Poly(ethylene oxide) electrolytes with tailored nitrile end-groups

Lithium salt-doped poly(ethylene oxide) (PEO) and PEO-derivatives have been widely investigated to develop polymer electrolytes for lithium-ion batteries. Nevertheless, low ionic conductivity, poor mechanical strength, and limited electrochemical stability of PEO electrolytes limited their practical uses in viable applications. Herein, we studied advanced PEO electrolytes by introducing tailored nitrile terminal moieties. Spectroscopic analyses and molecular dynamics simulations showed that nitrile end-groups changed Li⁺ solvation structures by forming –C≡N···Li⁺···(EO)_n interactions. This lowered coordination numbers and developed partially opened shell structures, enabling fast and selective Li⁺ relaxation dynamics. By doubling the local number density of terminal nitrile moieties, the effects of CN-driven interchain interactions increased, resulting in more than 2-fold increased lithium transference number and radically improved electrochemical stability window. To enhance mechanical strength in addition to improved electrochemical properties, nitrile-ended PEO electrolytes were blended with polystyrene-b-poly(ethylene oxide) (SEO) block copolymers. Intriguingly, even if the number density of nitrile end-groups was the same, nitrile termini at α- and ω-positions of PEO improved the blend electrolyte properties more effectively, compared to α, α-dinitrile PEO-blended counterparts.