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Synthesis and Characterization of Simultaneous Electronic and Ionic Conducting Block Copolymers for Lithium Battery Electrodes


Materials with nanostructured conducting domains are essential for a wide range of applications related to alternative energy. Active materials in battery and fuel cell electrodes such as LiFePO4, graphite, and platinum, are either electronic or ionic insulators. Nanoscale electron- and ion-conducting domains are necessary for enabling redox reactions in these materials. For example, a traditional porous lithium battery electrode consists of a redox-active material, carbon black for electronic conduction, and non-conductive binder that holds the particles in place. The pores are backfilled filled with organic electrolyte for ionic conduction. In some cases such as LiFePO4, electronic and ionic conductivities are so low that the active materials must be in nanoparticle form, and addressing such particles requires the transport of both kinds of charges to occur on nanometer length scales. Materials such as block copolymers can self-assemble and form co-continuous nanoscale domains. In this study, poly(3-alkylthiophene)-block-poly(ethylene oxide) (P3AT-PEO) copolymers are used to conducts both electronic and ionic charges. P3AT-PEO block copolymer molecules self-assemble on the nanometer length scale to yield P3AT-domains that conduct electronic charges and PEO-domains that conduct ionic charges. We propose to create a unique battery electrode where the LiFePO4 active material is dispersed in a nanostructured P3AT-PEO block copolymer, which functions simultaneously as the conductor of lithium ions and electronic charge, as well as the binder material in the electrode.

The first phase of this dissertation work involved the synthesis of P3AT-PEO block copolymers. Regioregular P3ATs were synthesized using the Grignard metathesis (GRIM) polymerization method where in-situ end-group functionalization was employed to obtain ethynyl-functionalized P3ATs. Azide-functionalized PEOs were obtained through end-group modification of PEO monomethyl ether. Ethynyl-functionalized P3ATs and azide-functionalized P3AT-PEO were coupled using 1,3-dipolar cycloaddition "click" reaction to obtain P3AT-PEO block copolymer. In particular, poly(3-hexylthiophene)-block-poly(ethylene oxide) (P3HT-b-PEO) copolymers and a poly(3-ethylhexylthiophene)-block-poly(ethylene oxide) (P3EHT-b-PEO) copolymer were synthesized in this study.

Next, the morphology of the P3AT-b-PEO copolymer was characterized using small angle X-ray scattering (SAXS). The morphologies of P3HT-b-PEO copolymers, where the P3HT block is the major component, are dominated by nanofibrils due to the crystallization of P3HT. In contrast, the nearly symmetric P3HT-b-PEO copolymers self-assemble into a lamellar phase. In addition, we show that P3EHT-b-PEO chains self-assemble to produce traditional nanoscale morphologies such as lamellae and gyroid in the melt-state. The segregation strength between the two blocks is controlled through the addition of lithium bis(trifluromethanesulfonyl) imide (LiTFSI). Our approach enables estimation of the "effective" Flory-Huggins interaction parameter, $chieff, using the random phase approximation (RPA). The $chieff trends with salt concentration suggest that the TFSI anion preferentially segregates into the P3EHT phase while Li+ remains in the PEO phase. For the salt-free sample, the gyroid morphology, obtained in the melt-state, is transformed into lamellae when the P3EHT block is crystallized. This is due to the "breaking out" of the crystalline phase. At high salt concentrations, P3EHT-b-PEO has a lamellar morphology in both melt and crystalline states (confined crystallization).

We present the first reported study on the relationship between morphology and electronic/ionic charge transport of P3HT-b-PEO/LiTFSI mixtures. Using ac impedance spectroscopy, we show that P3HT-b-PEO/LiTFSI mixtures can conduct electronic and ionic charges simultaneously. At 90 °C, the electronic conductivity of P3HT-b-PEO/LiTFSI mixtures ranged from 10-8 to 10-5 S/cm depending on the volume fraction of P3HT. The decoupled ionic conductivity is around ~10-4 S/cm. It was shown that LiTFSI partitions between P3HT and PEO microphases. In particular, LiTFSI only partitions between the microphases when the PEO block molecular weight is 2 kg/mol while we observe no partitioning when the PEO block molecular weight of 4.2 kg/mol. It thus appears that the chemical potential of LiTFSI in PEO is a function of the PEO block molecular weight. We propose that the higher chemical potential of LiTFSI for P3HT-b-PEO copolymers with PEO molecular weight of 2 kg/mol drives the LiTFSI into the P3HT rich microphase.

The electronic conductivity can be further increased by electrochemically chemically doping the P3HT chains with LiTFSI. Therefore, we quantified the electronic conductivity P3HT-b-PEO copolymers electrochemically oxidized with LiTFSI. We use a novel solid-state three-terminal electrochemical cell that enables simultaneous conductivity measurements and control over electrochemical doping of P3HT. At low oxidation levels, the electronic conductivity increases from 10-8 S/cm to 10-4 S/cm. At high oxidation levels, the electronic conductivity approaches 10-2 S/cm. These values match or exceed the ionic conductivity, which is important for enabling redox reactions in a battery as they involve equal moles of lithium ions and electronic charges.

A lithium metal battery was assembled where the positive electrode consisted of P3HT-b-PEO conductive binder and LiFePO4 active material. We were able to cycle batteries and obtain capacities approaching the theoretical limit of LiFePO4. Importantly, P3HT is electroactive within the voltage window of a charge/discharge cycle. The electronic conductivity of the P3HT-b-PEO copolymer binder is in the 10-4 to 10-2 S/cm range over most of the potential window of the charge/discharge cycle. This allows for efficient electronic conduction needed for the successful cycling of the batteries. However, at the end of the discharge cycle, the electronic conductivity decreases sharply to 10-7 S/cm, which means the "conductive" binder is now electronically insulating. The ability of our conductive binder to switch between electronically conducting and insulating states in the positive electrode provides an unprecedented route for automatic overdischarge protection in batteries.

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