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

Abstract

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.

Mixed Ionic–Electronic Conduction Increases the Rate Capability of Polynaphthalenediimide for Energy Storage

“…These measurements indicate that EG side chains significantly increase Li-ion diffusion and result in electrodes with mixed ionic and electron transport. This is consistent with recent reports that studied simultaneous ionic and electronic transport in different conjugated polymers, including conjugated polymers with oligoether side chains . These studies measured electronic and ionic conductivities simultaneously as a function of potential and demonstrated that oligoether side chains enhanced ionic conductivity while reducing electronic conductivity …”

“…This is consistent with recent reports that studied simultaneous ionic and electronic transport in different conjugated polymers, 34 including conjugated polymers with oligoether side chains. 35 These studies measured electronic and ionic conductivities simultaneously as a function of potential and demonstrated that oligoether side chains enhanced ionic conductivity while reducing electronic conductivity. 35 We were also interested in measuring the electronic conductivity of the PNDI-EG materials.…”

“…35 These studies measured electronic and ionic conductivities simultaneously as a function of potential and demonstrated that oligoether side chains enhanced ionic conductivity while reducing electronic conductivity. 35 We were also interested in measuring the electronic conductivity of the PNDI-EG materials. To quantify electronic conductivity, we measured the conductivity of pure PNDI-EGX electrodes both before and after chemical doping with 10 mol % 4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)-N,N-diphenylaniline (N-DPBI).…”

Impact of Side Chain Chemistry on Lithium Transport in Mixed Ion–Electron-Conducting Polymers

“…Of specific interest in recent literature is using MIECs as protective coatings and binders for Li-ion battery electrodes. − PVDF, the most widely used polymer binder, suffers from low electronic and Li-ion conductivities. This increases electrode impedance by interfering with charge transport during battery operation. − It stands to reason that utilizing a polymeric MIEC would lower this impedance and improve battery performance.…”

A Smart Polymeric Sol‐Binder for Building Healthy Active‐Material Microenvironment in High‐Energy‐Density Electrodes

“…With the help of the significance of ME@AM and its strong ties to the binder system, we can comprehensively summarize the past efforts, the challenges, and future opportunities for the studies of binders. In general, the past efforts on binder research can be divided into three categories: improvement of adhesion or component binding capability, [18][19][20][21][22][23][24][25] functionalization for ion/electron conduction or volume change accommodation, [16,17,23,[26][27][28][29][30][31] and the rational design of binder systems for electrode processing innovation as well as microstructure management. [2,[32][33][34][35] Specifically, to improve the adhesion property that is critical for stabilizing the structures/interfaces of ME@AM, lots of efforts can be found on replacing poly(vinylidene fluoride) (PVDF) by other polymers with abundant polar functional groups (e.g., COOH, CN, OH, etc.).…”