Enhancing the Ionic Conductivity of Poly(3,4-propylenedioxythiophenes) with Oligoether Side Chains for Use as Conductive Cathode Binders in Lithium-Ion Batteries

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-Hx(2)), 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-Hx(2), 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.