Ion Conducting Conduits with a Tortuosity of One from One Electrode to Another: The Role of Domain Connectivity and Tortuosity on Ion Conductivity in Block Copolymer Electrolyte Thin Films

Abstract

Block copolymer electrolytes (BCEs) are attractive candidates for electrochemical devices due to their ability to micro-phase segregate into two chemically distinct domains – one ion conducting and one ion insulating and mechanically resilient. BCEs can be simultaneously optimized for both ion conductivity and mechanical rigidity, variables inversely correlated in traditional polymer electrolytes. Recent reports correlate anisotropic orientation of the domains relative to the electrode to increase ionic conductivity, however it can be difficult to probe structure characteristics such as interconnectivity and tortuosity and the role of defects, like grain boundaries, on ion conductivity in bulk membrane materials1–3. This contribution disseminates a powerful, analytical platform for probing the relationship between BCE microphase separation and ion conductivity with unprecedented resolution via directed self-assembly (DSA) of BCE thin films into interdigitated electrode architectures. In this approach, we aligned the poly(styrene-b-2-vinylpyridine) (PSbP2VP) block copolymer to different degrees of orientation between two in-plane electrodes using topographic electrode guiding walls – a process known as graphoepitaxy. Using different wall designs, we oriented the block copolymer domains into pathways with varying transport lengths, tortuosities, and interconnectivities4. Following orientation, we modified the P2VP domain via a Menshutkin reaction to form an anion conducting domain5. The reaction is used to convert the 2-vinyl pyridine to n-methyl-2-vinylpyridinium iodide. The process is shown in Figure 1 along with a micrograph showing orthogonally oriented BCE domains relative to the electrode walls. The orientation and structures of the BCE were characterized using routine surface metrology tools such as SEM and AFM. Visual python code quantified the micrographs by extracting factors such as tortuosity and structure factors and correlated them to the ion conductivity performance. Structure variables such as ion conduction length, orientation to electrodes, tortuosity, and interconnectivity were characterized using electrochemical impedance spectroscopy (EIS). From the EIS, we inferred mechanisms of ion migration that contribute to the complex circuit behaviors of micro-phase segregated BCEs. We also report unexpected behaviors for interconnected and anisotropic domains that suggest further enhancements in solid state electrolyte systems and warrant future studies. Controlling the BCE architecture in thin film BCEs is envisioned as an attractive, unprecedented platform to understand the role structure plays in ion transport. Establishing accurate structure-property relationships will play a prominent role in the rationale design of BCE membrane separators for fuel cells, flow batteries, and electrolyzers with high ion conductivity without sacrificing mechanical integrity.