Dihexyl-Substituted Poly(3,4-Propylenedioxythiophene) as a Dual Ionic and Electronic Conductive Cathode Binder for Lithium-Ion Batteries

The polymer binders used in most lithium-ion batteries (LIBs) serve only a structural role, but there are exciting opportunities to increase performance by using polymers with combined electronic and ionic conductivity. To this end, here we examine dihexyl-substituted poly(3,4-propylenedioxythiophene) (PProDOT-Hx₂) as an electrochemically stable π-conjugated polymer that becomes electrically conductive (up to 0.1 S cm⁻¹) upon electrochemical doping in the potential range of 3.2 to 4.5 V (vs Li/Li⁺). Because this family of polymers is easy to functionalize, can be effectively fabricated into electrodes, and shows mixed electronic and ionic conductivity, PProDOT-Hx₂ shows promise for replacing the insulating polyvinylidene fluoride (PVDF) commonly used in commercial LIBs. A combined experimental and theoretical study is presented here to establish the fundamental mixed ionic and electronic conductivity of PProDOT-Hx₂. Electrochemical kinetics and electron spin resonance are first used to verify that the polymer can be readily electrochemically doped and is chemically stable in a potential range of interest for most cathode materials. A novel impedance method is then used to directly follow the evolution of both the electronic and ionic conductivity as a function of potential. Both values increase with electrochemical doping and stay high across the potential range of interest. A combination of optical ellipsometry and grazing incidence wide angle X-ray scattering is used to characterize both solvent swelling and structural changes that occur during electrochemical doping. These experimental results are used to calibrate molecular dynamics simulations, which show improved ionic conductivity upon solvent swelling. Simulations further attribute the improved ionic conductivity of PProDOT-Hx₂ to its open morphology and the increased solvation is possible because of the oxygen-containing propylenedioxythiophene backbone. Finally, the performance of PProDOT-Hx₂ as a conductive binder for the well-known cathode LiNi_(0.8)Co_(0.15)Al_(0.05)O₂ relative to PVDF is presented. PProDOT-Hx₂-based cells display a fivefold increase in capacity at high rates of discharge compared to PVDF-based electrodes at high rates and also show improved long-term cycling stability. The increased rate capability and cycling stability demonstrate the benefits of using binders such as PProDOT-Hx₂, which show good electronic and ionic conductivity, combined with electrochemical stability over the potential range for standard cathode operation

Evaluating the Impact of Conjugation Break Spacer Incorporation in Poly(3,4-propylenedioxythiophene)-Based Cathode Binders for Lithium-Ion Batteries

The driving force behind the significant advancement of conductive polymer binders for lithium-ion batteries (LIBs) stems from the poor binding strength, limited mechanical properties, and absence of electronic conductivity of the commonly used nonconjugated polymer binder, poly(vinylidene fluoride) (PVDF). With a goal to induce stretchability and deformability to the otherwise brittle conjugated backbone, we report here dihexyl-substituted poly(3,4-propylenedioxythiophene)-based (PProDOT-Hx2-based) conjugated polymers wherein conjugation break spacers (CBS, T-X-T) of varying alkyl spacer lengths (X = 6, 8, and 10) and varying contents (5, 10, and 20%) have been randomly incorporated into the PProDOT backbone, generating a family of nine random PProDOT-CBS copolymers. Electrochemical characterization revealed that three out of the nine PProDOT-CBS polymers (5% T-6-T, 5% T-8-T, and 10% T-6-T) are electrochemically stable over long-term cycling of 100 cycles. The electronic conductivity of the PProDOT-CBS polymers is consistent with previous literature reports on CBS polymers, where a decline in charge carrier mobility is observed with an increase in CBS content and spacer length, although no significant difference in ionic conductivity in these polymers was observed. This is supported by GIWAXS studies indicating a decrease in lamellar peak intensity with increasing CBS content and spacer length. Mechanical properties of the three selected PProDOT-CBS polymers were investigated by using the established “film-on-water” technique and a novel “film-on-solvent” technique that we report here for the first time, where the solvent used is the same as employed in the battery electrolyte. Both techniques showcase a generally lower tensile modulus (E) and higher crack onset strain (COS) of the PProDOT-CBS polymers relative to fully conjugated PProDOT-Hx2. Furthermore, significant enhancement in mechanical properties is observed with the “film-on-solvent” method, suggesting that electrolyte-induced swelling has a plasticizing effect on the polymers, accounting for their increased stretchability and deformability. Finally, cell testing of the PProDOT-CBS polymers with NCA cathodes aligned well with the electrochemical and mechanical studies, where a higher crack onset strain was crucial for higher capacity retention during long-term cycling. On the contrary, rate capability measurements proved that higher electronic conductivity is favored over mechanical properties during high rates of discharge. This work illustrates that the strategic introduction of CBS units into conjugated polymer binders is a viable method for the generation of stretchable conductive polymer binders for emerging high-capacity electrodes in LIBs

Measuring Heat Dissipation and Entropic Potential in Battery Cathodes Made with Conjugated and Conventional Polymer Binders Using <i>Operando</i> Calorimetry

