Surface Reconstruction Limited Conductivity in Block‐Copolymer Li Battery Electrolytes

• Published in: Advanced functional materials Vol. 29; no. 48
• Main Authors: Sutton, Preston, Bennington, Peter, Patel, Shrayesh N., Stefik, Morgan, Wiesner, Ulrich B., Nealey, Paul F., Steiner, Ullrich, Gunkel, Ilja
• Format: Journal Article
• Language: English
• Published: Hoboken Wiley Subscription Services, Inc 01.11.2019
• Subjects: Block copolymers; block‐copolymer electrolytes; Electrochemical impedance spectroscopy; Electrodes; electrode–electrolyte interface; Electrolytes; Ethylene oxide; Flux density; Ion currents; ionic conductivity; Isoprene; Lithium; Lithium batteries; Lithium-ion batteries; Low molecular weights; Materials science; Mechanical properties; Molten salt electrolytes; Polyethylene oxide; Product safety; Reconstruction; Solid electrolytes; Stainless steels; surface reconstruction
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.201905977

Solid polymer electrolytes for lithium batteries promise improvements in safety and energy density if their conductivity can be increased. Nanostructured block‐copolymer electrolytes specifically have the potential to provide both good ionic conductivity and good mechanical properties. This study shows that the previously neglected nanoscale composition of the polymer electrolyte close to the electrode surface has an important effect on impedance measurements, despite its negligible extent compared to the bulk electrolyte. Using standard stainless steel blocking electrodes, the impedance of lithium salt‐doped poly(isoprene‐b‐styrene‐b‐ethylene oxide) (ISO) exhibits a marked decrease upon thermal processing of the electrolyte. In contrast, covering the electrode surface with a low molecular weight poly(ethylene oxide) (PEO) brush results in higher and more reproducible conductivity values, which are insensitive to the thermal history of the device. A qualitative model of this effect is based on the hypothesis that ISO surface reconstruction at the different electrode surfaces leads to a change in the electrostatic double layer, affecting electrochemical impedance spectroscopy measurements. As a main result, PEO‐brush modification of electrode surfaces is beneficial for the robust electrolyte performance of PEO‐containing block‐copolymers and may be crucial for their accurate characterization and use in Li‐ion batteries. Block‐copolymer electrolytes have the potential to advance lithium battery technology if their ionic conductivity can be increased. This work uncovers that copolymer reconstruction at the electrode surface significantly reduces conductivity. It further demonstrates that conductivity drops can be prevented by specifically engineering the electrode surface, highlighting the importance of interfacial engineering in nanostructured polymer electrolytes.