Mixed Ion–Electron-Conducting Polymer Complexes as High-Rate Battery Binders

Next-generation battery binders must have both the adhesive properties of traditional binders [i.e., polyvinylidene fluoride (PVDF)] as well as high electronic and lithium ion conductivity to facilitate good rate performance. Here, we demonstrate a broadly applicable strategy of using electrostatic compatibilization between a conjugated and a nonconjugated polyelectrolyte, which provides intrinsic ionic and electronic conduction while also maintaining stability with conventional battery materials, resulting in high rate capability in LiFePO4 (LFP)-based cathodes. Three polythiophene-based polyelectrolyte complexes demonstrate electronic conductivities as high as 0.8 S/cm and room temperature ionic conductivities above 10–4 S/cm when swollen with an electrolyte (≈1 × 10–7 S/cm dry, without the presence of small-molecule electrolyte). Each complex proves to be an excellent binder when applied in LFP composite cathodes, enabling dramatic reductions in overpotential and improved performance at high cycling rates compared with cells utilizing a PVDF binder. The improved charge transport afforded by the conducting binders enables up to 70% of the cathode's capacity to be utilized at 6C, compared to only 1.4% when PVDF is the binder. These results demonstrate that complexation of conjugated polyelectrolytes not only is an effective design strategy for intrinsic mixed ion–electron conduction but also provides the stability and processability necessary for Li-ion battery binder applications, making them promising "pick and place" materials to achieve large performance improvements in LFP cathodes.