This study explores the influence of electronic and ionic conductivities on the behavior of conjugated polymer binders through the measurement of entropic potential and heat generation in an operating lithium-ion battery. Specifically, the traditional poly(vinylidene fluoride) (PVDF) binder in LiNi0.8Co0.15Al0.05O2 (NCA) cathode electrodes was replaced with semiconducting polymer binders based on poly(3,4-propylenedioxythiophene). Two conjugated polymers were explored: one is a homopolymer with all aliphatic side chains, and the other is a copolymer with both aliphatic and ethylene oxide side chains. We have shown previously that both polymers have high electronic conductivity in the potential range of NCA redox, but the copolymer has a higher ionic conductivity and a slightly lower electronic conductivity. Entropic potential measurements during battery cycling revealed consistent trends during delithiation for all of the binders, indicating that the binders did not modify the expected NCA solid solution deintercalation process. The entropic signature of polymer doping to form the conductive state could be clearly observed at potentials below NCA oxidation, however. Operando isothermal calorimetric measurements showed that the conductive binders resulted in less Joule heating compared to PVDF and that the net electrical energy was entirely dissipated as heat. In a comparison of the two conjugated polymer binders, the heat dissipation was lower for the homopolymer binder at lower C-rates, suggesting that electronic conductivity rather than ionic conductivity was the most important for reducing Joule heating at lower rates, but that ionic conductivity became more important at higher rates

Tunable Dopants with Intrinsic Counterion Separation Reveal the Effects of Electron Affinity on Dopant Intercalation and Free Carrier Production in Sequentially Doped Conjugated Polymer Films

Carrier mobility in doped conjugated polymers is limited by Coulomb interactions with dopant counterions. This complicates studying the effect of the dopant's oxidation potential on carrier generation because different dopants have different Coulomb interactions with polarons on the polymer backbone. Here, dodecaborane (DDB)-based dopants are used, which electrostatically shield counterions from carriers and have tunable redox potentials at constant size and shape. DDB dopants produce mobile carriers due to spatial separation of the counterion, and those with greater energetic offsets produce more carriers. Neutron reflectometry indicates that dopant infiltration into conjugated polymer films is redox-potential-driven. Remarkably, X-ray scattering shows that despite their large 2-nm size, DDBs intercalate into the crystalline polymer lamellae like small molecules, indicating that this is the preferred location for dopants of any size. These findings elucidate why doping conjugated polymers usually produces integer, rather than partial charge transfer: dopant counterions effectively intercalate into the lamellae, far from the polarons on the polymer backbone. Finally, it is shown that the IR spectrum provides a simple way to determine polaron mobility. Overall, higher oxidation potentials lead to higher doping efficiencies, with values reaching 100% for driving forces sufficient to dope poorly crystalline regions of the film

Room-temperature electrochemical fluoride (de)insertion into CsMnFeF6

We report on the reversible, electrochemical (de)fluorination of CsMnFeF6 at room temperature using a liquid electrolyte. CsMnFeF6 was synthesized via three methods (hydrothermal, ceramic, and mechanochemical), each of which yields products in a defect pyrochlore structure with varying particle sizes and phase purities. After three galvanostatic cycles, approximately one fluoride ion can be reversibly (de)inserted into mechanochemical CsMnFeF6 for multiple cycles. Ex situ X-ray absorption spectroscopy confirmed that both Mn2+ and Fe3+ are redox active. The cell impedance decreases after one cycle, suggesting that the formation of fluoride vacancies in early cycles generates mixed-valent Fe and enhances the material’s conductivity. Ex situ synchrotron diffraction revealed subtle expansion and contraction of the CsMnFeF6 cubic lattice on insertion and removal, respectively, during the first two cycles. New reflections intensify in the ex situ diffraction patterns from cycle 3, corresponding to a topotactic transformation of CsMnFeF6 from the pyrochlore structure into an orthorhombic polytype that continues cycling fluoride ions reversibly

Designing Amphiphilic Conjugated Polyelectrolytes for Self-Assembly into Straight-Chain Rod-like Micelles

Semiconducting polymers are a versatile class of materials that are used in many (opto)electronic applications, including organic photovoltaics. However, they are inherently disordered and suffer from poor conductivities due to bends and kinks in the polymer chains along the conjugated backbone, as well as disorder at grain boundaries. In an effort to reduce polymer disorder, we developed a method to straighten polymer chains by creating amphiphilic conjugated polyelectrolytes (CPEs) that self-assemble in water into worm-like micelles. The present work refines our design rules for self-assembly of CPEs. We present the synthesis and characterization of a straight, micelle-forming polymer, a derivative of poly(cyclopentadithiophene-alt-thiophene) (PCT) bearing two ammonium-charged groups per cyclopentadithiophene unit. Solution-phase self-assembly of PCT into micelles is observed by both small-angle X-ray scattering (SAXS) and cryo-electron microscopy (cryo-EM), while detailed SAXS fitting allows for characterization of intra-micellar interactions and inter-micelle aggregation. We find that PCT displays significant chain straightening thanks to the lack of steric hindrance between its alternating cyclopentadithiophene and thiophene subunits, which increases the propensity for the polymer to self-assemble into straight rod-like micelles. This work extends the availability of micelle-forming semiconducting polymers and points to further enhancements that can be made to obtain homogeneous nanostructured polymer assemblies based on cylindrical micelles