High Transference Number Polymer-Based Electrolytes for Lithium Batteries
The composition of modern electrolytes is key to the performance of lithium ion batteries. State-of-the-art electrolytes are based on lithium hexafluorophosphate (LiPF6) dissolved in a liquid carbonate solvent with stabilizing additives, which provide a sufficient combination of conductivity and stability towards the highly reactive electrode components. This electrolyte composition has been developed, and continues to evolve, to meet materials design and engineering requirements for high-performance energy storage, but work remains to enable the next generation of high energy density, fast charging batteries. While there are still challenges in electrode formulations and cell management, this dissertation focuses on an important remaining problem involving the electrolyte: concentration polarization as a result of the low lithium transference number (t+) of the electrolyte. t+ characterizes the relative motion of cations to anions within an electric field and is unity for an electrolyte where only lithium ions are mobile, and zero for the opposite case where only anions migrate. The standard liquid electrolyte discussed above has a transference number below 0.5, indicating the bulky anions move faster than lithium ions as a result of the large solvation shell of lithium ions. This high anion motion allows concentration gradients to form within a cell, limiting energy density and charge rates. In this dissertation, polymers are utilized in an effort to create higher transference number electrolytes by attaching the anion to the polymer backbone. This method has suffered from the key drawback of low conductivity for many years, and thus a primary concern of each section herein is improving electrolyte conductivity.
To study this class of electrolytes, initially a new polymer was synthesized based on polysulfone (PSF) condensation chemistry. This polymer allows incorporation of ion conducting poly(ethylene glycol) (PEG) segments, and ion containing sulfonate groups. This synthesis was an extension of existing sulfonated polysulfone and polysulfone-co-poly(ethylene glycol) polymers, but had never been combined into a single polymer before. This polymer, though not an ideal homogenous, low dispersity polymer, allows a wide range of compositions to be formed that could then be used in a variety of electrolytes.
In the first section, the wide accessible composition window of sulfonated PSF-co-PEG is employed to study the fundamentals of ion conduction in dry polymer electrolytes that have appended ions. Conductivity as a function of both PEG and sulfonate content is studied, demonstrating a tradeoff between ion content and segmental motion of the polymer backbone. This tradeoff has been observed in the past and typically in the literature is analyzed through the Vogel-Tammann-Fulcher (VTF) equation, a modified Arrhenius equation originally developed for polymer viscosity but also applied to conductivity. Here it is shown that careful fitting of this equation to conductivity data is crucial to interpret the results, and that a correlation may exist between the equation prefactor and activation energy. These parameters are usually fit to decouple the effects of ion content, related to the prefactor, and segmental motion, related to the activation energy. This correlation was found to exist in other polymer systems and implies that this equation does not necessarily decouple these effects, complicating any analysis based on it. Further, this correlation implies that decreasing the activation energy will also decrease the prefactor, significantly limiting potential design changes to improve conductivity. Blending of a short chain PEG to the dry polymer system is found to break the correlation, further motivating this common conductivity enhancing technique.
Following this discovery, this dissertation transitions to liquid state polyelectrolyte solutions utilizing the same sulfonated PSF-co-PEG polymer. Here the polymers are dissolved in a solvent such that lithium motion may be completely disconnected from polymer segmental motion. These polymer solutions were only recently suggested for battery application, with most prior polyelectrolyte work confined to water. The work here represents the first efforts to transition polyelectrolyte solutions into battery-relevant carbonate solvents. Comparison is first made between a highly polar solvent, dimethylsulfoxide (DMSO) and a carbonate blend solvent. It is shown through NMR characterization of peak width and diffusion measurements that the lithium does not dissociate from the sulfonate group in the carbonate blend solvent even though the polymer is fully dissolved and the dielectric constant of the carbonate blend is the same as DMSO. This demonstrates that new theories which do not solely utilize the dielectric constant to dictate ion interactions in solution will be necessary to predict polyelectrolyte behavior in these nonaqueous solvents.
A further challenge in polyelectrolyte solution design for battery applications is that the vast majority of polyelectrolyte literature focuses mainly on the behavior of the polymer, particularly from a structure perspective. Design of an electrolyte must primarily take transport into account, and for a battery the primary interest is in fact the counterion transport. Existing theory must therefore be understood in a new light to inform rational design of future electrolytes. Here, a fundamental study of transport in polyelectrolyte solutions with multiple different molecular weight polymers and as a function of solvent quality is undertaken. Fully sulfonated polysulfone without PEG is employed here because it is soluble in both DMSO and water, where DMSO represents a good solvent for the backbone and ions, while water is only a good solvent for the ions. It is demonstrated that many of the fundamental theories of polyelectrolyte solutions hold for this previously unstudied system, despite the relatively short chains. By comparing the diffusion of counterions and solvent with the same data for solutions of the monomer alone, the effect of the polymeric anion can be determined. It is found that the presence of a good solvent for the backbone causes an additional slowing of the solvent and lithium in DMSO, as opposed to water. This is despite much higher viscosity in the water systems. From this, several recommendations for polyelectrolyte solution design are made.
Taking inspiration from the current state of the art electrolyte, the final work contained herein discusses the use of additives to improve ion dissociation and conductivity in the carbonate blend solvent used previously. It is shown that crown ethers, and particularly 15-crown-5, are capable of achieving an order of magnitude increase in solution conductivity with the sulfonated PSF-co-PEG previously employed. This conductivity is shown to be sufficient to fabricate a full battery with commercial lithium iron phosphate and graphite electrodes. With the optimized electrolyte, nearly 90% of the theoretical capacity is achieved, three times as high as without additives, demonstrating the potential of these new electrolytes.