Increasing ionic conductivity of polymer–sodium salt complex by addition of a non-ionic plastic crystal

Abstract

Ionic conductivity and other physico-chemical properties of a soft matter composite electrolyte comprising of a polymer–sodium salt complex and a non-ionic plastic crystal are discussed here. The electrolyte under discussion comprises of polyethyleneoxide (PEO)–sodium triflate (NaCF₃SO₃) and succinonitrile (SN). Addition of SN to PEO–NaCF₃SO₃ resulted in significant enhancement in ionic conductivity. At 50% SN concentration (with respect to weight of polymer), the polymer–plastic composite electrolyte room temperature (= 25 °C) ionic conductivity was ∼1.1 × 10⁻⁴ Ω⁻¹ cm⁻¹, approximately 45 times higher than PEO–NaCF₃SO₃. Observations from ac-impedance spectroscopy along with X-ray diffraction, differential scanning calorimetry and Fourier transform infrared spectroscopy strongly suggest the enhancement in the composite is ionic conductivity due to enhanced ion mobility via decrease in crystallinity of PEO. The free standing composite polymer–plastic electrolytes were more compliable than PEO–NaCF₃SO₃ thus exhibiting no detrimental effects of succinonitrile addition on the mechanical stability of PEO–NaCF₃SO₃. We propose that the exploratory PEO–NaCF₃SO₃–SN system discussed here will eventually be developed as a prototype electrolyte for sodium–sulfur batteries capable of operating at ambient and sub-ambient conditions.