UC Berkeley

One of the major barriers to expanding the capacity of large-scale electrochemical energy storage within batteries is the threat of a catastrophic failure. Catastrophic battery pack failure can be initiated by a defect within a single battery cell. If the failure of a defective battery cell is not contained, the damage can spread and subsequently compromise the integrity of the entire battery back, as well as the safety of those in its surroundings. Replacing the volatile, flammable liquid electrolyte components found in most current lithium ion batteries with a solid polymer electrolyte (SPE) would significantly improve the cell-level safety of batteries; however, poor ionic conductivity and restricted operating temperatures compared to liquid electrolytes have plagued the practical application of SPEs. Rather than competing with the performance of liquid electrolytes directly, our approach to developing SPEs relies on increasing electrolyte functionality through the use of block copolymer architectures.

Block copolymers, wherein two or more chemically dissimilar polymer chains are covalently bound, have a propensity to microphase separate into nanoscale domains that have physical properties similar to those of each of the different polymer chains. For instance, the block copolymer, polystyrene-b-poly(ethylene oxide) (SEO), has often been employed as a solid polymer electrolyte because the nanoscale domains of polystyrene (PS) can provide mechanical reinforcement, while the poly(ethylene oxide) microphases can solvate and conduct lithium ions. Block copolymer electrolytes (BCEs) formed from SEO/salt mixtures result in a material with the bulk mechanical properties of a solid, but with the ion conducting properties of a viscoelastic fluid. The efficacy SEO-based BCEs has been demonstrated; the enhanced mechanical functionality provided by the PS domains resist the propagation of dendritic lithium structures during battery operation, thus enabling the use of a lithium metal anode. The increase in the specific energy of a battery upon replacing a graphite anode with lithium metal can offset the losses in performance due to the poor ion conduction of SPEs. However, BCEs that enable the use of a lithium anode and have improved performance would represent a major breakthrough for the development of high capacity batteries.

The electrochemical performance of BCEs has a complex relationship with the nature of the microphase separated domains, which is not well-understood. The objective of this dissertation is to provide fundamental insight into the nature of microphase separation and self-assembly of block copolymer electrolytes. Specifically, I will focus on how the ion-polymer interactions within a diverse set of BCEs dictate nanostructure. Combining such insight with knowledge of how nanostructure influences ion motion will enable the rational design of new BCEs with enhanced performance and functionality.

In order to facilitate the study of BCE nanostructure, synchrotron-based X-ray scattering techniques were used to study samples over a wide range of length-scales (i.e., from Angstroms to hundreds of nanometers) under conditions relevant to the battery environment. The development of the experimental aspects of the X-ray scattering techniques, as well as an improved treatment of scattering data, played a pivotal role in the success of this work. The dissemination of those developments will be the focus of the first section.

The thermodynamic impact of adding salt to a neutral diblock copolymer was studied in a model BCE composed of a low molecular weight SEO diblock copolymer mixed with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), a common salt used in lithium batteries. In neutral block copolymers (BCPs), self-assembly is a thermodynamically driven process governed by a balance between unfavorable monomer contacts (i.e., the enthalpic contribution) and the entropy of mixing. When the enthalpic and entropic contributions to free energy are similar in magnitude, a block copolymer can undergo a thermally reversible phase transition from an ordered to a disordered nanostructure (i.e., the order-to-disorder transition (ODT). We used temperature-dependent small angle X-ray scattering (SAXS) to observe this transition in the model SEO/LiTFSI system. Unlike neutral BCPs, which to a first approximation are single component systems, the SEO/LiTFSI system demonstrated the thermodynamically stable coexistence phases of ordered lamellae and disordered polymer over a finite temperature window. Analysis of the lamellar domains revealed an increase in salt concentration during the ODT, indicating local salt partitioning due to the presence of nanostructure. While the Gibbs phase rule predicts this behavior, this was the first result demonstrating a direct connection between ion-polymer interactions and block copolymer nanostructure.

We found evidence of salt redistribution in BCEs wherein self-assembly has been kinetically arrested. Through the structural analysis of BCEs formed from a high molecular weight SEO sample over a wide range of LiTFSI concentrations, it was revealed that in some cases, coexisting nanostructures were stable. While it is likely that the stability of these nanostructures was kinetic in nature, the relationship between nanostructure and salt partitioning revealed previously indicates that the salt could redistribute between the nanostructures to achieve the lowest energetic state. Unusual trends in the ionic conductivity with respect to salt concentration support this hypothesis. In some cases, high salt concentrations lead to significant improvements in ionic conductivity, representing a strong departure from the behavior of standard SPEs, and a possible route to improving the performance of BCEs.

The performance of BCEs can also be improved by chemically functionalizing one of the polymer blocks by covalently attaching the salt anion. Since the cation is the only mobile species, these materials are coined single-ion conducting block copolymers. Single ion conduction can improve the efficiency of battery operation. In order for cation motion to occur in single-ion conducting block copolymers, it must dissociate from the backbone of the anion-containing polymer block. Through the structural and electrochemical characterization of poly(ethylene oxide)-b-poly[(styrene-4-sulfonyltrifluoromethylsulfonyl)imide] (PEO-P(STFSI))-based single-ion conductors, we found that ion dissociation significantly influences nanostructure: when a large amount of ions are dissociated, the polymer blocks tend to mix, thus precluding microphase separation and the formation of nanostructure. This direct coupling of ion dissociation (and hence conduction) and nanostructure has interesting implications for BCE performance. For instance, without discreet microphases, the single-ion conducting polymers cannot provide the enhanced mechanical properties like those obtained in SEO/LiTFSI electrolytes. Future development in single-ion conducting block copolymers should investigate polymer architectures where a third polymer block, such as PS, facilitates microphase separation and improved mechanical properties.

Additional analysis of the single-ion conducting block copolymers revealed that ion dissociation from the charge-containing backbone (P(STFSI)) could also influence the crystallization of the neutral polymer block (PEO). Interestingly, ion dissociation did not disrupt PEO crystallization by directly interfering with the PEO chains, rather the homogeneity of the polymer melt prior to PEO crystallization led to differences in crystallization behavior. In the cases where ion dissociation lead to significant mixing of the polymer block, PEO crystallites grew unimpeded and formed well-ordered lamellar structures. When ion dissociation did not occur, fluctuations in concentration due to the demixing of PEO and P(STFSI) interrupted the growth of PEO crystallites, slowing the crystallization process and leading to less-ordered nanostructures.

The final study in this work highlights the capability of utilizing in situ electrochemical characterization techniques while monitoring polymer microstructure using synchrotron X-ray scattering. We studied the electrochemical oxidation (doping) of poly(3-hexylthiophene) (P3HT) in a block copolymer of poly(3-hexylthiophene)-b-poly(ethylene oxide) (P3HT-PEO) mixed with LiTFSI. During the doping process, we monitored the charge mobility electrochemically and the crystalline structure of P3HT using wide angle X-ray scattering (WAXS). Combining the structural analysis with the transport measurements in situ allowed the observation of a clear correlation between doping-induced changes in the P3HT crystal lattice and improvements in charge mobility. Since the doping-induced structural changes involve the intercalation of a salt anion into the P3HT crystal lattice, tuning the nature of the anion present during electrochemical oxidation might provide a new route to improving hole mobility in P3HT.