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

Rapid and Reversible Lithium Insertion with Multielectron Redox in the Wadsley-Roth Compound NaNb13O33

The development of high-performing battery materials is critical to meet the ever-increasing demand for portable energy storage for electronics and electric vehicles. Owing to their exceptionally high-rate capabilities, high volumetric capacities and long lifetimes, Wadsley-Roth compounds are promising lithium anode materials for fast-charging and high-powered devices. This study comprises an in-depth structural and initial electrochemical investigation of the Wadsley-Roth phase NaNb13O33 phase. To our knowledge, this is the first alkali-containing Wadsley-Roth compound tested for lithium insertion. Here, we report structural insights obtained from combined neutron and synchrotron diffraction as well as solid-state nuclear magnetic resonance (ss-NMR). We find that a variety of simple, solid-state methods reliably produce a ReO6-like base structure with periodic, ”shear” planes of edge-sharing NbO6 octahedra separating 5 x 3 octahedral blocks with square-planar Na+ occupying block corners. Through ss-NMR, we reveal the presence of sodium cations in block interior sites as well as square-planar block sites. Through combined experimental and computational studies, we demonstrate and rationalize the high-rate performance of this new anode material in lithium-ion half cells. Using X-ray photoelectron spectroscopy (XPS), we show the multi-electron redox of Nb, which enables capacities of 225 mA h g−1 at slow rates and anodic potentials. Without down-sizing or nano-scaling, 100 mA h g−1 of this capacity is retained at 20 C in micrometer-scale particles. By combining bond-valence mapping and DFT, we show that such excellent rate performance results from facile, multi-channel lithium diffusion down octahedral block interiors and from high electronic conductivity within shear planes. Finally, we utilize differential capacity analysis to identify optimal long-term cycling rates and achieve 80% capacity retention over 600 cycles with 30-minute charging and discharging intervals. Without optimization, these results place NaNb13O33 in the ranks of promising new high-rate lithium anode materials and warrant further research

Measuring Heat Dissipation and Entropic Potential in Battery Cathodes Made with Conjugated and Conventional Polymer Binders Using Operando 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

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