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 LiNi0.8Co0.15Al0.05O2 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.

Mixed electron- and ion-conducting polymers serve as excellent candidates for polymer binders in lithium-ion batteries (LIBs) because of an extension of functionality beyond simple mechanical adhesion. Such dual conduction was observed in our recent report on dihexyl-substituted poly­(3,4-propylenedioxythiophene) (PProDOT-Hx2), which showed excellent performance as a cathode binder for LiNi0.8Co0.15Al0.05O2 (NCA). However, ionic conductivity was found to be significantly lower than that of its electronic counterpart. To enhance mixed conduction, here we report a family of synthetically tunable, electrochemically stable, random copolymers based on PProDOT-Hx2, in which the hexyl (Hex) side chains are replaced to varying extents with oligoether (OE) side chains, generating a series of (Hex:OE) PProDOTs. When OE content was varied from 5 to 35%, the resulting copolymers were insoluble in the battery electrolyte and were stable after 100 electrochemical doping/dedoping cycles. Electron paramagnetic resonance and electrochemical kinetics studies were performed to illustrate the reversible and fast electrochemical doping process of (Hex:OE) PProDOTs. Electronic and ionic conductivity measurements as a function of electrochemical potential show a decrease in electronic conductivity and a concurrent increase in ionic conductivity with increasing incorporation of OE side chains. X-ray scattering studies on electrochemically doped polymers indicate a decline in crystalline ordering with the increase in OE content of the (Hex:OE) PProDOTs, suggesting that decreasing crystallinity is responsible for both the increased ionic and reduced electronic conductivity. Compounding these structural changes, swelling studies show a linear mass increase with OE content upon electrolyte exposure, indicating that solvent-induced swelling and electrolyte uptake play a significant role in the ability of these polymers to conduct ions. Finally, rigorous cell testing was performed by employing electrochemical impedance spectroscopy, galvanostatic charge–discharge, rate capability tests, and differential capacity vs voltage analysis, using NCA cathodes to understand the role of these polymers as mixed electron- and Li+-ion-conducting polymer binders in LIBs in comparison to the commonly used polyvinylidene fluoride. It is observed that (75:25) PProDOT containing 25% of OE side chains achieves the highest rate capability and fastest charging and discharging under symmetric testing conditions. The synthetic flexibility to fine-tune electronic and ionic conductivity makes (Hex:OE) PProDOTs a promising new class of mixed conducting polymers for electrochemical energy-storage application.

We report a unique solid-state lithium-sulfur cell based on a bilayer electrolyte and composite solid-state cathode. The bilayer electrolyte that contains a layer of mixed conduction membrane and a layer of polymer electrolyte eliminates the use of organic flammable liquid electrolytes and separators. The sulfur electrode is also a unique composite of sulfur with ionically-conducting intercalating nano-particulate material. Unlike many other solid-state batteries, this cell can be cycled at room temperature to utilize 85% of the active material at the sulfur electrode. The low internal resistance of the cell is comparable to that of a liquid electrolyte based lithium-sulfur cell. Impedance studies indicate that the low internal resistance results from the high ionic conductivity of the intercalating nano-particulate materials and the thin layer of polymeric electrolyte. While the volume changes at the cathode result in loss of inter-particle contact with repeated cycling, the addition of alumina to the polymer layer improved the capacity retention. This unique solid-state cell configuration opens a new pathway towards a safer high-energy lithium battery.

Developing lithium-ion batteries with both high specific energy and high-power capability is a challenging task because of the necessity for meeting conflicting design requirements. We show that high-energy and high-rate capability can be achieved by using various π-conjugated p-dopable polymers as binders at the cathode and by lowering the mass fraction of all the inactive components of the cell. We report a lithium-metal battery that can deliver 320 Wh kg−1 at C/2 using a mass-efficient cell design. To this end, three conducting polymers with different ionic and electronic conductivities have been studied; dihexyl-substituted poly(3,4-propylenedioxythiophene) (PProDOT-Hx2), poly(3-hexylthiophene) (P3HT), and a new Random Copolymer (Hex:OE)(80:20) PProDOT. These conducting polymers are compared against a conventional polymer binder, PVDF. We show that under the mass-efficient conditions required for achieving high specific energy and rate capability, the conducting polymers play a crucial role by providing electronic and ionic conductivity, protection against rapid growth of solid electrolyte interphase (SEI), and access to a large electrochemically active surface area. Thus, the use of conducting polymers with appropriate molecular structure as binders opens a viable pathway to maximizing the specific energy and rate capability of lithium-ion battery cathodes.

The performance of the sulfur-carbon composite cathode in a lithium-sulfur battery is determined by the nature of the carbon materials and the distribution of sulfur within the composite electrode. The role of these major factors is poorly understood. The present study uncovers the factors that contribute to the internal resistance and rate capability of Ketjenblack-carbon based sulfur cathodes. Despite their enormous surface area, Ketjenblack-carbon-based sulfur electrodes exhibit surprisingly poor discharge rate capability and a high polarization resistance, providing no more than 180 mAh g-1 at the C/20 rate and delivering a high specific capacity of 1200 mAh g-1 only at a very low discharge rate of C/50. We find that a large contribution to the polarization resistance arises from inter-particulate contacts and charge-transfer processes occurring close to 30% depth of discharge. Insulating sheets of sulfur on the Ketjenblack particles require an additional electron-percolation pathway for reducing the polarization. Addition of a low-surface area carbon, Super-P, provides such an electron conduction pathway resulting in an impressive and stable capacity of 950 mAh g-1 at C/5 rate. We anticipate that the detailed analysis of impedance and the new learnings will provide the insight for improving the design of the sulfur cathode.