  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) exhibited 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
resulted in higher and more reproducible conductivity values, which were
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.
