Thermal history effects on the ionic conductivity of PEO-salt electrolytes

The ionic conductivity of polyethylene oxide (PEO)-based electrolytes has been known to be greatly enhanced by filling with nano ceramic particles, though it is strongly dependent on thermal treatment schedule and decreases slowly for long aging. The unstableness of the conductivity was attributed to the change of crystallization kinetics caused by some kind of interactions among polymer chains and incorporated ceramic particles. In this paper, ceramic-free PEOnLiClO4 polymer electrolytes were studied by measuring ionic conductivity and optical micrographs. We show that the unstableness of the ionic conductivity is inherent in the semicrystalline state of ceramic-free PEO-salt electrolytes and the conductivity below melting temperature can be variant by almost two orders of magnitude according to thermal treatment and storage time, although it may be changed by introducing ceramic particles. The melting temperature Tm increases with storage time due to the stiffening of spherulites, and the increment of Tm is more pronounced for higher salt ratio. All these are primarily concerned with the two-phase nature and the slow re-crystallization kinetics of PEO.