A composition for solid polymer electrolyte which comprises a polyether polymer
and an electrolyte salt compound soluble in the polyether polymer; wherein said
polyether polymer is either (1) a polyether polymer having a toluene insoluble
content of not larger than 5 weight %, or (2) a polyether polymer having a Mooney
viscosity of 3 to 190, which has been prepared by applying a shearing force under
heated conditions using an extruder to a polyether polymer having a Mooney viscosity
of 10 to 200 so that the Mooney viscosity is reduced by 5 or more. This composition
exhibits good film-forming property, and gives a solid polymer electrolyte exhibiting
good and uniform ionic conductivity, and having high mechanical strength and surface
smoothness. The solid polymer electrolyte is suitable for batteries and capacitors,
especially for a lithium ion secondary battery.
