A Failure and Structural Analysis of Block Copolymer Electrolytes for Rechargeable Lithium Metal Batteries
The dissertation reports on the use of block copolymer electrolytes in rechargeable lithium metal batteries. The block copolymer studied is a polystyrene-block-poly(ethylene oxide) (SEO) block copolymer with roughly equal volume fractions of each block. A variety of molecular weights these symmetric SEO copolymers are reported on. A series of poly(ethylene oxide) (PEO) homopolymer electrolytes are also studied to serve as controls. The focus of this dissertation is on the failure of batteries with these block copolymer electrolytes and the structure of the block copolymer electrolytes during operation.
The failure mechanism of interest is dendrite formation on the lithium metal electrode during recharge. These dendrites grow through the electrolyte, reach the other electrode, and short-circuit the battery. A comparative study was performed with both the SEO and PEO electrolytes. The study focuses on the total operation time of these electrolytes before the short-circuit occurs. The SEO electrolytes microphase separated into a lamellar microstructure. The microstructure of the SEO electrolytes increase that amount of charge that can be passed before short-circuit by a factor of 11-48 over PEO electrolytes indicating an enhanced resistance to dendrite formation. A disordered SEO electrolyte (no microstructure) showed no improvement over PEO electrolytes. The applied current density and charging time were also varied to determine the effect these experimental conditions have on short-circuit due to dendrite formation in SEO electrolytes.
To further understand this failure mechanism, the coverage and size of nonuniform, dendritic structures is studied using scanning electron microscopy (SEM). For the homopolymer electrolyte after short-circuit, the fractional surface coverage of these nonuniform structures on the lithium metal electrode is low (0.11 ± 0.04). For the microstructured block copolymer electrolyte, the fractional surface coverage of dendritic structures remains low (ca. 0.07) even after a significant amount of cycling. After short-circuit in the microstructured block copolymer electrolyte cells, the fractional surface coverage dramatically increases to nearly full coverage (0.88 ± 0.05). The size of the smallest nonuniform growths on the surface is found to be ca. 500 nm in diameter and independent of electrolyte. This is significantly larger than the domain size of the block copolymer (30.4 nm). This indicates that the microstructure of the block copolymer does not limit the overall size of growing lithium metal dendrites.
The structure of the block copolymer electrolyte in a symmetric electrochemical cell with two lithium metal electrodes is also investigated in this dissertation. An initially disordered sample at elevated temperatures spontaneously orders with time. This spontaneous ordering appears to be salt concentration dependent with lower salt concentrations electrolytes not ordering within the experimental time frame. With the application of a constant voltage, a sample that is unambiguously disordered initially can be reversibly ordered. The ordering is the equivalent of changing the ODT by 50 °C. With the application of a constant current density, a sample is shown to undergo an order-to-disorder transition followed by an order-to-order transition to an unexpected morphology. The time to induce this morphological change is related to the applied current density with larger current densities leading to shorter induction times.