Enhancing Ion‐Electron Transport in Positive Electrode of Solid‐State Lithium Metal Batteries With Multifunctional Catholyte Made of Polymer Mixed Ionic‐Electronic Conductor PEDOT:PSSTFSI and Li 3 InCl 6

ABSTRACT The development of positive electrode composites (PECs) with improved ionic and electronic conductivity is critical for high‐performance all‐solid‐state batteries (ASSBs). Conventional strategies rely on tuning the ratio of cathode active material (CAM) to solid electrolyte (SE) or introducing electronic conductive additives, yet both often induce interfacial instabilities limiting long‐term performance. Here, mixed ionic‐electronic conductor, poly(3,4‐ethylenedioxythiophene): poly(4‐styrenesulfonyl(trifluoromethylsulfonyl)imide) (PEDOT:PSSTFSI), is incorporated as a functional polymer (FP) binder in Li 3 InCl 6 (LIC), forming multifunctional catholytes in a single step via one‐pot aqueous synthesis. Owing to π–π stacking and the TFSI − functional group, PEDOT:PSSTFSI enhances electronic and ionic conductivity within the catholyte. Compared to the polymer‐free NMC‐based PEC, the incorporation of 5 wt.% FP markedly increases electrode compactness, reducing the porosity by 41.8% and yielding a twelvefold increase in effective electronic conductivity. In parallel, ionic conductivity improves substantially—up to 15‐fold, with the highest reached at 0.6 wt.% of FP in the PEC. When tested in ASSB cells, PECs containing FP show enhanced rate capability, especially at 1 C rate with full reversibility, and deliver up to 30% higher discharge capacity at a C/20 rate compared to polymer‐free NMC‐LIC references. This study highlights mixed conducting binders as a versatile system to improve conducting pathways while enhancing electrode integrity.