transistors. [5] Polymer-based MIECs contain two principal components, one for electronic conduction and the other for ionic conduction. Each of these components should meet the molecular packing requirements for charge transport and both components should arrange in cocontinuous morphologies with percolation pathways for charge transport in three dimensions. Yet, achieving such structures relevant for MIECs has not been straightforward. At present, several different strategies are used to design and fabricate MIECs. They are: (1) mixing of conjugated polymers and polyelectrolytes [e.g., poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)], [5,6,8,9] (2) imbibing ionic fluids in conjugated polymer films, [11] (3) incorporating ion conducting side chains on conjugated polymers, [12][13][14][15][16] (4) ion-doped [17][18][19][20] or oxidized [21] conjugated polymers, and (5) diblock copoly mer with ion conducting and electronic conducting polymers as the constituent blocks [e.g., poly(3-hexylthiophene)-bpoly(ethylene oxide) (P3HT-b-PEO)]. [2,3] However, these methods do not afford separate and systematic control over the morphology and properties of the ionic and electronic conducting phases while keeping the overall morphology constant. The central problem is the lack of a general approach to control the molecular packing and morphology in MIECs. We report here an approach, based on polymer nanoparticle selfassembly, to fabricate polymer-based MIECs. This approach provides a pathway to achieve tailored molecular order for each component on the nanoscale (<100 nm) and targeted assembly of the components on the mesoscale (>100 nm-100 µm). We demonstrate the efficacy of this approach for fabricating MIECs through binary nanoparticle assemblies of electronically conducting and Li-ion conducting polymer nanoparticles (see Figure 1a).Nanoparticle self-assembly is emerging as a powerful tool to design and fabricate mesoscale assemblies and interfaces. [22][23][24][25] For polymer-based materials, they provide a unique pathway to generate complex structures and co-assemblies under nonequilibrium processing conditions with macroscopic homogeneity and compositional flexibility. In this strategy, we first fabricate polymer components as nanoparticles, and then use them as building blocks for further assembly to create Polymer-based mixed ionic-electronic conductors (MIECs) are desired for both bulk and interfacial materials in next-generation energy storage and electronic devices. Polymer-based MIECs contain two principal components, one for electronic conduction and the other for ionic conduction. The central problem is the lack of a general approach to control the molecular packing and morphology of the constituent components that will afford the ability to easily tune transport properties. This study demonstrates the efficacy of a modular method based on polymer nanoparticle self-assembly to achieve MIECs with tunable conductivity. This work uses poly(3-hexylthiophene) nanoparticles as the electronic conductor and li...