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Single-Ion-Conducting Block Copolymer Electrolytes for Lithium Batteries: Morphology, Ion Transport, and Mechanical Properties

  • Author(s): Rojas, Adriana Araceli;
  • Advisor(s): Balsara, Nitash P;
  • et al.

Lithium metal batteries have high theoretical specific energies, which make it a favorable candidate to meet our need for energy storage applications for electric vehicles and grid storage. However, there are significant safety concerns that limit our use of lithium metal electrodes. For this reason, polymer electrolytes have been a favorable choice of electrolyte, as they are they are more thermally and electrochemically stable against lithium metal. Block copolymer electrolytes are a promising candidate for these battery systems because of their ability to microphase separate into unique nanostructures. Given a high molecular weight block copolymer, the ion transport and moduli can be significantly improved relative to its hompolymer counterpart. As a result, block copolymers have been effective at slowing the growth of lithium dendrites. However, the main problem with block copolymer electrolytes where a salt was physically integrated, is the problem of concentration gradients that form over the length of the electrolyte. Concentration gradients are a result of low transference numbers, that is, the lithium ion of interest will carry a low fraction of current relative to the anion. To eliminate concentration gradients, single-ion-conducting block copolymer electrolytes were synthesized and characterized: poly(ethylene oxide)-b-poly(styrenesulfonyllithium(trifluoromethylsulfonyl)imide) (PEO-b-PSLiTFSI). In this class of copolymers, the anion (TFSI-1) was covalently bonded to the polystyrene backbone, allowing only the lithium ion to move.

The work enclosed elucidates the relationship between the morphology, ion transport, and mechanical properties of this single-ion-conducting block copolymer electrolyte. In the first phase of this dissertation, the synthesis of the monomer, the PEO macroinitiator, and the subsequent nitroxide mediated polymerization procedure are detailed. Improvements to the polymerization are described, and the characterization steps for ion-exchange and polymer structure are discussed.

The subsequent work discusses the relationship between ion transport and morphology using small angle X-ray scattering (SAXS) and impedance spectroscopy. It was demonstrated that the placement of the charged group in the non-ion-conducing block (PS) rendered fundamentally different nanostructure morphology. Unlike uncharged block copolymers, it was found that PEO-b-PSLiTFSI completely disordered (homogenized). There was no presence of concentration fluctuations. When the copolymer underwent an order-to-disorder transition, the ionic conductivity was found to increase three orders of magnitude. It was demonstrated that there are favorable interactions between the lithium ions and the ethyl ethers in PEO.

Next, the effect of ion concentration on morphology and ion transport were explored. It was found that copolymers of low ion concentration (r = [Li+][EO]-1) were microphase separated at room temperature. However, at high r, the copolymers were found to be disordered (homogenous) at low temperature. This was due to the effects of ion-entropy and the favorable interactions between lithium ion and the PEO block. Copolymers exhibited higher ionic conductivities at low temperature when copolymers were disordered. At high temperatures, all copolymers were disordered, and ionic conductivity peaked for r = 0.111.

In the next segment, the molecular weight of the block copolymer electrolytes were increased to understand its effect on block copolymer morphology and ion transport. It was found that these copolymers also disordered in the similar manner that the lower molecular weight copolymers disordered. However, a qualitatively different trend of ionic conductivity with r was observed. We owe the effects of lower ionic conductivity to the increase in the glass transition temperature, Tg. Preliminary studies in ion transport of lithium symmetric cells were shown. This was coupled with tomography studies.

Finally, a matched series of lithiated and magnesiated block copolymers were compared. It was found that the magnesiated block copolymers exhibited weak microphase separation for volume fractions of the ion-containing block, ϕPSTFSI, in the range 0.21 ≤ ϕPSTFSI ≤ 0.36. Unlike uncharged block copolymers, the tendency for microphase separation decreased with increasing ϕPSTFSI. Moreover, the magnesiated block copolymer with ϕPSTFSI = 0.38 was found to completely disorder in the similar manner as the lithiated copolymers. This loss of microstructure had significant influences on the resulting rheological and ion transport properties. The lithiated copolymers exhibited liquid-like rheological properties, characteristic of disordered copolymers. The magnesiated copolymers did not. Furthermore, the shear moduli of the magnesiated copolymers were several orders of magnitude higher than its lithiated pairs. The ionic conductivity of the lithiated copolymers was observed to be higher than its magnesiated pairs.

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