-b-PEO (SEO) / LiTFSI block copolymer electrolytes. In Chapter 7, we decouple grain growth from ionic conductivity during annealing, bringing to light the importance of thermal history and defect annihilation. In Chapter 8, we discover that strong magnetic fields macroscopically align the domains of a lamellar SEO / LiTFSI electrolyte, which produces 7Li NMR quadrupolar peak splitting which disappears at the order-to-disorder temperature, TODT, the temperature above which the polymer disorders. The presence of this peak splitting provides a new measure of TODT, which is typically detected using SAXS.
Chapters 9 and 10 combine SAXS and constant-current electrochemical polarization, which results in the salt accumulation at the positive electrode and depletion at the negative electrode. In Chapter 9, we study the local microstructure of a lamellar SEO / LiTFSI electrolyte, observing domain expansion at the salt-rich electrode and domain contraction at the salt-poor electrode. We discover that differently-oriented domains expand and contract to different extents, indicating that lamellae that do not provide direct conducting pathways between electrodes may still play an important role in ion transport by enabling the development of salt concentration gradients. In Chapter 10, we combine X-ray transmission with SAXS, and observe that the salt concentration of an SEO / LiTFSI electrolyte that initially has a body-centered cubic spherical (BCC) morphology fails to decrease below the salt concentration at which a transition to a hexagonally-packed cylindrical (HEX) morphology would have been expected. This suggests that the inability of polarization to induce morphological conditions can limit the development of salt concentration gradients, and therefore the applicable current.
This work provides new insights into the local interactions, morphological factors, and external forces that impact the performance of polymer electrolytes. The goal of this Dissertation is to provide information that will contribute to an increased depth of understanding of ion transport in polymer electrolytes, enabling the rational design of future high-performance electrolytes.