Investigating the effect of salt on polymer dynamics in block copolymer electrolytes through neutron spin-echo spectroscopy

Block copolymers have been studied for use in lithium metal solid-state batteries due to their ability to decouple ion transport and mechanical properties. While it is well known that the salt preferentially segregates into the conducting block, and that the motion of salt molecules is coupled to that of the polymer segments that solvate the ions, many questions about the nature of this coupling remain unanswered. Neutron spin-echo spectroscopy experiments were used to elucidate the nature of this coupling; the sample used was a mixture of a protonated and deuterated polystyrene-b-poly(ethylene oxide) doped with a lithium salt. The isotopic labeling allowed for only the dynamics of the ion-containing block, poly(ethylene oxide), to be measured. We quantified the dynamics, corresponding to length scales of about 5 nm, by comparing the data to predictions based on the Rouse model at low times as well as the standard tube model at long times. The crossover time between these modes was determined as a function of salt concentration. The tube shrinks with added salt and the segments slow down due to this. There is a direct relationship between segmental dynamics measured on the monomer length scale and ion transport under an applied field on macroscopic length scales.